Citation
Activity evoked in the visual system of human, rhesus monkey, and cat by spatially patterned and non-patterned visual stimuli

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Title:
Activity evoked in the visual system of human, rhesus monkey, and cat by spatially patterned and non-patterned visual stimuli
Creator:
Doddington, Harold William, 1942-
Publication Date:
Language:
English
Physical Description:
xi, 132 leaves : ill. ; 29 cm.

Subjects

Subjects / Keywords:
Checkerboards ( jstor )
Electrodes ( jstor )
Ganglia ( jstor )
Humans ( jstor )
Monkeys ( jstor )
Optic nerve ( jstor )
Optics ( jstor )
Retina ( jstor )
Visual cortex ( jstor )
Waveforms ( jstor )
Cats ( mesh )
Dissertations, Academic -- physiology -- UF ( mesh )
Haplorhini ( mesh )
Photic Stimulation ( mesh )
Physiology Thesis Ph.D ( mesh )
Visual Fields ( mesh )
Genre:
bibliography ( marcgt )
non-fiction ( marcgt )

Notes

Thesis:
Thesis (Ph.D.)--University of Florida, 1972.
Bibliography:
Bibliography: leaves 127-130.
General Note:
Typescript.
General Note:
Vita.
Statement of Responsibility:
by Harold William Doddington.

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University of Florida
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University of Florida
Rights Management:
Copyright [name of dissertation author]. Permission granted to the University of Florida to digitize, archive and distribute this item for non-profit research and educational purposes. Any reuse of this item in excess of fair use or other copyright exemptions requires permission of the copyright holder.
Resource Identifier:
023787256 ( ALEPH )
25427099 ( OCLC )
AEK3759 ( NOTIS )

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Full Text















ACTIVITY EVOKED IN THE VISUAL SYSTEM OF HUMAN,
RHESUS MONKEY, AND CAT BY SPATIALLY
PATTERNED AND NON-PATTERNED VISUAL STIMULI







By




HAROLD WILLIAM DODDINGTON


A DISSERTATION PRESENTED TO THE GRADUATE COUNCIL OF
THE UNIVERSITY OF FLORIDA
IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE
DEGREE OF DOCTOR OF PHILOSOPHY







UNIVERSITY OF FLORIDA
1972




ACTIVITY EVOKED IN THE VISUAL SYSTB1 OF HUMAN,
RHESUS MONKEY, AND CAT BY SPATIALLY
PATTERNED AND NON-PATTERNED VISUAL STIMULI
By
HAROLD WILLIAM DODDINGTON
A DISSERTATION PRESENTED TO THE GRADUATE COUNCIL OF
THE UNIVERSITY OF FLORIDA
IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE
DEGREE OF DOCTOR OF PHILOSOPHY
UNIVERSITY OF FLORIDA
1972


ACKNOWLEDGMENTS
V\
The author sincerely wishes to thank the members of his
supervisory committee, the faculty of the Physiology Department,
and his fellow graduate students for their encouragment and the
contributions which they have made to his education. The author
is especially appreciative of the guidance, assistance, and
provision of laboratory space by Dr. W. W. Dawson, his supervisory
committee chairman, and Dr. C. K. Adams for the development of the
research described in this dissertation. Most of all, the author
wishes to express to his wife his appreciation for her sharing of
the rewarding experiences and frustrations that have occurred
during this endeavor. This education and research was supported
by Training Grant GM 251 to the Physiology Department and Training
Grant MH 10321-06 to the Center for Neurobiological Sciences, both
from the National Institutes of Health.
11


TABLE OF CONTENTS
Page
ACKNOWLEDGMENTS ii
LIST OF FIGURES v
ABSTRACT ix
Chapter
I INTRODUCTION 1
Statement of the Problem 1
Review of Related Research 2
II EXPERIMENTAL METHODS AND PROCEDURES 15
Basic Considerations 15
Experiments with Behaving Animals
and Human Subjects 17
Experiments Using the Maxwellian View
Pattern Stimulator 23
Stimulator and Optics 23
Animals and Electrode
Implantation 29
Recording Sessions with Human
Subjects 31
Recording Sessions using Animals 32
III RESULTS 39
Human Subjects with the Behavioral Task 39
Monkeys with the Behavioral Task 43
Maxwellian View Stimulation of Humans 46
iii


Chapte Page
Maxwellian View Stimulation of Cats 51
Maxwellian View Stimulation of Monkey 81
Summary of Results 85
IV DISCUSSION 89
Human Subjects 89
Cat 89
Monkey 101
V CONCLUSIONS 103
Appendices
I BEHAVIORAL TASK TRAINING PROCEDURES
AND LOGIC CIRCUIT DESIGN 105
II USE OF DRUGS WITH PARALYZED ANIMALS 121
III RESPIRATION MONITOR AND ALARM CIRCUIT 123
BIBLIOGRAPHY 127
BIOGRAPHICAL SKETCH 131
IV


LIST OF FIGURES
Figure Page
1. Rhesus monkey performing the visual fixation task .... 21
2. Maxwellian view pattern stimulator 25
3. Schematic diagram of the Maxwellian view pattern stimulator
instrument 26
4. A test of the Maxwellian view pattern stimulator: checker
board patterns projected through a glass lens directly
onto photographic paper 27
5. A test of the Maxwellian view pattern stimulator: checker
board patterns projected through a glass lens onto a white
paper screen and photographed through the viewing channel
of the instrument 28
6. The ultimate test of the Maxwellian view stimulator:
checkerboard patterns projected onto the retina of a
paralyzed cat and photographed through the viewing channel
of the instrument 30
7. Photograph of an historical event: the experiment in which
cortical evoked responses sensitive to spatially patterned
visual stimuli were first recorded from cat 36
8. Human evoked responses to diffuse and checkerboard
patterned stimuli using the visual fixation task 40
9. Cummulative records of the performance of two human
subjects 42
10. Histogram of reaction times and cummulative record of
performance of a rhesus monkey 44
11. Evoked responses of rhesus monkey to diffuse and checker
board patterned stimuli using the visual fixation task. 45
12. Responses of humans to diffuse and patterned stimuli
presented by means of the Maxwellian view stimulator. . 48
v


Figure Page
13. Comparison of the responses evoked by checkerboard
patterned visual stimuli and a diffuse flash stimulus
which produces the same average retinal illumination. . 50
14. Repeatability of cat cortical evoked responses to diffuse
and patterned visual stimuli within one session 53
15. The effect of pattern feature size upon the cortical
evoked response of cat 1L3 55
16. Repeatability of cat cortical evoked responses over a
two-week period 56
17. Diffuse flash cortical evoked responses to a series of
stimulus intensities 57
18. The effect of pattern feature size upon the cortical
evoked responses of cat 1K9 59
19. The effect of defocus upon responses to a 31 minute of arc
per square checkerboard patterned stimulus 61
20. The effect of a positive 9 diopter defocus upon responses
to patterned stimuli with different feature sizes .... 63
21. Simultaneously recorded optic nerve and cortex responses
to diffuse and patterned stimuli 65
22. Repeatability of optic nerve and cortical evoked responses
from cat 1K9 within one session 67
23. The effect of pattern feature size upon the optic nerve
responses of cat H-9 71
24. Pattern sensitivity of optic nerve responses at several
combinations of adaptation level and flash intensity. . 73
25. Simultaneously recorded individual and average optic nerve
responses to diffuse and patterned stimuli 75
26. The effect of pattern feature size upon cortical evoked re
sponses of cat 1L1 76
27. The effect of pattern feature size upon cortical evoked
responses of cat 1L16 77
vi


Figure Page
28. The effect of pattern feature size upon optic nerve
responses of cat 1L16 78
29. The effect of pattern feature size upon optic nerve
responses of cat 1L6 80
30. Responses of a paralyzed rhesus monkey to spatially
patterned visual stimuli 83
31. Comparison of the evoked responses of rhesus monkey to an
11 minute pattern and to diffuse stimulation 84
32. Summary of the effects of pattern feature size upon the
visually evoked responses from the cortex of human,
rhesus monkey, and cat 87
33. Comparison of the spatial frequency range of the major
components of checkerboard patterns with modulation
transfer functions for the optics of the cat eye and the
human eye 91
34. Proportional amounts of defocus in terms of pattern
feature size 98
35. Behavioral training program number 1 110
36. Behavioral training program number 2 Ill
37. Fixation light control circuit for the visual fixation
task 113
38. Intertrial interval and reward control circuit for the
visual fixation task 114
39. Reward period timer circuit for the visual fixation
task 115
40. Early release and late release detection circuits for the
visual fixation task 116
41. Time-out timer and control circuit for the visual
fixation task 118
42. Stimulus flash control circuit for the visual fixation
task 119
vi 1


Figure Page
43. Reaction time ring counter circuit for the visual
fixation task 120
44. Schematic diagram of the respiration monitor and alarm
system 125
viii


Abstract of Dissertation Presented to the
Graduate Council of the University of Florida
in Partial Fulfillment of the Requirements for
the Degree of Doctor of Philosophy
ACTIVITY EVOKED IN THE VISUAL SYSTEM OF HUMAN,
RHESUS MONKEY, AND CAT BY SPATIALLY
PATTERNED AND NON-PATTERNED VISUAL STIMULI
By
Harold William Doddington
August, 1972
Chairman: Wm. W. Dawson, Ph.D.
Major Department: Physiology
Individual cells of visual cortex of cat and monkey have
been shown to be specifically responsive to the size, location, and
orientation of features in the visual field, while responses from
human scalp have been shown to be sensitive to spatially patterned
visual stimuli. To improve upon this loose relationship between
what is known about animal visual systems and the data from humans,
this research was designed to determine whether or not sensitivity
of evoked responses from several locations within the visual system
of animals (cat and rhesus monkey) to spatially patterned visual
stimuli could be observed and, if so, to describe it in terms of the
effect of pattern feature size, quality of focus, and repeatability.
Experiments were conducted with (1) attentive human, (2)
behaviorally trained monkey, and (3) cat and monkey paralyzed with
a neuromuscular blocking drug. Two methods were used. The behavioral
training program, and the Maxwellian view instrument for visual stimu
lation of human subjects and paralyzed animals are unique and are
described.
IX


Two monkeys were trained. A rhesus trained to the visual
fixation task well, and data from her indicated sensitivity of the
evoked response to checkerboard patterned stimuli. A sootie mangabey
did not train to the task well enough to be tested.
Data from human subjects demonstrated that the instruments
and procedures used were effective for producing spatially patterned
stimulation of the retina.
Cats with chronically implanted electrodes in the visual
system were paralyzed, stimulated by projecting onto the central
retinal area (1) an adaptation light with 30 degree diameter circu
lar field and (2) diffuse or spatially patterned flash stimuli cover
ing a 9 x 13.5 degree field, and their average responses recorded. On
occasion individual responses were examined. These were not acute
experiments. Recording sessions were 6 to 10 hours. Each animal
recovered and could undergo additional recording sessions days or
weeks later.
Results from paralyzed cats adapted to a dim photopic level
are that the evoked responses from optic nerve and visual cortex are
consistently sensitive to pattern stimulation. For equal energy
stimulation of the retina, the responses to checkerboard patterned
stimuli differ from those to non-pattemed (diffuse) stimuli primarily
in waveform, with small changes in overall amplitude. Response wave
forms changed progressively with pattern feature size. The optic
nerve response to 31, 55, and 79 minute patterns typically showed
increased positivity at latencies of 30 to 60 msec, which is the late
side of the primary positive peak. Difference waveforms (pattern
x


response diffuse response) were extracted. Using defocused patterned
stimulation, it was possible to obtain responses characteristic of
diffuse stimulation.
Responses from epidural cortex electrodes on a rhesus under
neuromuscular block demonstrate sensitivity to spatial pattern stimuli
more vividly than do responses from cats. This monkey responded very
well to a checkerboard pattern of size 11 minutes of arc per square.
Response waveform changes were sufficient to cause the difference
waveform to be larger in amplitude than the diffuse response. An
11 minute patterned stimulus defocused by 1.2 diopters produced a
diffuse response.
The major conclusion of this dissertation is that animals
can be used to study the sensitivity of evoked responses to spatially
patterned visual stimulation at several levels within the visual
system. Also, it was found that a given dioptric value of defocus
was less effective in degrading responses to patterns of larger
feature sizes than those of smaller feature sizes, which indicates
that feature size and/or quantity of contrast border are of greater
importance than contour sharpness in determining evoked activity.
xi


INTRODUCTION
Statement of the Problem
Human visual pattern evoked responses which are generated by
activity in the occipital cortex indicate that populations of cortical
neurons respond differently to patterns with features of different shape,
size, contrast, etc. It is unlikely that many single cell studies will
ever be done on humans. To develop an understanding of how evoked
responses which are sensitive to visual pattern information are generated,
and of how a cell relates to the populations of which it is a part, it
would be very desirable to determine whether or not an animal prepara
tion can be used as a model for studying pattern sensitive evoked
responses.
If such a model could be established, many experimental ques
tions which could not be asked with humans could be investigated.
Examples of these questions are: (1) From the receptive field struc
tures of retinal ganglion cells and geniculate cells, and the sensitivity
of ganglion cell responses to degree of focus of a retinal image, one
might expect evoked responses from these stages prior to cortex to be
modified by visual pattern information. Can such changes actually be
detected? (2) Are the "cortical sources" which generate certain features
of the evoked response localizable, or are they widely distributed
throughout visual cortex?
The biggest question in this research area is: can an animal
model for studying the pattern sensitivity of visual evoked responses
1


2
be established? This is the primary problem upon which the work of this
dissertation focuses. It is not surprising that there are no reports
of work on this problem by others. There are no simple approaches.
One must be able to prove that the visual stimulus results in a well
focused patterned retinal image. The animal must be restrained and
its muscle activity minimal at the time of stimulation, but the function
ing of its central nervous system must not be significantly degraded
by drugs.
The two preparations to be considered are (1) an animal trained
to a visual fixation task such that it can be stimulated properly and
drugs are not necessary for restraint, and (2) a paralyzed, unanesthe
tized animal that can be stimulated with a specially designed visual
pattern stimulator apparatus. For both of the above cases the instru
ments and procedures are designed so that a human subject can be tested
under the same conditions as the animals, with the major exception
that a neuromuscular blocking drug need not be administered to a human
subject.
Review of Related Research
Very little was known about the neural processing of visual
pattern information prior to about 1950, primarily because the tech
niques and instruments needed to record electrical activity from single
cells in the system or to record and extract low level evoked activity
from the on-going activity had not been available.
The first technique which provided focused stimuli to the retina
was published by Kuffler in 1953. His method for recording from single
ganglion cells of the "unopened" cat eye (cornea and lens not removed)


3
was to insert a hollow needle through the temporal side of the cat
eye and then to introduce a needle electrode through the hollow needle.
By not disturbing the normal optics of the eye, it was possible to
use a focused spot of light on a tangent screen to stimulate a par
ticular spot on the retina. With this technique, Kuffler studied the
receptive fields of individual retinal ganglion cells. He found these
receptive fields in light adapted animals to be approximately circular
areas which, with the stimulus spot placed in a central region, caused
the cell to respond with increased or decreased firing. Placing the
spot in a peripheral portion of the field caused a response opposite
to that of the central region. In other words, these receptive fields
have an antagonistic center-surround structure. Since this initial
work, a number of researchers have studied retinal cells. A good
review of this topic has been given by Witkovsky (1971).
There is an inverse relationship between ganglion cell density
and the size of the central regions of receptive fields (Rodieck and
Stone, 1965). Measurements of the diameters of central regions of
retinal ganglion cell receptive fields give a range of 0.5 to 8 degrees
of arc for cat, and 2 minutes to several degrees of arc for rhesus monkey.
The functional organization of these receptive fields is very dependent
on the adaptation state of the eye. Under dark adapted conditions,
the effect of a stimulus spot on the peripheral part of the recep
tive field is greatly weakened or absent, and the effective size of
the central region of the field is increased (Kuffler, 1953). .
By recording from single retinal ganglion cells of cat, it


4
has been shown that cells at this level are very sensitive to the quality
of focus of the retinal image (Hill and Ikeda, 1971; Ikeda and Wright,
1971). In the area centralis, slightly defocusing the image ( 2 diopters)
of the spot used to stimulate the central region of the receptive field
resulted in loss of responsiveness of the cell. The axons of retinal
ganglion cells form the optic nerve and proceed primarily to the
lateral geniculate nucleus (LGN). Single unit activity of cells of
the LGN has been recorded and analyzed in various ways. In cat and
monkey, these cells have been found to have receptive fields which
are in many ways similar to those of ganglion cells. The fields are
approximately circular, have antagonistic center and surround regions,
are not sensitive to direction of motion of a stimulus, etc. They
differ from those of ganglion cells in that the central region is, on
the average, smaller (Poggio et al., 1969). Hubei and Wiesel (1961)
have published evidence "that the surround mechanism has a considerably
stronger effect in opposing the activity evoked by stimulation of the
receptive-field center in the case of the geniculate neurons as com
pared with optic tract units."
Also, geniculate cells are the first in this system where
binocular interaction occurs and where general brain activity
governing excitability influences the processing of visual informa
tion (Angel et al., 1965; Pecci-Saevedra, 1965). Working in cat,
Poggio et a!. (1969) and Baker et al. (1969) investigated the time course
of excitatory response of geniculate cells. Poggio et al. found the size
of geniculate cell field centers to range from 0.3 to 4.5 degrees.


5
One might ask, do these cells with antagonistic, concentric
center-surround type fields respond if the whole field is illuminated
with a brief flash of light? The answer here is yes. Although there
is an antagonistic center-surround relationship, the response is not
completely negated. Both ganglion cells and geniculate cells do, in
general, show a change from their spontaneous rate of firing in this
case.
In addition to investigations of receptive field organization
for LGN cells, studies have been made to determine their responsive
ness to stimuli with definite spatial frequencies. Using moving
sinusoidal gratings displayed on a cathode-ray tube, cells in the cat
LGN were found to respond in the range 0.2 2.5 cycles/degree visual
angle. For the squirrel monkey, this range was 0.5-8 cycles/degree
(Campbell et al., 1969).
From the lateral geniculate nucleus, axons which are normally
referred to as the geniculo-striate fibers, pass via the optic radia
tions to visual cortex. Three distinct areas of visual cortex have
been identified anatomically and neurophysiologically. Broadmann's
area 17, the striate cortex, receives the majority of the geniculo-
cortical fibers. Areas 18 and 19 are the extrastriate cortex. They
receive afferent fibers from area 17, the LGN, and a variety of other
sources in both hemispheres.
In a series of papers, Hubei and Wiesel have described the
receptive fields and response characteristics of visual cortex cells
in areas 17, 18, and 19 of cat and monkey (Hubei and Wiesel, 1959;
1965; 1968). Their acute experiments were conducted with paralyzed,


6
lightly anesthetized animals, and they found it possible to classify
most of the cells which they recorded into one of three categories which
they defined; simple, complex, or hypercomplex. For complete defini
tions of these categories, it is best to refer to the original paper
(1965), but even the cells classified as simple show a more complicated
characteristic behavior than that of most ganglion cells and geniculate
cells. They found simple cells only in area 17. Area 18 contained
mostly complex cells according to their sampling, and in area 19
approximately 50 percent of the cells recorded were classified as
complex. Receptive fields of cells in the cortex are rarely circular,
usually elongated or rectangular. These cells are sensitive to shape,
size, orientation, intensity, contrast, motion, direction of motion,
and, to varying extents, location in the visual field of a light stimulus.
There is evidence that cortical cells in these areas are organized in
columns perpendicular to the cortical surface. Each column contains
cells which respond to the same location in the visual field, but
having slightly different specificities for orientation, shape, size,
etc. Such an arrangement would permit efficient interconnection of
simple cells to a complex cell, and complex cells to hypercomplex. Of
course, establishing a classification system with three categories
may well be artifical, in that there may actually be a continuum of
complexities of cell characteristics.
Since cells of the nervous system have in some cases been
known to behave differently under the influence of anesthetics and
other drugs than they do in alert, undrugged animals, it was logical
and necessary to attempt to verify the results from preparations of


7
the type Hubei and Wiesel used by designing an experiment to determine
the properties of cortical cells using alert, behaving, completely
undrugged animals. Wurtz (1969 a, b, c) carried out such experiments
with rhesus monkeys trained to a visual fixation task. His work includes
looking at the correlation between a cell's sensitivity to motion of
a stimulus and its rate of adaptation to a stationary stimulus and finds
that some units give a "vigorous and largely nonadapting response to
a stationary stimulus" of proper orientation and are not affected
by motion while other units adapt rapidly to a stationary stimulus
but are very sensitive to motion. With regard to the shape, etc.
of the receptive fields, he concludes that "the basic organization of
the simpler types of receptive fields of striate cortex neurons reported
for the paralyzed and anesthetized cat and monkey is found also in the
awake monkey."
The angular selectivity of single visual cortex cells of
paralyzed, anesthetized cats has been studied by Campbell et al. (1968)
using moving square wave gratings displayed on a cathode-ray tube.
Along with other criteria, the authors in general considered orienta
tion specific units to be cortical and orientation non-specific units
to be geniculo-cortical fibers. For cortical cells, they found that
"the distribution of preferred angles did not show any difference
between the oblique orientations and the vertical and horizontal
orientations."
The basic units from which the visual system is formed are the
cells. The form of this system, though its internal workings are still


8
fairly well concealed, can be seen at least in outline. On the average,
retinal receptor cells out-number retinal ganglion cells by at least
100 to 1 in human. The number of cells in lateral geniculate nucleus
is approximately equal to the number of optic nerve fibers which enter
the nucleus. And, the cells of striate cortex, let alone extrastriate
cortex, number at least one hundred times the cells of LGN. Thus, it
can be seen that there is considerable "convergence" and information
processing within the retina, some processing which is not very costly
in terms of the size of the system required at the level of LGN, and
a very large number of neurons in the organized structure of the
striate cortex.
Retinal ganglion cells are third order cells in the visual
afferent pathway which starts at the receptor cells. It is known that
receptors contain a photopigment in their outer segments; but the pro
cess by which absorption of the photons results in excitation of a
receptor, and eventually an active response of the second order cells,
bipolar and horizontal cells, is not understood. Apparently, the
meaningful activity of the first and second order cells of the retina
is all in terms of slow potentials because there is no significant
evidence of impluse generation by any retinal cells other than
ganglion cells.
Although the studies of the electrophysiological characteristics
of cells at the various levels of the primary visual pathway which
have been reviewed were all carried out by observing impulse responses,
activities other than impluses (frequency content 0 to less than 1000 Hz)
can be recorded at each level in the system. Neural impulses are


9
the coded form for transmission of information along axonal fibers
across distance, but transactions between cells require the exchange
of a chemical transmitter substance and their outcome is determined
by the weighted summation of post-synaptic potentials from all inputs
onto the cell which is receiving information, as seen at the axon
hillock. These timewise variations in cellular membrane potentials and
electric fields of volleys of activity in fiber tracts (slow compared
to the time course of single impulses) compose the slow potentials.
Slow potentials recorded in response to brief sensory or
electrical stimulation are commonly called evoked responses. This
term is not highly specific. For the purposes of this dissertation,
it shall refer primarily to transient electrical responses which have
fairly characteristic waveforms dependent upon the properties of the
stimulus, location recorded from, state of arousal of the animal, and,
to some extent, the individual animal.
Why are slow potentials or evoked responses of interest to
one who is studying the visual system? A brief look at the number of
channels into and out of the lateral geniculate (approximately 1
million) and the tremendous convergence and divergence present at
points, along with psychophysical phenomena, should convince one that
parallel processing of information is very important in this system.
In order to understand such a system, it is necessary to determine
what is the relationship of an individual unit to the local population,
and to the overall population. Slow potentials and evoked responses
should be valuable in this respect because they contain information
primarily related to a population, the size of which depends upon the


