Group Title: visual evoked potential
Title: The visual evoked potential
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Title: The visual evoked potential cortical correlates of sensory ocular dominance
Physical Description: v, 84 leaves : ill. ; 28 cm.
Language: English
Creator: Nelson, Gregory Hugh, 1947-
Publication Date: 1983
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Subjects / Keywords: Vision   ( lcsh )
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Thesis: Thesis (Ph. D.)--University of Florida, 1983.
Bibliography: Includes bibliographical references (leaves 72-80).
Statement of Responsibility: by Gregory Hugh Nelson.
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THE VISUAL EVOKED POTENTIAL: CORTICAL CORRELATES OF
SENSORY OCULAR DOMINANCE






















By

GREGORY HUGH NELSON


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


1983















ACKNOWLEDGMENTS


I would like to express heartfelt appreciation and

gratitude to my wife, Anne.










TABLE OF CONTENTS


PAGE

ACKNOWLEDGEMENTS................................. ii

ABSTRACT............................ .. .. ............. iv

CHAPTERS

I INTRODUCTION............................... 1
The Visual System .......................... 1
Fusion, Suppression, and Binocular
Rivalry ................................. 9
Psychophysiological Measures of
Binocular Vision ........................ 17
The Visual Evoked Potential................ 18
The VEP and Suppression.................... 23
The VEP and Stereoscopic Vision............ 28
Handedness and Eye Dominance............... 31
A Behavioral Measure of Sensory
Dominance................................ 34
A Neurophysiological Model of
Suppression............................... 37
The Present Study.......................... 45

II METHOD............ ... ....................... 46
Subjects..... ... ......................... .. 46
Preliminary Procedures..................... 46
VEP Collection............................ 47
Stimuli..................................... 48
Procedure I................................ 51
Procedure II........................ ........ 52

III RESULTS.................................. 53
Behavioral Data............................. 53
VEP Measures............................... 53
VEP/Sensory Dominance Correlation.......... 55
Gender, Handedness, and Sighting Dominance. 57
Sighting Dominance/Sensory Dominance....... 60

IV DISCUSSION................................. 61

REFERENCES............................... ............ 72

APPENDIX A..... ................ ................... 81

BIOGRAPHICAL SKETCH............................... 84


iii














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



THE VISUAL EVOKED POTENTIAL: CORTICAL CORRELATES OF
SENSORY OCULAR DOMINANCE

By

Gregory Hugh Nelson

April, 1983


Chairman: Nathan W. Perry, Jr.
Major Department: Clinical Psychology


Sensory ocular dominance is the preference for one

image over another when the two eyes are presented dichoptic

stimuli. In a manner similar to other lateralized functions

of human behavior, ocular dominance has traditionally been

a dichotomous measure and an individual has been designated

either left or right eye dominant. The present experiment

utilized a measure of sensory dominance which was not

limited to a left or right designation, but rather, was a

continuous dimension which allowed a dominance designation

of any intermediate point between the two extremes of

completely left or completely right eye dominant.

The cortical mechanisms of sensory dominance have

not been clearly delineated, but the process of visual

suppression is instrumental to sensory dominance. The








neurophysiology of suppression is not known at this time,

but suppression has been measured and described with both

psychophysical and electrophysiological techniques. The

visual evoked potential (VEP) has been used to examine

the cortical electrophysiological properties of suppression.

When the presentation of dichoptic stimuli results in

suppression there is a decrease in the amplitude of the

VEP compared to binocular stimulation. The present study

was designed to compare the sensory ocular dominance

measure established by dichoptic stimulation to the

amplitude of the VEP acquired during the same dichoptic

stimulation.

Fifteen male and female volunteers between 18 and

31 years of age were presented pairs of different letters

dichoptically. The verbal reports of what the subjects saw

were tabulated to determine their sensory dominance scores.

VEPs were acquired simultaneously for each pair. The VEPs

to dichoptic stimulation were compared to VEPs acquired

during binocular stimulation. It was the hypothesis of

this paper that the amount of reduction in the VEP from

binocular to dichoptic conditions would correlate to the

corresponding sensory dominance measure. This hypothesis

was not supported by the data. No systematic relationship

was seen between the reduction in VEP amplitude and the

sensory dominance measure. These results were discussed in

relation to the variability of the verbal reports of the

subjects and the issue of variability within the VEP. The

neurophysiological aspects of suppression were discussed.












CHAPTER I
INTRODUCTION


Ocular dominance is an extremely complex phenomenon

whose mechanisms remain something of a mystery. Once

thought to be of a unitary nature, it has proven to be a

multifacted and formidable area of scientific investigation.

Sensory ocular dominance is demonstrated by the preference

for one image over another when the two eyes are presented

two disparate images. Sensory dominance is related to the

visual mechanisms of fusion, suppression,and stereopsis

in some manner that is not clearly understood at the present

time. The visual evoked potential (VEP) has been utilized

as a research tool to investigate these processes (fusion,

suppression, and stereopsis) and some of the systematic

changes which occur within the VEP in relation to these

have been described and corroborated in the literature. An

analysis of the VEP for correlates of sensory dominance has

not been reported in the literature and this is the focus

of this study.

