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

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Activity evoked in the visual system of human, rhesus monkey, and cat by spatially patterned and non-patterned visual stimuli
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Doddington, Harold William, 1942-
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Thesis:
Thesis (Ph.D.)--University of Florida, 1972.
Bibliography:
Bibliography: leaves 127-130.
Statement of Responsibility:
by Harold William Doddington.
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Typescript.
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Vita.

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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













ACKNOWLEDGMENTS


The author sincerely wishes to thank the members of his
I
supervisory committee, the faculty of the Physiology Department,

and his fellow graduate students for their encouragement 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.













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













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













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. Cumulative records of the performance of two human
subjects. . . ... ...... 42

10. Histogram of reaction times and cumulative 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












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













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 ... .. 111

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













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.









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-patterned (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










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.












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









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)









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









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). Hubel 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.









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, Hubel and Wiesel have described the

receptive fields and response characteristics of visual cortex cells

in areas 17, 18, and 19 of cat and monkey (Hubel and Wiesel, 1959;

1965; 1968). Their acute experiments were conducted with paralyzed,










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 artificial, 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










the type Hubel 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










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










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










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









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









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










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.









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










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 (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 nonpatterned 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










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.









Ophthalmoscopic examination, measurement of intraocular pressures,

and retinoscopy 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 cumulative

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).










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.










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














































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 from the drinking tube through
which sips of water were issued as rewards for correct completion
of trials.










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.










Experiments Using the Maxwellian View Pattern Stimulator


Stimulator and Optics

To study the evoke< ;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










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
































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.


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A B C D


EXAMPLES OF THE QUALITY OF PATTERN WHICH THE PATTERN STIMULATOR
INSTRUMENT IS CAPABLE OF PROJECTING.

A. 0.8 mm/sq. pattern = 17.5 min. of arc = 68 p 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.




















3/ U Eili.















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 funds at the standard
adaptation light level chosen for use in these experiments (780
trolands retinal illumination).










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








































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.










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 wasto 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.









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










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 CO2

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 02, 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.










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 CO2 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 funds 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).









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











































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.










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 psec 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 signs 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










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.

















































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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 5 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










































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Control


I If I _


0 100


200 300 400
Time in Milliseconds


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


500










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. phencyclidinee hydrochloride (Sernylan)), 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


























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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.










79'
55'

31' v %1'11V^W *A
S= PS17



11'


Diffuse

79'
55'

31'
S= CD
17.5


11'

Diffuse

79'
55'

31'
S = GD
17.5 1


110 v 11'

Diffuse

0 Milliseconds 500

AVERAGE RESPONSES


0 Milliseconds 500

DIFFERENCE WAVEFORMS










(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













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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 shown 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




























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79,


55'


31'


17.5'


11'


5'


Diffuse



Control
i I I I I
0 100 200 300 400 500
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


Cat 1L3


Neutral I>nsity
Attenuation
in log Units
Diffuse
(0.3)
0.5


0.9
Control








40 pv


0
Neutral Density
Attenuation
in Log Units
Diffuse
(0.3)
0.5


0.9

Control


3 100 200
Time in Milliseconds
DETRASIRIATE CORTEX A\W t-G. RESPONSE






















0 100 200
Time in Milliseconds


STRIATE CORTEX AVERAGE EJ-.riL''


Figure 17. Diffuse flash cortical evoked responses to a series of
stimulus intensities. Striate and extrastriate responses were
recorded simultaneously. Llectrode locations: striate P3.0, L3.0;
extrastriate P3.0, L8.0 (both epidural).





















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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 wavefom are influenced by quality of focus and to what
extent. Cat 1K9 striate electrode. 64 sweeps/average response.




















Diffuse


+9.0


+4.0




+2.0





+1.25


Defocus

in +0.12

Diopters


-2.0


Control


Cat 1K9


I I I
40 i80 120 160
Time in Milliseconds
DIFFERENCE WAVEFORM


I I I A 1
0 40 80 120 160 200 0
Time in Milliseconds
31 MINUTE PATTERN RESPONSE















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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.




















79'




55'




31'




17.5'




Diffuse




Control


AVERAGE RESPONSE


79'




55'




31'




17.5'




Diffuse



Control


40 juv Cat 1K9



79'




55,



31'




17.5'




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DIFFERENCE WAVEFORM


STRIATE CORTEX


8 pv




79'




55'




31'




17.5'




Diffuse


AVERAGE RESPONSE


DIFFERENCE WAVEFORM


OPTIC NERVE






























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



















79'
6:10 pm



31'

4:48 pm
4:56 pm
5:28 pm

7:50 pm



Diffuse

4:40 pm
5:18 pm
6:00 pm


Cat 1K9


50 msec.
I I


Defocused by +4 D.


OPTIC NERVE


79'
6:20 pm






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5:06 pm





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79'
6:20 pm





31'

5:06 pm


79'
6:10 pm



31'

4:48 pm
4:56 pm
5:28 pm
7:50 pm



Diffuse

4:40 pm

5:18 pm
6:00 pm









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 shown 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










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 shown

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.























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
























A.

10X Standard
Adaptation
Level


31'


17.5'


11i


2X Normal
Flash Intensity
Intensity
Diffuse






55'


B.

Standard
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0.5X Normal
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Standard
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2X Normal
Flash
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31'


17.5'


Diffuse


31'


17.5'


Diffuse


0 100 200
Time in Milliseconds

CAT OPTIC NERVE AVERAGE RESPONSES










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 largeold,

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.










Cat H-9






10 Pv


-6 34 114


194 Msec.


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114 194 Msec.


Diffuse 31' Pattern

INDIVIDUAL OPTIC NERVE RESPONSES


0 40


80 120
Time in Milliseconds


200


AVERAGE OPTIC NERVE RESPONSES


Figure 25. Simultaneously recorded individual and
nerve responses to diffuse and patterned stimuli.


average optic
Cat H-9.


31'





Diffuse












































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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





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(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 7 hour recording

session within which the animal heart rate, body temperature, and

expired CO2 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



























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Difference
Waveform



11' Pattern





Diffuse


300
Milliseconds


0 500
400 500


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-patterned 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.


200
Time in


|










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

200 msec.







HUMAN








60 uv

200 msec.








MONKEY


48 uv

80 msec.










CAT


DIFFERENCE WAVEFORM


87


79'
55'

31'


17.5'


11'


Diffuse






79'

55'


31'


17.5'


11'

Diffuse


79'


55'


31'


17.5'


Diffuse


AVERAGE RESPONSE










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 two 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 two

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.