Pattern evoked responses--spatial and temporal frequency interactions

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Pattern evoked responses--spatial and temporal frequency interactions
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Thesis (Ph.D.)--University of Florida, 1980.
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Bibliography: leaves 72-89.
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by Elmar Thorwaldt Schmeisser.
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Typescript.
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Vita.

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PATTERN EVOKED RESPONSES--
SPATIAL AND TEMPORAL FREQUENCY INTERACTIONS





BY

ELMAR THORWALDT SCHMEISSER


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


1980































Copyright 1980

by

Elmar Thorwaldt Schmeisser















ACKNOWLEDGMENTS


It is a pleasure to acknowledge the aid of my

Chairman, Dr. William W. Dawson, not only for providing

the seeds for a sequence of projects which saw publication

and eventually lead to this dissertation, but for

invaluable aid in expanding otherwise too terse statements

as well as deftly managing occasional lapses into

convoluted german grammar. Both the facilities and the

guidance provided are much appreciated. I must also thank

Dr. Jay Enoch for insuring a manageable project and clear

definition of objectives in the planning and execution of

this work. Dr. John Munson, Dr. Arthur Otis and Dr.

Wendell Stainsby all not only shepherded me through the

comprehensive exams, thus building a fund of physiological

training, but also aided in maintaining the relationship

between abstract research results and physiological

reality. Also, I wish to thank Dr. Keith White for ably

stepping in for Dr. Enoch after his departure for

California and providing pointed and interested feedback,

thus leading to a stronger project.

Lastly, I wish to thank my wife who had to put up

with conflicts between courting and lab but came through

with me to the end.


iii



















TABLE OF CONTENTS


CHAPTER


ACKNOWLEDGEMENTS .

LIST OF FIGURES .

ABSTRACT . .

CHAPTER I
INTRODUCTION .

CHAPTER II
REVEIW OF SELECTED LITERATURE

Psychophysics .

Electrophysiology *

Binocular Data .

Summary . .

CHAPTER III
EXPERIMENTAL RATIONALE .

CHAPTER IV
METHODS .. . .

Subjects .

Stimulus .

Recording .

Analysis .


. . iii

* . vi

* . viii



. . 1



. . 4




o o 9

S . 15

. . 18



. . 19



. . 21

. 21

S . 22

. . 29

. . 30


PAGE









CHAPTER V
RESULTS . . 33

Phase Calibration . ... 33

Spectral Power . ... 33

Amplitude Analyses . 54

Phase Analyses . ....... 57

Correlation Analysis . 58

Results Summary. . 60

CHAPTER VI
DISCUSSION. . . 61

APPENDIX
CIRCUIT DESCRIPTION AND PARTS LIST ... 66

Voltage Regulators. . 69

Current Regulators . .. 69

External Units . . 70

Switches . . 70

Integrated Circuits . 70

Transformer . ... 70

Capacitors ... . 70

Resistors (Values in Ohms) ... 71

LITERATURE CITED . ..... 72


. . 90


BIOGRAPHICAL SKETCH















LIST OF FIGURES


FIGURE PAGE

1 Schematic illustration of stimulus and
recording arrangement. Al, A2:
preamplifiers; C: computer; F:
band-pass filter; M: first-surface
mirrors; 0: observer; 01: master
display oscillosocpe; 02: slave display
oscilloscope; P: prism array; S:
shield room; SG: signal generator; ST:
stimulus control circuitry; T:
oscilloscope trigger input; Tl:
oscilloscope trigger signals; T2:
computer trigger signal; X: master
oscilloscope horizontal control; Y:
oscilloscope vertical control; Z:
oscilloscope luminance control input;
Z1, Z2: luminance control signals to 01
and 02 respectively . ... 24

2 Stimulus luminance profile metrics for
gratings used . 27

3 Phase run of subject A, session #1. A:
unfiltered data; B: filtered data (5-9
Hz); C: counterphase trace of the
grating luminance contrast; D:
zero-crossing signal. The numbers
between C and D indicate stimulus
events; the numbers over the peaks in A
and B indicate the respective response
positivities . . 35

4 Phase run of subject B, session #3.
Otherwise as in Fig. 3 . 37

5 Grand unweighted, non-normalized digital
sum of all 15 subject-session phase
runs. Otherwise as in Fig. 3. Absolute
amplitude is arbitrary . 39









6 Responses to a monocular presentation of
a complex grating with the components at
a phase angle of 00. A: unfiltered
data; B: filtered data; C: stimulus
traces (sine or square mode
counterphase) as well as time and
amplitude calibration on square wave; D:
power spectra of responses. Note a
factor of two scale shift of the sine
mode versus square mode power spectra 42

7 Responses to a binocular presentation of
a complex grating with the components at
a phase angle of 00. Otherwise as in
Fig. 6 . . 44

8 Responses to a binocular presentation of
a simple (Fl: 2 cycles/degree) grating.
Otherwise as in Fig. 6 . 46

9 Responses to a dichoptically presented
gratings (Fl+F3) with the components at
a phase angle of 1800. Otherwise as in
Fig. 6 .. . . 48

10 Bar graph of response power to component
fundamental (Fl) and second harmonic
(F2) by mode sine (SN) and square (SQ)
mode counterphase and stimulus; means
and standard error of the means (n=15) 51

11 Bar graph of the component power ratio
(fundamental/second harmonic) by mode
and stimulus. -Means and standard error
of the means (n=15) . 53

12 A: means and standard error of the
means of response amplitude, both sine
and square mode, by stimulus:
unfiltered data; B: filtered data; C:
means and standard error of the means of
response phase, unfiltered data; D:
filtered data (n=30) . 56

13 Schematic diagram of the stimulus
circuitry labelled "ST" in Fig. 1 68


vii















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


PATTERN EVOKED RESPONSES--
SPATIAL AND TEMPORAL FREQUENCY INTERACTIONS

BY

Elmar Thorwaldt Schmeisser

December 1980

Chairman: William W. Dawson, Ph.D.
Major Department: Physiology


Visual evoked potentials from the scalps of five

human volunteers were recorded in response to 1800 phase

shifts of simple (single spatial frequency) and complex

(sum of two spatial frequencies) sinusoidal gratings of

two and six cycles/degree in a 50 field. Steady-state

responses were obtained by reversing the contrast of the

gratings (counterphasing) at seven shifts/second, either

abruptly (square mode) or smoothly (sine mode). Each eye

always received the same total amount of light during

stimulus presentations, whether monocular, binocular or

dichoptic. Sine mode counterphasing enhances the second

temporal harmonic component of the evoked waveform,

leading to a doubled response peak per stimulus shift.

Square mode counterphasing generates a response with

essentially all the power at the fundamental temporal

viii









frequency. With complex gratings (2 + 6 cycles/degree),

the temporal fundamental to second harmonic ratio is

enhanced when the two grating spatial components are at 00

relative phase angle. Gross amplitude did not differ

between sine and square mode evoked responses, nor between

two and six cycles/degree gratings. Hypothetical

(synthetic) responses to complex gratings were computed by

digitally adding the observed responses to simple gratings

presented alone. These synthetic responses were

significantly greater in amplitude than any observed

response to the presented complex gratings, indicating

that the observed response is not a linear superposition

of the responses to the individual grating components.

Response amplitudes ranked significantly by condition:

binocular>dichoptic>monoptic, indicating that masking of

one grating by another simultaneously presented in the

fellow eye did not occur. Pearson correlation

coefficients calculated between all possible pairs showed

that, in the best possible case, only 61% of the variance

of the observed dichoptic sum of gratings response can be

accounted for by the contribution of the responses to the

simple gratings alone. It is concluded that (a)

sinusoidal counterphasing stimuli lead to two stimulus

events per shift as the grating fades into and out of

visibility (an observed grating onset and offset) which

produces peak doubling in the response waveform, (b)

spatial frequency channels that are harmonically related









in a 1:3 ratio are partially additive in the

suprathreshold steady-state case, and (c) the location of

this interaction is probably cortical rather than

precortical as the monocularly presented complex grating

response does not differ from the dichoptic response.















CHAPTER I
INTRODUCTION


Since the demise of the homunculus in the pineal body

after Descartes around 1600, modeling of the function of

the visual system has been based on a mechanistic analysis

or decomposition of a visual scene into simpler elements.