10
recording conditions. For at least the following reasons, models of
the visual system mechanism for processing spatial pattern information
should be based on more than data from microanatomical and single unit
studies. There are no techniques for simultaneously studying impulses
from more than two or three cells out of a very small functionally related
group of cells (Glaser, 1971). In addition, there is no guarantee
that all neurons in geniculate or cortex are capable of generating im
pulses, and single units are usually selected for study because accept
able impulses can be recorded from them. Also, single unit studies
are admitted to be biased against the smaller cells in a population.
What work has been done relative to evoked responses to
diffuse and patterned visual stimuli and their relationship to anatomy
and single unit activity?
Because slow potential activity recorded in response to a
sensory stimulus on an EEG-type record is the combination of on-going
background activity and stimulus related activity, and because the
background activity is often the larger of the two, it was very difficult
to study the stimulus related activity until Dawson (1954) published
his photographic superposition technique for extracting this stimulus
related activity. By 1961, a special purpose averaging computer had
been developed to perform this extraction process (Clark, 1961).
Given tools to work with, researchers began to look at sensory evoked
responses.
The first paper concerned with both diffuse and patterned stimuli
came in 1965 (Spehlmann). In 1967, a paper by Rietveld et al. addressed
this topic in greater detail. Recording differentially from an electrode


11
on the scalp 1% cm above the inion and on the midline with reference
to an electrode on an earlobe, they compared the responses of 8 dark
adapted human subjects to diffuse and checkerboard patterned light
flashes of 2 microsecond duration. This study includes experiments
to determine the effect of the size of squares in the black and
white checkerboard pattern, the effect of flash intensity, the contribu
tion of central and peripheral retina to the pattern response, and the
comparison of responses to checkerboard patterns with those to line
gratings and diamond patterns. The results show that for most human
subjects checkerboard patterns containing squares of 10 to 60 minutes
of arc (visual angle) width evoke responses that are larger in ampli
tude and noticeably different in waveform than responses to diffuse
flashes from which the same light flux enters the eye. Checkerboard
pattern responses fall into a continuum with respect to pattern size,
with the largest, most distinct pattern responses being generated by
patterns with approximately 12 to 15 minute squares while responses
to patterns with larger or smaller features are smaller in amplitude
and their waveforms become indistinguishable from diffuse flash
responses as pattern features reach extreme sizes. Responses to patterns
of diamonds are approximately the same as to checkerboards. Line
gratings evoke responses which are different in waveshape than, and
intermediate in amplitude to those for diffuse or checkerboard patterns.
For patterned stimuli the response amplitude is proportional to the
contrast of the pattern presented. A pattern of 20 minute squares
produces much larger responses when presented to the fovea than when
presented to the peripheral retina. There is no data on this point


12
for other partem sizes.
Another report, Harter and White (1968), describes research
which utilized diffuse and checkerboard patterned flashes to look
at the effect of defocusing the patterned flashes upon the evoked
responses of human subjects. The findings of this experiment were
that defocusing by 3 diopters effectively made the responses to
a pattern of 12 minute squares indistinguishable from responses to
a diffuse flash, but that a larger amount of defocusing was necessary
to degrade responses to patterns of larger squares by the same amount.
A study of the binocular addition of visual responses
evoked by dichoptic presentation of diffuse and patterned stimuli has
been done by Ciganek (1971). It was found that the response to stimu
lation of one eye with a white field or a large (4.5 degrees) checker
board pattern was suppressed by the simultaneous stimulation of the
other eye with a small (1 degree) checkerboard pattern.
Very recent publications by Jeffreys (1968, 1970) discuss
responses he has obtained from scalp electrodes on human subjects
by stimulating specific regions of the visual field with checkerboard
patterned stimuli, and what he thinks these responses indicate
about the locations of the source regions within the cortex which
generate the activity seen as the significant peaks within the first
200 milliseconds of the response. Of course, it is not known how
concentrated or diffuse these "source regions" might be.
To obtain evidence for spatially selective and orientationally
selective "channels" in the human visual system, Campbell and Maffei
(1970) have recorded a steady-state driven evoked response to


13
sinusoidal grating patterns which undergo 180 degrees phase reversal
at a rate of 8 times per second. They found that the slope for the
graphical curve of evoked response amplitude versus the log of the
pattern contrast could be significantly increased by simultaneously
presenting more than one spatial frequency or orientation while
maintaining the same total light from the stimulus. These authors
found that spatial frequencies for which "channels" of human central
vision are sensitive are all above 3 cycles/degree of visual angle.
So, visual evoked potentials from human subjects are sensitive
to type of visual pattern presented, size of features, orientation,
contrast, and what region of the retina is stimulated. They are also
effected by whether or not the subject is performing a task which
directs attention toward or away from the visual stimulus (Kopell
et al., 1969).
To determine the relationship of evoked responses to the
anatomy and to single unit activity, animal research has been done,
but it has all been done with diffuse flash stimuli. Fox and O'Brien
(1965) demonstrated that the curve of probability of firing for a single
cortical cell can duplicate rather well the evoked response which can
be recorded from the same electrode after death of the cell.
Creutzfeldt et al. (1969) published a study of simultaneously recorded
surface evoked potentials and intracellular potentials from area 17
of cat in response to flash stimulation. Their findings and discussion
of the relationships of cortical cell activity to evoked potentials
are interesting, and, hopefully, it will someday be possible to do
similar experiments with both pattern and diffuse flash stimuli.


14
Evoked potentials have been recorded from the scalp and cortex of
unanesthetized monkeys (Hughes, 1964; Spinelli, 1967; Vaughan and
Gross, 1969). In none of these experiments were the monkeys trained
to a task. They were simply placed in a primate chair and faced
toward the flashing stimulus. Hughes, and Vaughan and Gross were
interested in the effects of visual system lesions. Spinelli had
used simple patterned stimuli, but showed no significant effect.
This review of literature, although it has been selective,
hopefully does help to point out the situation at this time. Research
to determine what spatial features in the visual field cells at
different levels in the visual system respond to has been done in
animal experiments by single unit recording of impulses. Other investi
gations, which look at the responses of populations of neurons to
patterned visual stimuli, have been done only with human subjects.


EXPERIMENTAL METHODS AND PROCEDURES
Basic Considerations
A primary requisite to studying responses of an animal or
human subject to spatially patterned visual stimuli is a means of
producing a retinal image of as high a quality as the optics of the
given eye will permit. In general, there are two possible approaches
to satisfying this condition which do not constrain the possible stimuli
to particular geometric forms (such as interference patterns) or
colors. The first possible approach is to obtain the cooperation of
the animal (or human subject) in fixating and focusing upon a pattern
within the visual field. The second approach is to maintain the eye
in a fixed position and project the desired pattern directly onto the
retina. The two groups of experiments described in this work relate
to these two approaches.
A corequisite of the need to produce a well focused retinal
image is that of acquiring an excellent quality original or master
copy of the pattern to be used. With human subjects, the most effec
tive patterned stimulus yet described, in terms of response amplitude
and extent of waveform change, is a checkerboard pattern composed of
squares having a side length of 10 to 15 minutes of arc of visual
angle. To produce a master checkerboard pattern, a two layer paper
assembly, one black sheet adhered on top of a white sheet with wax,
was lightly criss-crossed with a scalpel blade and every other black
15


16
square lifted off. From this master pattern several series of photo
graphic transparencies were produced, each transparency in a series
containing a different number of squares.
To produce stimulation of a human subject or animal, a flash
of light was projected through one of these transparencies in an
appropriate projection system. Since half of the squares are trans
parent and half of them are approximately opaque black, the light
transmitted by such a pattern is equal to ((light of the flash surface
reflection) x h (loss due to the optical density of the film base)).
The optical density of the "opaque" features in these patterns is
slightly greater than 3.0 log units.
To produce nonpattemed stimulation which delivers the same
quantity of light to the retina, a "diffuse" pattern which is a piece
of clear film base plus a 0.3 log unit neutral density gelatin filter
has been assembled. By use of diffuse and patterned stimuli which
provided equal energy to the retina, it was expected that responses
to stimulation with a diffuse pattern, a pattern of features too small
to be resolved by the subject, or a completely defocused pattern
should be the same.
An additional constraint imposed by the author upon the design
of these experiments was that insofar as practical the apparatus to
deliver the stimulus flashes to animals was designed to work equally
well with human subjects. This allowed (1) recording of responses
from human subjects which can be compared with published data, and (2)
recording of animal responses which can, to some extent, be compared


17
to human responses.
Experiments with Behaving Animals and Human Subjects
When one wishes to study activity of the nervous system,
experiments with behaving animals have some distinct advantages over
other types of experimental procedures. Two of these are that (1)
the experiments can be completely free from the effects of drugs and
(2) that a large amount of data.can be collected from each animal.
The disadvantage of such experiments is the relatively large invest
ment which must be made for each animal and the resulting limitation
on the number of animals which can be studied.
Because of the similarity of its retina and the remainder of
its visual system to that of human, a rhesus monkey (Macaca mulatta)
was chosen for this experiment. In addition, since the human evoked
response indicates that patterned stimulation is most effective when
presented to the macular region of the retina, and this is the region
of highest cone density, it is possible that a primate having a higher
cone density throughout the retina than either human or rhesus would
show an exaggerated example of sensitivity to patterned stimulation.
Therefore, a behaving sootie mangabey (Cercocebus torquatus) was
chosen as a second animal to be studied with this procedure (Kolmer,
1930). The animals used were (1) a young adult female rhesus and
(2) a young adult female sootie mangabey. Neither had previously
been in a primate chair.
The rhesus had never been behaviorally trained. She was
6 years old and weighed 13 lb 14 oz (6.3 Kg) before training started.


18
Ophthalmoscopic examination, measurement of intraocular pressures,
and retinos copy showed her eyes to be normal with less than ^
diopter spherical myopic error each. The mangabey was approximately
6 years old and weighed 13 lb 0 oz (5.9 Kg). She had previously
been trained to a behavioral task. An eye examination, as above,
showed her eyes to be normal except for 2 diopters spherical myopic
error in each.
These animals were kept in individual cages and, by the method
of Glassman et al., 1969, were transferred to a primate chair daily
for training or recording sessions. Free food was supplied in the
cages, but water was provided only while an animal was in the chair.
At the beginning of every training session, the animal was weighed.
To obtain visual evoked response data from these animals,
a visual fixation task (Wurtz, 1969a) was designed. For training and
testing the animals, a programmable logic system to control the behav
ioral task and an acoustic chamber (IAC model AC-5) with a ground
glass projection screen window in one wall were available. A ceiling
light illuminated the inside of the chamber. The luminance of the
white chamber wall around the projection screen was 5 to 10 candela/square
meter and that of the screen itself was 1.1 candela/square meter.
Two 35 mm slide projectors with solenoid operated shutters projected
through a light-tight tunnel onto the projection screen. A cummulative
recorder and a bank of decimal counters were available to monitor
performance. Using these facilities, the final task was designed and
then the several stages of training necessary to train the animal up
to the final program were created (see Appendix I).


19
The specific purpose of the final task was to obtain the
cooperation of the animal in sitting quietly, fixating and focusing
upon a small spot of light in the center of the projection screen
for a period during each trial when a stimulus flash might or
might not be presented. The task devised required the animal to
operate a single manipulandum with its hand. The animal received
two types of cues, audible and visual, and was rewarded for a correct
trial by a sip of water. For a correct trial the sequence of events
was as follows: When no audible or visual cues were being given, the
animal was allowed to pull and hold the manipulandum lever. Upon
pulling the lever, a small spot of light (the fixation light) was
projected on the center of the screen and remained there for a period
controlled by a variable interval timing system. If the animal held
the manipulandum and then released it within a short time after the
offset of the fixation light (the reward period), then a tone which
signified a correct response and indicated the duration of the intertrial
interval was turned on and after a brief delay the sip of water was
delivered. When the intertrial interval ended, after 10 or 15 seconds,
the animal was permitted to start a new trial. Within such a trial
a stimulus flash which covered the whole projection window was some
times delivered. The flash was semirandomly positioned within the
fixation light period. A stimulus flash of 600 msec duration was
used because the system was not able to produce very brief flashes.
The response recorded was that to the onset of the flash (Harter, 1971).
Use of a limited hold procedure (Moody, 1970) assured that the animal
must watch the fixation light very carefully in order to consistently
complete trials correctly.


20
As a penalty for an incorrect response, a time-out accompanied
by a warbling audible error signal was imposed. The basic time-out
period was of fixed duration, but could be restarted during the time
out period if the animal operated the manipulandum. An incorrect
trial occurred when the animal released the manipulandum before the
offset of the fixation light or too long afterwards, or pulled the
manipulandum during the intertrial interval.
Normal durations for the time periods in the task were:
fixation light period was variable with a uniform probability density
distribution over the range 1% seconds to 8 seconds, reward period
was 500 milliseconds, intertrial interval was 10 seconds, and a time
out was approximately 30 seconds.
To prevent any possibility that sounds from the system out
side the acoustic chamber might be used by the animal as cues, audio
white noise up to 50 KHz was produced at a moderate sound level
inside the chamber.
The rhesus monkey was trained from adapting to the primate
chair to performing the final task well in a period of 5 months,
training 6 days per week. Figure 1 shows this monkey while she was
performing the task. The animal was always provided a measured amount
of water on days when training sessions were not conducted. Using the
same procedures, the mangabey required more than one year to train.
For recording, a small region of the scalp over striate cortex
just anterior to the nuchal ridge and approximately 2 cm from the mid
line was locally anesthetized with xylocaine and a wound clip with
a recording lead was attached to the scalp at this location, which


21
Figure 1. Rhesus monkey performing the visual fixation task. The
monkey is seated in a primate chair, viewing the projection screen
window in the chamber wall, and operating the manipulandum with
one hand. Her mouth is not far iron the drinking tube through
which sips of water were issued as rewards for correct completion
of trials.


22
is within 2 degrees of the cortical representation of the fovea.
This served as the active electrode. For differential recording,
the indifferent electrodes were a pair of ear clips tied together
through equal resistances. These electrodes were worn by the animal
for several training sessions before an actual attempt to record
responses was made.
During the programming and testing of the behavioral task,
several humans performed the task under the same conditions which
the animals were later to encounter except that the water reward
was not issued. When development of the system was complete, responses
to diffuse and patterned visual stimuli were recorded from several
subjects. For differential recording, the active electrode was placed
on the scalp using electrode paste at 1.5 cm above the inion on the
midline and an ear clip was used as the indifferent electrode.
The recording equipment used included a special low-noise
differential input preamplifier (similar to Schuler et al., 1966)
with a fixed gain of 80, followed by another amplifier with adjustable
gain and usually set to 150. Both amplifiers were designed and constructed
by the author, and had a bandwidth of 0.1 Hz to greater than 30 KHz,
3 db. The amplified signal was FM tape recorded (0.1 to 1000 Hz) and
a sync pulse was recorded on the same tape on a direct record channel.
Recordings were played into a Fabritek model 1052 averaging computer.
Individual responses could also be examined by displaying them on the
computer. Averaged and individual responses were printed from the
computer by a strip-chart recorder.


23
Experiments Using the Maxwellian View Pattern Stimulator
Stimulator and Optics
To study the evokec ^.ponses of paralyzed animals to spatially
patterned visual stimulation, one could refract an animal and then
present the patterned visual stimulus on a tangent screen in front of
the animal. A superior technique, however, is to refract the animal,
then project the spatially patterned visual stimulus directly on the
retina, and simultaneously view the retinal image and focus it for
maximum sharpness. This is the technique which has been developed
and utilized in this dissertation. It permits the experimenter to
know exactly what region of the retina is being stimulated and be
confident that in each experiment the image of the stimulus pattern
is well focused upon the retina. Since the visual pattern stimulator
apparatus which has been used is unique, it is described here. The
instrument is basically a three optical channel instrument. It consists
of one ophthalmoscopic viewing channel, one Maxwellian view adaptation
light channel, and one Maxwellian view patterned stimulus channel.
The pattern stimulating channel is based upon a design described by
Westheimer (1966). This channel is combined with the other two channels
using a beam splitter in front of the eye to be stimulated. This
particular pattern stimulating channel has the distinct advantage that
the pattern can be continuously focused through a 5 diopter range
without changing either the overall size of the retinal region being
stimulated or the size of any feature in the pattern in terms of
visual angle. On the other side of the coin, once a setting for
optimum focus has been determined a pattern can be used as its own


24
control by defocusing it through several diopters and not changing
the retinal region being stimulated or the size of any feature in it.
Figure 2 is a photograph of the pattern stimulating instrument.
The original binocular viewing apparatus has been replaced by a monocular
one to improve light efficiency of the viewing channel. Another
modification, which is not visible, was the addition of a cross hair
with a bead at its midpoint in an image plane within the viewing
tube. This provided a fixation point for human subjects and an aid
to the experimenter in centering and focusing the ophthalmoscope on
an animal retina. A schematic diagram of the instrument is given in
figure 3.
Extensive testing of the pattern stimulating instrument has
been carried out both optically and by conducting evoked response
experiments with human subjects. To demonstrate that the instrument
is capable of producing an excellent patterned image on the retina, a
lens was placed in front of the instrument and a piece of photographic
paper was then exposed by flashes through the pattern stimulating channel.
The results of this test are shown in figure 4. With the same lens and
white paper screen, the patterned images were photographed through the
viewing channel (with eyepiece and erecting lens removed) to demonstrate
the effect of the viewing channel upon the image seen by the experimenter.
The results of this test are shown in figure 5. As a final demonstra
tion of the ability of the instrument to project a patterned image
upon the retina, a cat was paralyzed, fitted with a contact lens,
refracted, and several patterns were projected onto its retina and
photographed through the viewing channel of the instrument. Two of


25
Figure 2. Maxwellian view pattern stimulator. The pattern
stimulating channel is contained in the case which is mounted
on top of the ophthalmoscope. 'The lid on top of the case allows
insertion of a patterned transparency, optical filter, or an
occluder. A calibrated scale for the range of focus is inside
the case and the knob to control pattern focus is on the side of
the case not seen.


BS1,BS2 Beam splitters
M First surface mirrors
ND Neutral density filter
P Pattern transparency
XX Range of focus
Figure 3. Schematic diagram of the Maxwellian view pattern stimulator instrument


¡V,
w
V IB B B
.-Z-I iV
: ij
.v
.>
" *
.V.V
B B B 1
* -V-

:=$=: w
B
EXAMPLES OF THE QUALITY OF PATTERN WHICH THE PATTERN STIMULATOR
INSTRUMENT IS CAPABLE OF PROJECTING.
A.
0.8
nun/sq .
pattern = 17
. 5 min
i. of arc
= 68 /u on the
: retina
B.
1.4
mm/sq.
pattern = 31
min.
of arc =
120
p on
the
retina
C.
2.5
mm/sq.
pattern = 55
min.
of arc =
212
p on
the
retina
D.
3.6
mm/sq.
pattern = 79
min.
of arc =
306
p on
the
retina
Figure 4. A test of the Maxwellian view pattern stimulator:
checkerboard patterns projected through a glass lens directly
onto photographic paper.


28
Figure 5. A test of the Maxwellian view pattern stimulator:
checkerboard patterns projected through a glass lens onto a white
paper screen and photographed through the viewing channel of the
instrument. A modification of the original design of the viewing
channel to improve light efficiency at the expense of image
quality was made to allow viewing of the fundus at the standard
adaptation light level chosen for use in these experiments (780
trolands retinal illumination).


29
these photographs are shown in figure 6.
Obviously, to study responses of a paralyzed animal to
spatially patterned visual stimuli, one must have high quality contact
lenses available because the surface of the cornea of a paralyzed
animal dries, becomes irregular, and does not permit the projection
of a high quality image onto the retina. In agreement with Vakkur
et al. (1963), for cat a supply of zero diopter plastic lenses of
radius 8.7 mm has served nicely. Additional correction, when necessary,
was obtained by first utilizing the range of focus available in the
instrument and then by placing a lens in the spectacle plane.
The patterns projected by this instrument were on trans
parencies that were mounted in 35 mm slide frames.
Animals and Electrode Implantation
Six cats were used in these experiments. They were all
healthy adults and weighed between 2.5 and 4.3 Kg. One young adult
rhesus monkey (Macaca mulatta) was used. He weighed 3.8 Kg.
The operations to place chronically implanted electrodes into
these animals were conducted as follows. Ten mil insulated stain
less steel wire electrodes with sharpened tips were prepared in advance.
An animal was anesthetized with sodium pentobarbital (Nembutal). A
surgical anesthetic level was achieved with an initial dose of 15 to
20 mg/kg administered i.v.. Additional anesthetic was given later
in the procedure, if necessary. Using sterile surgical techniques,
bipolar electrodes were placed in optic nerve and lateral geniculate
nucleus stereotaxically while photically evoked responses were


30
Figure 6. The ultimate test of the Maxwellian view stimulator:
checkerboard patterns projected onto the retina of a paralyzed
cat and photographed through the viewing channel of the
instrument. In order to be photographed, it was necessary
that the light pass through the optics of the cat eye twice
and the viewing channel of the instrument once. Also, it
should be noted that while light entered the eye through only
a portion of the cornea and pupil, the whole pupil was used
in viewing the retina. Therefore, the actual retinal images
were superior to the images photographed.


31
recorded from them to insure that the desired structure had been
located. One mm diameter stainless steel ball electrodes were placed
epidurally over striate and extrastriate visual cortex (Bilge et al.,
1967; Daniel and Whitteridge, 1961). As an indifferent electrode,
on cats a loop of stainless steel wire was mounted on top of the skull
over the frontal sinus. Electrodes were held in position by fastening
their shafts to the skull with a fast-setting acrylic plastic. A
block of plastic into which two holes had been drilled and tapped
was cemented onto the skull centered on the midline at approximately
anterior 24 mm. The electrode leads were brought to a miniature elec
trical connector socket and this socket was mounted in a mound of
plastic on top of the skull. The animal was allowed to recover from
the anesthetic and returned to its cage.
Recording Sessions with Human Subjects
An effective way to test the Maxwellian view pattern stimulator
instrument was to use it to record evoked responses from human subjects.
Three subjects were used. They were all between 20 and 30 years old.
Two were emmetropic and had not been recorded from previously. CD wore
corrective lenses. An active electrode placed on the scalp at 1.5 cm
above the inion on the midline and an ear clip indifferent electrode
were used. A subject was placed in a well darkened electrically
shielded room, given a chin rest in front of the stimulator, and asked
to fixate a tiny bead at the midpoint of a vertical hairline through
the center of the adaption light field. Patterns, light levels, and
recording instruments were as described in the next section for animals.