The Visual System

Ocular Dominance

Dominance,as it applies to human physiology,implies

predominant, priority, or preferential activity of one side

of the bilateral systems of the body. To always, or most

often, write with the same hand is to have a dominant hand.










To always, or most often,kick a ball with the same foot is

to have a dominant foot. A hand or foot preference is easily

identifiable and the consequence of this preference can be

seen in almost every aspect of human behavior. Handedness

is just one of the more apparent lateralized functions of

human physiology and it has served as something of a model

for research in the areas of dominance and lateralization.

The concept of a dominant cerebral hemisphere is generally

accepted and is used to explain the various physiological

preferences which are demonstrated in normal human behavior.

Ocular dominance, the preference or priority of one

eye over the other, was believed to reflect this character-

istic of generalized laterality, i.e., cerebral dominance.

Porta (1593), seemingly the first to comment on ocular

dominance in the literature, hypothesized that just as most

people are right-handed and right-footed, they would be

found to be right-eyed. This postulation found support in

numerous research projects (Parsons, 1924; Coons & Mathias,

1928; Miles, 1930; Updegraff, 1932; Eyre & Schmeckle, 1933;

Berner & Berner, 1953; Klemm, Gibbons, Allen, & Richey,

1980), but the evidence which has failed to support the

hand dominance/eye dominance correlation suggests that the

phenomenon of a preferred eye is not merely a consequence

of cerebral dominance (Cuff, 1931; Smith, 1933; Eason,

Groves, White, & Oden, 1967a; Culver, Tanley, & Eason,

1970; Gronwall & Sampson, 1971; Seyal, Sato, White, &

Porter, 1981).










The human anatomy demonstrates a remarkable symmetry,

a design which is essentially represented in the central

nervous system (CNS). Anatomic decussations of afferent

and efferent pathways of the nervous systems result in the

left and right cerebral hemisphere "controlling" the

contralateral function. Thus, a right-handed person suggests

a left hemisphere dominant for movement, and further, centers

for other motor, auditory and visual speech, verbal auditory

memory, and perhaps primary auditory sensation are presumed

dominant in the left hemisphere. To intuitively extend this

schema to include the visual system may be a gross generali-

zation, though, for it ignores the anatomical uniqueness of

the visual pathways. Each eye has neural projections which

terminate in the left and right hemispheres. The retina of

each eye is divided such that there is a representation of

only a visual half-field to each hemisphere. There is a

crossing of exactly half of the retinal image to the contra-

lateral side and the two hemispheres must collaborate for

the perception of a full retinal image. This anatomy

suggests that if there is a correlation between cerebral

dominance and ocular dominance there is much about this

relationship which is concealed from us and the correlations

may be an artifact of other independent mechanisms.

In general, ocular dominance is rather easy to

establish within a sample population. Using primarily

sighting or aiming tasks, researchers have found the

majority of their subjects to be right eye dominant; fewer










of them were left eye dominant, and a very small percentage

were mixed dominant. Miles (1929) found 5% to be mixed

dominant while Cuff (1930) saw 9%. Crider (1944), in a care-

fully controlled study of 830 visually normal subjects,

found 7% to be ambiocular. Of the researchers who report

ocular dominance, the generally accepted proportions seem

to center about 65% right dominant, 35% left dominant, and

some small percentage demonstrating mixed dominance (Duke-

Elder, 1949: Spong, 1962; Gronwall & Sampson, 1971). These

results have been further corroborated across three different

age groups. Coren (1974) evaluated ocular dominance in three

subject populations with mean ages of 44.6 weeks, 9.0 years,

and 25.2 years. The pattern of right eye dominance demon-

strated by these age groups was 61.8%, 64.5%, and 65.1%,

respectively. These results clearly indicate, with 68

subjects in the youngest age group, that the adult pattern

of eye dominance is already established by the age of 10

months. The data suggest that the dominance measured in

these reports is not a concomitant of cerebral laterali-

zation or cerebral dominance. The data also continue to

fulfill Porta's (1593) prophecy that just as most people

are right-handed they are also right-eyed. Why the right

eye tends to be preferred is not known at this time, but

becoming more clear is that ocular dominance is not a

unitary function.

In contrast to the discussion on the nature of a

dominant hand or dominant foot, investigation into the










nature of ocular dominance suffers from numerous qualifi-

cations and widely disparate definitions. A simple unitary

definition of ocular dominance is inadequate, and as a

consequence, as many definitions prevail as there are

investigators. Many tests were devised and subsequently

standardized which measured ocular dominance, but the

precise aspect of ocular dominance which they were measuring

was not always clear. This issue became so progressively

confusing that researchers were forced to examine more

closely the various ocular dominance which were being

measured by distinctly different methodologies.

The complexity of the issue can be appreciated with

only a brief description of the visual system. Each retina

in the two eyes is basically a complete organ system. Each

has primary sensory projections to the visual cortex via

the lateral geniculate bodies. Complex motor feedback loops

mediate accommodation, pupillary reflex, and convergence.

The two eyes normally are directed towards and focus on the

same object but image a slightly disparate view of this

object as a consequence of the distance between the eyes.