These elements have been conceived in two major groupings:

"features" such as edges, colors, corners etc., and

"components" in a mathematical or "Fourier" sense. The

putative detectors of these elements are neurons at the

various levels in the quasi-hierarchical structure of the

nervous system.

Pattern analysis by apparent feature detection has

been demonstrated in monkey visual cortex (Hubel and

Wiesel, 1977). An order array of cortical cells seems to

respond optimally to differing orientations and locations

of bar or edge stimuli. A visual scene could be conceived

of as an ensemble of such bars or edges. This type of

analysis would, therefore, be a plausible method for

mapping visual space into patterns of neural activity,

especially considering the interconnectivity of those

neurons (Finette, Harth and Csermely, 1978).

A visual scene can be analyzed alternatively, by

analogy, along the lines of the mathematical decomposition

1









of a complex waveform into its components (Gaskill, 1978).

This has been done optically (Switkes, Mayer and Sloan,

1978) and so raises the question of whether it is also

done neurally (Kelly, 1977; Graham, 1979; Ochs, 1979).

The simplest stimulus in two dimensional visual

space, in a Fourier, or spatial frequency sense, is a

grating pattern whose luminosity varies sinusoidally along

a single dimension (Ramirez, 1974; Kelly and Magnuski,

1975; Weisstein et al., 1977; DeValois, DeValois and Yund,

1979). In this case, there is a single component (the

fundamental) at one orientation, neglecting symmetrical

reflection. Gratings with different luminance

profiles--square, triangular, etc.--will have multiple

Fourier components. The visibility or detectability of

gratings having various luminance profiles can be simply

related to the strengths (power) of their spatial

frequency components (Campbell and Robson, 1968). Such

research, in conjunction with work using compound gratings

synthesized from known spatial frequencies (Graham and

Nachmias, 1971) has led to the concept of independent

information "channels" in the visual system, each

sensitive to a small range of spatial frequencies, and

operating independently (Riggs, 1974; Quick and Reichert,

1975; Abel and Quick, 1978; Watson and Nachmias, 1980) and

has led as well to clinical applications (e.g., Arden,

1979). These channels are generally conceived of as

theoretical constructs, each receiving the same









inputs--the visual scene segment from the fields of its

receptors--but abstracting different information to pass

on to higher processors. Interactions of information

within a channel can be used to model results from spatial

masking experiments. An ensemble of independent channels

can be used to model adaptation effects. Whether there

exists a neural unit identifiable with a channel remains

unclear (see Review below).

Temporal frequency channels are a logical extension

of this concept; i.e., that time-varying stimuli may be

detected as components (Tolhurst, Sharpe and Hart, 1973).

Results from psychophysical masking experiments support a

temporal sustained versus transient dichotomy, paralleling

neural findings (Breitmeyer and Ganz, 1976). Such masking

experiments show interactions between these presumed

channels, and so have questioned the independence of

Fourier channels and the Fourier model in general (e.g.,

Graham and Rogowitz, 1976; Legge, 1976; Bergen, Wilson and

Cowan, 1979; Julesz and Caelli, 1979; Tangney, Weisstein

and Berbaum, 1979). The literature review in this

dissertation will examine the evidence which has been

offered by both psychophysical and electrophysiological

techniques. This will establish a framework for a set of

experiments designed to probe spatiotemporal interactions

as reflected by the pattern visual-evoked potential

recorded from humans.















CHAPTER II
REVIEW OF SELECTED LITERATURE

Psychophysics


Psychophysical exploration of spatial frequency

channels has developed evidence from both detection and

matching paradigms. Detection experiments, such as those

by Campbell and Robson (1968) and Graham and Nachmias

(1971) have shown that the contrast sensitivity function

defines a set of mutually independent thresholds for

different spatial frequencies. By using a compound

grating formed of more than one spatial frequency, a

grating is detected whenever any one component exceeds its

independently determined threshold. Further, gratings

will be differentiated only when a component is above

threshold for one grating, but not for the comparison

grating (Nachmias et al., 1973; Nachmias and Weber, 1975;

Isono, 1979; Ginsburg, Cannon and Nelson, 1980). Graham

and Nachmias' (1971) results also show a phase

independence between spatial frequencies when the

components are separated by more than one octave in the

high frequency ranges. These results have been extended

to circular targets as well (Kelly and Magnuski, 1975).

The hypothetical structure of such channels in terms

of bandwidth and sensitivity has been related to the

-4








psychophysical thresholds for detection (Kelly, 1972;

Abadi and Kulikowski, 1973; Mostafavi and Sakrison, 1976)

as well as to receptive field structure (Legge, 1978b).

The channels also seem to be orientation selective, as

adaptation by a grating at one orientation does not raise

the threshold for detection of a grating at another

(Carlson, Cohen and Gorog, 1977). Such tuning, however,

seems quite broad, being several minutes of arc, compared

to the observed vernier accuracy of the visual system, on

the order of 10 sec of arc (Blakemore and Hague, 1972;

!,Testheimer, 1978).

Detectability or threshold level experiments, while

providing much information, do not necessarily predict the

behavior of the visual system in its normal, non-threshold

operating ranges. Suprathreshold experiments in contrast

matching and size perception have to some extent also

supported the multiple parallel narrow band channels model

(Watanabe et al., 1968; Prome et al., 1979; Levinson and

From, 1979; Arend and Lange, 1980), but complications

indicating some interactions between channels have been

found (Stromeyer and Klein, 1975; Tangney, Weissten and

Berbaum, 1979). Composite pattern adaptation effects have

indicated an inhibition between channels (Stecher, Sigel

and Lange, 1973). Nonindependence has also been found

with masking patterns two octaves away (Henning, Hertz and

Broadbent, 1975; Cavanaugh, 1978; Klein and Stromeyer,

1980), with phase perception (Stromeyer, Lange and canz,









1973; Atkinson and Campbell, 1974; Arend and Lange, 1979;

Burr, 1980) and with sensitivity increases by certain

masks (Tolhurst and Barfield, 1978).

Alternative models for pattern perception have

assumed some form of broad band channel or channels in

parallel or replacing the narrow band array (Stromeyer and

Klein, 1974; Kulikowski and King-Smith, 1973; Kulikowski,

Abadi and King-Smith, 1973; Spitzberg and Richards, 1975;

Snyder and Srinivasen, 1979; Arend and Lange, 1979).

Wilson and Bergen (1979) and Bergen, Wilson and Cowan

(1979) reduce the number of mechanisms to four but invoke

large nonlinearities to fit the data. Two-dimensional

periodic patterns seem to require either size-tuned

detectors or extensive interchannel interaction (Burton,

1973, 1976; Carlson, Cohen and Gorog, 1977; Tyler, 1978a).

On the other hand, phase channels as well as spatial

frequency channels have been proposed (Bacon, 1976).

Interactions are also demonstrated with binocular stimuli

(Blake and Levinson, 1977; Legge, 1979; Walker and Powell,

1979; Blake and Rush, 1980; Cormack and Blake, 1980) and

dichoptic masking experiments (Abadi, 1976).

Cortical location of these channel interactions, and

specifically inhibition of neighboring columns as

postulated by Abadi (1976), while often assumed, need not

be the only determinant of spatial frequency selectivity,

although there seems to be a progressive tuning to sharper

selectivity by each stage in the visual pathway









(Armington, Gaarder and Schick, 1967; Armington, Corwin

and Marsetta, 1971; Maffei and Fiorentini, 1973;

Armington, 1977). Kelly (1975), by postulating receptive

field coherence, has presented a retinal model. Retinal

field size for channels has been described by Quick and

Reichert (1975) who maintained that the channels had a

constant retinal area independent of center frequency.

This is flatly contradicted by Hoekstra et al. (1974) and

by Howell and Hess (1978), who found a critical response

amplitude dependence on the number of cycles present in

the display to avoid artifactual decreases in recorded

sensitivity, and also in part by a series of papers by

Koenderink et al. (1978) in which target extent in the

visual field had a critical effect on detection. Kelly's

(1976) specific attempt to infer a postretinal location

for Fourier channels showed that for the mechanisms he

tested, the achromatic and opponent-color pathways were

equally affected by pattern adaptation, and so presumably

the effects occurred prior to a neural locus at which

these two pathways specialize, although color

discrimination by spatial frequency channels remains in

debate (Ingling, 1978; King-Smith and Carden, 1978).