32
Recording Sessions using Animals
In a brief sketch, during a recording session an animal was
anesthetized with halothane gas anesthetic long enough to intubate
it with a tracheal cannula and administer the initial dose of neuro
muscular blocking drug i.v., then it was moved into an electrically
shielded room, placed on a continuous infusion of neuromuscular block
ing drug, respirated, refracted and corrected, allowed to adapt to
a fixed background field (adaptation light), and then it was stimu
lated and recorded from for a period of from 6 to 8 hours. At the end
of this period the animal recovered from the paralysis and was returned
to its cage.
The priorities within these recording sessions were (1) to
determine whether or not the visual evoked responses of cat or monkey
are sensitive to spatially patterned visual stimulation and if so,
then (2) the effect of pattern feature size, (3) whether or not the
responses are repeatable over hours and weeks, and (4) the effect of
quality focus of the pattern. In accordance with these priorities,
the general plan for recording sessions was to obtain responses from
the animal to diffuse flashes and to a series of 4 or 5 checkerboard
patterns of different feature size, to run this series in forward
and reverse pattern feature size order 2 or 3 times within one session,
to run selected patterns additional times, and using the range of
focus built into the stimulator, or external lenses, to compare
responses to a given pattern when it was well focused and when it was
defocused by different amounts.
The choice and order of averaging runs within each session
was generally in agreement with the priorities stated. This project


33
was essentially exploratory, and therefore a rigid design or order
for the collection of the responses was not appropriate.
The use of drugs was carefully limited to the minimum necessary
to restrain the animal and maintain it in a good physiological state.
Appendix II provides details of the use of drugs in this project.
Over all sessions with cats, the average amount of gallamine triethi-
odide (Flaxedil) administered was 9 mg/kg/hr.
To obtain an early warning of any significant change in the
operation of the animal's respiratory or cardiovascular systems, a
respiration monitor and alarm system to monitor end expiratory (X>2
level was designed and developed by the author. It was used in con
junction with a Beckman LB-1 CO2 meter. The details of this system
are given in Appendix III.
To conduct a recording session after all the necessary instru
ments were ready, a chronically implanted animal was given an injec
tion of atropine i.m., and received one drop of one percent cyclopentolate
hydrochloride (Cyclogyl) topically applied to each cornea. After 30
minutes, halothane gas anesthetic was administered to the animal.
The anesthesia was continued while respiratory and cardiac activity
were watched and reflexes were tested. When proper anesthetic level
was reached, the animal was intubated with a tracheal cannula (inflat
able cuff type). The 23 gauge needle of a scalp infusion set which
was attached to a syringe was inserted into a vein. The animal was
switched from the C^, halothane mixture to air and allowed to breath
off the anesthetic as the initial dose of neuromuscular blocking drug,
gallamine triethiodide, was administered slowly through the i.v. catheter.


34
At this point the animal was connected to a positive pressure respirator
pump. The animal was wrapped with an automatically controlled heating
blanket and placed on a canvas bed which has a metal frame and, on the
front end, an adjustable head-holding bracket which can be screwed
to the plastic block on top of the animal's head. The animal was then
connected to a syringe pump. The CC>2 monitor and alarm system was
now in operation, and the respirator was adjusted to give a steady 4
percent end expiratory CC>2 level. The EEG was continuously monitored
with an oscilloscope. Using palpation or a stethoscope, a check on
the rate and strength of the heartbeat was made frequently. Each
cornea was treated topically with a steroid solution (dexamethasone),
and a few minutes later a solution of methylcellulose was applied to
each cornea and to the inside surface of the contact lenses. The
contact lenses were placed on the eyes and examined with an ophthal
moscope to insure that the fit was acceptable and no air bubbles were
trapped under the lenses. The eyelids and nictitating membrane were
held open by an opaque ring cemented to the outside surface of the
contact lenses. Normally the eye to be stimulated was fitted with a
high quality transparent plastic lens and the other eye with an
opaque black plastic lens. Next the fundus of the eye to be stimulated
was then studied with a hand-held ophthalmoscope to become familiar with
the pattern of blood vessels, especially, for cats, those in the
neighborhood of the area centralis. Area centralis was located using
the pattern of retinal vessels and the coordinates, 14.7 degrees radi
ally from the center of the optic disc at an angle of 22 degrees above
horizontal and temporal to the optic disc (Vakkur et al., 1963).


35
In the paralyzed animal, because the eye maintains a fixed position
in the orbit, it was possible to orient the eye to be stimulated
by orienting the head. Therefore, the bracket to which the animal's
head was attached was adjusted to align the visual axis of this eye
to a straightforward horizontal gaze.
A streak retinoscope was used to refract the eye and any
corrective lens necessary was mounted in the spectacle plane on the
front of the stimulator instrument. The pattern stimulator instrument
was then positioned in front of the eye and aligned such that the
area centralis (macular region for monkey) was in the center of the
field of view, which for this instrument was approximately 30
degrees in diameter. The ophthalmoscopic viewing channel was focused
and the adaptation light level adjusted. A patterned stimulus trans
parency was placed in the instrument and with a continuous light source
temporarily attached to this channel the pattern was projected, viewed,
and focused on the retina. The focus was tested with several trans
parencies with different feature sizes and the focus adjustments were
made as many times as necessary in order for the investigator to be
confident that the best possible focus had been achieved. The light
source to the pattern stimulating channel was then changed from con
tinuous to xenon flash. Figure 7 shows a cat during an actual recording
session.
The standard adaptation light intensity used produced a retinal
illumination of 780 trolands. A standard flash intensity was chosen.
Using the diffuse pattern, the investigator when at the standard adapta
tion level found the standard flash intensity to be 2.5 log units above


36
Figure 7. Photograph of an historical event: the experiment
in which cortical evoked responses sensitive to spatially
patterned visual stimuli were first recorded from cat. The
cat is paralyzed, artificially respirated, and warmed by an
automatically controlled heating blanket which has been laid
aside for the photograph. Its head is supported and positioned
by a plastic bracket. The animal is wearing a contact lens in
its right eye and is being visually stimulated (left eye
occluded). Recording is from chronically implanted electrodes.


37
threshold for detection of the flash.
Trigger pulses to both stimulation and recording equipment
were normally spaced 2.25 seconds apart. The xenon flashes (approxi
mately 10 ysec duration) for the stimulator were obtained from a Grass
model PS2 photostimulator and conducted to the pattern stimulator
through a 1/8 inch diameter fiber-optic bundle. Recording channel
amplification was obtained from Tektronix type 122 differential ampli
fiers with (-3 db) bandwidth normally set at 0.2 to 1000 Hz. No
additional filters were used during paralyzed animal experiments.
The amplified individual responses were sometimes FM tape recorded
simultaneously with the averaging process which was performed using
a Fabritek model 1052 averaging computer.
The stimulator, respirator, infusion pump, recording equip
ment, and data logs were carefully attended to between runs. Appropri
ate action was taken if any sign.-, of excessive wetness in the lungs
or mucous in the trachea were encountered. The volume of drug remain
ing in the syringe pump was watched closely, and a record of the
quantity of drug infused was kept. The retina was viewed periodically
to insure that the position of the retinal landmarks had not changed.
The time interval between averaging runs ranged from 4 to 15 minutes.
Averaged responses normally contained 64 individual responses, but on
occasion contained 32, 128, or 256 responses.
When all of the desired runs for a recording session had been
collected (or up to 30 minutes before), the infusion was switched to
glucose saline with no gallamine before the stimulator was moved away
from the animal. A final check of the retinal position and corneal


38
transparency was made to insure that no change had occurred during
the experiment, and then the contact lenses were removed. The animal
was observed until a diaphragm reflex or movement of the jaw was
seen, and then neostigmine was administered slowly through the i.v.
catheter up to a maximum of 25 nanograms per kilogram. When the
animal's ability to respirate itself was adequate, it was disconnected
from the apparatus. After a period for observation, the animal was
returned to its cage.
The procedure for recording from paralyzed monkey was
essentially the same as that for cat with the exceptions of different
drug dosages, different contact lens curvatures, and different retinal
landmarks.
All animals used in these experiments survived and were
healthy afterwards.


RESULTS
Human Subjects with the Behavioral Task
When given verbal instructions, human subjects were able to
learn the behavioral task in a small number of trials. Two human sub
jects served in this experiment. The evoked responses recorded from
them are shown in figure 8, and the simultaneously recorded cumulative
records of their performance are shown in figure 9.
The subjects were attentive and very few errors were made.
At latencies between 100 and 300 milliseconds, the response waveforms
were sensitive to pattern feature size, but the waveform changes were
not large. In the pattern responses of subject HD, an increase in the
negativity of the peak at 120 milliseconds was the largest waveform
change. For subject CD, the difference between pattern and diffuse
responses is a positive-negative-positive sequence which starts at a
latency of 170 milliseconds in the 15 minute of arc pattern response
and translates to noticeably shorter latencies in the responses to
patterns with larger feature sizes.
In this research, responses were recorded from human subjects
in order to provide data for comparison of the two methods used with
each other and with responses in the literature. The effectiveness
of each of these methods for producing spatially patterned visual stimu
lation of animals should be proportional to its effectiveness with
human subjects.
39


Subject = HD TilKe in Milliseconds Subject CD Time in Milliseconds
Figure 8. Human evoked responses to diffuse and checkerboard patterned stimuli using the visual
fixation task. 32 individual responses/average response.


Figure 9. Cummulative records of the performance of two human subjects. In the records of trials and
responses, each upward step marks the beginning of a trial. For subject HD, each downward stroke indicates
the correct completion of a trial. In the records of time-out periods, the duration of the negative going
pulse indicates the duration of the period.


time between
TRIALS
AND
RESPONSES
Start
TIME-OUT
PERIODS
TRIALS
AND
RESPONSES
TIME-OUT
PERIODS
Start
Finish


43
Monkeys with the Behavioral Task
In training, the rhesus made steady progress, especially
from the point where she was given the manipulandum and discovered
that it controlled the task. After 5 months of training, she was
correctly completing greater than 80 percent of all trials started.
At this time, fixation light periods required some lever holds up to
10 seconds and the fixation light spot was dim, usually colored (red
or green) and subtended 25 minutes of arc (visual angle). For two
weeks, ear clip electrodes were placed on her during training sessions.
By Sh months, 95 percent of all trials started were being completed
correctly. A record of the animal's performance and the distribution
of her reaction times to the offset of the fixation light is given
in figure 10. Evoked responses recorded simultaneously with that
performance are given in figure 11. These responses have positive
peaks at 90, 130, and 150 milliseconds and a distinct negative peak
at approximately 107 milliseconds. The responses to 15 and 20 minutes
per square patterns are noticeably larger in amplitude than the response
to diffuse flash stimulation. Pattern sizes were not repeated during
that recording session. Visual comparison of the set of 32 individual
responses to the 15 minute per square pattern with those to the diffuse
stimulus shows that the responses to the patterned stimulus were more
consistent and larger than those to the diffuse stimulus.
In several other sessions when some responses were recorded
from this animal, recording procedures and electrodes were being
developed and no usable sets of data were obtained. All data collection
from the scalp electrode was preliminary to data that was to be


Trials
and
Responses
Time-Out
Periods
w \f\j\tu1 it "trvw""- y- v
Finish
Figure 10. Histogram of reaction times and cummulative record of performance of
rhesus monkey. (A) The average is based on 5 consecutive sessions and contains
the session shown in B as the 3rd session. (B) Same type record as in figure 9.
Note that all but a few of the time-outs were caused by a lever pull during an
intertrial interval (exceeding the speed limit) rather than an incorrect response


45
Time in Milliseconds
Figure 11. Evoked responses of rhesus monkey to diffuse and checkerboard
patterned stimuli using the visual fixation task. 32 individual
respcnses/average response.


46
recorded from an array of implanted ball type epidural electrodes
over striate cortex. The implant operation was performed under sodium
pentobarbital anesthesia. Near the end of the operation the veter
inarian who was present administered a small additional dose of
anesthetic, i.m. (phencyclidine hydrochloride (Semylan)), and its
facilitatory interaction with the pentobarbital was fatal to the
animal.
The increase in amplitude of the 15 and 20 minute per square
pattern responses over that of the diffuse response was interpreted
as a possible indication of sensitivity of the rhesus evoked response
to pattern.
The mangabey trained well until the manipulandum was intro
duced. From that point on, her progress was not steady. After several
months of training, she knew the task well enough to perform it very
well on some days, but her performance was not consistent. Her motiva
tion to work seemed not to be very well related to deprivation or weight
loss. Additional problems developed and when the rhesus died, it was
decided by the author not to pursue the training of this animal further.
The mangabey's training has been continued by another student, but no
data has been recorded at this time.
Maxwellian View Stimulation of Humans
The visually evoked responses of three human subjects to
diffuse and patterned stimuli are shown in figure 12. They demonstrate
sensitivity of the human evoked response to spatially patterned stimuli.
Responses from these same subjects are presented in figure 13 along with
"difference waveforms" derived by subtracting from the given response


Figure 12. Responses of humans to diffuse and patterned stimuli presented by means of the Maxwellian view
stimulator. Averages of 64 responses with 2.25 seconds between stimulus flashes.


100 200 300 400
500 0 100 200 300 400
Time in Milliseconds
500 0


Figure 13. Comparison of the responses evoked by checkerboard patterned
visual stimuli and a diffuse flash stimulus which produces the same
average retinal illumination. Difference waveforms (pattern response -
diffuse response) are given for 3 human subjects. Stimulation was by
means of the Maxwellian view stimulator.


50
S = PS
79'
55' vV^
31
17.5
Diffuse
79' hvvw/^V
S = CD
55'
31'
17.5
11'
Diffuse
u^Vyv\v^/V'/v^^
vV
hUM,
..M n' iw V*AH
}f/VV
DIFFERENCE WAVEFORMS


51
(pattern or diffuse) a diffuse response. Pattern sizes were repeated
for each of these subjects and the responses were consistent. The
effects of patterned stimulation relative to diffuse stimulation, and
pattern feature size, as seen in these responses, are in agreement with
the findings of Rietveld et al., 1967, and Harter and White, 1968.
Maxwellian View Stimulation of Cats
The cortical responses of the first two cats from which
responses were recorded, were observed as they were averaged on line
in an attempt to identify any changes in waveform which correlated with
a change from diffuse to patterned stimulation or vice versa. Such
changes were identified and the stimulus conditions were alternated
a number of times to determine whether or not the changes were repeat-
able and consistent. Figure 14 shows such data for cat 1L3.
In the upper half of figure 14, average responses were recorded
for a diffuse stimulus and a checkerboard patterned stimulus, alternately,
over a 70 minute period, and they showed consistent waveform features
in the response to pattern which were not found in the response to
diffuse stimulation. To demonstrate the consistency of this data, the
averages of all five average responses for the diffuse stimulus and
for the 31 minute patterned stimulus are also given in figure 14.
Using the terms defined by Creutzfeldt, et al., (1969) for the components
of the cat cortical visual evoked response, the response to 31 minutes
of arc pattern stimulation has a more distinct wave I-wave II complex,
a double peaked wave III, and a larger wave IV than the response to
diffuse stimulation. Also, in the lower portion of figure 14, are


Figure 14. Repeatability of cat cortical evoked responses to diffuse and patterned visual stimuli within
one session. Time of run is printed along the ordinate. In the upper portion of the figure runs were alter
nated between the diffuse and the 31 minute pattern stimuli. An ensemble average of 5 average responses is
given for each of these 2 cases. In the lower portion of this figure, responses to 2 other pattern sizes
are compared. Using terms defined by Creutzfeldt et al. (1969), waves I, II, III, IV, and V are labeled on
the ensemble averages. Each average response contains 64 individual responses to stimulus flashes 2.25
seconds apart.


Cat 1L3
6:39 pm
6 :25 pm
6:12 pm
5:57 pm
5:43 pm
Average
0 100 200
Time in Milliseconds
on


54
responses to two patterns with different feature sizes obtained in
an alternating series of averaging runs. It can be seen that the
waveform changed with pattern feature size and that the changes were
consistent with this series.
For cat 1L3, the array of different response waveforms obtained
with pattern feature size is displayed along with difference waveforms
(pattern minus diffuse) in figure 15. These responses were practically
duplicated by responses recorded from the same animal thirteen days later
and displayed in figure 16.
A series of responses to different intensities of diffuse
flash were recorded. It is shown in figure 17 and is evidence that
the waveforms seen with patterned stimuli are not obtained with
different intensities of diffuse stimulation.
One chronically implanted optic nerve electrode was in place
in cat 1L3, but was not usable in this experiment because in a prelim
inary recording session the cornea of the eye ipsilateral to the im
planted nerve was damaged by a tight fitting contact lens. Therefore,
stimulation for the data described above was to the eye contralateral
to the implanted nerve.
Another animal, cat 1K9, possessed usable electrodes both in
optic nerve and on cortex (epidural). Responses recorded from this
animal are shorn in figures 18 through 22. During the first recording
session with this animal, problems with the recording equipment limited
the data collection and only responses from cortex were recorded.
Figure 18 shows the effect of pattern feature size. Responses from
epidural striate and extrastriate electrodes were quite similar and


EXTRASTRIATE CORTEX AVERAGE RESPONSE DIFFERENCE WAVEFORM
Figure 15. The effect of pattern feature size upon the cortical evoked response of cat 1L3.
64.individual responses/average response. 2.25 seconds between stimulus flashes.


56
Time in Milliseconds
Figure 16. Repeatability of cat cortical evoked responses
over a two-week period. Compare these responses with
those which were recorded 13 days earlier and shown in
figure 15. The recording conditions were the same as for
figure 15.


57
LXTRASTR1ATE CORTEX AVERAGE RESPONSE
i 1 L .... .1 L.
0 100 200
Time in Milliseconds
STRIATE CORTEX AVERAGE RESPONSE
Figure 17. Diffuse flash cortical evoked responses to a series of
stimulus intensities. Striate and extrastriate responses were
recorded simultaneously. Electrode locations: striate P3.0, L3.0;
extrastriate P3.0, L8.0 (both epidural).


Figure 18. The effect of pattern feature size upon the cortical evoked responses of
cat 1K9. Both the striate and extrastriate responses are sensitive to spatial pattern.
64 sweeps/average. 2.25 seconds between stimulus flashes. Electrode locations:
striate P3.0, L3.0; extrastriate P3.0, L8.0 (both epidural).
* Difference waveforms are left extrastriate responses minus the upper diffuse response.


Cat 1K9
Control
i i i i i i
0 40 80 120 160 200
i i i i i i i i 1 1 1 >
0 40 80 120 160 200 0 40 80 120 160 200
Time in Milliseconds
on
oo
LEFT STRIATE AVERAGE RESPONSE
LEFT EXTRASTRIATE AVERAGE RESPONSE
DIFFERENCE WAVEFORM *


Figure 19. The effect of defocus upon responses to a 31 minute of arc per
square checkerboard patterned stimulus. Difference waveforms (+0.12 D
response other response) are helpful in determining which features in
the response waveform are influenced by quality of focus and to what
extent. Cat 1K9 striate electrode. 64 sweeps/average response.


61
O 40 80 120 160 200 0 40 i80 120 160 200
Time in Milliseconds Time in Milliseconds
31 MINUTE PATTERN RESPONSE DIFFERENCE WAVEFORM


Figure 20. The effect of a positive 9 diopter defocus upon responses to patterned stimuli with different
feature sizes. Note that there is very little sensitivity of response to pattern when the pattern is
defocused by 9 diopters. 64 sweeps/average response.


LEFT
I I I I L.
0 40 80 120 160
FOCUSED
0
40
80
120
160
*
DIFFERENCE WAVEFORM


Figure 21. Simultaneously recorded optic nerve and cortex responses to
diffuse and patterned stimuli. In the cortical responses note that the
peak of wave III is at a different latency in the 79 minute pattern
response than in the diffuse response.


65
STRIATE CORTEX
DIFFERENCE WAVEFORM
OPTIC NERVE


Figure 22. Repeatability of optic nerve and cortical evoked responses
frcrn cat 1K9 within one session. In addition, an example of the effect of
4 diopters of defocus is given.


67
OPTIC NERVE


68
the shape of waves III, IV, and V was influenced by feature size in
both cases. Figure 19 shows the effect of various degrees of defocus
upon the response to the 31 minute of arc pattern where defocus was
achieved by use of an external lens in the spectacle plane. Figure
20 describes the effect of a constant +9 diopter defocus upon patterns
of different feature size. It demonstrates that when well defocused,
the feature size of a patterned stimulus did not influence the waveform
of the response and that the difference waveform between responses to
a focused and a defocused patterned stimulus was very similar to that
between responses to a focused patterned stimulus and a diffuse stimulus
as shorn in figure 18 for data recorded one hour earlier. The external
lens was used because the large (1 to 2 degrees of arc per square)
patterns to which the evoked responses from the cats in these experi
ments were found to be sensitive were not well defocused by the maximum
use of the range of focus built into the stimulator. Use of different
external lenses introduces small changes in pattern feature size on
the retina and size of retinal region stimulated, in the form of
spectacle magnifications, which will be discussed later.
In a second recording session, one week later, both optic
nerve and cortex responses were recorded from this same animal. The
effect of pattern feature size upon these responses can be seen in
figure 21. The agreement of these cortex responses with those recorded
one week earlier is evidence for repeatability of this effect within
one animal. Also, it is very important to note that the optic nerve
responses are sensitive to pattern stimulation and to pattern feature
size. Most noticeable is an increase in the positivity of the wave
form at a latency of 25 to 35 milliseconds, which is on the late side


69
of the primary positive peak of an optic nerve response to diffuse
stimulation. The repeatability of the responses within one session
with this animal was also tested and the results are shown in figure
22. Included in the same figure is an example of the effect of
+4 diopter defocus with zero spectacle magnification upon the response
to a 31 minute of arc per square pattern and a 79 minute of arc per
square pattern. This data indicates that an equal amount of blurring
of contrast borders for patterns with different size features does
not cause the evoked responses to revert equally well to the waveform
characteristic of diffuse stimulation.
A third animal, cat H9, provided excellent data on the optic
nerve. For this animal, the effect of pattern feature size is shorn
in figure 23. There is good agreement between data shown in this figure
and that in figure 21. Also, responses to the 11 minute of arc per
square pattern are included in this figure and it can be seen that the
difference between responses to this pattern and to diffuse is either
insignificant or very small. This was a consistent finding from
several animals. The control run was taken late in the session and
shows that the animal had developed some synchronous EEG activity which
was not evident earlier in the session.
To be assured that the effects of pattern feature size upon
the visually evoked optic nerve potential were not absolutely peculiar
to one set of adaptation level and stimulus intensity conditions, the
recordings given in figure 24 were made with half the normal flash
intensity, twice the normal flash intensity, and twice the normal flash
intensity at ten times standard adaptation level. Pattern sensitivity
of the response was evident under each of these conditions.


Figure 23. The effect of pattern feature size upon the optic nerve responses of cat H-9. Time of run and
feature size are given for each response. 128 sweeps/average response.


79
8:48 pm
55'
8:34 pm
31'
8:24 pm
17.5'
8:15 pm
11'
8:06 pm
Diffuse
8:58 pm
Control
9:15 pm
9:09 pm
Time
CAT OPTIC NERVE AVERAGE RESPONSES


Figure 24. Pattern sensitivity of optic nerve responses at several
combinations of adaptation level and flash intensity.


73
o 100 200
Time in Milliseconds
CAT OPTIC NERVE AVERAGE RESPONSES


74
A difference in the average response waveforms to two different
stimuli implies a difference (on the average) in the individual responses
to these stimuli. In figure 25, sets of typical individual optic nerve
responses are given. The differences between these sets of individual
responses can be seen. For diffuse responses, the largest peak of the
primary positivity is at a latency of 30 to 34 milliseconds and a smaller
peak or an inflection is usually seen at 55 milliseconds. For the 31
minutes of arc pattern the largest peak of the primary positivity occurs
between 40 and 50 milliseconds. In general the agreement between indivi
dual and average responses in this set of data is quite good.
There were no useable cortex electrodes on this animal.
Of the remaining three cats, two provided data on the cortex
and two on the optic nerve. These three animals were very large^old,
male cats and their responsiveness to pattern was not as clear and
vivid as that in the data already given. They were, however, sensitive
to pattern. For cat 1L1, typical cortical responses to diffuse and
patterned stimuli are given in figure 26 along with difference wave
forms. These responses are unusual in that wave IV is small and in
the 31 minute of arc response the second peak of wave III is quite
large.
Cortex responses of animal 1L16 are shown in figure 27.
They are repeatable within one session. A negative-positive-negative
sequence is seen in these responses in the time period normally occupied
by a single negative-going wave IV. Optic nerve responses from this
same animal are shown in figure 28 along with their difference wave
forms.