The two images are integrated or "fused" by a central

cortical process to result in a single perception accompanied

by the appreciation of depth. The many tests which

evaluate the phenomenon of ocular dominance may tap into

any of various way-points along the circuitry, and at the

same time, probably demonstrate a hierarchy of dominance

within the complete sensorimotor visual system. It thus










seems essential to abandon any expectations that cerebral

lateralization will correlate to eye dominance and that the

measures of eye dominance will correlate with one another.

Ocular dominance or "eye preference" can easily be

determined for any individual. A simple procedure is to

ask a subject to perform several tasks which require the

use of only one eye. If a distant target is aligned with

an extended finger only one eye will intercept this line.

The eye selected by the subject to peep through a small

aperture or to sight through a tube is the dominant eye.

The data generated by these types of tasks have been

interpreted as suggesting there is a controlling eye which

has superiority over the other in directing the line of

vision. Walls (1951) cataloged 25 criteria for tests of

ocular dominance and concluded that the primary type of

dominance was a consequence of one eye dominating the

directional capacity of vision. Thus, as the eyes move

throughout the field of vision, efferent innervation is

directed to only one eye and the other is yoked to this

movement. This explanation is adequate for sighting or

aiming tests, but the theory does not hold up for all tests

or conditions of dominance.

In a clinical setting, one eye may be found to

dominate due to different acuity. Duke-Elder (1938, p.

1056) reported, "when vision in the two eyes is unequal

for some pathological or refractive reason or when

strabismus exists, the better eye obtains a position of









marked supremacy, but when the two are approximately equal

in visual acuity there may be little evidence of dominance."

It is rather easy to imagine that if one eye had a slight

measure of superior acuity, it may ultimately come to be

relied upon preferentially during a sighting test. Amblyopia

is a clinical example of one eye assuming the position of

dominance due to superior acuity. The weaker eye will be

nonpreferred during a sighting task because of its comparatively

inferior capabilities. This situation suggests how inextri-

cable the sensory and motor aspects of ocular dominance

may be, but the real complexity of this phenomenon may be

best demonstrated in a nonclinical situation.

If we examine the rather simple process of viewing

through a monocular microscope, we can appreciate more than

what meets the eye. Students of science are taught to keep

both eyes open while using only one to look through the

lens of the monocular microscope. Each student must go

through some process of determining which eye to use at the

lens, and eventually a preference for one eye over the other

is established. A casual explanation would be that

the acuity of the preferred eye gives it a slight

advantage. This could be true, but let us examine

another aspect of the process. What is going on in the

other eye? To be successful with the microscope the image

from the nonpreferred eye must be disregarded or "suppressed."

Does this then determine the preferred eye? Does the eye

which is most easily suppressed become the nonpreferred









eye? There is no ostensible oculomotor component in this

situation. Given equal acuity in the two eyes, this then

must reflect the operation of some central process within

the visual system and represents yet another type of ocular

dominance.

Several researchers have pursued a factor-analytic

process in an attempt to delineate and operationalize the

many types of ocular dominance which have been suggested

(Crider, 1944; Walls, 1951; Cohen, 1952; Lederer, 1961;

Gronwall & Sampson, 1971). The most convincing research

reported and the one which seems to be surviving the

guantlet of following research was published by Coren and

Kaplan (1973). Thirteen selected tests for eye dominance

were administered to 57 subjects with normal vision. The

results of these tests were correlated and subjected to a

varimax factor analysis. Three factors emerged which were

labeled by the authors: sighting dominance, acuity dominance,

and sensory dominance.

Sighting dominance was the most significant factor

with the majority of the tests loading on it. These were

the sighting and aiming tests plus a convergence test which

is considered to be motoric in nature. Acuity dominance

was identified by those tests which demanded a response to

a degraded or ambiguous stimulus. Under these conditions

the eye with the better visual acuity is favored. The

third type of dominance described was sensory dominance.

This type of dominance appeared in situations where the









visual system has to select between two different inputs

to the two eyes. Sighting and therefore sighting dominance

most likely occurs more frequently in the normal repertoire

of human behavior, while acuity dominance may be involved

in those systems experiencing visual anomalies. Sensory

dominance, on the other hand, may have only very subtle

effects during normal vision, but to understand the

mechanisms of this process may be to understand a significant

portion of the central processing which occurs during normal

visual perception.


Fusion, Suppression, and Binocular Rivalry

Sensory dominance can be evaluated in the laboratory

by presenting the subject with distinctly different stimuli

to the two eyes. The subject will report one, the other,

or an alternating pattern of the two depending on the

specific characteristics of the stimuli. In normal vision

we are aware of only single objects in spite of the fact

that there are two retinal images. Perceptual integration

or "fusion" prevents a double image fromoccurring. If the

two images are too dissimilar,fusion will not be possible

and the two images will be in rivalry. As in using the

monocular microscope, the image from one eye is ignored or

"suppressed" during rivalry while the image of the other

eye is in favor. Rivalry, in the case of the microscope

is also influenced by content saliency and directional

attention, while the experimental situation evaluates the

self-determining nature of rivalry. During rivalry the









image which is not perceived is said to be suppressed while

the image which is perceived is said to be dominant.