Orientation effects are usually assumed to require

cortical processing, however, since it is in the cortical

cells that orientation selectivity of neural elements

seems most strongly expressed, as determined by cat and

monkey studies (Bisti and Maffei, 1974; Boltz, Harwerth








and Smith, 1979; Bauer et al., 1979; Berkeley, Kitterle

and Watkins, 1975; Hubel and Wiesel, 1977; among others),

and also apparent from studies with pattern evoked retinal

responses (Maffei and Campbell, 1970).

Modulating the stimuli in the temporal domain by

flickering or counterphasing the grating leads to changes

in contrast sensitivity (Kelly, 1977; Koenderink and van

Doom, 1979. Such temporal stimuli can also be examined

from a Fourier analysis viewpoint. The interdependence of

temporal and spatial channels can be seen in the effect of

temporal modulation of a grating on the "oblique effect,"

a decrease of response or of sensitivity to stimuli in

non-horizontal or -vertical presentations (May et al.,

1979); the effect is apparent only at low temporal

frequencies (Camisa, Blake and Lema, 1977). Selectively

masking "sustained" channels appears to uncover several

"transient" channels with differing peak sensitivities

(Legge, 1978a). Kelly (1979a) using stabilized-image

stimuli has shown, that without some temporal modulation

such as that provided by the slow drift of a "fixated"

eye, thresholds are elevated by a factor of 20. Using

inverse Fourier transforms, Kelly constructed line spread

functions from the stabilized velocity data that closely

resemble those derived by localized but unstabilized

stimuli and with electrophysiologically determined

receptive field structures (Braddick, Campbell and

Atkinson, 1978; Legge, 1978b; Kelly, 1979b).









Electrophy siology


With the advent of signal averaging computers,

electrophysiological data from humans could be taken with

high enough signal-to-noise ratios to be generally useful,

both as a research tool (Riggs and Wooten, 1972; MacKay

and Jeffreys, 1973; Riggs, 1974, 1977), and as a clinical

diagnostic technique (Sokol, 1976; Sherman, 1979). Such

data, generally stimulus-evoked potentials, occupy an

intermediate area in stimulus-response analysis, between

cellular or unit recording in neural networks, as in

retina (Dowling, 1970) or cortex (Hubel and Wiesel, 1977),

and whole organism responses, generally explored

psychophysically. Evoked potentials sum the responses of

a population of cells within the electrical range of the

recording electrode, integrating them according to their

strength and distance (Verzeano et al., 1968; Creutzfeld

et al., 1969; Lesevre and Joseph, 1979; Legatt, Arezzo and

Vaughan, Jr., 1980). Sharp depolarizations correlate well

with surface positivity (Creutzfeld and Kuhnt, 1973) and

reflect processes that can be isolated psychophysically

(e.g., cone mechanisms: Gouras and Padmos, 1974; masking:

Samoilovich and Trush, 1979) or electrophysiologically

(e.g., internal/conditioned inhibition: Shul'gina, 1979;

DeToledo-Morrell, Hoeppner and Morrell, 1979; pattern

selective adaptation: Movshon and Lennie, 1979).









Evoked potentials, by their definition, are locked to

a stimulus event, rather than a stimulus condition,

although various conditions [not only experimental (Uren,

Stewart and Crosby, 1979)] can have profound effects on

such potentials, as a vast clinical literature atests

(e.g., Ashworth, Maloney and Townsend, 1978; Ikeda,

Tremain and Sanders, 1978; Kadobayashi et al., 1978;

Noebels, Roth and Kopell, 1978; Rover et al., 1978; Von

Knorring, 1978; Andreassi et al, 1979; Arseni et al.,

1979; Facciolla et al., 1979; Frank and Torres, 1979;

Kataoka et al., 1979; Klassen et al., 1979; Naparstek et

al., 1979; Seppalainen, Raitta and Huuskonen, 1979;

Shagrass et al., 1979; Sokol and Nadler, 1979; Wolfe,

1979; Wright, Craggs and Sergejew, 1979; Lee et al., 1980;

Regan et al., 1980; among others). Visual acuity testing

has used the evoked potential in addition to

psychophysical tests to assess objectively spatial

frequency characteristics of the visual system and to

prescribe corrective refractions (Sokol, 1972; Arden and

Lewis, 1973; Bodis-Wollner and Diamond, 1976; Bostrom,

Keller and Marg, 1978; Hess, Howell and Kitchin, 1978;

Hofmann, Bartl and Gruber, 1978; Enoch, Bedell and

Kaufman, 1979; Enoch, Campos, Greer and Trobe, 1979;

Enoch, Ohzu and Itoi, 1979; Enoch, Yamade and Namba, 1979;

Klingaman and Moskowitz-Cook, 1979; Tyler et al., 1979;

Dobson and Davison, 1980; Julesz, Kropfl and Petrig, 1980;

King-Smith and Kulikowski, 1980).









Evoked potential research to demonstrate spatial

frequency channels has used both masking and adaptation

paradigms, as in similar psychophysical work (Ochs and

Aminoff, 1980). The classic experiment by Campbell and

Maffei (1970) used a grating of a specified spatial

frequency to adapt a channel, and by sampling spatial

frequencies near it with counterphasing gratings, Campbell

and Maffei demonstrated a selective suppression of the

evoked response until the test grating was at least an

octave away from the adapting grating. Similarly,

rotation of a test grating at the same spatial frequency

as the adapting grating showed a 150 specificity for

orientation of a particular channel, paralleling

psychophysical results. Some differences from

psychophysical data have also been demonstrated; recovery

of the depressed evoked potential response is much faster

than the recovery of the contrast detection threshold

(Mecacci and Spinelli, 1976). Other work, however, shows

close correspondences with psychophysics (e.g., contrast

thresholds: Adachi-Usami, 1979; Kojima and Zrenner,

1980). Direct comparison of electrophysiologic and

psychophysiologic measures of apparent contrast shows

close correlations with each other and indicates that the

evoked potential can be an accurate reflection of

perceptual experience (Bodis-Wollner, Hendley and

Kulikowski, 1972; Pranzen and Berkeley, 1975).








Temporal frequency channels, usually conceived as a

dichotomous set of parallel transient and sustained

mechanisms, have been implied by evoked responses to both

slow reversal rate stimuli and by temporal tuning curves

using higher rates of stimulus presentation (Lennie,

1980). Such stimuli are sorted in evoked potential

research into two broad overlapping classes according to

stimulation rate: "steady-state" and "transient"

response recording. Steady-state recording presupposes a

temporally periodic stimulus presented for a sufficiently

long period of time that the effects of the abrupt onset
of stimulation are no longer apparent (Regan, 1977).

Transient evoked potentials, by contrast, deliver one

stimulus and wait until the response to that stimulus has

returned to background activity before presenting the next

(Kinney, 1977). In nonlinear signal processing systems,

the steady-state response is not necessarily equivalent to

the superposition of the transient responses at the same

rate. In this regard, the evoked potentials do not behave

as a linear system. Patterns of signal buildup and decay

with various time courses and differing arousal states

have demonstrated this essential nonlinearity in cortex

(Steinberg, 1965; van der Tweel and Verduyn Lunel, 1965).

The analysis of steady-state activity also gives

information on the visual pathways in the form of phase

lag and permits the modeling of the signal processing

system as a resistor-capacitor (R-C) network or frequency









filter (Bode, 1945). Amplitude (or power) alone can only

provide information on the resistive elements of a system;

phase information is required in addition to permit a full

description of a system's behavior. Additionally,

steady-state recording permits the separation of low

versus high temporal frequency "channels," i.e., the

modeling of the visual "processor" as a temporal Fourier

analyzer (Jones and Keck, 1978; Graham, 1979). Such

separation shows a strong interaction with spatial

frequency. By using transient stimuli, Jones and Keck

(1978) were able to show that there were at least two

mechanisms responding to the stimuli. One had a short

latency component and was strongest at 1 or 2

cycles/degree, saturated at low contrast, and appeared

insensitive to precise retinal location. The major

component was tuned to higher spatial frequencies and

showed a linear response with log contrast. This type of

response was also found by Regan (1978) using an online

analog Fourier analyzer and steady-state recording.