75
-6 34 114 194 Msec. -6 34 114 194 Msec.
Diffuse 31' Pattern
INDIVIDUAL OPTIC NERVE RESPONSES
AVERAGE OPTIC NERVE RESPONSES
Figure 25. Simultaneously recorded individual and average optic
nerve responses to diffuse and patterned stimuli. Cat H-9.


O 100 200
Time in Milliseconds
STRIATE CORTEX
i ¡ i : ¡_
0 100 200
Time in Milliseconds
DIFFERENCE WAVEFORM
Figure 26. The effect of pattern feature size upon cortical evoked responses of cat 1L1.


O 100 200
Time in Milliseconds
STRIATE CORTEX
0 100 200
Tir.e in Milliseconds
DIFFERENCE WAVEFORM
'-j
Figure 27. The effect of pattern feature size upon cortical evoked responses of cat 1L16.


OPTIC NERVE DIFFERENCE KAVEFORM
Figure 28. The effect of pattern feature size upon optic nerve responses of cat 1L16.


79
From the sixth cat, 1L6, optic nerve responses are given in
figure 29. All responses from this animal were taken with standard
adaptation level and twice normal flash intensity.
In general, for all four cats from which optic nerve data were
obtained, the effect of patterned stimulation as opposed to diffuse
stimulation on the optic nerve response was consistent. Observation
of the responses and the difference waveforms shows that either the
slope of the rise to the primary positive peak, or the amplitude of
the early side of the peak, is slightly reduced, and either the ampli
tude of the late side of this peak increases, or a second positive peak
or inflection develops on the downward slope. The effect of pattern
feature size appears to modulate the extent to which the changes in
waveform just described occur.
There is considerable variation between cats in the cortical
evoked response to diffuse flash. For this reason, the effect of
patterned stimulation must be judged with respect to the diffuse
flash response of the same animal. In the response to a pattern of
medium sized features (approximately 30 minutes of arc per square),
waves I and II are usually unchanged, a double peak on wave III is
either developed or enhanced, and when they can be identified, waves
IV and V usually increase in size.
The smallest effective pattern feature size used was 17.5
minutes of arc per square. The largest feature size used (79 minutes)
was effective. The most noticeable effect of pattern feature sizes
within this range was to modify the amplitude and latency of the peaks
of wave III.
Responses of the lateral geniculate nucleus were recorded


79'
55'
31'
17.5
Diffuse
Control
OETIC NERVE
DIFFERENCE WAVEFORM
Figure 29. The effect of pattern feature size upon optic nerve responses of cat 1L6.


81
(using bipolar wire electrodes) from five of these cats simultaneously
with those from optic nerve or cortex. No consistent effect of patterned
stimulation was identified.
Maxwellian View Stimulation of Monkey
Visually evoked responses from an epidural electrode over
striate cortex of a paralyzed rhesus monkey are shown in figures 30
and 31. These responses were obtained during a lh hour recording
session within which the animal heart rate, body temperature, and
expired CC>2 were very stable at normal values. At the end of the
session this animal recovered rapidly from the neuromuscular blockade
and was in good health. There was a clearly visible sensitivity of
this response to pattern feature size. Within this session, pattern
feature sizes were repeated and the responses were consistent.
In terms of their polarity and latency (in milliseconds),
the peaks which are starred in figure 31 are P39, N46, P67, P96,
P124, and N240. The difference between a pattern response and a diffuse
response is best described by the difference waveform in figure 31.
It is important to recognize that the difference waveform is not small;
it is larger in amplitude than the diffuse response. Responses from
this animal indicate that the effect of pattern feature size is to con
trol the magnitude of the activity represented by the difference wave
form shown, without changing the latency of the difference waveform
peaks.
A response to a defocused patterned stimulus is less different
from a diffuse response than is a response to a well focused patterned
stimulus. To defocus a patterned stimulus to the extent that the


Figure 30. Responses of a paralyzed rhesus monkey to spatially patterned visual
stimuli. Each stimulus was projected onto the macular region using the Maxwellian
view pattern stimulator. The effects of pattern feature size, and +1.2 and +2.5
diopters of defocus are displayed. These recordings are from a chronically
implanted epidural electrode within 1 degree of the foveal representation in
striate cortex. The indifferent electrode was a stainless steel wire loop under
the scalp over frontal cortex. An average response contains 64 individual
responses to flashes 2.25 seconds apart.


O 100 200 300 400 500
Time in Milliseconds
4:03 pm
3:36 pm
3:13 pm
2:53 pm
2:33 pm
I L
0 100
FOCUSED STIMULI
+
32 jjv
I
1 I 1 L_
200 300 400 500
Time in Milliseconds
DEFOCUSED BY +1.2 D.
I i i i i
0 100 200 300 400
Time in Milliseconds
DEFOCUSED BY +2.5 D.
500


84
Difference
Waveform
11' Pattern
Diffuse
0 100 200 300 400 500
Time in Milliseconds
Figure 31. Comparison of the evoked responses of rhesus monkey
to an 11 minute pattern and to diffuse stimulation. The
difference waveform (pattern diffuse) indicates that the
change in the activity evoked within the visual system between
these two cases is large and that rhesus monkey is a good
animal in which to study the generation of visual evoked
potentials using both patterned and non-pattemed visual
stimuli. Asterisks mark peaks at latencies of 39, 46, 67, 96,
124, and 240 milliseconds. Recording conditions are the same
as in figure 30.


85
related response is identical to a diffuse response, fewer diopters
of defocus are required for a pattern of small feature size than for
a pattern of larger feature size, at least within the feature size
range used in this experiment. Note that for this animal, the minimum
dioptric value of defocus necessary to effectively eliminate sensitivity
to a patterned stimulus was less than or equal to 1.2 diopters for
an 11 minute pattern and approximately 2.5 diopters for a 55 minute
pattern.
Summary of Results
The results which have been presented show that the visually
evoked responses of human, cat, and rhesus monkey cortex and cat optic
nerve are sensitive to spatial pattern. Since the sensitivity of
human visually evoked responses to checkerboard patterned stimuli had
already been established, responses were recorded from human subjects
primarily to demonstrate that the procedures and instruments used were
effective. Difference waveforms which are helpful in appreciating and
comparing the sensitivity of the response to each of the stimulus
conditions were derived from the recorded responses. For cat and
monkey, evidence for sensitivity to pattern was obtained by comparison
of responses to diffuse stimulation and patterned stimulation which
provided the same average retinal illumination. Responses to the
diffuse stimulus and to patterns of different feature size were different
and repeatable (see figure 32). The effect of quality of focus upon
responses to patterned stimuli was studied within this project primarily
as a control measure to obtain evidence that pattern features and not


Figure 32. Summary of the effects of pattern feature size upon
the visually evoked responses from the cortex of human, rhesus
monkey, and cat. The responses of each species are pattern
sensitive. 64 sweeps/average response.


20 uv
79'
55'
200 msec. 31'
17.5'
11'
HUMAN
Diffuse
60 uv
79'
55'
200 msec.
31'
17.5'
MONKEY
11'
Diffuse
48 uv
80 msec.
55'
31'
17.5'
CAT
Diffuse


88
some minute intensity differences or other factors were the cause of
the waveform changes observed in the evoked responses. For cat, it was
also shown that cortical responses were sensitive to pattern under
several conditions of adaptation level and flash intensity, and that
responses characteristic of a patterned stimulus were not obtained
using diffuse stimulation with flash intensities greater or less than
the intensity normally used. Sensitivity of optic nerve responses of
cat to patterned stimuli was observed in individual as well as average
responses.


Full Text
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UNIVERSITY OF FLORIDA
3 1262 08554 3626



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FILES


ACTIVITY EVOKED IN THE VISUAL SYSTB1 OF HUMAN,
RHESUS MONKEY, AND CAT BY SPATIALLY
PATTERNED AND NON-PATTERNED VISUAL STIMULI
By
HAROLD WILLIAM DODDINGTON
A DISSERTATION PRESENTED TO THE GRADUATE COUNCIL OF
THE UNIVERSITY OF FLORIDA
IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE
DEGREE OF DOCTOR OF PHILOSOPHY
UNIVERSITY OF FLORIDA
1972

ACKNOWLEDGMENTS
The author sincerely wishes to thank the members of his
supervisory committee, the faculty of the Physiology Department,
and his fellow graduate students for their encouragment and the
contributions which they have made to his education. The author
is especially appreciative of the guidance, assistance, and
provision of laboratory space by Dr. W. W. Dawson, his supervisory
committee chairman, and Dr. C. K. Adams for the development of the
research described in this dissertation. Most of all, the author
wishes to express to his wife his appreciation for her sharing of
the rewarding experiences and frustrations that have occurred
during this endeavor. This education and research was supported
by Training Grant GM 251 to the Physiology Department and Training
Grant MH 10321-06 to the Center for Neurobiological Sciences, both
from the National Institutes of Health.
11

TABLE OF CONTENTS
Page
ACKNOWLEDGMENTS ii
LIST OF FIGURES v
ABSTRACT ix
Chapter
I INTRODUCTION 1
Statement of the Problem 1
Review of Related Research 2
II EXPERIMENTAL METHODS AND PROCEDURES . 15
Basic Considerations 15
Experiments with Behaving Animals
and Human Subjects 17
Experiments Using the Maxwellian View
Pattern Stimulator 23
Stimulator and Optics 23
Animals and Electrode
Implantation 29
Recording Sessions with Human
Subjects 31
Recording Sessions using Animals 32
III RESULTS 39
Human Subjects with the Behavioral Task 39
Monkeys with the Behavioral Task 43
Maxwellian View Stimulation of Humans 46
iii

Chapte Page
Maxwellian View Stimulation of Cats 51
Maxwellian View Stimulation of Monkey 81
Summary of Results 85
IV DISCUSSION 89
Human Subjects 89
Cat 89
Monkey 101
V CONCLUSIONS 103
Appendices
I BEHAVIORAL TASK TRAINING PROCEDURES
AND LOGIC CIRCUIT DESIGN 105
II USE OF DRUGS WITH PARALYZED ANIMALS 121
III RESPIRATION MONITOR AND ALARM CIRCUIT 123
BIBLIOGRAPHY 127
BIOGRAPHICAL SKETCH 131
IV

LIST OF FIGURES
Figure Page
1. Rhesus monkey performing the visual fixation task .... 21
2. Maxwellian view pattern stimulator 25
3. Schematic diagram of the Maxwellian view pattern stimulator
instrument 26
4. A test of the Maxwellian view pattern stimulator: checker¬
board patterns projected through a glass lens directly
onto photographic paper 27
5. A test of the Maxwellian view pattern stimulator: checker¬
board patterns projected through a glass lens onto a white
paper screen and photographed through the viewing channel
of the instrument 28
6. The ultimate test of the Maxwellian view stimulator:
checkerboard patterns projected onto the retina of a
paralyzed cat and photographed through the viewing channel
of the instrument 30
7. Photograph of an historical event: the experiment in which
cortical evoked responses sensitive to spatially patterned
visual stimuli were first recorded from cat 36
8. Human evoked responses to diffuse and checkerboard
patterned stimuli using the visual fixation task 40
9. Cummulative records of the performance of two human
subjects 42
10. Histogram of reaction times and cummulative record of
performance of a rhesus monkey 44
11. Evoked responses of rhesus monkey to diffuse and checker¬
board patterned stimuli using the visual fixation task. . 45
12. Responses of humans to diffuse and patterned stimuli
presented by means of the Maxwellian view stimulator. . . 48
v

Figure Page
13. Comparison of the responses evoked by checkerboard
patterned visual stimuli and a diffuse flash stimulus
which produces the same average retinal illumination. . . 50
14. Repeatability of cat cortical evoked responses to diffuse
and patterned visual stimuli within one session 53
15. The effect of pattern feature size upon the cortical
evoked response of cat 1L3 55
16. Repeatability of cat cortical evoked responses over a
two-week period 56
17. Diffuse flash cortical evoked responses to a series of
stimulus intensities 57
18. The effect of pattern feature size upon the cortical
evoked responses of cat 1K9 59
19. The effect of defocus upon responses to a 31 minute of arc
per square checkerboard patterned stimulus 61
20. The effect of a positive 9 diopter defocus upon responses
to patterned stimuli with different feature sizes .... 63
21. Simultaneously recorded optic nerve and cortex responses
to diffuse and patterned stimuli 65
22. Repeatability of optic nerve and cortical evoked responses
from cat 1K9 within one session 67
23. The effect of pattern feature size upon the optic nerve
responses of cat H-9 71
24. Pattern sensitivity of optic nerve responses at several
combinations of adaptation level and flash intensity. . . 73
25. Simultaneously recorded individual and average optic nerve
responses to diffuse and patterned stimuli 75
26. The effect of pattern feature size upon cortical evoked re¬
sponses of cat 1L1 76
27. The effect of pattern feature size upon cortical evoked
responses of cat 1L16 77
vi

Figure Page
28. The effect of pattern feature size upon optic nerve
responses of cat 1L16 78
29. The effect of pattern feature size upon optic nerve
responses of cat 1L6 80
30. Responses of a paralyzed rhesus monkey to spatially
patterned visual stimuli 83
31. Comparison of the evoked responses of rhesus monkey to an
11 minute pattern and to diffuse stimulation 84
32. Summary of the effects of pattern feature size upon the
visually evoked responses from the cortex of human,
rhesus monkey, and cat 87
33. Comparison of the spatial frequency range of the major
components of checkerboard patterns with modulation
transfer functions for the optics of the cat eye and the
human eye 91
34. Proportional amounts of defocus in terms of pattern
feature size 98
35. Behavioral training program number 1 110
36. Behavioral training program number 2 Ill
37. Fixation light control circuit for the visual fixation
task 113
38. Intertrial interval and reward control circuit for the
visual fixation task 114
39. Reward period timer circuit for the visual fixation
task 115
40. Early release and late release detection circuits for the
visual fixation task 116
41. Time-out timer and control circuit for the visual
fixation task 118
42. Stimulus flash control circuit for the visual fixation
task 119
vi 1

Figure Page
43. Reaction time ring counter circuit for the visual
fixation task 120
44. Schematic diagram of the respiration monitor and alarm
system 125
viii

Abstract of Dissertation Presented to the
Graduate Council of the University of Florida
in Partial Fulfillment of the Requirements for
the Degree of Doctor of Philosophy
ACTIVITY EVOKED IN THE VISUAL SYSTEM OF HUMAN,
RHESUS MONKEY, AND CAT BY SPATIALLY
PATTERNED AND NON-PATTERNED VISUAL STIMULI
By
Harold William Doddington
August, 1972
Chairman: Wm. W. Dawson, Ph.D.
Major Department: Physiology
Individual cells of visual cortex of cat and monkey have
been shown to be specifically responsive to the size, location, and
orientation of features in the visual field, while responses from
human scalp have been shown to be sensitive to spatially patterned
visual stimuli. To improve upon this loose relationship between
what is known about animal visual systems and the data from humans,
this research was designed to determine whether or not sensitivity
of evoked responses from several locations within the visual system
of animals (cat and rhesus monkey) to spatially patterned visual
stimuli could be observed and, if so, to describe it in terms of the
effect of pattern feature size, quality of focus, and repeatability.
Experiments were conducted with (1) attentive human, (2)
behaviorally trained monkey, and (3) cat and monkey paralyzed with
a neuromuscular blocking drug. Two methods were used. The behavioral
training program, and the Maxwellian view instrument for visual stimu¬
lation of human subjects and paralyzed animals are unique and are
described.
IX

Two monkeys were trained. A rhesus trained to the visual
fixation task well, and data from her indicated sensitivity of the
evoked response to checkerboard patterned stimuli. A sootie mangabey
did not train to the task well enough to be tested.
Data from human subjects demonstrated that the instruments
and procedures used were effective for producing spatially patterned
stimulation of the retina.
Cats with chronically implanted electrodes in the visual
system were paralyzed, stimulated by projecting onto the central
retinal area (1) an adaptation light with 30 degree diameter circu¬
lar field and (2) diffuse or spatially patterned flash stimuli cover¬
ing a 9 x 13.5 degree field, and their average responses recorded. On
occasion individual responses were examined. These were not acute
experiments. Recording sessions were 6 to 10 hours. Each animal
recovered and could undergo additional recording sessions days or
weeks later.
Results from paralyzed cats adapted to a dim photopic level
are that the evoked responses from optic nerve and visual cortex are
consistently sensitive to pattern stimulation. For equal energy
stimulation of the retina, the responses to checkerboard patterned
stimuli differ from those to non-pattemed (diffuse) stimuli primarily
in waveform, with small changes in overall amplitude. Response wave¬
forms changed progressively with pattern feature size. The optic
nerve response to 31, 55, and 79 minute patterns typically showed
increased positivity at latencies of 30 to 60 msec, which is the late
side of the primary positive peak. Difference waveforms (pattern
x

response - diffuse response) were extracted. Using defocused patterned
stimulation, it was possible to obtain responses characteristic of
diffuse stimulation.
Responses from epidural cortex electrodes on a rhesus under
neuromuscular block demonstrate sensitivity to spatial pattern stimuli
more vividly than do responses from cats. This monkey responded very
well to a checkerboard pattern of size 11 minutes of arc per square.
Response waveform changes were sufficient to cause the difference
waveform to be larger in amplitude than the diffuse response. An
11 minute patterned stimulus defocused by 1.2 diopters produced a
diffuse response.
The major conclusion of this dissertation is that animals
can be used to study the sensitivity of evoked responses to spatially
patterned visual stimulation at several levels within the visual
system. Also, it was found that a given dioptric value of defocus
was less effective in degrading responses to patterns of larger
feature sizes than those of smaller feature sizes, which indicates
that feature size and/or quantity of contrast border are of greater
importance than contour sharpness in determining evoked activity.
xi

INTRODUCTION
Statement of the Problem
Human visual pattern evoked responses which are generated by
activity in the occipital cortex indicate that populations of cortical
neurons respond differently to patterns with features of different shape,
size, contrast, etc. It is unlikely that many single cell studies will
ever be done on humans. To develop an understanding of how evoked
responses which are sensitive to visual pattern information are generated,
and of how a cell relates to the populations of which it is a part, it
would be very desirable to determine whether or not an animal prepara¬
tion can be used as a model for studying pattern sensitive evoked
responses.
If such a model could be established, many experimental ques¬
tions which could not be asked with humans could be investigated.
Examples of these questions are: (1) From the receptive field struc¬
tures of retinal ganglion cells and geniculate cells, and the sensitivity
of ganglion cell responses to degree of focus of a retinal image, one
might expect evoked responses from these stages prior to cortex to be
modified by visual pattern information. Can such changes actually be
detected? (2) Are the "cortical sources" which generate certain features
of the evoked response localizable, or are they widely distributed
throughout visual cortex?
The biggest question in this research area is: can an animal
model for studying the pattern sensitivity of visual evoked responses
1

2
be established? This is the primary problem upon which the work of this
dissertation focuses. It is not surprising that there are no reports
of work on this problem by others. There are no simple approaches.
One must be able to prove that the visual stimulus results in a well
focused patterned retinal image. The animal must be restrained and
its muscle activity minimal at the time of stimulation, but the function¬
ing of its central nervous system must not be significantly degraded
by drugs.
The two preparations to be considered are (1) an animal trained
to a visual fixation task such that it can be stimulated properly and
drugs are not necessary for restraint, and (2) a paralyzed, unanesthe¬
tized animal that can be stimulated with a specially designed visual
pattern stimulator apparatus. For both of the above cases the instru¬
ments and procedures are designed so that a human subject can be tested
under the same conditions as the animals, with the major exception
that a neuromuscular blocking drug need not be administered to a human
subject.
Review of Related Research
Very little was known about the neural processing of visual
pattern information prior to about 1950, primarily because the tech¬
niques and instruments needed to record electrical activity from single
cells in the system or to record and extract low level evoked activity
from the on-going activity had not been available.
The first technique which provided focused stimuli to the retina
was published by Kuffler in 1953. His method for recording from single
ganglion cells of the "unopened" cat eye (cornea and lens not removed)

3
was to insert a hollow needle through the temporal side of the cat
eye and then to introduce a needle electrode through the hollow needle.
By not disturbing the normal optics of the eye, it was possible to
use a focused spot of light on a tangent screen to stimulate a par¬
ticular spot on the retina. With this technique, Kuffler studied the
receptive fields of individual retinal ganglion cells. He found these
receptive fields in light adapted animals to be approximately circular
areas which, with the stimulus spot placed in a central region, caused
the cell to respond with increased or decreased firing. Placing the
spot in a peripheral portion of the field caused a response opposite
to that of the central region. In other words, these receptive fields
have an antagonistic center-surround structure. Since this initial
work, a number of researchers have studied retinal cells. A good
review of this topic has been given by Witkovsky (1971).
There is an inverse relationship between ganglion cell density
and the size of the central regions of receptive fields (Rodieck and
Stone, 1965). Measurements of the diameters of central regions of
retinal ganglion cell receptive fields give a range of 0.5 to 8 degrees
of arc for cat, and 2 minutes to several degrees of arc for rhesus monkey.
The functional organization of these receptive fields is very dependent
on the adaptation state of the eye. Under dark adapted conditions,
the effect of a stimulus spot on the peripheral part of the recep¬
tive field is greatly weakened or absent, and the effective size of
the central region of the field is increased (Kuffler, 1953). .
By recording from single retinal ganglion cells of cat, it

4
has been shown that cells at this level are very sensitive to the quality
of focus of the retinal image (Hill and Ikeda, 1971; Ikeda and Wright,
1971). In the area centralis, slightly defocusing the image (± 2 diopters)
of the spot used to stimulate the central region of the receptive field
resulted in loss of responsiveness of the cell. The axons of retinal
ganglion cells form the optic nerve and proceed primarily to the
lateral geniculate nucleus (LGN). Single unit activity of cells of
the LGN has been recorded and analyzed in various ways. In cat and
monkey, these cells have been found to have receptive fields which
are in many ways similar to those of ganglion cells. The fields are
approximately circular, have antagonistic center and surround regions,
are not sensitive to direction of motion of a stimulus, etc. They
differ from those of ganglion cells in that the central region is, on
the average, smaller (Poggio et al., 1969). Hubei and Wiesel (1961)
have published evidence "that the surround mechanism has a considerably
stronger effect in opposing the activity evoked by stimulation of the
receptive-field center in the case of the geniculate neurons as com¬
pared with optic tract units."
Also, geniculate cells are the first in this system where
binocular interaction occurs , and where general brain activity
governing excitability influences the processing of visual informa¬
tion (Angel et al., 1965; Pecci-Saevedra, 1965). Working in cat,
Poggio et a!. (1969) and Baker et al. (1969) investigated the time course
of excitatory response of geniculate cells. Poggio et al. found the size
of geniculate cell field centers to range from 0.3 to 4.5 degrees.