Experimental conditions can be manipulated so that one or

the other stimulus dominates or so that perception alter-

nates between the two stimuli. The effects of rivalry and

suppression are easily visible when the classical orthogonal

grids of Figure 1 are presented to the right and left eyes

independently (dichoptically). Under these conditions,

the observer sees an alternation between the two inputs and

generally the input of one or the other is seen for longer

durations.

In the natural environment, it is not often that such

disparate images fall upon the retina to create a rival

situation. We normally perceive and respond to the visual

environment as if seen through a single "cyclopean eye"

(Julesz, 1971). This cyclopean perception is the neuro-

logical product of fusion. An extremely powerful mechanism,

the two images do not necessarily have to fall on exactly

corresponding areas of the two retinae for a single image

to be formed. A certain degree of disparity is tolerated

and fusional or Panum areas are thus defined. At the

fovea, the center of the visual axis line for each eye, the

Panum area is approximately 6-8 minutes of arc on the

horizontal axis and 4-6 minutes of arc on the vertical

axis (Ogle, 1964). Panum areas can vary in size according

to the characteristics of the stimulus.












































Figure 1. Orthogonal grids presented dichoptically to
establish binocular rivalry.









Two stimuli which differ only in brightness can be

fused as long as the contrast of the two are not reversed

and as long as the brightness difference is not too large.

Two stimuli with differences in reflected wave length (color)

can be fused if they are of equal luminance and the exposure

time is sufficiently long. The perceived color will be a

mixture of the two. Two stimuli whose contours do not

differ by a factor greater than Panum's area can be fused

(Levelt, 1968). The visual system clearly exhibits a

"compulsion for fusion." Although the term is technically

used to describe a motor reflex, it does summarize in a

rhetorical manner the effect of various characteristics of

fusion. There is a striving for fusion for the purpose of

preventing the appearance of double images. When fusion

is not possible, suppression of one or part of the images

can/will occur.

Strabismus is a condition in which the two eyes are not

able to fixate on the same object. One eye maintains a line

of sight which deviates from fixation. Usually this means

that the corresponding retinal elements are being differ-

entially stimulated and diplopia should result. For the

most part this does not occur because the misaligned image

is suppressed. The misaligned image usually is weaker

(amblyopic) due to the attentional component of accommodation

bringing into focus the image of the normal eye. Suppression

will act upon the weaker image. Researchers have demon-

strated that the contribution of the amblyopic eye to









binocular perception is less than that of the normal eye,

reflecting the suppression it undergoes (Perry & Childers,

1972; Schor, 1977). The treatment of strabismic amblyopia

often includes training the patient to concentrate on using

the weaker eye more often. The positive correlations

between the effects of this training on amblyopic patients

and on normals undergoing suppression during rivalry have

led some researchers to feel that the mechanisms are one

and the same. Suppression experienced by normal subjects

during rivalry and the suppression experienced by amblyopic

patients appear to be the same mechanisms (Coren & Duckman,

1975; Porac, 1975; Porac & Coren, 1975).

Suppression, as manifested in binocular rivalry, is

a function of the differences in the physical characteristics

between the two stimuli. Differences in contour, luminance,

contrast, and color will affect suppression. If we refer

to Figure 1 once again, the grids have equal size bars and

spaces (spatial frequency) and they are constructed with an

equal amount of contrast. If we assume that they are

presented with equal luminance to an observer, they will

differ only in the way the contours or edges strike the

two retinae. Corresponding elements (cells, cell assemblies,

receptor field) of the two eyes will receive disparate input

and will rival. In this situation, the observer will see

an alternation or cycling between the two patterns as

suppression acts upon one, then the other, eye. There may

be short periods where the complete visual field is not










suppressed, but there is instead a shifting mosiac of

elements of both patterns. The time that either stimulus

is suppressed should be approximately equal to the time of

suppression for the other while viewing these two stimuli.

If, on the other hand, we present stimuli similar to those

in Figure 2a to an observer, the time of suppression will

not be equivalent for the two eyes. The stimuli differ in

contrast as well, as contour and the stimulus on the right

would predominate. If the luminance intensity of the left

hand stimulus was increased, the dominance ratio could be

altered in its favor. In Figure 2b, the stimuli differ in

their spatial frequency but are similar in contrast and

intensity. Research has shown that the stimulus on the

right will be dominant. There is evidence that the visual

system is possibly "tuned" to spatial frequency and fre-

quencies in the 2-4 cycles/degree range are preferred in a

rivalry situation (Wade, 1975; Fahle, 1982). If a

refractive lens was placed between the observer and the

stimulus on the right in Figure 2b so as to blur the image,

this would have the effect of modifying the spatial frequency

and the contrast and it would thereby alter the dominance

ratio in the direction of the stimulus on the left.

As we have seen, various stimuli will dominate or

be suppressed according to their "strength," i.e., contours,

luminance, contrast, and the dominance ratio between two

stimuli can be altered by changing the strength of one of

them. This system functions as if some form of contralateral





























































b.