Responses to low spatial frequencies seemed enhanced with

fast stimulus presentation rates, and high spatial

frequencies with low reversal rates. These data support a

model of the visual system with many parallel channels

that are specific to spatial frequency, temporal frequency

and orientation, all with relatively narrow bandwidths,

and permit the description of the system response in terms

of a spatiotemporal response surface defined on purely









electrophysiological criteria (Tyler, Apkarian and

Nakayama., 1978). This surface has been shown to be

homologous to the data derived from psychophysical

contrast detection threshold studies (Barris and Dawson,

1980). Relative spatial phase detection, however, seems

to require either channel interaction, separate broadly

tuned channels, or an additional class of mechanisms

specifically sensitive to phase (e.g., Burr, 1980; Ross

and Johnstone, 1980).

Analysis of cat and monkey cortical neurons has

revealed a possible cellular basis for the Fourier model

of visual processing (Maffei and Fiorentini, 1973). Hubel

and Wiesel's (1977) model of monkey cortex as an ensemble

of bar detectors has been recast as an ensemble of spatial

frequency detectors (Noda et al., 1971; Maffei, 1978;

Movshon, Thompson and Tolhurst, 1978a, 1978b, 1978c;

Kulikowski, 1979). This model seems tc. fit the tuning

curves for the neurons better as the cells show a greater

selectivity when tested by gratings rather than bars

(Albrecht, DeValois and Thorell, 1979), although the

concept has been extensively debated (see for example

Glezer, Ivanoff and Tscherbach, 1973; Tyler, 1975; Glezer,

1975; Glezer et al., 1976; Tyler, 1978b; Glezer, 1979).

DeValois, DeValois and Yund (1979) directly tested the

concept of "Fourier" tuning in ccrtical neurons and found

that both simple and complex cells were sensitive to the

orientation of the optical Fourier transform components of









a pattern, rather than the orientation of the edges. This

sensitivity applied also to the spatial frequency axis, as

the response to the higher harmonics of square edged and

missing fundamental patterns was predicted by the Fourier

components (Maffei et al., 1979a). At lower spatial

frequencies, however, some edge detection does occur

(Maffei et al., 1979b; Kulikowski and Bishop, 1980; Ross

and Johnstone, 1980).



Binocular Data


The existence of two functional "simultaneously"

seeing eyes in the normal human, and the assumption of

singular independent information channels immediately

raises the problem of binocular integration. Do the two

eyes constitute separate visual channels (Cormack and

Blake, 1980)? By using the performance degrading effects

of stimulus uncertainty on grating detection, Cormack and

Blake (1980) were able to show that eye-of-origin

uncertainty was unable to degrade detection in

stereo-normal subjects, but did lower performance in

stereo-blind subjects, thus indicating that the two eyes,

at threshold, normally behave as a single channel.

However, especially at suprathreshold levels, the

separateness of information from the two eyes is not lost

at higher levels. Binocular rivalry, or alternating

monocular suppression during dichoptic viewing, compounds

the problem of spatial frequency channels and perceptual









fusion (Julesz, 1978). However, even in nondichoptic

situations, i.e., with perceptual fusion, Makous and

Sanders (1978) were able to show that some monocular

suppression remained.

The neural location of this suppression remains

unclear as neither evoked potentials need be reduced, nor

McCollough color aftereffects weakened, by a perceptual

invisibility of the inducing stimuli (Cobb, Morton and

Ettlinger, 1967; White et al., 1978). Binocular

suppression, or facilitation, when measured by evoked

potentials, seems critically dependent on the stimulus

parameters, and has generated apparently contradictory

results. If the images from the two eyes are not

perceptually fused, suppression results; if the images are

disparate, but do fuse, enhancement results; if only a

blank field is presented to one eye, that eye is

apparently ignored (Ciganek, 1971; Harter, 1977).

Individual components of the transient evoked potential

behave differently with differing spatial frequencies and

orientations (Harter, Seiple and Salmon, 1973; Harter,

Seiple and Musso, 1974; Harter, Condor and Towle, 1980).

Active suppression of an artificially blurred image in one

eye has been demonstrated with steady-state evoked

potentials (Fiorentini et al., 1978) and has been

correlated with similar results found in human amblyopia

(Srebro, 1978; Wanger and Nilsson, 1978; Levi, Harwerth

and Smith, 1979). Temporally disjoint but spatially









identical and fused stimuli produce a reduction of

steady-state evoked potentials in stereo-normals but a

smaller reduction in humans with impaired binocular vision

(Lennerstrand, 1978a). Inducing greater binocular rivalry

by changing the color and pattern presented to each eye

resulted in a perceptual suppression/alternation, but the

evoked response did not reflect the rivalry (Lennerstrand,

1978b). In fact, the signal depression was greater with

no spatial disparity, supporting the hypothesis that the

depression due to the temporally disjunct stimulation was

producing interference in a single spatial frequency

channel, as a form of stimulus masking. Unit analysis has

shown neurons "tuned" for interocular disparity (Fischer

and Kruger, 1979; Poggio, 1979) but the variability of the

response is so great that a "variability" code has been

proposed (Crawford and Cool, 1970) since occlusions of

response as well as facilitation have been found (Noda,

Creutzfeld and Freeman, 1971). Binocular response of

striate cells seems dependent on the corpus callosum

(Payne et al., 1980) and would imply a separable evoked

response for each eye. In sum, these data would seem to

indicate that at least some aspects of depressed detection

reside in the neural elements of the primary visual

cortex, even if perceptual rivalry may not (Martin, 1970).









'Summary


To recapitulate the main findings detailed above:

(a) psychophysical data from detection, discrimination and

masking experiments have been modelled as spatial and/or

temporal frequency channels, responsive to Fourier

components of stimuli; (b) electrophysiological recordings

at the single unit level in area 17 are consistent with

activation by spatio-temporal Fourier components; (c)

stimulus evoked potentials, although recording neural

discharges en masse, can be manipulated meaningfully by

appropriate changes in stimulus Fourier components; and

(d) data from psychophysical, evoked potential and

single-unit recording experiments suggest that

eye-of-origin might be an additional characteristic of the

spatiotemporal frequency channels.















CHAPTER III
EXPERIMENTAL RATIONALE


The multiple channel model of visual spatial

frequency analysis (Campbell and Robson, 1968) predicts

independence between channels more than an octave apart.

If this independence holds in the visual cortex, the

response to the complex gratings will be a linear

superposition of the response to the grating's components

presented separately. Any interaction, constructive or

destructive, will be apparent from the evoked waveform (by

correlation analysis) as well as by amplitude and/or phase

changes. Phase independence of sum of sinusoid gratings,

predicted by the same detection experiments, will also be

tested.

If the spatial frequency channels are fully

independent at area 17, i.e., independent cortical columns

(Hubel and Wiesel, 1972), then the mode of stimulus

component addition should not change the response, i.e.,

haploscopic (dichoptic) presentation of a complex grating

should equal monocular presentation. If there is

interaction, then these two modes of stimulation will

indicate whether the interaction is cortical or

precortical (if the response to the complex monocular









stimulus is less than that for the dichoptic input

assuming destructive masking).

Lastly, by adding harmonics to a steady-state

temporal stimulus sinusoidall counterphase vs. square wave

at seven shifts/second), this experiment will show whether

temporal frequency channels also interact to change any of

the above results. If there is no difference, the visual

system is using only the fundamental stimulus frequency.

If enhancement or interference occurs, the model of the

visual system as a strict Fourier analyzer in the temporal

domain cannot be maintained.

In conclusion, these experiments will test the degree

of independence of spatial and temporal frequency channels

in steady-state suprathreshold response of the human

visual system. Should interaction between the channels be

found, these experiments should also indicate a possible

site for them (retina/LGN or primary cortex) i.e., does

the eye-of-origin constitute an appropriate characteristic

of a spatial frequency channel. The results link the

threshold detection experiments in psychophysical research

and suprathreshold visual evoked responses.