5
One might ask, do these cells with antagonistic, concentric
center-surround type fields respond if the whole field is illuminated
with a brief flash of light? The answer here is yes. Although there
is an antagonistic center-surround relationship, the response is not
completely negated. Both ganglion cells and geniculate cells do, in
general, show a change from their spontaneous rate of firing in this
case.
In addition to investigations of receptive field organization
for LGN cells, studies have been made to determine their responsive¬
ness to stimuli with definite spatial frequencies. Using moving
sinusoidal gratings displayed on a cathode-ray tube, cells in the cat
LGN were found to respond in the range 0.2 - 2.5 cycles/degree visual
angle. For the squirrel monkey, this range was 0.5-8 cycles/degree
(Campbell et al., 1969).
From the lateral geniculate nucleus, axons which are normally
referred to as the geniculo-striate fibers, pass via the optic radia¬
tions to visual cortex. Three distinct areas of visual cortex have
been identified anatomically and neurophysiologically. Broadmann's
area 17, the striate cortex, receives the majority of the geniculo-
cortical fibers. Areas 18 and 19 are the extrastriate cortex. They
receive afferent fibers from area 17, the LGN, and a variety of other
sources in both hemispheres.
In a series of papers, Hubei and Wiesel have described the
receptive fields and response characteristics of visual cortex cells
in areas 17, 18, and 19 of cat and monkey (Hubei and Wiesel, 1959;
1965; 1968). Their acute experiments were conducted with paralyzed,

6
lightly anesthetized animals, and they found it possible to classify
most of the cells which they recorded into one of three categories which
they defined; simple, complex, or hypercomplex. For complete defini¬
tions of these categories, it is best to refer to the original paper
(1965), but even the cells classified as simple show a more complicated
characteristic behavior than that of most ganglion cells and geniculate
cells. They found simple cells only in area 17. Area 18 contained
mostly complex cells according to their sampling, and in area 19
approximately 50 percent of the cells recorded were classified as
complex. Receptive fields of cells in the cortex are rarely circular,
usually elongated or rectangular. These cells are sensitive to shape,
size, orientation, intensity, contrast, motion, direction of motion,
and, to varying extents, location in the visual field of a light stimulus.
There is evidence that cortical cells in these areas are organized in
columns perpendicular to the cortical surface. Each column contains
cells which respond to the same location in the visual field, but
having slightly different specificities for orientation, shape, size,
etc. Such an arrangement would permit efficient interconnection of
simple cells to a complex cell, and complex cells to hypercomplex. Of
course, establishing a classification system with three categories
may well be artifical, in that there may actually be a continuum of
complexities of cell characteristics.
Since cells of the nervous system have in some cases been
known to behave differently under the influence of anesthetics and
other drugs than they do in alert, undrugged animals, it was logical
and necessary to attempt to verify the results from preparations of

7
the type Hubei and Wiesel used by designing an experiment to determine
the properties of cortical cells using alert, behaving, completely
undrugged animals. Wurtz (1969 a, b, c) carried out such experiments
with rhesus monkeys trained to a visual fixation task. His work includes
looking at the correlation between a cell's sensitivity to motion of
a stimulus and its rate of adaptation to a stationary stimulus and finds
that some units give a "vigorous and largely nonadapting response to
a stationary stimulus" of proper orientation and are not affected
by motion while other units adapt rapidly to a stationary stimulus
but are very sensitive to motion. With regard to the shape, etc.
of the receptive fields, he concludes that "the basic organization of
the simpler types of receptive fields of striate cortex neurons reported
for the paralyzed and anesthetized cat and monkey is found also in the
awake monkey."
The angular selectivity of single visual cortex cells of
paralyzed, anesthetized cats has been studied by Campbell et al. (1968)
using moving square wave gratings displayed on a cathode-ray tube.
Along with other criteria, the authors in general considered orienta¬
tion specific units to be cortical and orientation non-specific units
to be geniculo-cortical fibers. For cortical cells, they found that
"the distribution of preferred angles did not show any difference
between the oblique orientations and the vertical and horizontal
orientations."
The basic units from which the visual system is formed are the
cells. The form of this system, though its internal workings are still

8
fairly well concealed, can be seen at least in outline. On the average,
retinal receptor cells out-number retinal ganglion cells by at least
100 to 1 in human. The number of cells in lateral geniculate nucleus
is approximately equal to the number of optic nerve fibers which enter
the nucleus. And, the cells of striate cortex, let alone extrastriate
cortex, number at least one hundred times the cells of LGN. Thus, it
can be seen that there is considerable "convergence" and information
processing within the retina, some processing which is not very costly
in terms of the size of the system required at the level of LGN, and
a very large number of neurons in the organized structure of the
striate cortex.
Retinal ganglion cells are third order cells in the visual
afferent pathway which starts at the receptor cells. It is known that
receptors contain a photopigment in their outer segments; but the pro¬
cess by which absorption of the photons results in excitation of a
receptor, and eventually an active response of the second order cells,
bipolar and horizontal cells, is not understood. Apparently, the
meaningful activity of the first and second order cells of the retina
is all in terms of slow potentials because there is no significant
evidence of impluse generation by any retinal cells other than
ganglion cells.
Although the studies of the electrophysiological characteristics
of cells at the various levels of the primary visual pathway which
have been reviewed were all carried out by observing impulse responses,
activities other than impluses (frequency content 0 to less than 1000 Hz)
can be recorded at each level in the system. Neural impulses are

9
the coded form for transmission of information along axonal fibers
across distance, but transactions between cells require the exchange
of a chemical transmitter substance and their outcome is determined
by the weighted summation of post-synaptic potentials from all inputs
onto the cell which is receiving information, as seen at the axon
hillock. These timewise variations in cellular membrane potentials and
electric fields of volleys of activity in fiber tracts (slow compared
to the time course of single impulses) compose the slow potentials.
Slow potentials recorded in response to brief sensory or
electrical stimulation are commonly called evoked responses. This
term is not highly specific. For the purposes of this dissertation,
it shall refer primarily to transient electrical responses which have
fairly characteristic waveforms dependent upon the properties of the
stimulus, location recorded from, state of arousal of the animal, and,
to some extent, the individual animal.
Why are slow potentials or evoked responses of interest to
one who is studying the visual system? A brief look at the number of
channels into and out of the lateral geniculate (approximately 1
million) and the tremendous convergence and divergence present at
points, along with psychophysical phenomena, should convince one that
parallel processing of information is very important in this system.
In order to understand such a system, it is necessary to determine
what is the relationship of an individual unit to the local population,
and to the overall population. Slow potentials and evoked responses
should be valuable in this respect because they contain information
primarily related to a population, the size of which depends upon the

10
recording conditions. For at least the following reasons, models of
the visual system mechanism for processing spatial pattern information
should be based on more than data from microanatomical and single unit
studies. There are no techniques for simultaneously studying impulses
from more than two or three cells out of a very small functionally related
group of cells (Glaser, 1971). In addition, there is no guarantee
that all neurons in geniculate or cortex are capable of generating im¬
pulses, and single units are usually selected for study because accept¬
able impulses can be recorded from them. Also, single unit studies
are admitted to be biased against the smaller cells in a population.
What work has been done relative to evoked responses to
diffuse and patterned visual stimuli and their relationship to anatomy
and single unit activity?
Because slow potential activity recorded in response to a
sensory stimulus on an EEG-type record is the combination of on-going
background activity and stimulus related activity, and because the
background activity is often the larger of the two, it was very difficult
to study the stimulus related activity until Dawson (1954) published
his photographic superposition technique for extracting this stimulus
related activity. By 1961, a special purpose averaging computer had
been developed to perform this extraction process (Clark, 1961).
Given tools to work with, researchers began to look at sensory evoked
responses.
The first paper concerned with both diffuse and patterned stimuli
came in 1965 (Spehlmann). In 1967, a paper by Rietveld et al. addressed
this topic in greater detail. Recording differentially from an electrode

11
on the scalp 1% cm above the inion and on the midline with reference
to an electrode on an earlobe, they compared the responses of 8 dark
adapted human subjects to diffuse and checkerboard patterned light
flashes of 2 microsecond duration. This study includes experiments
to determine the effect of the size of squares in the black and
white checkerboard pattern, the effect of flash intensity, the contribu¬
tion of central and peripheral retina to the pattern response, and the
comparison of responses to checkerboard patterns with those to line
gratings and diamond patterns. The results show that for most human
subjects checkerboard patterns containing squares of 10 to 60 minutes
of arc (visual angle) width evoke responses that are larger in ampli¬
tude and noticeably different in waveform than responses to diffuse
flashes from which the same light flux enters the eye. Checkerboard
pattern responses fall into a continuum with respect to pattern size,
with the largest, most distinct pattern responses being generated by
patterns with approximately 12 to 15 minute squares while responses
to patterns with larger or smaller features are smaller in amplitude
and their waveforms become indistinguishable from diffuse flash
responses as pattern features reach extreme sizes. Responses to patterns
of diamonds are approximately the same as to checkerboards. Line
gratings evoke responses which are different in waveshape than, and
intermediate in amplitude to those for diffuse or checkerboard patterns.
For patterned stimuli the response amplitude is proportional to the
contrast of the pattern presented. A pattern of 20 minute squares
produces much larger responses when presented to the fovea than when
presented to the peripheral retina. There is no data on this point

12
for other pattern sizes.
Another report, Harter and White (1968), describes research
which utilized diffuse and checkerboard patterned flashes to look
at the effect of defocusing the patterned flashes upon the evoked
responses of human subjects. The findings of this experiment were
that defocusing by ± 3 diopters effectively made the responses to
a pattern of 12 minute squares indistinguishable from responses to
a diffuse flash, but that a larger amount of defocusing was necessary
to degrade responses to patterns of larger squares by the same amount.
A study of the binocular addition of visual responses
evoked by dichoptic presentation of diffuse and patterned stimuli has
been done by Ciganek (1971). It was found that the response to stimu¬
lation of one eye with a white field or a large (4.5 degrees) checker¬
board pattern was suppressed by the simultaneous stimulation of the
other eye with a small (1 degree) checkerboard pattern.
Very recent publications by Jeffreys (1968, 1970) discuss
responses he has obtained from scalp electrodes on human subjects
by stimulating specific regions of the visual field with checkerboard
patterned stimuli, and what he thinks these responses indicate
about the locations of the source regions within the cortex which
generate the activity seen as the significant peaks within the first
200 milliseconds of the response. Of course, it is not known how
concentrated or diffuse these "source regions" might be.
To obtain evidence for spatially selective and orientationally
selective "channels" in the human visual system, Campbell and Maffei
(1970) have recorded a steady-state driven evoked response to

13
sinusoidal grating patterns which undergo 180 degrees phase reversal
at a rate of 8 times per second. They found that the slope for the
graphical curve of evoked response amplitude versus the log of the
pattern contrast could be significantly increased by simultaneously
presenting more than one spatial frequency or orientation while
maintaining the same total light from the stimulus. These authors
found that spatial frequencies for which "channels" of human central
vision are sensitive are all above 3 cycles/degree of visual angle.
So, visual evoked potentials from human subjects are sensitive
to type of visual pattern presented, size of features, orientation,
contrast, and what region of the retina is stimulated. They are also
effected by whether or not the subject is performing a task which
directs attention toward or away from the visual stimulus (Kopell
et al., 1969).
To determine the relationship of evoked responses to the
anatomy and to single unit activity, animal research has been done,
but it has all been done with diffuse flash stimuli. Fox and O'Brien
(1965) demonstrated that the curve of probability of firing for a single
cortical cell can duplicate rather well the evoked response which can
be recorded from the same electrode after death of the cell.
Creutzfeldt et al. (1969) published a study of simultaneously recorded
surface evoked potentials and intracellular potentials from area 17
of cat in response to flash stimulation. Their findings and discussion
of the relationships of cortical cell activity to evoked potentials
are interesting, and, hopefully, it will someday be possible to do
similar experiments with both pattern and diffuse flash stimuli.

14
Evoked potentials have been recorded from the scalp and cortex of
unanesthetized monkeys (Hughes, 1964; Spinelli, 1967; Vaughan and
Gross, 1969). In none of these experiments were the monkeys trained
to a task. They were simply placed in a primate chair and faced
toward the flashing stimulus. Hughes, and Vaughan and Gross were
interested in the effects of visual system lesions. Spinelli had
used simple patterned stimuli, but showed no significant effect.
This review of literature, although it has been selective,
hopefully does help to point out the situation at this time. Research
to determine what spatial features in the visual field cells at
different levels in the visual system respond to has been done in
animal experiments by single unit recording of impulses. Other investi¬
gations, which look at the responses of populations of neurons to
patterned visual stimuli, have been done only with human subjects.

EXPERIMENTAL METHODS AND PROCEDURES
Basic Considerations
A primary requisite to studying responses of an animal or
human subject to spatially patterned visual stimuli is a means of
producing a retinal image of as high a quality as the optics of the
given eye will permit. In general, there are two possible approaches
to satisfying this condition which do not constrain the possible stimuli
to particular geometric forms (such as interference patterns) or
colors. The first possible approach is to obtain the cooperation of
the animal (or human subject) in fixating and focusing upon a pattern
within the visual field. The second approach is to maintain the eye
in a fixed position and project the desired pattern directly onto the
retina. The two groups of experiments described in this work relate
to these two approaches.
A corequisite of the need to produce a well focused retinal
image is that of acquiring an excellent quality original or master
copy of the pattern to be used. With human subjects, the most effec¬
tive patterned stimulus yet described, in terms of response amplitude
and extent of waveform change, is a checkerboard pattern composed of
squares having a side length of 10 to 15 minutes of arc of visual
angle. To produce a master checkerboard pattern, a two layer paper
assembly, one black sheet adhered on top of a white sheet with wax,
was lightly criss-crossed with a scalpel blade and every other black
15

16
square lifted off. From this master pattern several series of photo¬
graphic transparencies were produced, each transparency in a series
containing a different number of squares.
To produce stimulation of a human subject or animal, a flash
of light was projected through one of these transparencies in an
appropriate projection system. Since half of the squares are trans¬
parent and half of them are approximately opaque black, the light
transmitted by such a pattern is equal to ((light of the flash - surface
reflection) x h - (loss due to the optical density of the film base)).
The optical density of the "opaque" features in these patterns is
slightly greater than 3.0 log units.
To produce nonpattemed stimulation which delivers the same
quantity of light to the retina, a "diffuse" pattern which is a piece
of clear film base plus a 0.3 log unit neutral density gelatin filter
has been assembled. By use of diffuse and patterned stimuli which
provided equal energy to the retina, it was expected that responses
to stimulation with a diffuse pattern, a pattern of features too small
to be resolved by the subject, or a completely defocused pattern
should be the same.
An additional constraint imposed by the author upon the design
of these experiments was that insofar as practical the apparatus to
deliver the stimulus flashes to animals was designed to work equally
well with human subjects. This allowed (1) recording of responses
from human subjects which can be compared with published data, and (2)
recording of animal responses which can, to some extent, be compared

17
to human responses.
Experiments with Behaving Animals and Human Subjects
When one wishes to study activity of the nervous system,
experiments with behaving animals have some distinct advantages over
other types of experimental procedures. Two of these are that (1)
the experiments can be completely free from the effects of drugs and
(2) that a large amount of data.can be collected from each animal.
The disadvantage of such experiments is the relatively large invest¬
ment which must be made for each animal and the resulting limitation
on the number of animals which can be studied.
Because of the similarity of its retina and the remainder of
its visual system to that of human, a rhesus monkey (Macaca mulatta)
was chosen for this experiment. In addition, since the human evoked
response indicates that patterned stimulation is most effective when
presented to the macular region of the retina, and this is the region
of highest cone density, it is possible that a primate having a higher
cone density throughout the retina than either human or rhesus would
show an exaggerated example of sensitivity to patterned stimulation.
Therefore, a behaving sootie mangabey (Cercocebus torquatus) was
chosen as a second animal to be studied with this procedure (Kolmer,
1930). The animals used were (1) a young adult female rhesus and
(2) a young adult female sootie mangabey. Neither had previously
been in a primate chair.
The rhesus had never been behaviorally trained. She was
6 years old and weighed 13 lb 14 oz (6.3 Kg) before training started.

18
Ophthalmoscopic examination, measurement of intraocular pressures,
and retinos copy showed her eyes to be normal with less than ^
diopter spherical myopic error each. The mangabey was approximately
6 years old and weighed 13 lb 0 oz (5.9 Kg). She had previously
been trained to a behavioral task. An eye examination, as above,
showed her eyes to be normal except for 2 diopters spherical myopic
error in each.
These animals were kept in individual cages and, by the method
of Glassman et al., 1969, were transferred to a primate chair daily
for training or recording sessions. Free food was supplied in the
cages, but water was provided only while an animal was in the chair.
At the beginning of every training session, the animal was weighed.
To obtain visual evoked response data from these animals,
a visual fixation task (Wurtz, 1969a) was designed. For training and
testing the animals, a programmable logic system to control the behav¬
ioral task and an acoustic chamber (IAC model AC-5) with a ground
glass projection screen window in one wall were available. A ceiling
light illuminated the inside of the chamber. The luminance of the
white chamber wall around the projection screen was 5 to 10 candela/square
meter and that of the screen itself was 1.1 candela/square meter.
Two 35 mm slide projectors with solenoid operated shutters projected
through a light-tight tunnel onto the projection screen. A cummulative
recorder and a bank of decimal counters were available to monitor
performance. Using these facilities, the final task was designed and
then the several stages of training necessary to train the animal up
to the final program were created (see Appendix I).

19
The specific purpose of the final task was to obtain the
cooperation of the animal in sitting quietly, fixating and focusing
upon a small spot of light in the center of the projection screen
for a period during each trial when a stimulus flash might or
might not be presented. The task devised required the animal to
operate a single manipulandum with its hand. The animal received
two types of cues, audible and visual, and was rewarded for a correct
trial by a sip of water. For a correct trial the sequence of events
was as follows: When no audible or visual cues were being given, the
animal was allowed to pull and hold the manipulandum lever. Upon
pulling the lever, a small spot of light (the fixation light) was
projected on the center of the screen and remained there for a period
controlled by a variable interval timing system. If the animal held
the manipulandum and then released it within a short time after the
offset of the fixation light (the reward period), then a tone which
signified a correct response and indicated the duration of the intertrial
interval was turned on and after a brief delay the sip of water was
delivered. When the intertrial interval ended, after 10 or 15 seconds,
the animal was permitted to start a new trial. Within such a trial
a stimulus flash which covered the whole projection window was some¬
times delivered. The flash was semirandomly positioned within the
fixation light period. A stimulus flash of 600 msec duration was
used because the system was not able to produce very brief flashes.
The response recorded was that to the onset of the flash (Harter, 1971).
Use of a limited hold procedure (Moody, 1970) assured that the animal
must watch the fixation light very carefully in order to consistently
complete trials correctly.

20
As a penalty for an incorrect response, a time-out accompanied
by a warbling audible error signal was imposed. The basic time-out
period was of fixed duration, but could be restarted during the time¬
out period if the animal operated the manipulandum. An incorrect
trial occurred when the animal released the manipulandum before the
offset of the fixation light or too long afterwards, or pulled the
manipulandum during the intertrial interval.
Normal durations for the time periods in the task were:
fixation light period was variable with a uniform probability density
distribution over the range 1% seconds to 8 seconds, reward period
was 500 milliseconds, intertrial interval was 10 seconds, and a time¬
out was approximately 30 seconds.
To prevent any possibility that sounds from the system out¬
side the acoustic chamber might be used by the animal as cues, audio
white noise up to 50 KHz was produced at a moderate sound level
inside the chamber.
The rhesus monkey was trained from adapting to the primate
chair to performing the final task well in a period of 5 months,
training 6 days per week. Figure 1 shows this monkey while she was
performing the task. The animal was always provided a measured amount
of water on days when training sessions were not conducted. Using the
same procedures, the mangabey required more than one year to train.
For recording, a small region of the scalp over striate cortex
just anterior to the nuchal ridge and approximately 2 cm from the mid¬
line was locally anesthetized with xylocaine and a wound clip with
a recording lead was attached to the scalp at this location, which

21
Figure 1. Rhesus monkey performing tire visual fixation task. The
monkey is seated in a primate chair, viewing the projection screen
window in the chamber wall, and operating the manipulandum with
one hand. Her mouth is not far iron the drinking tube through
which sips of water were issued as rewards for correct completion
of trials.

22
is within 2 degrees of the cortical representation of the fovea.
This served as the active electrode. For differential recording,
the indifferent electrodes were a pair of ear clips tied together
through equal resistances. These electrodes were worn by the animal
for several training sessions before an actual attempt to record
responses was made.
During the programming and testing of the behavioral task,
several humans performed the task under the same conditions which
the animals were later to encounter except that the water reward
was not issued. When development of the system was complete, responses
to diffuse and patterned visual stimuli were recorded from several
subjects. For differential recording, the active electrode was placed
on the scalp using electrode paste at 1.5 cm above the inion on the
midline and an ear clip was used as the indifferent electrode.
The recording equipment used included a special low-noise
differential input preamplifier (similar to Schuler et al., 1966)
with a fixed gain of 80, followed by another amplifier with adjustable
gain and usually set to 150. Both amplifiers were designed and constructed
by the author, and had a bandwidth of 0.1 Hz to greater than 30 KHz,
3 db. The amplified signal was FM tape recorded (0.1 to 1000 Hz) and
a sync pulse was recorded on the same tape on a direct record channel.
Recordings were played into a Fabritek model 1052 averaging computer.
Individual responses could also be examined by displaying them on the
computer. Averaged and individual responses were printed from the
computer by a strip-chart recorder.

23
Experiments Using the Maxwellian View Pattern Stimulator
Stimulator and Optics
To study the evokec ^.ponses of paralyzed animals to spatially
patterned visual stimulation, one could refract an animal and then
present the patterned visual stimulus on a tangent screen in front of
the animal. A superior technique, however, is to refract the animal,
then project the spatially patterned visual stimulus directly on the
retina, and simultaneously view the retinal image and focus it for
maximum sharpness. This is the technique which has been developed
and utilized in this dissertation. It permits the experimenter to
know exactly what region of the retina is being stimulated and be
confident that in each experiment the image of the stimulus pattern
is well focused upon the retina. Since the visual pattern stimulator
apparatus which has been used is unique, it is described here. The
instrument is basically a three optical channel instrument. It consists
of one ophthalmoscopic viewing channel, one Maxwellian view adaptation
light channel, and one Maxwellian view patterned stimulus channel.
The pattern stimulating channel is based upon a design described by
Westheimer (1966). This channel is combined with the other two channels
using a beam splitter in front of the eye to be stimulated. This
particular pattern stimulating channel has the distinct advantage that
the pattern can be continuously focused through a 5 diopter range
without changing either the overall size of the retinal region being
stimulated or the size of any feature in the pattern in terms of
visual angle. On the other side of the coin, once a setting for
optimum focus has been determined a pattern can be used as its own

24
control by defocusing it through several diopters and not changing
the retinal region being stimulated or the size of any feature in it.
Figure 2 is a photograph of the pattern stimulating instrument.
The original binocular viewing apparatus has been replaced by a monocular
one to improve light efficiency of the viewing channel. Another
modification, which is not visible, was the addition of a cross hair
with a bead at its midpoint in an image plane within the viewing
tube. This provided a fixation point for human subjects and an aid
to the experimenter in centering and focusing the ophthalmoscope on
an animal retina. A schematic diagram of the instrument is given in
figure 3.
Extensive testing of the pattern stimulating instrument has
been carried out both optically and by conducting evoked response
experiments with human subjects. To demonstrate that the instrument
is capable of producing an excellent patterned image on the retina, a
lens was placed in front of the instrument and a piece of photographic
paper was then exposed by flashes through the pattern stimulating channel.
The results of this test are shown in figure 4. With the same lens and
white paper screen, the patterned images were photographed through the
viewing channel (with eyepiece and erecting lens removed) to demonstrate
the effect of the viewing channel upon the image seen by the experimenter.
The results of this test are shown in figure 5. As a final demonstra¬
tion of the ability of the instrument to project a patterned image
upon the retina, a cat was paralyzed, fitted with a contact lens,
refracted, and several patterns were projected onto its retina and
photographed through the viewing channel of the instrument. Two of

25
Figure 2. Maxwellian view pattern stimulator. The pattern
stimulating channel is contained in the case which is mounted
on top of the ophthalmoscope. 'The lid on top of the case allows
insertion of a patterned transparency, optical filter, or an
occluder. A calibrated scale for the range of focus is inside
the case and the knob to control pattern focus is on the side of
the case not seen.