Figure 2. Dichoptic stimuli presented to establish binocular
rivalry.










recriprocal inhibition was occurring between the two

monocular channels prior to the level where the channels

merge. Such does not seem to be the case, though (Walker,

1978). A basic observation of those working with binocular

rivalry is that increasing the stimulus strength of an

image in rivalry causes a change in the duration of

dominance of the contralateral image only. The duration

of dominance of the changed stimulus remains unaffected

(Levelt, 1968; Fox & Rasche, 1969; Walker, 1975). The

typical methodology will have an observer view rivalry

stimuli and indicate whether one, the other, or parts of

both are perceived. With increases in the strength of

one stimulus there is a concomitant reduction in the

duration of dominance of the other. The strength of the

suppressed stimulus determines the relative dominance of

the contralateral stimulus.

In this review we have seen the conceptualization of

ocular dominance shift from a simple undimensional process

to a multifaceted mechanism with its own complex and yet

undetermined processes. Ocular dominance which was once

accepted to be a function of general cerebral dominance

now appears to be a consequence of the structure and

neurophysiology of the visual system and somewhat independent

of cerebral dominance. Sensory dominance is related to the

properties of fusion and suppression, and even though the

functional characteristics of these mechanisms are being

described with remarkable detail, the underlying neural










processes are not yet known. The binocular visual system

expects disparaties on a specifically small scale and the

"compulsion for fusion" functions as part of the process

which allows three-dimensional perception of the three-

dimensional world we live in. When disparaties which

exceed the limitations of fusion are encountered, the

visual system reduces the confusion and the double image

via suppression of one of the images. Suppression occurs

as a consequence of the physical characteristics of the

stimuli interacting with the fundamental properties of

the binocular visual system. An important step in under-

standing suppression and sensory dominance would be the

assessment of the distribution of the relative contributions

made by the right and left eyes under dichoptic conditions.


Psychophysioloqical Measures of
Binocular Vision

There have been physiological studies of normal and

abnormal binocular vision where direct recordings were

obtained from cortical cells in animals. This type of

procedure is not easily applied to human subjects, however,

and the visual evoked potential (VEP) has been utilized as

an effective assessment tool of cortical functioning.

Cortical correlates of binocular rivalry, suppression, and

fusion have been examined with the VEP with a moderate

degree of success. The effects of binocular rivalry and

suppression can be seen in the VEP. Some of the stimulus

characteristics which were seen as important variables









in suppression have been seen to affect the VEP. The issue

of ocular dominance, usually measured as sighting dominance,

is much less clear within the VEP data and no systematic

pattern has been described. The relationship between

cerebral dominance, ocular dominance, and the VEP is parti-

cularly confusing as there are data to support each

alternative description. The VEP as it relates to the

distribution of sensory dominance has not been reported

in the literature. The VEP methodology will be described

and some of the data which describe the relationship

between the VEP and ocular dominance, fusion, and suppression

will be reviewed.


The Visual Evoked Potential

The electroencephalogram (EEG) of man has been

recorded and studied with an optimistic and determined

intensity since Hans Berger published his research in 1929.

It did not take long before relationships were established

between the EEG and certain types of brain functioning

or states. Certain types of organic brain disorders, e.g.,

seizure disorders and brain lesions, were correlated to

specific characteristics of the EEG such as frequency,

wave-shape, or general aberrant patterns. One of the

limitations of the EEG data, though, is that their sources)

or origins) is not unitary. The EEG reflects the aggregate

or sum electrical activity of the myriad of processes which

may be occurring simultaneously within the cortex. Added









to this confusion is the fashion in which these potentials

are differentially conducted throughout the cortex and the

cranium. It is extremely difficult for one neuronal

process to be evident within this collective and cryptic

electrical activity.

It was, however, noticed that the on-going EEG would

normally demonstrate a detectable response to the onset of

a specific sensory stimulus. This EEG response was parti-

cularly gross in nature, revealing little, as if to indicate

only that something had occurred. With repeated stimulation

under identical conditions, it was found that this ambiguous

electrophysiological response could be enhanced. The concept

of a technique whereby the recorded potential change is

locked in time with a stimulus is basic to all methodologies

involving average evoked potentials (AEP).

The typical AEP is obtained by repeatedly presenting

a stimulus to the subject or patient and algebraically

summing the samples of EEG which are time-locked to the

presentation of the stimulus. The resultant waveform is

expected to represent primarily activity involved in the

central processing (cortical processing) of the stimulus.

Activity contained within the EEG which is not related to

the stimulus is considered "random" and is expected to

average to an insignificant level in comparison to the

event-related cortical activity. A typical sample may

represent 500 msec of EEG averaged over 50-100 repetitions

of the stimulus. Among the first investigators to record









AEP data was Dawson (1954). He used a method of recording

his samples on photographic material where each repetition

would overlay the preceding ones. He would have a "hard

copy" of the resultant waveform from which to take his

measurements. Analog and then digital computers soon

became commercially available which allowed discrete

establishment of the waveform and fast and precise measure-

ments of the results. The typical visual evoked potential

(VEP) is extracted from the EEG by repeated stimulation

with a flashing light. The light may be direct, reflected,

or filtered. It may be a homogeneous field or patterned

in a very complex manner. The eyes need not be open.