CHAPTER IV
METHODS

Subjects


Three male and two female volunteers, Caucasian,

between 22 and 30 years old, who gave their informed

consent, served as subjects. The females were between

menses and ovulation in their menstrual cycles. All

subjects had corrected 20/20 Snellen acuities in each eye,

perfect Farnsworth D-15 color discrimination tests in each

eye, Titmus stereoacuity of 50 sec of arc or better, and

normal binocular fusion ranges as tested by prismatic

displacement of a 50 target with a phoropter. Monocular

contrast sensitivity curves were obtained for each eye by

inspection of photographs of Arden type sine wave grating

plates (Arden, 1979) and'were normal in shape and

approximately equal between the two eyes for each subject.

All subjects practiced viewing the stimulus display before

recording sessions were started. No difficulty was

reported in aligning the haploscopically presented

display. Subjects wore their normal corrections, and no

artificial pupils were used. All subjects were tested

three times on separate days, producing 15

subject-sessions of 20 stimuli each (see below).









Stimulus


Two Tektronics RM503 oscilloscopes with blue-green

(P2) phosphors were mounted to face each other (Fig. 1).

Two first surface mirrors were mounted between them to

direct the respective images to a set of prisms mounted on

rotating sleeves, permitting adjustment for individual

interpupillary distances. The circular oscilloscope

screens subtended 50 of arc at the eye. Fixation and

fusion were aided by providing a black fixation spot of

22'48" in the center of each screen (two subjects) or

diagonal crosshairs of 00 black silk of 1'53" diameter

(three subjects). No significant differences were noted

in control runs with one subject in response wave form,

amplitude or phase with the different fixation marks in

distinction to some psychophysical results (Weisstein, et

al. 1977). The horizontal deflection plates of CRO#2

(slave) were connected in parallel to those of CRO#1

(master) to insure perfect synchrony of sweep, triggering

and unblanking as well as phase lock between the two

displays.

Vertical sine wave gratings were generated on the

oscilloscope faces by modulating the z (luminance) axes

with the appropriate waveform (Campbell and Robson, 1968).

A mean luminance field was established with a 0.55 MHz

signal from an external function generator fed to the

x-inputs of the oscilloscopes. The field was refreshed at

1.56 KHz and provided approximately 140 traces per degree









Figure 1.


Schematic illustration of stimulus and recording
arrangement. Al, A2: preamplifiers;
C: computer; F: band-pass filter; M: first-
surface mirrors; 0: observer; 01: master
display oscillosocpe; 02: slave display
oscilloscope; P: prism array; S: shield room;
SG: signal generator; ST: stimulus control
circuitry; T: oscilloscope trigger input;
Tl: ocilloscope trigger signals; T2: computer
trigger signal; X: master oscilloscope
horizontal control; Y: oscilloscope vertical
control; Z: oscilloscope luminance control
input; Zl, Z2: luminance control signals to 01
and 02 respectively.

















Z2:


I--
I
T1 Z1 Z2
A A


ST


T2


Z1









of visual angle. Due to the line spread of the phosphor,

the individual traces were completely unresolvable. The

brightness was adjusted to 5.16 cd/m2 at the eye although

this level is not critical for contrast evoked potentials

(van der Tweel, Estevez and Cavonius, 1979). The AC

signal from the stimulus circuitry (see Appendix)

modulated the luminance of the screens around this value.

The high refresh rate and the stable triggering of the

oscilloscopes prevented variability of the counterphasing

stimulus, such as found in television systems (van Lith,

van Marle and van Dok-Mak, 1978; Meienberg et al., 1979).

Luminance profiles of four shapes were used (Fig. 2): (1)

a fundamental sine wave, (2) its third harmonic, (3) the

sum of 1 and 2 at a phase angle of 00 (peaks subtract),

and (4) the sum of 1 and 2 at a phase angle of 1800 (peaks

add) (Atkinson and Campbell, 1974). A linearity check of

the screens with a photoresistor showed a distortion of

0.58% (power of the fundamental to the first harmonic) of

voltage versus luminance with a 32 V sine wave grating.

All stimulus waveform luminances were calibrated with a

Pritchard photometer at the plane of the subject's eye and

confirmed the display contrast linearity. A linear

regression calculated between peak to peak voltage of the

stimulus waveforms and per cent contrast

(L -L /2L X100%) yielded a correlation coefficient
max min mean
for the line of 0.9985.










Figure 2. Stimulus luminance profile metrics for gratings
used.
















VOLTS
(PP)
18


CD/M2
(MAX/MIN)
6.22/2.70


STIM

F1




F 3





S 0







S 1


CONTRAST
( AKx10 0%)
34




34





45







55


0 5.16/5.16


18 6.22/2.70





26 7.20/2.51







32 8.03/2.33


DC









The fundamental spatial frequency was set at two

cycles per degree, producing 10 iterations of the waveform

on the screen (Hoekstra et al, 1974; Howell and Hess,

1978; McCann, Savoy and Hall, 1978). The stimulus

circuitry permits these waveforms to be presented

monocularly (mean luminance field to the fellow eye),

binocularly, and dichoptically (Fl to one eye, F3 to the

other, either at 00 or 1800 phase angle). Finally, the

circuit permits the waveforms to be counterphased in two

modes: sinusoidal (continuous) and square-wave (abrupt).

The fundamental temporal stimulus frequency was set at

seven shifts (contrast reversals) per second and modulated

both the simple and complex grating phase transitions at

the same rate, as suggested by Pantle (1973). These

spatial and temporal frequency values are near the human

contrast sensitivity peak both psychophysically (Kelly,

1977) and electrophysiologically (Bodis-Wollner, Hendley

and Kulikowski, 1972; Regan, 1978). All 20 stimuli (10

sine mode and 10 square mode) were presented in each

session but in different random orders and to different

eyes. The stimuli were:

FlM, 2 cycles/degree, monocular presentation
FlB, 2 cycles/degree, binocular presentation
F3M, 6 cycles/degree, monocular presentation
F3B, 6 cycles/degree, binocular presentation
SOM, 2+6 cycles/degree at 00 relative phase angle,
monocular presentation
SOB, the same as SOM but binocular presentation
SOD, 2 cycles/degree presented to one eye, 6
cycles/degree presented to the fellow eye, the
fused gratings at 00 relative phase angle
(dichoptic)









SIM, the same as SOM but a relative phase angle of
1800
S1B, the same as SIM but presented binocularly
S1D, the same as SOD but with the fused dichoptic
grating components at a phase angle of 1800.



Recording


Subjects were seated in a darkened radio-frequency

shielded room and viewed the display through the opened

door. The laboratory was dimmed to reduce distraction.

VERs were recorded from a point on the scalp over the

visual cortex, 2.5 cm above the inion on the mid-saggital

plane with a Beckman Agc-AgCl disk electrode attached with

electroconductive paste and referenced to yoked Ag-AgCl

earclip electrodes; ground was established with another

Ag-AgCl disk electrode on the forehead. Response signals

were preamplified (0.8-50 Hz, -3 dB points) and fed to the

input of a Nicolet MED-80 computer. The signals were also

narrow band filtered (Krohn-hite. band pass filter: 5-9

Hz) and fed simultaneously to the computer. Both raw and

filtered data as well as the stimulus trigger were

recorded during acquisition on FM tape (Tandberg

Instrumentation Recorder) for later off-line analysis.

The bandpass filter was calibrated for gain (0.9) and

phase lag (1060 at 7 Hz) to correct the filtered data

amplitude and latency. Two stimulus shifts were recorded

per averaging epoch; the time occupied by a third shift

was used by the computer for artifact reject, baseline

correction and variance calculations on the just completed









sweep. Thus, both "left" and "right" shifts were averaged

together in the final record. Two hundred sweeps were

taken per stimulus and averaged with a time resolution of

280 lUsec per acquired point and a vertical resolution of

0.05 volts.



Analysis


Both filtered and unfiltered averaged signals were

evaluated for amplitude between the most positive point

following a stimulus shift (Pl) and the most negative

point preceding that Pl (Nl). This gave two values per

record for both the filtered and unfiltered signals.