BS1,BS2 - Beam splitters
M - First surface mirrors
ND - Neutral density filter
P - Pattern transparency
X—X - Range of focus
Figure 3. Schematic diagram of the Maxwellian view pattern stimulator instrument

27
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EXAMPLES OF THE QUALITY OF PATTERN WHICH THE PATTERN STIMULATOR
INSTRUMENT IS CAPABLE OF PROJECTING.
A.
00
o
nun/sq .
pattern = 17
. 5 min
i. of arc
= 68 /u on the
: retina
B.
1.4
mm/sq.
pattern = 31
min.
of arc =
120
p on
the
retina
C.
2.5
mm/sq.
pattern = 55
min.
of arc =
212
p on
the
retina
D.
3.6
mm/sq.
pattern = 79
min.
of arc =
306
p on
the
retina
Figure 4. A test of the Maxwellian view pattern stimulator:
checkerboard patterns projected through a glass lens directly
onto photographic paper.

28
Figure 5. A test of the Maxwellian view pattern stimulator:
checkerboard patterns projected through a glass lens onto a white
paper screen and photographed through the viewing channel of the
instrument. A modification of the original design of the viewing
channel to improve light efficiency at the expense of image
quality was made to allow viewing of the fundus at the standard
adaptation light level chosen for use in these experiments (780
trolands retinal illumination).

29
these photographs are shown in figure 6.
Obviously, to study responses of a paralyzed animal to
spatially patterned visual stimuli, one must have high quality contact
lenses available because the surface of the cornea of a paralyzed
animal dries, becomes irregular, and does not permit the projection
of a high quality image onto the retina. In agreement with Vakkur
et al. (1963), for cat a supply of zero diopter plastic lenses of
radius 8.7 mm has served nicely. Additional correction, when necessary
was obtained by first utilizing the range of focus available in the
instrument and then by placing a lens in the spectacle plane.
The patterns projected by this instrument were on trans¬
parencies that were mounted in 35 mm slide frames.
Animals and Electrode Implantation
Six cats were used in these experiments. They were all
healthy adults and weighed between 2.5 and 4.3 Kg. One young adult
rhesus monkey (Macaca mulatta) was used. He weighed 3.8 Kg.
The operations to place chronically implanted electrodes into
these animals were conducted as follows. Ten mil insulated stain¬
less steel wire electrodes with sharpened tips were prepared in advance
An animal was anesthetized with sodium pentobarbital (Nembutal). A
surgical anesthetic level was achieved with an initial dose of 15 to
20 mg/kg administered i.v.. Additional anesthetic was given later
in the procedure, if necessary. Using sterile surgical techniques,
bipolar electrodes were placed in optic nerve and lateral geniculate
nucleus stereotaxically while photically evoked responses were

30
Figure 6. The ultimate test of the Maxwellian view stimulator:
checkerboard patterns projected onto the retina of a paralyzed
cat and photographed through the viewing channel of the
instrument. In order to be photographed, it was necessary
that the light pass through the optics of the cat eye twice
and the viewing channel of the instrument once. Also, it
should be noted that while light entered the eye through only
a portion of the cornea and pupil, the whole pupil was used
in viewing the retina. Therefore, the actual retinal images
were superior to the images photographed.

31
recorded from them to insure that the desired structure had been
located. One mm diameter stainless steel ball electrodes were placed
epidurally over striate and extrastriate visual cortex (Bilge et al.,
1967; Daniel and Whitteridge, 1961). As an indifferent electrode,
on cats a loop of stainless steel wire was mounted on top of the skull
over the frontal sinus. Electrodes were held in position by fastening
their shafts to the skull with a fast-setting acrylic plastic. A
block of plastic into which two holes had been drilled and tapped
was cemented onto the skull centered on the midline at approximately
anterior 24 mm. The electrode leads were brought to a miniature elec¬
trical connector socket and this socket was mounted in a mound of
plastic on top of the skull. The animal was allowed to recover from
the anesthetic and returned to its cage.
Recording Sessions with Human Subjects
An effective way to test the Maxwellian view pattern stimulator
instrument was to use it to record evoked responses from human subjects.
Three subjects were used. They were all between 20 and 30 years old.
Two were emmetropic and had not been recorded from previously. CD wore
corrective lenses. An active electrode placed on the scalp at 1.5 cm
above the inion on the midline and an ear clip indifferent electrode
were used. A subject was placed in a well darkened electrically
shielded room, given a chin rest in front of the stimulator, and asked
to fixate a tiny bead at the midpoint of a vertical hairline through
the center of the adaption light field. Patterns, light levels, and
recording instruments were as described in the next section for animals.

32
Recording Sessions using Animals
In a brief sketch, during a recording session an animal was
anesthetized with halothane gas anesthetic long enough to intubate
it with a tracheal cannula and administer the initial dose of neuro¬
muscular blocking drug i.v., then it was moved into an electrically
shielded room, placed on a continuous infusion of neuromuscular block¬
ing drug, respirated, refracted and corrected, allowed to adapt to
a fixed background field (adaptation light), and then it was stimu¬
lated and recorded from for a period of from 6 to 8 hours. At the end
of this period the animal recovered from the paralysis and was returned
to its cage.
The priorities within these recording sessions were (1) to
determine whether or not the visual evoked responses of cat or monkey
are sensitive to spatially patterned visual stimulation and if so,
then (2) the effect of pattern feature size, (3) whether or not the
responses are repeatable over hours and weeks, and (4) the effect of
quality focus of the pattern. In accordance with these priorities,
the general plan for recording sessions was to obtain responses from
the animal to diffuse flashes and to a series of 4 or 5 checkerboard
patterns of different feature size, to run this series in forward
and reverse pattern feature size order 2 or 3 times within one session,
to run selected patterns additional times, and using the range of
focus built into the stimulator, or external lenses, to compare
responses to a given pattern when it was well focused and when it was
defocused by different amounts.
The choice and order of averaging runs within each session
was generally in agreement with the priorities stated. This project

33
was essentially exploratory, and therefore a rigid design or order
for the collection of the responses was not appropriate.
The use of drugs was carefully limited to the minimum necessary
to restrain the animal and maintain it in a good physiological state.
Appendix II provides details of the use of drugs in this project.
Over all sessions with cats, the average amount of gallamine triethi-
odide (Flaxedil) administered was 9 mg/kg/hr.
To obtain an early warning of any significant change in the
operation of the animal's respiratory or cardiovascular systems, a
respiration monitor and alarm system to monitor end expiratory (X>2
level was designed and developed by the author. It was used in con¬
junction with a Beckman LB-1 CO2 meter. The details of this system
are given in Appendix III.
To conduct a recording session after all the necessary instru¬
ments were ready, a chronically implanted animal was given an injec¬
tion of atropine i.m., and received one drop of one percent cyclopentolate
hydrochloride (Cyclogyl) topically applied to each cornea. After 30
minutes, halothane gas anesthetic was administered to the animal.
The anesthesia was continued while respiratory and cardiac activity
were watched and reflexes were tested. When proper anesthetic level
was reached, the animal was intubated with a tracheal cannula (inflat¬
able cuff type). The 23 gauge needle of a scalp infusion set which
was attached to a syringe was inserted into a vein. The animal was
switched from the C^, halothane mixture to air and allowed to breath
off the anesthetic as the initial dose of neuromuscular blocking drug,
gallamine triethiodide, was administered slowly through the i.v. catheter.

34
At this point the animal was connected to a positive pressure respirator
pump. The animal was wrapped with an automatically controlled heating
blanket and placed on a canvas bed which has a metal frame and, on the
front end, an adjustable head-holding bracket which can be screwed
to the plastic block on top of the animal's head. The animal was then
connected to a syringe pump. The CC>2 monitor and alarm system was
now in operation, and the respirator was adjusted to give a steady 4
percent end expiratory CO2 level. The EEG was continuously monitored
with an oscilloscope. Using palpation or a stethoscope, a check on
the rate and strength of the heartbeat was made frequently. Each
cornea was treated topically with a steroid solution (dexamethasone),
and a few minutes later a solution of methylcellulose was applied to
each cornea and to the inside surface of the contact lenses. The
contact lenses were placed on the eyes and examined with an ophthal¬
moscope to insure that the fit was acceptable and no air bubbles were
trapped under the lenses. The eyelids and nictitating membrane were
held open by an opaque ring cemented to the outside surface of the
contact lenses. Normally the eye to be stimulated was fitted with a
high quality transparent plastic lens and the other eye with an
opaque black plastic lens. Next the fundus of the eye to be stimulated
was then studied with a hand-held ophthalmoscope to become familiar with
the pattern of blood vessels, especially, for cats, those in the
neighborhood of the area centralis. Area centralis was located using
the pattern of retinal vessels and the coordinates, 14.7 degrees radi¬
ally from the center of the optic disc at an angle of 22 degrees above
horizontal and temporal to the optic disc (Vakkur et al., 1963).

35
In the paralyzed animal, because the eye maintains a fixed position
in the orbit, it was possible to orient the eye to be stimulated
by orienting the head. Therefore, the bracket to which the animal’s
head was attached was adjusted to align the visual axis of this eye
to a straightforward horizontal gaze.
A streak retinoscope was used to refract the eye and any
corrective lens necessary was mounted in the spectacle plane on the
front of the stimulator instrument. The pattern stimulator instrument
was then positioned in front of the eye and aligned such that the
area centralis (macular region for monkey) was in the center of the
field of view, which for this instrument was approximately 30
degrees in diameter. The ophthalmoscopic viewing channel was focused
and the adaptation light level adjusted. A patterned stimulus trans¬
parency was placed in the instrument and with a continuous light source
temporarily attached to this channel the pattern was projected, viewed,
and focused on the retina. The focus was tested with several trans¬
parencies with different feature sizes and the focus adjustments were
made as many times as necessary in order for the investigator to be
confident that the best possible focus had been achieved. The light
source to the pattern stimulating channel was then changed from con¬
tinuous to xenon flash. Figure 7 shows a cat during an actual recording
session.
The standard adaptation light intensity used produced a retinal
illumination of 780 trolands. A standard flash intensity was chosen.
Using the diffuse pattern, the investigator when at the standard adapta¬
tion level found the standard flash intensity to be 2.5 log units above

36
Figure 7. Photograph of an historical event: the experiment
in which cortical evoked responses sensitive to spatially
patterned visual stimuli were first recorded from cat. The
cat is paralyzed, artificially respirated, and warmed by an
automatically controlled heating blanket which has been laid
aside for the photograph. Its head is supported and positioned
by a plastic bracket. The animal is wearing a contact lens in
its right eye and is being visually stimulated (left eye
occluded). Recording is from chronically implanted electrodes.

37
threshold for detection of the flash.
Trigger pulses to both stimulation and recording equipment
were normally spaced 2.25 seconds apart. The xenon flashes (approxi¬
mately 10 ysec duration) for the stimulator were obtained from a Grass
model PS2 photostimulator and conducted to the pattern stimulator
through a 1/8 inch diameter fiber-optic bundle. Recording channel
amplification was obtained from Tektronix type 122 differential ampli¬
fiers with (-3 db) bandwidth normally set at 0.2 to 1000 Hz. No
additional filters were used during paralyzed animal experiments.
The amplified individual responses were sometimes FM tape recorded
simultaneously with the averaging process which was performed using
a Fabritek model 1052 averaging computer.
The stimulator, respirator, infusion pump, recording equip¬
ment, and data logs were carefully attended to between runs. Appropri¬
ate action was taken if any sign.-, of excessive wetness in the lungs
or mucous in the trachea were encountered. The volume of drug remain¬
ing in the syringe pump was watched closely, and a record of the
quantity of drug infused was kept. The retina was viewed periodically
to insure that the position of the retinal landmarks had not changed.
The time interval between averaging runs ranged from 4 to 15 minutes.
Averaged responses normally contained 64 individual responses, but on
occasion contained 32, 128, or 256 responses.
When all of the desired runs for a recording session had been
collected (or up to 30 minutes before), the infusion was switched to
glucose saline with no gallamine before the stimulator was moved away
from the animal. A final check of the retinal position and corneal

38
transparency was made to insure that no change had occurred during
the experiment, and then the contact lenses were removed. The animal
was observed until a diaphragm reflex or movement of the jaw was
seen, and then neostigmine was administered slowly through the i.v.
catheter up to a maximum of 25 nanograms per kilogram. When the
animal's ability to respirate itself was adequate, it was disconnected
from the apparatus. After a period for observation, the animal was
returned to its cage.
The procedure for recording from paralyzed monkey was
essentially the same as that for cat with the exceptions of different
drug dosages, different contact lens curvatures, and different retinal
landmarks.
All animals used in these experiments survived and were
healthy afterwards.

RESULTS
Human Subjects with the Behavioral Task
When given verbal instructions, human subjects were able to
learn the behavioral task in a small number of trials. Two human sub¬
jects served in this experiment. The evoked responses recorded from
them are shown in figure 8, and the simultaneously recorded cumulative
records of their performance are shown in figure 9.
The subjects were attentive and very few errors were made.
At latencies between 100 and 300 milliseconds, the response waveforms
were sensitive to pattern feature size, but the waveform changes were
not large. In the pattern responses of subject HD, an increase in the
negativity of the peak at 120 milliseconds was the largest waveform
change. For subject CD, the difference between pattern and diffuse
responses is a positive-negative-positive sequence which starts at a
latency of 170 milliseconds in the 15 minute of arc pattern response
and translates to noticeably shorter latencies in the responses to
patterns with larger feature sizes.
In this research, responses were recorded from human subjects
in order to provide data for comparison of the two methods used with
each other and with responses in the literature. The effectiveness
of each of these methods for producing spatially patterned visual stimu¬
lation of animals should be proportional to its effectiveness with
human subjects.
39

Subject = HD * TilKe in Milliseconds Subject » CD Time in Milliseconds
Figure 8. Human evoked responses to diffuse and checkerboard patterned stimuli using the visual
fixation task. 32 individual responses/average response.

Figure 9. Cummulative records of the performance of two human subjects. In the records of trials and
responses, each upward step marks the beginning of a trial. For subject HD, each downward stroke indicates
the correct completion of a trial. In the records of time-out periods, the duration of the negative going
pulse indicates the duration of the period.

time between
TRIALS
AND
RESPONSES
Start
TIME-OUT
PERIODS
TRIALS
AND
RESPONSES
TIME-OUT
PERIODS
Start
Finish

43
Monkeys with the Behavioral Task
In training, the rhesus made steady progress, especially
from the point where she was given the manipulandum and discovered
that it controlled the task. After 5 months of training, she was
correctly completing greater than 80 percent of all trials started.
At this time, fixation light periods required some lever holds up to
10 seconds and the fixation light spot was dim, usually colored (red
or green) and subtended 25 minutes of arc (visual angle). For two
weeks, ear clip electrodes were placed on her during training sessions.
By Sh months, 95 percent of all trials started were being completed
correctly. A record of the animal's performance and the distribution
of her reaction times to the offset of the fixation light is given
in figure 10. Evoked responses recorded simultaneously with that
performance are given in figure 11. These responses have positive
peaks at 90, 130, and 150 milliseconds and a distinct negative peak
at approximately 107 milliseconds. The responses to 15 and 20 minutes
per square patterns are noticeably larger in amplitude than the response
to diffuse flash stimulation. Pattern sizes were not repeated during
that recording session. Visual comparison of the set of 32 individual
responses to the 15 minute per square pattern with those to the diffuse
stimulus shows that the responses to the patterned stimulus were more
consistent and larger than those to the diffuse stimulus.
In several other sessions when some responses were recorded
from this animal, recording procedures and electrodes were being
developed and no usable sets of data were obtained. All data collection
from the scalp electrode was preliminary to data that was to be

Trials
and
Responses
Time-Out
Periods
wv—u—\y— vr~ ■ - ■ trww"" ■ v~ 1 u
Finish
Figure 10. Histogram o£ reaction times and cummulative record of performance of
rhesus monkey. (A) The average is based on 5 consecutive sessions and contains
the session shown in B as the 3rd session. (B) Same type record as in figure 9.
Note that all but a few of the time-outs were caused by a lever pull during an
intertrial interval (exceeding the speed limit) rather than an incorrect response

45
Time in Milliseconds
Figure 11. Evoked responses of rhesus monkey to diffuse and checkerboard
patterned stimuli using the visual fixation task. 32 individual
respcnses/average response.

46
recorded from an array of implanted ball type epidural electrodes
over striate cortex. The implant operation was performed under sodium
pentobarbital anesthesia. Near the end of the operation the veter¬
inarian who was present administered a small additional dose of
anesthetic, i.m. (phencyclidine hydrochloride (Semylan)), and its
facilitatory interaction with the pentobarbital was fatal to the
animal.
The increase in amplitude of the 15 and 20 minute per square
pattern responses over that of the diffuse response was interpreted
as a possible indication of sensitivity of the rhesus evoked response
to pattern.
The mangabey trained well until the manipulandum was intro¬
duced. From that point on, her progress was not steady. After several
months of training, she knew the task well enough to perform it very
well on some days, but her performance was not consistent. Her motiva¬
tion to work seemed not to be very well related to deprivation or weight
loss. Additional problems developed and when the rhesus died, it was
decided by the author not to pursue the training of this animal further.
The mangabey's training has been continued by another student, but no
data has been recorded at this time.
Maxwellian View Stimulation of Humans
The visually evoked responses of three human subjects to
diffuse and patterned stimuli are shown in figure 12. They demonstrate
sensitivity of the human evoked response to spatially patterned stimuli.
Responses from these same subjects are presented in figure 13 along with
"difference waveforms" derived by subtracting from the given response

Figure 12. Responses of humans to diffuse and patterned stimuli presented by means of the Maxwellian view
stimulator. Averages of 64 responses with 2.25 seconds between stimulus flashes.

100 200 300 400
500 0 100 200 300 400
Time in Milliseconds
500 0

Figure 13. Comparison of the responses evoked by checkerboard patterned
visual stimuli and a diffuse flash stimulus which produces the same
average retinal illumination. Difference waveforms (pattern response -
diffuse response) are given for 3 human subjects. Stimulation was by
means of the Maxwellian view stimulator.

50
55
31
S = PS
79'
17.5
11
Diffuse
S = CD
Diffuse
v^Vyv\v^/V'/^^
vV
w¥\
J I I
u*jk
?yMíí!!y''l,,,H
yvW'H/
i i i !j i .- j

51
(pattern or diffuse) a diffuse response. Pattern sizes were repeated
for each of these subjects and the responses were consistent. The
effects of patterned stimulation relative to diffuse stimulation, and
pattern feature size, as seen in these responses, are in agreement with
the findings of Rietveld et al., 1967, and Harter and White, 1968.
Maxwellian View Stimulation of Cats
The cortical responses of the first two cats from which
responses were recorded, were observed as they were averaged on line
in an attempt to identify any changes in waveform which correlated with
a change from diffuse to patterned stimulation or vice versa. Such
changes were identified and the stimulus conditions were alternated
a number of times to determine whether or not the changes were repeat-
able and consistent. Figure 14 shows such data for cat 1L3.
In the upper half of figure 14, average responses were recorded
for a diffuse stimulus and a checkerboard patterned stimulus, alternately,
over a 70 minute period, and they showed consistent waveform features
in the response to pattern which were not found in the response to
diffuse stimulation. To demonstrate the consistency of this data, the
averages of all five average responses for the diffuse stimulus and
for the 31 minute patterned stimulus are also given in figure 14.
Using the terms defined by Creutzfeldt, et al., (1969) for the components
of the cat cortical visual evoked response, the response to 31 minutes
of arc pattern stimulation has a more distinct wave I-wave II complex,
a double peaked wave III, and a larger wave IV than the response to
diffuse stimulation. Also, in the lower portion of figure 14, are

Figure 14. Repeatability of cat cortical evoked responses to diffuse and patterned visual stimuli within
one session. Time of run is printed along the ordinate. In the upper portion of the figure runs were alter¬
nated between the diffuse and the 31 minute pattern stimuli. An ensemble average of 5 average responses is
given for each of these 2 cases. In the lower portion of this figure, responses to 2 other pattern sizes
are compared. Using terms defined by Creutzfeldt et al. (1969), waves I, II, III, IV, and V are labeled on
the ensemble averages. Each average response contains 64 individual responses to stimulus flashes 2.25
seconds apart.

Cat 1L3
6:39 pm
6 :25 pm
6:12 pm
5:57 pm
5:43 pm
Average
0 100 200
Time in Milliseconds
on

54
responses to two patterns with different feature sizes obtained in
an alternating series of averaging runs. It can be seen that the
waveform changed with pattern feature size and that the changes were
consistent with this series.
For cat 1L3, the array of different response waveforms obtained
with pattern feature size is displayed along with difference waveforms
(pattern minus diffuse) in figure 15. These responses were practically
duplicated by responses recorded from the same animal thirteen days later
and displayed in figure 16.
A series of responses to different intensities of diffuse
flash were recorded. It is shown in figure 17 and is evidence that
the waveforms seen with patterned stimuli are not obtained with
different intensities of diffuse stimulation.
One chronically implanted optic nerve electrode was in place
in cat 1L3, but was not usable in this experiment because in a prelim¬
inary recording session the cornea of the eye ipsilateral to the im¬
planted nerve was damaged by a tight fitting contact lens. Therefore,
stimulation for the data described above was to the eye contralateral
to the implanted nerve.
Another animal, cat 1K9, possessed usable electrodes both in
optic nerve and on cortex (epidural). Responses recorded from this
animal are shorn in figures 18 through 22. During the first recording
session with this animal, problems with the recording equipment limited
the data collection and only responses from cortex were recorded.
Figure 18 shows the effect of pattern feature size. Responses from
epidural striate and extrastriate electrodes were quite similar and

EXTRASTRIATE CORTEX AVERAGE RESPONSE DIFFERENCE WAVEFORM
Figure 15. The effect of pattern feature size upon the cortical evoked response of cat 1L3.
64.individual responses/average response. 2.25 seconds between stimulus flashes.

56
Time in Milliseconds
Figure 16. Repeatability of cat cortical evoked responses
over a two-week period. Compare these responses with
those which were recorded 13 days earlier and shown in
figure 15. The recording conditions were the same as for
figure 15.

57
EXTRASTRIATE CORTEX AVERAGE RESPONSE
i 1 i i u
0 100 200
Time in Milliseconds
STRIATE CORTEX AVERAGE RESPONSE
Figure 17. Diffuse flash cortical evoked responses to a series of
stimulus intensities. Striate and extrastriate responses were
recorded simultaneously. Electrode locations: striate - P3.0, L3.0;
extrastriate - P3.0, L8.0 (both epidural).