A representative VER is presented in Figure 3. The

duration of this response is 500 msec. Negative polarity

is indicated by an upward deflection of the waveform.

Important overall characteristics of the waveform are its

amplitude and its shape, though there are numerous other

descriptive characteristics of the VEP utilized. Amplitude

is customarily described as the distance between the

largest negative and the largest positive peaks. Individual

waves (components) are labeled according to their polarity

and their latency from the time of stimulation. Within

subject variability of the VEP is surprisingly small,

although highly dependent on the conditions of stimulation,

on electrode location, and on the state of the subject

(Callaway, 1975). There is significant variation between

subjects (Regan, 1972; Perry & Childers, 1969). Because























N90


N290


N480


Amplitude


P220


100 200 300 400


msec


Figure 3. Representative Visual Evoked Potential.


500


J









of intersubject variability, normative data for VEP have

been extremely difficult to establish and there is a need

for standardization within the research field to aid in

the comparison of data across laboratories (Aunon & Cantor,

1977). Despite these handicaps, this rapidly expanding

and prolific methodology is allowing significant advances

in the study of CNS functioning and of human behavior.

Some of the handicaps are merely indicative of the infancy

status of the paradigm while other obstacles are demon-

strative of the complex nature of the system under

investigation.

The investigation of binocular rivalry with the visual

evoked potential has come to span several avenues of research

and generates much data which are important to the under-

standing of binocular vision in general. Suppression,

fusion, and dominance have been discussed in describing

binocular rivalry and these as well as processes such as

stereopsis and binocular facilitation seem to be

inextricably bound together or linked in such a way that

no single process functions independently. It is because

of these types of relationships that the research of

seemingly independent processes converge upon one another.

VEP research involving binocular rivalry, suppression,

fusion, stereopsis, and binocular facilitation experiences

significant overlap and each contributes to a fund of

knowledge which is relevant to the investigation of the

other. The link between the processes seems to be the









neurological substrate of binocular vision and the cortical

integration of two independent sensory inputs. A review

of the research in these various areas is necessary in

order to construct a model or framework within which the

results of the present study can be explained. Studies

which have examined the VEP in relation to suppression,

dominance stereopsis, and facilitation will be reviewed.


The VEP and Suppression

Suppression is the inhibition, attenuation, or

fading of the features of one monocular input in contrast

to the other monocular input during binocular vision.

Binocular facilitation, on the other hand, is the enhance-

ment of the monocular perception via the mechanisms of

binocular vision. Enchancement (beyond depth perception)

is not the case under all viewing conditions and it is

more often found under unusual or extreme visual conditions

(Home, 1978; Blake & Rush, 1980). In psychophysical

measures, binocular facilitation may be seen as a lowered

detection threshold under binocular conditions compared

to levels under monocular conditions. In electrophysio-

logical measures, specifically the VEP, binocular

facilitation may be represented by an increase in the

VEP amplitude during binocular stimulation in comparison

to monocular stimulation. Suppression has also been seen

to have an influence on the VEP and this effect is an

attentuation or a decrease in the VEP amplitude from










levels established by binocular facilitation to levels

equal to that of monocular stimulation. After Lansing

(1964) published his findings of a correlation between

the amplitude of the on-going EEG and suppression, the

evoked potential methodology has been applied to this area

of research with often conflicting results. Rivalry and

suppression seemed to have no effect upon the VEP

(Kaufman, Pitblado, & Miller, 1965; Riggs & Whittle,

1967; Cobb, Ettlinger, & Morton, 1967a). This contrasted

with findings that suppression during rivalry correlated

with a definitive change in the VEP (van Balen, 1964;

Lehmann & Fender, 1967; MacKay, 1968; Cobb, Morton &

Ettlinger,1967b, 1968; Lawwill & Biersdorf, 1968). These

conflicting results seem to reflect the manner in which

suppression was examined and do not necessarily reflect

the nature of the mechanism. The stimuli utilized to

evoke a response varied considerably between researchers.

Checkerboard patterns, bars, spatial gradients, diffused

light, geometric patterns, flashed patterns, and pattern

reversals were all utilized in establishing the assumed

conditions of retinal disparity and the VEP in response

to this condition.

The typical procedure used to establish binocular

rivalry and to assess interocular suppression can be

outlined with a brief description of the work of Cobb

et al. (1967a, 1967b, 1968). The stimuli utilized in these

experiments were horizontal bars and vertical bars of a










fixed spatial frequency which were presented dichoptically

to their subjects. Initially the stimuli were "flashed"

to the subject while he was indicating the left eye

dominance or right eye dominance to continuous presentation

of these patterns. No correlation was seen between the

VEP and the dominant or suppressed conditions. The

authors then changed the method of presenting these same

patterns. Instead of flashing the horizontal or

vertical bars, each pattern was moved linearly in the

vertical or horizontal direction a distance equal to the

width of the bars. The effect of this movement was the

reversal of the bars and spaces without apparent movement

of the entire stimulus. The evoked potential was time-

locked to the pattern reversal. Under these conditions,

the amplitude of the VEP to the suppressed eye was less

than when the same eye was dominant. Similar interocular

suppression has been demonstrated when the two eyes have

respectively viewed diffuse and patterned light (Lehmann

& Fender, 1967, 1968), a small spot of light and diffused

light (Shipley, 1969; Franceschetti & Burian, 1971),

patterns of identical spatial frequencies (Spekreijse,

van der Tweel, & Regan, 1972), line grids and checks with

different contrasts (Ciganek, 1971; Harter, Towle, &

Zakrzewski, 1977; Srebro, 1978), and line grids with

different orientation (Harter, Conder, & Towle, 1980).