Latencies were calculated from the Pl and referenced to

the zero crossing of the stimulus, thus giving a phase lag

figure in degrees. The absolute phase lag for the visual

system was calculated from the following test run, also

performed at each subject-session. A sinusoidal grating

at F1 was presented binocularly, and by gating the

temporal waveform generator, sinusoidally counterphased

six times, followed by an equivalent quiet period of time

with the grating stationary. Two hundred sweeps were

taken of this 1.4 second epoch. The average phase lag of

the resulting six Pl peaks was taken to represent the

response time of the visual system to a 7 Hz shifts/sec

phase translation. The phase lag figures of the

continuous stimulation runs were referenced to this lag.









DC "noise" runs showed signal to noise ratios of 2-1 or

better in all subjects.

Pearson correlation coefficients were calculated

between the 512 point records of all possible stimulus

pairs, both filtered and unfiltered, and averaged across

all subject-sessions for each stimulus. Under the

assumption of linear superposition, expected response

waveforms, which will be referred to as "synthetic"

responses, were created by digital addition of the

responses to the Fl and F3 stimulation (FSM=F1M+F3M;

FSB=FlB+F3B). These synthetic responses were also

compared to the observed sum of sines responses.

Unfiltered data were reacquired from tape on a long

time base (1.44 sec=10 stimulus shifts per computer sweep)

to avoid low frequency window effects (leakage) in the

subsequent Fourier analysis. The Nyquist sample frequency

cutoff was 358 Hz, which is well above the 50 Hz upper

limit of the recorded data, thus preventing aliasing

(Ramirez, 1974). Power spectra were calculated for each

stimulus condition, and the three bins surrounding the

fundamental (bin width=0.7 Hz) were summed to give the

power of the VER at the stimulus frequency. The second

and third harmonics were similarly quantified.

Numerical amplitude and phase data as well as the

power figures and Pearson r correlation matrix were

evaluated by Repeated Measures Analysis of Variance and






32

Duncan's Multiple Range Test by the Statistical Analysis

System of the Northeast Regional Data Center.















CHAPTER V
RESULTS

Phase Calibration


Figures 3 and 4 show phase runs of two of the five

subjects. Each zero-crossing of the stimulus produces a

response peak that lags by at least 3600 at this stimulus

frequency, depending on the individual. A double response

can clearly he seen in the unfiltered traces, indicating a

strong second harmonic component in the response. The

overall stationarity and reproducibility of the response

can be seen from the grand unweighted non-normalized

digital sum of all 15 subject-session phase runs in the

study (Fig. 5). This method of gated stimuli permits,

therefore, an absolute calibration of the phase lag in the

human visual system to (quasi-) steady state stimuli, in

agreement with Diamond's (1977) rejection of Regan but

without his complex methodology. The values thus obtained

provide the baseline latencies against which the

steady-state responses are measured.



Spectral Power


The "doubling" of the response peaks to sinusoidally

as opposed to abruptly (square mode) counterphasing

33









Figure 3.


Phase run of subject A, session #1.
A: unfiltered data; B: filtered data (5-9
Hz); C: counterphase trace of the grating
luminance contrast; D: zero-crossing signal.
The numbers between C and D indicate stimulus
events; the numbers over the peaks in A and B
indicate the respective response positivities.
















E2

uV 1
0


45 6


0 200
MS


1 2 3 4 5 6


.I-n..









Figure 4. Phase run of subject B, session #3, Otherwise
as in Fig. 3.











2 3


v V V \l


223
uV 1



0 2002
B





123456
O U U L o









Figure 5. Grand unweighted, non-normalized digital sum of
all 15 subject-session phase runs. Otherwise
as in Fig. 3. Absolute amplitude is arbitrary.












4 5


5 6


0 200


1 2 3 4 5 6

" Ur UL -









stimuli can be seen to occur in monocular, binocular and

dichoptic presentations, whether of simple or complex

gratings (Figs. 6, 7, 8 and 9). The enhancement of the

second harmonic by the sinusoidal mode can be clearly seen

in the power spectra of each response.

Means of the fundamental and second harmonic power

are graphed in Pig. 10; their ratios in Fig. 11.

Statistical analysis showed that for square mode

counterphasing stimuli, the fundamental frequency of the

response is greater than the second harmonic (p=0.0016)

for all stimuli, but that the fundamental and second

harmonic are not significantly different in the sinusoidal

case (p>0.05 by Duncan's). The third harmonic showed

essentially no mode or stimulus differences. The analysis

of the frequency component power ratios confirmed this,

but the large variability of the non-normalized data

allowed only four of the 10 stimuli (FlM, SlB, SOB and

SOM) to show a significant mode difference (p<0.05, SQ>SN)

although all the others also tended in the same direction.

The response power of all stimuli showed overlapping

ranges by Duncan's test in the sine mode, but SOM proved a

significantly higher fundamental to second harmonic power

ratio in the square mode presentation than all other

stimuli (see below).

Analysis of the monocular vs binocular subsets of the

power data for "simple" gratings (FlM, FlB, F3M, F3B)

showed no significant differences in the fundamental,









Figure 6.


Responses to a monocular presentation of a
complex grating with the components at a phase
angle of 00o. A: unfiltered data; B: filtered
data; C: stimulus traces (sine or square mode
counterphase) as well as time and amplitude
calibration on square wave; D: power spectra
of responses. Note a factor of two scale shift
of the sine mode versus square mode power
spectra.





42










A


590


7 14 21
FREQ (Hz)


7 14 21


L'
C-I
b.


233


143.92 Ms 2.44 uV









Figure 7. Responses to a binocular presentation of a
complex grating with the components at a phase
angle of 00. Otherwise as in Fig. 6,

































143.92 Ms


LA
vI
v-


7 14 21
FREQ (Hz)


794


224


7 14 21


SOB


2.44 uV









Figure 8. Responses to a binocular presentation of a
simple (Fl: 2 cycles/degree) grating.
Otherwise as in Fig. 6.











FIB














C





D


561


7 14 21
FREQ (Hz)


MS 12.44 uV


1535

1


~IU17 34
7 14 21



ui
I-
C"J









Figure 9. Responses to a dichoptically presented gratings
(F1+F3) with the components at a phase angle of
1800. Otherwise as in Fig. 6.













S D


B


143.92 MS


987


7 14 21
FREQ (Hz)


D 621




7 14 21


2.44 uV


CO
L1
.-I
o^









second harmonic or their ratio in either sine or square

modes. For the complex stimuli (SOM, SOB, SOD, SlM, SlB,

SlD), the fundamentals were not significantly different

(p>0.05) but the second harmonic showed a significant

difference due to condition (p=0.0074); i.e., binocular

and dichoptic values are both significantly above

monocular values. The frequency component ratio showed

complex interactional effects (phase by condition by mode:

p=0.0002); however, the pattern of significance in the

breakdown showed merely that monocular ratios tended to be

greater than binocular and dichoptic ratios, and that

square mode ratios tended to be greater than sine mode

ratios, echoing the earlier analysis. Also, a significant

difference due to component phase appeared in the square

mode monocular power ratio (p=0.0049) indicating that

complex stimuli with a phase angle of 00 between the first

and third harmonic components (SOM) show a higher temporal

response component power ratio (fundamental/second

harmonic) than with a phase angle of 1800 (SIM), i.e., an

enhancement of the fundamental compared to the second

harmonic when the components are in a square wave phase

relationship; but as neither component alone is

significantly affected, the significance of this result

remains unclear.

















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(fundamental/second harmonic) by mode and
stimulus. Means and standard error of the
means (n=15).














































F3B SOM SOB S


ESINE MODE

SQUARE MODE


SIB SID


STIMULUS


20r


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.Amplitude Axalyses


The mean unfiltered and filtered response amplitude

values are graphed in Figs. 12 A and B for all stimuli as

well as for the synthetic responses. Immediately apparent

is that the synthetic binocular sum (FSB) is significantly

higher than all other stimuli in both sine and square

modes, unfiltered, and in the filtered square mode (all

p's=0.0001); the filtered sine mode values showed no

significant ranking of stimuli (p=0.5142). Actual

responses to complex (sum of sinusoid) gratings tended to

cluster in the center of the ranking and indicate that the

response to input on "separate" spatial frequency channels

is in general not a linear addition of the responses to

the components presented separately. Relative stimulus

component phase angle had no significant effect on the

response metrics.