Figure 18. The effect of pattern feature size upon the cortical evoked responses of
cat 1K9. Both the striate and extrastriate responses are sensitive to spatial pattern.
64 sweeps/average. 2.25 seconds between stimulus flashes. Electrode locations:
striate - P3.0, L3.0; extrastriate - P3.0, L8.0 (both epidural).
* Difference waveforms are left extrastriate responses minus the upper diffuse response.

Cat 1K9
Control
i i i i i i
0 40 80 120 160 200
i i i i i i i i 1 1 1 >
0 40 80 120 160 200 0 40 80 120 160 200
Time in Milliseconds
on
oo
LEFT STRIATE AVERAGE RESPONSE
LEFT EXTRASTRIATE AVERAGE RESPONSE
DIFFERENCE WAVEFORM *

Figure 19. The effect of defocus upon responses to a 31 minute of arc per
square checkerboard patterned stimulus. Difference waveforms (+0.12 D
response - other response) are helpful in determining which features in
the response waveform are influenced by quality of focus and to what
extent. Cat 1K9 striate electrode. 64 sweeps/average response.

61
O 40 80 120 160 200 0 40 i80 120 160 200
Time in Milliseconds Time in Milliseconds
31 MINUTE PATTERN RESPONSE DIFFERENCE WAVEFORM

Figure 20. The effect of a positive 9 diopter defocus upon responses to patterned stimuli with different
feature sizes. Note that there is very little sensitivity of response to pattern when the pattern is
defocused by 9 diopters. 64 sweeps/average response.

LEFT
I I I I L.
0 40 80 120 160
FOCUSED
0
40
80
120
160
*
DIFFERENCE WAVEFORM

Figure 21. Simultaneously recorded optic nerve and cortex responses to
diffuse and patterned stimuli. In the cortical responses note that the
peak of wave III is at a different latency in the 79 minute pattern
response than in the diffuse response.

65
STRIATE CORTEX
DIFFERENCE WAVEFORM
OPTIC NERVE

Figure 22. Repeatability of optic nerve and cortical evoked responses
frcrn cat 1K9 within one session. In addition, an example of the effect of
4 diopters of defocus is given.

67
OPTIC NERVE

68
the shape of waves III, IV, and V was influenced by feature size in
both cases. Figure 19 shows the effect of various degrees of defocus
upon the response to the 31 minute of arc pattern where defocus was
achieved by use of an external lens in the spectacle plane. Figure
20 describes the effect of a constant +9 diopter defocus upon patterns
of different feature size. It demonstrates that when well defocused,
the feature size of a patterned stimulus did not influence the waveform
of the response and that the difference waveform between responses to
a focused and a defocused patterned stimulus was very similar to that
between responses to a focused patterned stimulus and a diffuse stimulus
as shorn in figure 18 for data recorded one hour earlier. The external
lens was used because the large (1 to 2 degrees of arc per square)
patterns to which the evoked responses from the cats in these experi¬
ments were found to be sensitive were not well defocused by the maximum
use of the range of focus built into the stimulator. Use of different
external lenses introduces small changes in pattern feature size on
the retina and size of retinal region stimulated, in the form of
spectacle magnifications, which will be discussed later.
In a second recording session, one week later, both optic
nerve and cortex responses were recorded from this same animal. The
effect of pattern feature size upon these responses can be seen in
figure 21. The agreement of these cortex responses with those recorded
one week earlier is evidence for repeatability of this effect within
one animal. Also, it is very important to note that the optic nerve
responses are sensitive to pattern stimulation and to pattern feature
size. Most noticeable is an increase in the positivity of the wave¬
form at a latency of 25 to 35 milliseconds, which is on the late side

69
of the primary positive peak of an optic nerve response to diffuse
stimulation. The repeatability of the responses within one session
with this animal was also tested and the results are shown in figure
22. Included in the same figure is an example of the effect of
+4 diopter defocus with zero spectacle magnification upon the response
to a 31 minute of arc per square pattern and a 79 minute of arc per
square pattern. This data indicates that an equal amount of blurring
of contrast borders for patterns with different size features does
not cause the evoked responses to revert equally well to the waveform
characteristic of diffuse stimulation.
A third animal, cat H9, provided excellent data on the optic
nerve. For this animal, the effect of pattern feature size is shorn
in figure 23. There is good agreement between data shown in this figure
and that in figure 21. Also, responses to the 11 minute of arc per
square pattern are included in this figure and it can be seen that the
difference between responses to this pattern and to diffuse is either
insignificant or very small. This was a consistent finding from
several animals. The control run was taken late in the session and
shows that the animal had developed some synchronous EEG activity which
was not evident earlier in the session.
To be assured that the effects of pattern feature size upon
the visually evoked optic nerve potential were not absolutely peculiar
to one set of adaptation level and stimulus intensity conditions, the
recordings given in figure 24 were made with half the normal flash
intensity, twice the normal flash intensity, and twice the normal flash
intensity at ten times standard adaptation level. Pattern sensitivity
of the response was evident under each of these conditions.

Figure 23. The effect of pattern feature size upon the optic nerve responses of cat H-9. Time of run and
feature size are given for each response. 128 sweeps/average response.

CAT OPTIC NERVE - AVERAGE RESPONSES

Figure 24. Pattern sensitivity of optic nerve responses at several
combinations of adaptation level and flash intensity.

73
o 100 200
Time in Milliseconds
CAT OPTIC NERVE - AVERAGE RESPONSES

74
A difference in the average response waveforms to two different
stimuli implies a difference (on the average) in the individual responses
to these stimuli. In figure 25, sets of typical individual optic nerve
responses are given. The differences between these sets of individual
responses can be seen. For diffuse responses, the largest peak of the
primary positivity is at a latency of 30 to 34 milliseconds and a smaller
peak or an inflection is usually seen at 55 milliseconds. For the 31
minutes of arc pattern the largest peak of the primary positivity occurs
between 40 and 50 milliseconds. In general the agreement between indivi¬
dual and average responses in this set of data is quite good.
There were no useable cortex electrodes on this animal.
Of the remaining three cats, two provided data on the cortex
and two on the optic nerve. These three animals were very large^old,
male cats and their responsiveness to pattern was not as clear and
vivid as that in the data already given. They were, however, sensitive
to pattern. For cat 1L1, typical cortical responses to diffuse and
patterned stimuli are given in figure 26 along with difference wave¬
forms. These responses are unusual in that wave TV is small and in
the 31 minute of arc response the second peak of wave III is quite
large.
Cortex responses of animal 1L16 are shown in figure 27.
They are repeatable within one session. A negative-positive-negative
sequence is seen in these responses in the time period normally occupied
by a single negative-going wave IV. Optic nerve responses from this
same animal are shown in figure 28 along with their difference wave¬
forms.

75
-6 34 114 194 Msec.
Diffuse
-6 34 114 194 Msec.
31' Pattern
INDIVIDUAL OPTIC NERVE RESPONSES
AVERAGE OPTIC NERVE RESPONSES
Figure 25. Simultaneously recorded individual and average optic
nerve responses to diffuse and patterned stimuli. Cat H-9.

O 100 200 0 100 200
Tine in Milliseconds Tine in Milliseconds
STRIATE CORTEX DIFFERENCE WAVEFORM
Figure 26. The effect of pattern feature size upon cortical evoked responses of cat 1L1.

O 100 200
Time in Milliseconds
STRIATE CORTEX
0 100 200
Tir.e in Milliseconds
DIFFERENCE WAVEFORM
'-j
Figure 27. The effect of pattern feature size upon cortical evoked responses of cat 1L16.

OPTIC NERVE DIFFERENCE WAVEFORM
Figure 28. The effect of pattern feature size upon optic nerve responses of cat 1L16.

79
From the sixth cat, 1L6, optic nerve responses are given in
figure 29. All responses from this animal were taken with standard
adaptation level and twice normal flash intensity.
In general, for all four cats from which optic nerve data were
obtained, the effect of patterned stimulation as opposed to diffuse
stimulation on the optic nerve response was consistent. Observation
of the responses and the difference waveforms shows that either the
slope of the rise to the primary positive peak, or the amplitude of
the early side of the peak, is slightly reduced, and either the ampli¬
tude of the late side of this peak increases, or a second positive peak
or inflection develops on the downward slope. The effect of pattern
feature size appears to modulate the extent to which the changes in
waveform just described occur.
There is considerable variation between cats in the cortical
evoked response to diffuse flash. For this reason, the effect of
patterned stimulation must be judged with respect to the diffuse
flash response of the same animal. In the response to a pattern of
medium sized features (approximately 30 minutes of arc per square),
waves I and II are usually unchanged, a double peak on wave III is
either developed or enhanced, and when they can be identified, waves
IV and V usually increase in size.
The smallest effective pattern feature size used was 17.5
minutes of arc per square. The largest feature size used (79 minutes)
was effective. The most noticeable effect of pattern feature sizes
within this range was to modify the amplitude and latency of the peaks
of wave III.
Responses of the lateral geniculate nucleus were recorded

O 100 200
Tine in Milliseconds
OPTIC NERVE
0
100 200
Time in Milliseconds
DIFFERENCE WAVEFORM
00
o
Figure 29. The effect of pattern feature size upon optic nerve responses of cat 1L6.

81
(using bipolar wire electrodes) from five of these cats simultaneously
with those from optic nerve or cortex. No consistent effect of patterned
stimulation was identified.
Maxwellian View Stimulation of Monkey
Visually evoked responses from an epidural electrode over
striate cortex of a paralyzed rhesus monkey are shown in figures 30
and 31. These responses were obtained during a lh hour recording
session within which the animal heart rate, body temperature, and
expired CC>2 were very stable at normal values. At the end of the
session this animal recovered rapidly from the neuromuscular blockade
and was in good health. There was a clearly visible sensitivity of
this response to pattern feature size. Within this session, pattern
feature sizes were repeated and the responses were consistent.
In terms of their polarity and latency (in milliseconds),
the peaks which are starred in figure 31 are P39, N46, P67, P96,
P124, and N240. The difference between a pattern response and a diffuse
response is best described by the difference waveform in figure 31.
It is important to recognize that the difference waveform is not small;
it is larger in amplitude than the diffuse response. Responses from
this animal indicate that the effect of pattern feature size is to con¬
trol the magnitude of the activity represented by the difference wave¬
form shown, without changing the latency of the difference waveform
peaks.
A response to a defocused patterned stimulus is less different
from a diffuse response than is a response to a well focused patterned
stimulus. To defocus a patterned stimulus to the extent that the

Figure 30. Responses of a paralyzed rhesus monkey to spatially patterned visual
stimuli. Each stimulus was projected onto the macular region using the Maxwellian
view pattern stimulator. The effects of pattern feature size, and +1.2 and +2.5
diopters of defocus are displayed. These recordings are from a chronically
implanted epidural electrode within 1 degree of the foveal representation in
striate cortex. The indifferent electrode was a stainless steel wire loop under
the scalp over frontal cortex. An average response contains 64 individual
responses to flashes 2.25 seconds apart.

O 100 200 300 400 500
Time in Milliseconds
4:03 pm
3:36 pm
3:13 pm
2:53 pm
2:33 pm
i L
0 100
FOCUSED STIMULI
200
+
32 jjv
I
1 1 L_
300 400 500
Time in Milliseconds
DEFOCUSED BY +1.2 D.
I i i i i—
0 100 200 300 400
Time in Milliseconds
DEFOCUSED BY +2.5 D.
500

84
Difference
Waveform
11' Pattern
Diffuse
0 100 200 300 400 500
Time in Milliseconds
Figure 31. Comparison of the evoked responses of rhesus monkey
to an 11 minute pattern and to diffuse stimulation. The
difference waveform (pattern - diffuse) indicates that the
change in the activity evoked within the visual system between
these two cases is large and that rhesus monkey is a good
animal in which to study the generation of visual evoked
potentials using both patterned and non-pattemed visual
stimuli. Asterisks mark peaks at latencies of 39, 46, 67, 96,
124, and 240 milliseconds. Recording conditions are the same
as in figure 30.

85
related response is identical to a diffuse response, fewer diopters
of defocus are required for a pattern of small feature size than for
a pattern of larger feature size, at least within the feature size
range used in this experiment. Note that for this animal, the minimum
dioptric value of defocus necessary to effectively eliminate sensitivity
to a patterned stimulus was less than or equal to 1.2 diopters for
an 11 minute pattern and approximately 2.5 diopters for a 55 minute
pattern.
Summary of Results
The results which have been presented show that the visually
evoked responses of human, cat, and rhesus monkey cortex and cat optic
nerve are sensitive to spatial pattern. Since the sensitivity of
human visually evoked responses to checkerboard patterned stimuli had
already been established, responses were recorded from human subjects
primarily to demonstrate that the procedures and instruments used were
effective. Difference waveforms which are helpful in appreciating and
comparing the sensitivity of the response to each of the stimulus
conditions \\Tere derived from the recorded responses. For cat and
monkey, evidence for sensitivity to pattern was obtained by comparison
of responses to diffuse stimulation and patterned stimulation which
provided the same average retinal illumination. Responses to the
diffuse stimulus and to patterns of different feature size were different
and repeatable (see figure 32). The effect of quality of focus upon
responses to patterned stimuli was studied within this project primarily
as a control measure to obtain evidence that pattern features and not

Figure 32. Summary of the effects of pattern feature size upon
the visually evoked responses from the cortex of human, rhesus
monkey, and cat. The responses of each species are pattern
sensitive. 64 sweeps/average response.

87
20 uv
200 msec.
HUMAN
/V^v
SY^v,‘■^-v,"','M^Vv*V*'',^^
60 uv
200 msec.
MONKEY
48 uv
80 msec.
CAT
AVERAGE RESPONSE
DIFFERENCE WAVEFORM

88
some minute intensity differences or other factors were the cause of
the waveform changes observed in the evoked responses. For cat, it was
also shown that cortical responses were sensitive to pattern under
several conditions of adaptation level and flash intensity, and that
responses characteristic of a patterned stimulus were not obtained
using diffuse stimulation with flash intensities greater or less than
the intensity normally used. Sensitivity of optic nerve responses of
cat to patterned stimuli was observed in individual as well as average
responses.

DISCUSSION
Human Subjects
The evoked responses of human subjects to diffuse and patterned
visual stimuli were recorded by two methods, the visual fixation task
and the Maxwellian view stimulation. By comparison of these too sets
of data it is evident that the two methods were not equally effective.
Only one of the four subjects was recorded from by both methods. The
results were consistent; the responses obtained using the Maxwellian
view stimulator were more vividly responsive to pattern than those
obtained using the behavioral task. It is now evident that although
the responses obtained during use of the behavioral task were pattern
sensitive, additional development of this technique is needed to improve
its effectiveness. The ultimate goal should be to make these too
methods equally effective so that working with monkeys, the same
animals could be tested first with the behavioral task and second
when stimulated by the Maxwellian view method.
Cat
Data presented in the results of this dissertation indicate
that the visually evoked responses of cat optic nerve and visual
cortex are sensitive to spatially patterned stimuli. It is reasonable
to expect the pattern sensitivity of these responses to be related to
the quality of the optics of the eye, retinal grain or ganglion cell
density, and ganglion cell dendritic field sizes.
89

90
The most recent and very well done work on the quality of optics
of the cat eye is that of Wassle (1971). In figure 33 the modulation
transfer function (MTF) determined by Wassle is compared with the
feature sizes of checkerboard patterns available for use with the Max¬
wellian view stimulator. The MTF is based upon sinusoidal frequency
components. An intensity profile obtained by a scan across a checker¬
board pattern would be a squarewave. From the analysis of a square-
wave, it is known that given a system with a squarewave applied to its
input, the system's transfer function must pass the fundamental, third,
and fifth harmonics reasonably well in order for the output to approxi¬
mate a squarewave. In this figure horizontal line segments that are
terminated at the left end with vertical dashes which indicate the
fundamental spatial frequency, and which are terminated at the right
end with a triangle which indicates the fifth harmonic frequency, are
used to indicate the spatial frequency range necessary to transmit
a checkerboard pattern with the feature size indicated onto the retina
and maintain reasonably sharp contrast borders on the features.
Data from this research shows the evoked responses of cat to
be sensitive to patterns with feature sizes from 79 minutes of arc
per square (the largest pattern used) down to 17.5 minutes of arc per
square. It was a consistent observation that stimulation with the
pattern having a feature size 11 minutes of arc per square produced
responses which were not noticeably different from diffuse flash
responses. The pattern of feature size 5 minutes of arc per square
was rarely used with cats. These features are smaller than the visual
acuity of cats as determined by behavioral testing (Dews and Wiesel,

f-r i- h3 i
¡O3 /O' ¡O •, /
SpdiAÍ Frehuehcj ¡H CjcIes/ Minuta
Figure 33. Comparison of the spatial frequency range of the major components of checkerboard
patterns with modulation transfer functions for the optics of the cat eye and the human eye
(MFT's from Wassle, 1971). The frequency range indicated for each pattern feature size_
extends from the fundamental indicated by the vertical dash to the 5th harmonic undicated by
the triangle.

92
1970; Smith, 1936). Thus the finest pattern for which a sensitivity
of the evoked response was noticeable, that is 17.5 minutes of arc
per square, in terms of the fundamental, third, and fifth harmonics of
the spatial frequency of the pattern should be represented in the
retinal image at relative contrasts of 0.85, 0.57, and 0.27 respectively.
For the 11 minute pattern, these values are 0.75, 0.35, and 0.0; which
indicates that for the retinal image of the 11 minute pattern, the
amount of blur due to non-ideal optics of the eye is quite signifi¬
cant.
The largest pattern feature size used in these experiments did
produce responses which were sensitive to patterned stimulation, there¬
fore no statement can be made from this data concerning the largest
pattern feature size that is effective for producing responses which
are noticeably different from those to diffuse flash. Since for the
larger patterns, the frequency components which are important to the
production of a good retinal image reach the retina with relative
contrasts close to 1.0, the optics of the eye do not limit the
sensitivity of the visual system to pattern feature sizes of 1.5 degrees
of arc per square and larger. Ganglion cell density in the cat retina
has been mapped by Stone (1965) and the distribution is unimodal
with its maximum at the center of area centralis.
The region of retina with finest retinal grain, area centralis,
should be the region which responds best to small pattern feature sizes.
The effect of retinal location was not deliberately studied in this
research. The retinal region stimulated was always centered on area

93
centralis, but the overall size of the region stimulated was large
enough to allow the question, where within the region are the cells
which respond most vigorously to each of the effective pattern feature
sizes located, to arise.
Dendritic field sizes for the giant ganglion cells in the
retina of cat have been determined by Honrubia and Elliot (1970) using
silver stained whole retina flat mounts. They found the dendritic
fields for cells in the center of area centralis to be approximately
70 microns in diameter. Field size increases as distance from the
center of area centralis increases. It is interesting to observe
the correspondence between the smallest field size measured by
Honrubia and Elliot and the size on the retina of the smallest pattern
feature which was effective in modifying the waveform of the evoked
response (17.5 minutes of arc = 68 microns on the retina). Leicester
and Stone (1967) have measured dendritic field diameters of the smaller
(not giant) deep multidendrite ganglion cells of the cat retina and
report the range to be 70 to 710 microns, the same as found for the
giant ganglion cells. Rodieck and Stone (1965) have reported the
electrophysiologically determined receptive field centers of giant
ganglion cells of cat to range from 85 to 880 microns.
In the present dissertation research, the retinal region
stimulated was rectangular, 9 by 13.5 degrees, and centered on the area
centralis. Therefore, the perimeter of the retinal region stimulated
was 4 to 7 degrees from the center of area centralis. At this distance,
Honrubia and Elliot show giant ganglion cell dendritic field diameters
to range between 200 and 300 microns. The largest pattern feature size

94
used in these experiments was 306 microns per square and therefore
squares of this size at the perimeter of the pattern should have
stimulated ganglion cells quite effectively. For each of the 4
effective pattern feature sizes used, an annulus of ganglion cells
in the retina had dendritic fields which matched in size the features
of the pattern.
If the set of ganglion cells responding most strongly to each
of these patterns was different, there are several possible approaches
for testing this hypothesis. One could stimulate a smaller retinal
region and compare the pattern feature size which evokes the best
response to pattern when area centralis is stimulated with the feature
size which is most effective when the region stimulated is 4 to 7
degrees from the center of area centralis or even more peripheral.
Another interesting approach would be to create a patterned stimulus
containing a series of feature sizes with the smallest feature size
in the center of the pattern and matched in size to the known size of
ganglion cell dendritic fields in the area centralis, and progressively
larger features in the more peripheral portions of the pattern. Responses
to this pattern could be compared to responses to patterns each con¬
taining only one feature size.
Data presented in the results of this dissertation indicate
that there is a consistent effect of patterned stimulation upon the
optic nerve response: the slight reduction in the slope or amplitude
of the rise to the primary positive peak of the response and usually
quite noticeable increase in the positivity on the late side of this

95
peak. This finding may relate to the observation of Rodieck and Stone
(1965) and Barlow et al. (1964) that stimulation of ganglion cell
receptive field centers produces activity with a shorter latency than
stimulation of the receptive field surround. Rodieck and Stone note
that the activity in response to surround stimulation may be up to
60 milliseconds later than that evoked by center stimulation alone.
Since stimulation of the whole receptive field normally produces a
response characteristic of the center, and diffuse flash stimulation
of the retina does stimulate entire receptive fields, the optic nerve
response to diffuse flash would be expected to result from ganglion
cell activity characteristic of center stimulation. A diffuse flash
should produce virtually no ganglion cell activity characteristic
of stimulation of surround alone. Checkerboard patterned stimulation
with patterns having a feature size reasonably well matched to recep¬
tive field center size should produce ganglion cell activity character¬
istic of surround stimulation in a number of cells which is approximately
equal to the number of cells that respond in a manner characteristic
of receptive field center stimulation, and many cells which receive
receptive field center stimulation and very little surround stimulation
should respond more vigorously than to diffuse flash. Therefore the
waveform of the optic nerve response to patterned stimulation can be
interpreted as indicating that retinal ganglion cells respond either
in smaller number or less vigorously in a manner characteristic of
receptive field center stimulation and that the increase in positivity
on the late side of the primary positive peak is an indication that a
reasonable percentage of the cells which do respond to the stimulation

96
respond in a manner characteristic of surround stimulation.
The cortical response waveforms for the effective patterned
stimuli, 17.5, 31, 55, and 79 minutes of arc per square were each
different. The amplitudes of several peaks in the response were
effected so as to cause a change in waveshape without any striking
change in the overall amplitude of the response. If a different set
of retinal ganglion cells in an annular region is stimulated most
effectively by each pattern feature size, perhaps the changes in wave¬
shape of the cortical responses to different pattern feature sizes
relate to the different locations of the cortical representations of
the most stimulated ganglion cells.
Another topic which warrants discussion is that of the effect
of defocus of the retinal image upon patterned stimulation. Basically,
defocusing a patterned stimulus converts each point of light in the
focused image into a blur (spot) circle, the radius of which is pro¬
portional to the amount of defocus and the size of the pupil of the
eye or, in the case of Maxwellian view stimulation, the diameter of
the focal spot at the pupil of the eye. For a given Maxwellian view
pattern stimulating instrument, the radius of blur circles produced
by a certain dioptric value of defocus is constant and independent
of feature size, therefore contrast borders of pattern features are
blurred by the same amount independent of the feature size. An amount
of blur sufficient to reduce a pattern of small feature size to approxi¬
mately diffuse stimulation will not reduce a pattern of larger feature
size to approximately diffuse stimulation. .An example of the effect
of a constant amount of defocus upon patterns of different feature

97
size can be seen in figure 22. Four diopters of defocus were sufficient
to reduce the response to a 31 minute of arc pattern to an approximately
diffuse response, but the same 4 diopters were not equally effective
with a 79 minute of arc pattern.
From the data collected for this dissertation, it was clear
that an amount of blurring of contrast borders which is small relative
to the size of the features in a patterned stimulus was very ineffec¬
tive in reducing the response to the patterned stimulus, and that
pattern feature size was effective in determining the waveform of the
evoked response whether the patterned stimulus was well focused or
slightly defocused. There is no evidence in the literature on responses
of human subjects to checkerboard patterned stimulation in which several
dioptric values of defocus were used (Harter and White, 1968; Harter,
1971) that any consideration was given to the actual relationship of
the size of blur circles given by a dioptric value of defocus to
the pattern feature sizes used. To determine, for cat, the relation¬
ship between blur circle radius and pattern feature size over the whole
range of realistic dioptric values of defocus, the schematic eye
derived by Vakkur et al. (1963) and a focal spot diameter of 3 mm at
the pupil were used to calculate the points through which the curves
in figure 34 are drawn. An assumption that the optics of the eye were
ideal (MTF = 1.0) was used. Actually, in teims of visual angle, these
curves are also valid for experiments with human or monkey when using
the same stimulator. Each curve in this figure represents a constant
ratio between blur circle radius and pattern feature size. These rela¬
tionships can be used to determine the effect of a certain dioptric
value of defocus which is necessary to create a proportional defocus

Figure 34. Proportional amounts of Defocus in terms of pattern feature
size. Defocusing changes each point of light in a focused image into a
blur spot which is proportional in size to the extent of defocus. Each
curve represents a constant ratio of blur spot radius to pattern
feature size.