It is now generally accepted that interocular suppression

has a cortical correlate detectable in the VEP. The










primary feature of this effect is reduction of the amplitude

of the-VEP (Harter, 1977).

Binocular facilitation, sometimes referred to as

binocular summation in the VEP literature, is the finding

that the VEP amplitude of binocular stimulation is greater

than either monocular VEP to the same stimulus. The bino-

cular facilitation reported by researchers ranges from

25%-100% increase over the amplitude of the monocular VEP

(Perry, Childers, & McCoy, 1968; Ciganek, 1970; Harter,

Seiple, & Salmon, 1973; Srebro, 1978; Fiorentini, Maffei,

Pirchio, & Spinelli, 1978; Hoeppner, 1980; Trick, Dawson,

& Compton, 1982). The range of facilitation observed

seems to depend on stimulus characteristics and methodology.

The amount of facilitation has been shown to be influenced

by cortical recording location (Perry et al., 1968),

luminance intensity (White & Bonelli, 1970; Ciganek, 1970;

Harter et al., 1977; Trick et al., 1982), contrast

(Ciganek, 1971; Harter et al., 1973; Srebro, 1978), and

spatial frequency (Harter et al., 1980; Apkarian,

Nakayama, & Tyler, 1981). As we have seen, these are

the same factors which have been described as influencing

binocular rivalry and suppression. This in itself is not

all that surprising because the visual system responds

to light intensity, contrast, and spatial frequency by

intrinsic design. What is interesting, however, is that

these variables seem to interact with these different

mechanisms (fusion, suppression, stereopsis) in a very









similar manner. The implication is that the same neuronal

substrate serves all of these processes up to a point where

some higher level of cortical functioning is necessary for

integration and synthesis of the visual percept.

Binocular facilitation and binocular suppression as

manifested in the VEP have been evaluated within the same

experiment (Trick et al., 1982). Using 11 subjects with

equal acuity and normal stereopsis,these authors first

compared the binocular VEP to the two monocular VEPs for

the same stimulus. These data indicated an average of 40%

increase in the amplitude of the binocular over the mono-

cular VEPs. The amplitude of the binocular VEPs was always

less than the sum of the two monocular VEPs. A luminance

difference was then introduced between the monocular

stimuli and the amplitude of the VEP was seen to decrease

with increasing disparity in luminance. The amplitude

could be suppressed to a level equal to that of the

monocular responses.

Similar results have also been seen in the investi-

gation of retinal disparity, stereopsis, and the VEP.

Under normal viewing conditions, virtually identical

images fall upon corresponding retinal areas. Under

atypical binocular viewing conditions, conditions which

may be associated with disorders of binocular vision, the

images falling on the two retinae may differ in terms of

sharpness of focus, size, orientation, luminance intensity,

and contrast. Under these disparate conditions, fusion of





28


the two images is difficult if not impossible. An

interesting study which looked simply at the fusion of

the two images was reported by Kawasaki, Hirose,

Jacobson, and Cordella (1970). These authors established

VEPs in six subjects to fusable binocular stimuli. One of

the stimuli was then rotated in relation to the other so

as to make the two stimuli nonfusable. The VEPs to the

nonfusable stimuli were suppressed in amplitude compared

to the fused condition. An observation which was made

during the procedure was that a certain amount of rotation

was possible without breaking fusion. The VEP followed

this perceptual phenomenon and was not affected by

rotation until the point at which fusion was interrupted.


The VEP and Stereoscopic Vision

Stereopsis, stereoscopic vision resulting in depth

perception, results from the two eyes having a slightly

different angle of view of the three-dimensional world of

objects in front of them. This results in slightly

different images impinging on the two retinae and it is

this disparity which instigates the perception of depth.

Stereopsis occurs over the entire binocular visual field

and is not restricted to foveal vision. There is continuing

evidence which suggests that depth perception is possible

even in the absence of binocular fusion (Ogle, 1964;

Walker, 1978). The binocular visual system is significantly

more sensitive to disparities along the horizontal axis









than to those along the vertical axis. There appears to

be approximately a ten-fold difference in the sensitivity

between the vertical and horizontal orientation with

vertical disparities contributing little to depth

perception during normal binocular viewing (Westheimer,

1978; Fahle, 1982). The VEP recorded during stimulation

with either horizontal or vertical grid lines shows a

marked increase in facilitation with horizontal disparities

accompanied by depth perception as compared to vertical

disparities and little realization of depth (Regan &

Spekreijse, 1970; Apkarian et al., 1981). It has been

shown that the VEP,in response to disparate stimuli which

produced depth perception,is larger in amplitude than the

binocular VEP to stimuli not resulting in a perception of

depth. A positive linear function of the amplitude of

the VEP in relation to the perception of depth has been

described (Fiorentini & Maffei, 1970; Regan & Spekreijse,

1970; Harter, 1977). The amplitude of the VEP increases

with the amount of perceived depth in the stimuli. This

function is maintained until the disparity no longer

provides cues for depth perception and the amplitude of

the VEP then decreases as one or the other stimulus

undergoes suppression.