With one exception .(F3M), square and sine mode

unfiltered amplitudes do not differ significantly; square

mode responses tend to be larger than sine mode responses

in the filtered case but not strongly so (p<0.05 in F3B,

F3M and SlM; other stimuli rank square>sine but have

p<0.05). Also as expected, unfiltered data tend to be

larger than filtered data for all modes and stimuli

(p<0.05).

Rearranging the amplitude data for simple gratings

into subsets, the responses to Fl and F3 grating stimuli

(2 and 6 cycles/degree) do not differ significantly;













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however, they are both less than the synthetic responses

(FSM and FSB: p=0.0001 for monocular, p=0.0003 for

binocular conditions). For all stimulus frequencies (Fl,

F3 and FS) and both modes (sine and square), binocular

responses were higher than monocular but not significantly

so (0.18>p>0.05). With complex stimuli (SO and Sl), the

sine mode unfiltered responses did show a significant

ranking (p=0.0148: binocular>dichoptic>monocular, with

overlapping Duncan's ranges), which was weakly confirmed

with the square mode responses (p=0.0709). This indicates

that the monocular response is not depressed by "masking"

in the fellow eye when the "mask" is of a (square wave)

harmonically related frequency but rather the response is

enhanced to fall between monocular and binocular levels.



Phase Analvses


Figures 12 C and D show the phase values of the

filtered and unfiltered responses respectively. In

general, the square mode phase is unaffected by filtering

(p=0.2220), whereas the sine mode is, although the

difference reaches significant levels for only two stimuli

(FlM and SiB, p<0.05: unfiltered>filtered); the other

stimuli follow the same pattern. Also, in general, the

trend is for unfiltered sine mode responses to have

larger,- i.e., longer phase lags than square mode

(p=0.0772) whereas the opposite holds true for filtered

responses (mode by stimulus: p=0.0228; SlD and SlM:









p<0.05, square>sine; other stimuli nonsignificant but in

the same pattern). The differences are due to the

separate phase of the second harmonic in the sine case.

The stimuli did not rank in any significant order as the

Duncan's ranges overlapped extensively. Grouping the data

by condition (monocular, binocular and dichoptic) also

revealed no significant relationships.



Correlation Analysi s


Pearson correlation coefficients were calculated

between all possible response pairs for each subject

session, both filtered and unfiltered, to define a data

base for comparing individual mean correlations to the

general mean. A selection of mean correlations across

subject-session comparing the synthetic sum of spatial

frequency responses to the actual responses is given in

Table 1. Of the entire matrix of 276 mean correlations of

unfiltered responses, only the correlation of FSB-SQ vs

SlD-SO (unfiltered) fell more than one standard deviation

above the mean (neglecting the trivial correlations of

*e,.g., FlM with FSM, as FlM+F3M=FSM). None of the 276

filtered data correlations fell above one standard

deviation over the mean. The analysis of variance on the

matrices indicated that the ranking of correlations was

significant (p=0.0001 for both filtered and unfiltered

cases), but all the Duncan's ranges overlapped

extensively. However, even in this one case, only 61% of









Table 1.


Selected Pearson Correlation Coefficients
comparing Synthetic sum of component grating
responses (FSM and FSB) to actual responses
(SOM, SOBSOD, SIM SLB, SID); U: unfiltered
F: filtered data; SN: sine mode; : square
mode. sne ode; SQ: square


FSM
SN SQ SN FSB SQ

.5102 .1620 .5032 .1969 SN SM
.5853 .2273 .4608 .2633 F


.2814
.3985

.4362
.4544

.3493
.2984

.4424
.4925

.3559
.4299

.4713
.3149

.3222
.2148

.5227
.6388

.4125
.5501

.4821
.3868


.6701
.8673

.1656
.2292

.5792
.7409

.2237
.2007

.6205
.7716

.0450
-.0530

.5942
.6561

.2675
.2522

.6514
.8905

.2889
.2581


.2149
.3390

.5883
.6227

.3898
.3267

.4470
.3424

.2595
.2273

.5513
.4418

.2804
.2985

.5523
.4952

.4444
.4560

.5554
.4477


.4827 .7231 .4324 .7833 U SQ
.4928 .9191 .3951 .9336 F

Mean S.D. ean D Mean S
+ -S.D* Mean S.D ---


.7003
.8318

.2121
.2424

.6255
.7316

.1674
.1438

.5762
.7063

.1057
-.0636

.6270
.6526

.2562
.2329

.6462
.7904

.3580
.2962


U
F

U
F

U
F

U
F

U
F

U
F

U
F

U
F

U
F


.3830
.4047


.3599
.5803


.7429 ~ -.1756
---------023 --


SOB


SIM


S1B


SCD


S1D


.7429
0a cn


.0231









the variance in the observed dichoptic sum of spatial

frequencies response is accounted for by the individual

responses to the single spatial frequencies. This

indicates a definite nonlinearity, 'i.e., interaction, at

suprathreshold levels, between these spatial frequency

channels.



Results Summary


In short, therefore, the power spectral analysis has

shown a significant enhancement of the second temporal

harmonic component of the evoked waveform to the

sinusoidal counterphase mode of stimulation. The

amplitude analysis has refuted the concept of a linear

superposition of spatial frequency channels greater than

one octave apart. The phase analysis shows complex

effects that are not easily relatable to either

stimulation mode or spatial stimulus waveform. Lastly,

the correlation analysis has shown a partial prediction of

the response waveforms to complex grating stimulation by

the summed responses to their simpler components, that

does not depend on eye-of-origin information.















CHAPTER VI
DISCUSSION


Sinusoidally counterphased gratings physically

present stimulus power at only one temporal frequency.

Abrupt counterphasing presents energy at all the odd

harmonics of the fundamental as well. Thus, square wave

counterphased gratings, having only one stimulus event per

shift, will necessarily show power in the fundamental

frequency in the response waveform, and may show power in

the odd harmonics as well. Regrettably, whether the third

harmonic of the stimulus is also generating a response

waveform, the non-normalized data are too variable to

show. The unexpected finding of peak splitting of the

response waveform to sinusoidal stimulation, however,

indicates that the visual system is not responding in a

linear fashion in the temporal domain. An explanation can

be proposed based on the contrast sensitivity curve,

namely that the grating is not being perceived as

counterphasing, but rather that two separate gratings are

alternately appearing and vanishing, with each onset into

visibility and offset into the period of perceived

"non-grating" mean luminance acting as a stimulus event

and generating its own peak (Spekreijse, van der Tweel and

Zuidema, 1973). This would explain the appearance of a

61









"second harmonic" to the presumed stimulation frequency.

An alternative explanation could be proposed from the

observation that a sinusoidally counterphasing grating can

be decomposed into two gratings drifting across one

another (Sekuler, Pantle and Levinson, 1978). This would

require the hypothesis that the mechanism responding to

e.g., the rightward drifting grating respond with a

different latency to that responding to the leftward

drifting grating; however, such differential directional

responses have not been noted in the evoked potential

literature, unlike spatial frequency differences which do

correlate with latency changes (Vassilev and Strashimirov,

1979).

Channel independence in the spatial frequency domain

assumes independently responding mechanisms, and thus,

presumably, independent evoked responses. Psychophysical

masking experiments have postulated a destructive

interaction between channels that suppresses the response

below monocular levels in a dichoptic paradigm; however,

the data reported here support neither of these two

positions. If the two and six cycles/degree channels were

independent, the digital sum of the monocular responses

would be of the same amplitude and phase as well as highly

correlated with the observed dichoptic sum of gratings

response. The dichoptic response not only is lower than

the synthetic response whether monocular or binocular,

indicating some occlusion, but the correlation









coefficients are also low (see Table 1), for example,

FSM/SOD, square mode, unfiltered, shows only 43% of the

variance accounted for. The filtered cases, of course,

have higher values, but even with the data reduced to the

response fundamental, only 79% of the variance is

accounted for. Neither of these values falls more than

one standard deviation above the mean correlation of the

entire data matrix, and therefore is of doubtful

significance. But it is also clear that a suppression of

the evoked potential by the dichoptic condition did not

occur either. Separate channels are still, however, a

valid model of some of the data, since the relative phase

angle of the stimulus components does not affect the

response fundamental or amplitude, despite a 10%

difference in overall contrast. What remains, therefore,

is that in the steady-state, suprathreshold case, gratings

that are square wave related,' i.e., in a 1:3 harmonic

relationship, and phase locked, do interact and enhance

the evoked potential approximately 15% above the monoptic

case but not to the level of the pure binocular

stimulation response (approximately 24%).