99
effect for a different pattern feature size.
By comparison of figure 19 with the proportional defocus
figure, it can be seen that 4 diopters of defocus, which produced blur
spots of radius S//2", were necessary to effectively eliminate the
sensitivity of the cortical evoked response to a 31 minute of arc
pattern. A defocus of 2 diopters, which produced a blur spot radius
of S/3 or greater, was only partially effective.
In the paper titled "Effects of Contour Sharpness and Check
Size on Visually Evoked Potentials" by Harter and White, human subjects
were seated in a darkened chamber and viewed stimuli presented on a
translucent screen, under these conditions the diameter of a subject’s
pupil would be approximately 6 mm and the radius of a blur spot
created by a given dioptric value of defocus would be twice as large as
the blur spot radius which can be determined from figure 34 (proportional
defocus). Therefore, even assuming that the eye has perfect quality
optics, the minimum blur spot radius in the retinal images were 10.3
minutes of arc for 1 D (diopter) of error, 20.6 for 2D, 30.9 for 3 D,
and approximately 62 for 6 D. The results published in that paper
indicate that visually evoked responses to a 20 minute checkerboard
pattern were only slightly effected by a 1 D error, were moderately
effected by a 2 D error, and were greatly effected by errors of 3 D
or larger. In other words, to produce a moderate degradation of the
visually evoked response to checkerboard pattern stimulation these data
indicate that an amount of defocus which produces a blur spot radius
approximately equal to the pattern check size is necessary. Data from
responses to 12 minute, and 46 minute patterns support this finding also.

100
When an image of a checkerboard pattern is defocused to the extent
that light from each point on a contrast border is spread completely
across the squares adjacent to the border, it is difficult to speak
about contour sharpness since pattern features are practically eliminated
in this condition.
From their data Harter and White conclude "the results of this
study indicate that certain components of the averaged evoked cortical
potential are very sensitive to the sharpness of contour of a patterned
visual stimulus and to the size of the elements of the pattern." In
terms of the quality of contour sharpness in the retinal image and on
the basis of the data from human and from cat it is not possible to
agree with this statement. From the point of view of one who is
interested in the relationship between the quality of the retinal image
and the evoked response to a patterned stimulus, the evoked response is
amazingly insensitive to the degree of contour sharpness. It appears
that the cortical evoked response is primarily sensitive to pattern
feature size and that as feature size is modified or practically eliminated
by reduction of contour sharpness the cortical evoked response becomes
less characteristic of that to a patterned stimulus and more character¬
istic of that to a diffuse stimulus. Two points need to be mentioned:
(1) Harter and White state that accommodation was not completely con¬
trolled during their experiment and (2) certainly perceived contour
sharpness and contour sharpness in the retinal image are not identical.
Physiologically, one must influence the activity of retinal
ganglion cells before activity is evoked in visual cortex or an image
can be perceived. In this sense it is reasonable to interpret the

101
finding that a relatively large blur of contrast borders is necessary
in order to significantly effect the cortical evoked response as an
indication that pattern feature size is of greater importance in
determining the activity of a retinal ganglion cell than is contour
sharpness. It should be noted that in response to defocus of a
patterned stimulus the optic nerve response is influenced to approxi¬
mately the same extent as the cortical evoked response (see figure 22).
Monkey
The quantity of data obtained from the behaving rhesus monkey
was very small and can be taken only as an indication of pattern sensi¬
tivity of the cortical evoked response. The data obtained from the
paralyzed rhesus monkey is strong evidence that the evoked responses
of monkey are sensitive to pattern stimulation, at least at the level
of visual cortex. Since the same stimulator was used with this animal
as with the paralyzed cats the information in the figure concerning
the proportional defocus is directly applicable. In general the optics
of the eye of man and rhesus monkey are of similar quality and it is
significant to note that in this experiment where the animal was adapted
to a low photopic level rather than being dark adapted, and.where
no accommodation was possible, the amount of defocus of contrast
borders relative to pattern feature size which was required to eliminate
the sensitivity of the evoked response to the patterned stimulus was
less than for the human subjects in the work of Harter and White. For
example, their results show that 1 D of defocus produced a sight to
moderate degradation of the cortical evoked response to a 12 minute
checkerboard pattern with human subjects, while the cortical evoked

102
response of this rhesus to an 11 minute checkerboard pattern was
completely degraded by 1.2 D of defocus. A defocus of 1.2 D with an
11 minute checkerboard pattern produced blur spots of radius 0.7
times the side length of a pattern feature, and physically this
should be sufficient to effectively eliminate pattern features.

CONCLUSIONS
The primary conclusion of this dissertation is that cat and
monkey can be used to study the sensitivity of evoked responses to
spatially patterned visual stimulation. This conclusion is based
upon the findings that (1) the evoked responses of cat optic nerve and
cortex, and monkey cortex are modified in waveform in response to a
checkerboard patterned stimulus compared to diffuse stimulation of the
same average intensity, (2) these responses are repeatable over hours
and weeks, (3) the response waveforms are sensitive to pattern feature
size, and (4) the response waveform characteristic of a given patterned
stimulus can be reduced to that characteristic of a diffuse stimulus
by defocusing the patterned stimulus.
In addition, there is a remarkable correlation between (1) the
smallest pattern feature size which is effective in producing evoked
responses which are different in waveform from diffuse flash responses,
(2) the smallest checkerboard pattern feature size for which any of the
fifth harmonic of the fundamental spatial frequency passes through the
optics of the cat eye, (3) the histologically determined dendritic field
size of retinal ganglion cells in area centralis, and (4) the electro-
physiological ly determined receptive field center size for ganglion
cells in area centralis. This indicates that the spatial selectivity
demonstrated by the relatively small sample of ganglion cells from
which single unit activity has been recorded is reasonably character¬
istic of the cells in the population as a whole.
103

104
It was also concluded that there is no evidence at the
present time to support the statement that the averaged cortical evoked
response to a patterned visual stimulus is very sensitive to the sharp¬
ness of contrast borders. In terms of the contrast borders in the
retinal image, the amount of defocus necessary to have more than a slight
effect on an average response to a checkerboard pattern must be sufficient
to blur each border approximately half way across each adjacent square
and,for different feature sizes, the dioptric value of defocus necessary
to obtain comparable effects is proportional to the pattern feature
size. A study specifically designed to examine this issue carefully
should be performed.
In general, this was an exploratory study. Its positive
results provide the basis for many related and/or more detailed pro¬
jects to gain knowledge of the processing of spatial pattern informa¬
tion in the visual system and to investigate the relationship of single
unit activity and evoked responses. The two methods of visual pattern
stimulation developed in this research are valuable tools. Specifically,
in this project Maxwellian view visual pattern stimulation of paralyzed
animals has proven to be a very effective technique. Further develop¬
ment of these methods will provide the means to approach a number of
experimental questions.

APPENDIX I
Behavioral Task Training Procedures and Logic Circuit Diagrams
A basic description of the visual fixation task used is given
in the methods section of this dissertation. The early training of a
monkey is described here.
To train an animal up to the final task after it had accepted
sitting and eating or drinking while in a primate chair, the chaired
animal was set in the acoustic chamber for longer periods each day for
approximately one week. At this point the magazine training of the
animal was initiated. During each training session, the chaired animal
was placed in the acoustic chamber and viewed through a peep hole by
the experimenter. Water rewards were issued to the animal via the drink
tube in the chamber when the experimenter judged that the animal was
sitting sufficiently still facing forward toward the projection screen.
The water rewards were always accompanied by the audible tone which
would later be used to signify a correct release of the manipulandum
and the duration of the intertrial interval period. The length of
time for which the animal was required to sit still facing forward
was gradually increased. After several days, program I, which is
manually controlled by the experimenter but provides a slight delay
between the onset of the tone and an automatically measured reward,
was instituted. Only a few training sessions were necessary on program
I before transferring to program II. Program II starts with the tone
and measured reward as in program I, but in this case the period of
the tone is automatically timed and as the tone is turned off, a large
105

106
spot of light is presented on the screen. This spot of light remains
on the screen until turned off by the experimenter at the end of the
trial. As in the previous program, each trial was initiated by the
experimenter when the monkey was sitting quietly facing the screen.
When the monkey was familiar with this program and sat quietly for
periods of up to 30 seconds while the light was on, a new element was
added to the system. The manipulandum was placed in the chamber in
front of the primate chair. By the method of successive approximations
the monkey was taught to reach and pull the manipulandum lever. The
experimenter no longer freely initiated trials, but only in response to
an attempt as good or better than previous attempts to reach out and
pull the manipulandum. Thus at this stage of training the monkey
learned that it could initiate a trial which could result in a reward
by pulling the manipulandum when all audible and visible cues were
absent. Additional pulls during the intertrial interval and light
period were not rewarded. There was no penalty time-out at this
stage of training, but the light could be left on until the monkey kept
its hand off the lever. It was necessary to avoid allowing in any way
the animal to associate the manipulandum with turning off the light.
When performance with this program was adequate, the monkey was trans¬
ferred to a simplified version of the final task. The logic program for
the final task was modified only in that the variable interval fixation
light period was temporarily replaced by a very short fixed interval
fixation light period (approximately 100 milliseconds) and the reward
period was relatively long. During program II the animal was pulling
and very quickly releasing the manipulandum to initiate a trial. It

107
was now necessary to train the animal to pull and hold the manipulandum
until the end of the fixation light (F.L.) period. This was done by
making the F.L. period short enough that the animal could not release
the lever before the light went off, and then, gradually increasing the
duration of the F.L. period so that occasionally when the animal
released the lever too soon, it received a time-out. From this the
animal discovered that longer holds on the lever were rewarded and
very short holds were not. As soon as the animal would perform holds
of one-half second with reasonable success, the fixed interval F.L.
timing circuit was replaced with the variable interval F.L. timing
circuit so as to avoid training the animal to judge any fixed time
interval and force it to associate the offset of the F.L. with the
proper time to release the lever. The chief element of the variable
timing circuit was a film programmer which senses holes punched in a
loop of photographic movie film. At first the intervals punched in
the film loops were of necessity very short and little variation was
permissible if the monkey was to continue to perform with reasonable
success, but session after session film loops with slightly longer
intervals and a greater range of variability were used until the monkey
eventually learned to perform holds of up to 8 seconds duration or
greater and to respond accurately to the offset of the fixation light
regardless of the variability between intervals. As the animal was
being trained to longer and more variable F.L. intervals, the size
and intensity of the F.L. spot were gradually reduced and the color of
the spot was some days red, some days green, some days white so that
finally the animal was forced to fixate very well a small dim spot and

108
to disregard its color. At this point, all that remained was to
gradually introduce the stimulus flashes during the F.L. period. The
first stimulus flashes which were presented were dim and infrequent.
Gradually they were made more frequent and eventually more intense.
Diffuse flashes and checkerboard patterned flashes were used and the
animal discovered that the presence or absence of a stimulus flash
during the F.L. period had no effect upon its success in performing
the task for which it was being rewarded. Always, the time of occur¬
rence of the stimulus flash within the F.L. period was variable.
The occurrence and timing of stimulus flashes were controlled by
a film loop on a second film programmer. This film loop was con¬
structed so that it could be synchronized with the F.L. period film
loop.
A key to logic circuit labels precedes the diagrams which
describe the logic of the training programs and the final task. Program
I is shown in figure 35, program II in figure 36, and the final visual
fixation task program in figures 37 through 43. A subject performed
the visual fixation task by operating the manipulandum shown in the
fixation light control circuit. The experimenters normal control of
this task consisted of choosing the appropriate punched films for the
programmers; adjusting the reward system flow resistance; and setting
switches to select the reward period, intertrial interval, and time¬
out durations.

109
KEY TO LOGIC CIRCUIT LABELS
Abbreviation Logic Circuit Function
AND And Gate
B C Binary Counter
C T Decimal Counter with Readout
C X Clean Control Switch
F Feedback Relay
F F Flip Flop
F P Film Programmer
HA Logic Amplifier
INV Inverter
M Manipulandum Switch
MV Multivibrator
OR Or Gate
O S One-Shot
P Projector
PC Photocell and Amplifier
RS Reset Pulse Generator
Switch Ordinary Switch
SA Sonalert Tone Generator
SV Solenoid Valve
Z Cummulative Recorder Input

Figure 35. Behavioral training program number 1.
110

Figure 36. Behavioral training program number 2.

Figure 37. Fixation light control circuit for the visual fixation task.

£11

* A17
Figure 38. Intertrial interval and reward control circuit for the visual fixation task.

Figure 39. Reward period timer circuit for the visual fixation task.
115

Figure 4Q. Early release and late release detection circuits for the visual fixation task.
r±n

Figure 41. Time-out timer and control circuit for the visual fixation task.


Figure 42. Stimulus flash, control circuit for the visual fixation task.
119

Figure 43, Reaction time ring counter circuit for the visual fixation task.
120

APPENDIX II
Use of Drugs with Paralyzed Animals
Throughout these experiments an attempt was made to limit the
use of drugs to the minimum necessary to restrain the animal and to
maintain a steady physiological state as much as possible. A neuro¬
muscular blocking drug was necessary and gallamine triethiodide (Flaxedil)
was chosen for this purpose. Since these were not acute experiments,
particular care was taken so as not to accumulate excessive amounts of
this drug in the animal's body. The disappearance of this drug from
the plasma has been shown to follow a 3 time-constant exponential curve
(Kalow, 1959; Walts and Dillon, 1968) with the first 20 to 30 minute
period after the initial dose having the fastest time-constant, the
next approximately 1 hour having the intermediate time-constant, and
the period following lh hours having the slowest time-constant. In
addition, it was known from experience that an initial dose of 5 mg
per kg of gallamine administered intravenously was sufficient to block
skeletal muscles including the respiratory muscles. For cats, the
method which worked well for all experiments included in this disserta¬
tion was to use an initial dose of 5 mg per kg administered i.v. followed
within approximately 10 minutes by the beginning of an infusion at the
rate of 15 mg per kg per hour for 40 minutes to 1 hour, at which time
the infusion rate was reduced to approximately 10 mg per kg per hour
and maintained for another hour, and then the infusion rate could be
reduced to 5 to 7 mg per kg per hour and maintained for the remainder
121

122
of the experiment (up to 10 hours duration). The author’s limited
experience with the use of this drug on rhesus monkey indicates that
doses of 70 to 80 percent of that used for cat are sufficient.
As mentioned by Cleland and Enroth-Cugell (1970), the activity
and responsiveness of retinal ganglion cells may fall to a low level after
several hours if glucose is not contained in the drug solution that
is infused. The volume of solution infused is also significant in the
sense that it determines to what extent the animal's kidneys filter the
drug and its breakdown products from the blood during the experiment
(Cohen et al., 1967). The solution infused during these recording
sessions was normal or slightly hypotonic saline with 5 percent dex¬
trose and 4 mg per ml of gallamine.
Atropine in doses of 0.3 to 0.4 mg per kg (for cat) was given
and repeated after 4 to 5 hours to minimize salivary and respiratory
tract secretions while the animal was paralyzed and being respirated.
As a CNS stimulant to prevent the animal from sleeping,
amphetamine sulfate was available, but it was rarely used and any data
taken after administration of this drug has been so labeled. When used,
it was only in mild doses (0.5 to 1.0 mg per kg) and did not produce
any noticeable increase in response amplitude. The main effect was to
reduce the amplitude of on-going background activity.
At the end of the recording session, an infusion containing
the acetylcholine esterase blocker, neostigmine, was used to antagonize
the effect of gallamine and accelerate the animal's recovery from
paralysis. The maximum dosage given was 25 ng per kg.

APPENDIX III
Respiration Monitor and Alarm Circuit
This instrument was designed and developed to perform the
function that a technician devoting 100 percent of his time and
attention to monitoring breath-by-breath indications of end expiratory
CO2 level would serve; it detects CO2 levels outside of the normal
range in either direction. Its notable characteristics are that it
is ever present, makes accurate decisions, and alerts the experimenter
of possible difficulty at the earliest possible moment after checking
several indications to avoid producing unnecessary alarms. When used
properly, it has proven to be a valuable and reliable instrument.
A schematic diagram of the circuit used is given in figure 44.
An analog signal proportional to CO2 content of the expired gases is
necessary as an input. A signal level of approximately 0.3 volt/percent
CO2 is expected (available from a Beckman model LB-1 CO2 meter). The
signal input terminal leads to a high input impedance differential ampli¬
fier containing Ql, Q2, IC1, and related components. The amplified
signal is made available to two adjustable threshold comparator circuits
(IC2, IC4) and to test point 1 (TP1). IC2 detects crossings of the set
lower limit of the normal end expiratory CO2 range. Each crossing from
below to above this limit charges the 20 mfd capacitor following D2
to the maximum it can attain in this circuit. This capacitor continu¬
ously discharges at a rate controlled by R2. If the potential across
this capacitor ever becomes less than a fixed value (approximately 1/3
123

Figure 44. Schematic diagram of the respiration monitor and alarm system.

»II Wll«
PARTS
IC1 â–  uA741 operational amplifier
IC2,3,4,S * uA710 comparator
IC6 = uA711 dual comparator
Ql,2,3,4,7,8 = 2N3643
Q5,6,9 = 2N5163
Dl,2,3,4,5,6 - 1N4008
SCR1.2 = C106A1
S} = DPOT (reset switch
showniin operate position)
Ü0NTKÜ1.S
R1 sets lower limit.
R2 sets time allowed below
lower limit before
activating the alarm.
R3 sets upper limit.
R4 sets number of violations
of the upper limit tolerable
per unit time.
RS controls volunc of audio
from sonalert.
RESPIRATION MONITOR S ALARM
dotignotf by:
H W Ood dlnf ton
125

126
maximum potential), it is detected by comparator IC3 and the low
level warning light and audible alarm are activated.
IC4 detects crossings of the set upper limit of the normal
range. Each crossing from below to above this limit triggers a one-
shot (IC5, etc.) to deliver a pulse of current to the 20 mfd capacitor
which is attached to the gate of FET Q9. The magnitude of this current
pulse is controlled by R4. Current pulses are integrated on this
capacitor and the stored charge decays at a rate determined by the
10 megohm parallel resistance. If the potential across this capacitor
ever rises to a fixed value (approximately 1/3 of the potential avail¬
able at the cathode of D6 during a pulse), it is detected by comparator
IC6 and the audible alarm is activated without the light.
The outputs of the lower and upper limit crossing detectors
are available at TP2 and TP3 respectively for ease in adjusting these
settings. Depending upon the settings of R2 and R4, temporary high
or low CO¿ conditions can be accepted without generating an alarm. Once
activated, the light and/or alarm remain on until the reset switch is
operated.

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BIOGRAPHICAL SKETCH
Harold William Doddington was bom in St. Joseph, Michigan,
on January 18, 1942. He is the son of Harold E. and Charlotte K.
Doddington. He grew up in Hallandale, Florida, and attended South
Broward High School. He played trombone in the high school band for
4 years and was an amateur radio operator. Upon graduation from
high school in 1959, he entered the University of Florida to major
in electrical engineering. From 1960 through 1963, he participated
in the cooperative engineering education program working with the
Federal Communications Commission at the district field office in
Miami, Florida, as a student engineer.
He received the first Sustained Superior Performance Award
ever issued by the FCC to a student employee. He spent the summer
of 1962 youth hosteling in Western Europe. In August, 1964, he
received the degree Bachelor of Electrical Engineering from the
University of Florida and was employed as a member of the technical
staff by ITT Caribbean Manufacturing, Research and Development
Laboratory, Rio Piedras, Puerto Rico. In September of 1965, he
returned to the University of Florida to accept a Graduate School
Fellowship and in August of 1966 he was awarded the degree Master
of Engineering. Because of a strong interest in the interaction
between applied physical sciences and medical sciences, he entered
a doctoral program in the Physiology Department of the College of
Medicine. He became associated with the Visual Sciences Laboratory,
131

132
an interdisciplinary group, and developed research related to the
processing of spatial pattern information in the visual system.
He received a one year traineeship in physiology and then a pre-
doctoral fellowship in the Center for Neurobiological Sciences.
In 1965, Harold William Doddington married the former
Claudia Anna Walker. She holds the degree Master of Science in
Engineering. They have one daughter, Hally Katheryn.

I certify that I have read this study and that in my opinion
it conforms to acceptable standards of scholarly presentation and is
fully adequate, in scope and quality, as a dissertation for the degree
of Doctor of Philosophy.
William W. Dawson, Chairman
Professor of Physiology
I certify that I have read this study and that in my opinion
it conforms to acceptable standards of scholarly presentation and is
fully adequate, in scope and quality, as a dissertation for the degree
of Doctor of Philosophy.
Arthur B. Otis
Professor of Physiology
I certify that I have read this study and that in my opinion
it conforms to acceptable standards of scholarly presentation and is
fully adequate, in scope and quality, as a dissertation for the degree
of Doctor of Philosophy.
\—
Assistant Professor of Physiology
I certify that I have read this study and that in my opinion
it conforms to acceptable standards of scholarly presentation and is
fully adequate, in scope and quality, as a dissertation for the degree
of Doctor of Philosophy.
Calvin K. Adams
Assistant Professor of Psychology

I certify that I have read this study and that in my opinion
it conforms to acceptable standards of scholarly presentation and is
fully adequate, in scope and quality, as a dissertation for the degree
of Doctor of Philosophy.
Professor of Electrical Engineering
This dissertation was submitted to the Dean of the College of Medicine
and to the Graduate Council, and was accepted as partial fulfillment
of the requirements for the degree of Doctor of Philosophy.
August, 1972
Dean,
Cóliege of Me die ini
Dean, Graduate School

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UNIVER'SITY OF FLORIDA
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