Comparing the responses to two-spatial-frequency-

channel-stimulation by condition of stimulus (monoptic vs.

dichoptic) leads to evidence suggesting the site of

interaction of these channels. The data for simple

gratings showed a small, if non-significant, binocular

versus monocular enhancement (approximately 17%),









presumably explained by the fact that the monocular case

in this study was quasi-binocular. There was always the

same total luminance field presented to each eye at all

times, rather than a dark field to one eye as in classical

binocular facilitation studies which demonstrate 1.4 fold

enhancement. Thus, one can assume that at least some

aspects of the binocular mechanisms were also activated in

the "monocular" case, i.e.,* the various conditions of

stimulus presentation did not radically alter the

responding population of cortical neurons giving rise to

the evoked signal. If the interaction between the two and

six cycle/degree channels is cortical, the cortical

population of neurons can "see" a difference between the

monoptic and dichoptic sum of gratings only with a

monocular suppression (or by integrating the information

from purely monocular cells), not otherwise (Braccini,

Gambardella and Suetta, 1980). This implies that SOM=SOD

and S1M=S1D in the fused case. On the other hand, if the

interaction is precortical, i.e., before integration of

information of the two eyes, not only would FSM=SOD, but

SOMySOD. Since the Duncan's ranges showed a monocular and

dichoptic overlap in agreement with Sturr and Teller's

(1973) dichoptic disk/anulus sensitization experiments (as

well as a binocular-dichoptic overlap), none of the

conditions for a precortical locus are satisfied, whereas

those for a cortical one are. Additionally, eye-of-origin

information does not appear to be an important






65


characteristic of spatial frequency channels when examined

with an equal-luminance paradigm.















APPENDIX
CIRCUIT DESCRIPTION AND PARTS LIST


The circuit diagrammed in Fig. 13 consists of four

major sections:

1. a fundamental plus third harmonic spatial

waveform synthesizer,

2. a logic switch pair to abruptly replace this

waveform with its inverse

3. a temporal waveform synchronizer, and

4. a programmable variable gain amplifier for

amplitude modulation.

The first section consists of FG1, IC1 and IC2 with

their associated discrete components. The F3 frequency is

set by C4, and FG1 is tuned to 1/3 this value. The SYNC

pulse from FG1 locks the peaks of the two waveforms

together. Phase selectivity (00 or 1800) is determined by

which SYNC pulse (peak or valley) is seen by IC1.

Switches Sl and S2 determine whether a composite waveform

(Fl+F3=Z) is seen by the remainder of the circuit, or only

the simple waveforms.

The second section (ICs 3-7) creates the inverted

waveforms, and in response to IC8's output, switches

between them and the noninverted signals. Final gain

adjustment for the square wave mode counterphase output is

accomplished by ICs 6 and 7.
66







































*-i
54



















0
-4




















































4
tI
r1




.It

v-I







'-.

U








'-I





*Hr






68



r-8





g m




LI -- 2 r
a




CIc A
a a .




YV
cuc
V. ccA







-m --- H -ID'ce *-A -
A* T>-




0 cc... a
^^g. 1I .i ..





N ..t








N.N
> t I ,


ur ----I
Lu c


no in
Q ra Q
aI












ILE I
uj__ h CC r^
Y -I

I, r^ n- 1 r+






U, C C ,


.P I
I"~ ~ ~ t -- J L? a ^
c3 oT'1^'ti 1'?A
2_Ja -- ?o
__ ___\^,ll









The third section has two parts: a zero-crossing

detector (IC8) and an ideal rectifier (IC9). The input

waveform comes from FG2 and determines the counterphase

rate as well as the programming waveform for the last

stage. FG2 also produces a square wave, synched to its

sine wave output, to generate a computer trigger pulse

through Tl, CR1 and CR2.

The final stage (ICs 10-13) takes the rectified

temporal signal to control the gain of the square wave

mode counterphasing signal of section two and sinusoidally

varies the amplitude. As the TTL switches are

counterphasing at the zero-crossing of the temporal

waveform from FG2, the output will change phase by 1800 at

that point at which the instantaneous output voltage from

ICs 10 and 11 are zero. This produces the sine-mode

counterphased signal.

Square- and sine-mode outputs are selected by S3 and

fed to the Z-axes of the two oscilloscopes comprising the

stimulus display.



Voltage Regulators

21 1N761A 4.9 V zeener diode
Z2 1N754Z 6.8 V zeener diode


Current Regulators

CR1 SA 1181/631-1 Diode
CR2 SA 1181/631-1 Diode
CR3 1N4067 Diode
CR4 1N4067 Diode
CR5 1N4146 Diode
CR6 1N4148 Diode









External Unit s


WAVETEC model 110
EXACT model 505
Power supply


Function generator
Function generator
(+15 volts/-15 volts)


Switches


Slide switch
Slide switch
Toggle switch


Integrated Circuits


Function generator
Op amp
Op amp
Op amp
TTL DPDT switch
TTL DPDT switch
Op amp
Op amp
Dual comparator
Op amp
Op amp
Transconductance amp
Transconductance amp
Op amp
Op amp


Transformer

Audio transformer


Capacitors


microfarad
microfarad
microfarad
microfarad
microfarad
microfarad
microfarad
picofarad
picofarad
microfarad
microfarad
picofarad
picofarad


FG1
FG2
+V/-V


DPST
DPST
DPDT


ICl
IC2
IC3a
IC3b
IC4
IC5
IC6
IC7
IC8
IC9a
IC9b
IC10
IC11
IC12
IC13


Cl
C2
C3
C4
C5
C6
C7
C8
C9
C10
Cli
C12
C13


XR205
741
1/2 747
1/2 747
LH5042
LH5042
741
741
LH711
1/2 747
1/2 747
CA3090
CA3090
741
741


0.1
0.1
0.1
0.01
0.1
0.1
0.1
300
300
4.7
4.7
300
300









Resistors (Values in Ohms)


R1
R2
R3
R4
R6
R7
R8
R9
R10
R11
R12
R13
R14
R15
R16
R17
R18
R19
R20
R21
R22
R23
R24
R25
R26
R27
R28*
R29
R30
R31
R32
R33
R34
R35
R36
R37
R38
R39
R40
R41
R42
R43
R44
R45
R46
R47
R48
R49
R50
R51
R52


10K
20K
10K
5K
4K
5K
1K
1K
15K
10K
10K
10K
5K
5K
200K
220
5K
5K
5K
5K
200K
5K
5K
220
10K
33K
5K
5K
5K
5K
4.7K
18K
27K
47K
47K
47K
50K
2.37K
4.7K
18K
27K
47K
47K
47K
50K
2.3K
5K
5K
5K
5K
100


Trim pot



Trim pot






Trim pot

Balance pot


Trim pot
Trim pot

Balance pot

Trim pot




Trim pot








Balance pot







Balance pot


Trim pot

Trim pot















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BIOGRAPHICAL SKETCH


The author was born on October 3, 1950 in Stuttgart,

West Germany to Gerhardt Heinz and Evelyn Martha (Poenitz)

Schmeisser. His parents now live in San Mateo,

California, and are naturalized U.S. citizens.

After emmigrating first to Canada in 1951 and then to

California in 1958, the author attended Menlo School in

Menlo Park, California, graduating from the high school in

1968. Thereafter, he attended the University of

California at San Diego, receiving the Bachelor of Arts

degree in 1972 with a major in biology. In 1974, he

received the Master of Science degree from the University

of Massachusetts at Amherst from the Department of

Zoology. After working for two.years at the Stanford

University Medical Center as a research technician and,

later, research assistant, he entered the University of

Florida in 1976 for further graduate training in the

Department of Physiology.

The author married the former Jean Marie Wollenberg

in September 1980.




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