Research with patient populations contributed early

evidence that the right cerebral hemisphere plays a more

important role in stereopsis than the left hemisphere.

Patients with right unilateral cerebral lesions were









compared with a group of patients with left cerebral

lesions and a group of patients with no CNS involvement

(Carmon & Bechtoldt, 1969; Benton & Hecaen, 1970). The

three patient groups were administered a battery of tests

for stereopsis, one of which was the random-dot stereogram

(RDS) (Julesz, 1960, 1971). The RDS consists of two

stimuli presented binocularly which individually appeared

to be a random array of dots. The array of random dots

contains a restricted region of dots which are correlated

so that when viewed binocularly these regions can be fused.

A horizontal disparity is calculated into the "correlated

regions." When viewed binocularly, the RDS appears as a

static background with a region within that appears to be

floating in a plane above or below depending on the nature

of the disparity. The importance of this method of

stimulating depth perception is that monocular depths cues

(figure-ground contours, overlap, size, and form) are

avoided. The two monocular images must be "compared" at

some presumably cortical level in order for the correlated

region to be utilized. The results of the patient groups

with these tests for stereopsis showed the right hemispheric

lesion group performing significantly below the level of

the left hemisphere lesion and the non-CNS groups. These

two latter groups were not significantly different from

one another in their perception of depth. The authors of

these two studies felt the results provided strong support

for right cerebral dominance in stereopsis.









VEP corroboration of these studies has certainly

not been categorical and the results of studies examining

hemispheric lateralization with the VEP are equivocal.

Nonetheless, Harter (1977) in discussing binocular inter-

action and the VEP hypothesized that the right hemisphere

has a greater involvement in binocular interaction than

the left hemisphere. Harter was drawing upon the results

of the patient studies as well as his own work in which he

frequently saw greater VEPs from the right versus the left

hemisphere. Opposing this point of view is data strongly

suggesting that there is little lateralization for

stereopsis (Breitmeyer, Julesz, & Kropfl, 1975; Julesz,

Breitmeyer, & Kropfl, 1976; Lehmann & Julesz, 1978).

Lehmann and Julesz (1978) stimulated visual half-fields

with RDSs and found that each hemisphere has equal capacity

for stereopsis as measured behaviorally and with the VEP.

The anatomy of the visual path-ways is such that the nasal

half of each retina is projected onto the contralateral

hemisphere. Thus, stimulation of the same visual half-

field innervates only one cerebral hemisphere. These

authors found each hemisphere equally responsive to depth

perception cues utilizing the RDS.


Handedness and Eye Dominance

Laterality or predominance of the operations of one

hemisphere within the visual system is expected because of

the lateralization of function of other systems. The









intuitive approach would suggest that cerebral dominance

would generalize across most all cerebral functions; thus,

a right-handed individual's dominant left hemisphere

would be dominant for motor functioning, language

functioning, auditory functioning, visual functioning, etc.

The reverse would be true for a left-handed individual's

dominant right hemisphere. In reality, though, precise

rules for delineation of functions do not seem possible.

Most individuals are left hemisphere dominant for

language regardless of handedness and many people exhibit

a "mix" of dominance for various functions (Milner,

Branch, & Rasmussen, 1964; Satz, Achenback, & Fennell,

1967; Zurif & Bryden, 1969). Failure of the cerebral

cortex to demonstrate a lawful bilateral division of labor

is continued in the occiptal cortex and the VEP studies

which examined laterality of visual functions produce such

a wide range of results that no clear definitive statements

are possible at this time. The enigmatic nature of how

cerebral dominance and ocular dominance influence VEP is

clearly demonstrated in some of the data presented by

Perry et al. (1968).

In their examination of binocular facilitation and

cortical recording of patients, Perry et al. (1968) also

determined sighting dominance for each subject. There

was no consistent relationship between VEP size and the

eye dominance measured. The authors went on to report

individual data on several subjects. Two subjects









differed from the remainder of the sample in that their

VEPs to monocular stimulation recorded from the two

hemispheres were significantly different in size from

one another. The differences seen in these subjects

were as great as those seen in patients with unilateral

eye disease (Copenhaver & Perry, 1964). One of these two

subjects was further evaluated for eye and hand dominance.

This subject proved to be ambidextrous on two tasks, right-

handed on five tasks, and left-handed on three tasks. He

had normal stereopsis and neither eye was dominant as a

controlling eye. The left eye was dominant for far vision

and the right eye was dominant for near vision. If any

conclusion are going to be drawn from these data they must

be formulated in the same context as data from two other

subjects of this same study. They were, by every measure,

right-eye dominant and had equal monocular VEPs.

Studies which have examined handedness, eye dominance,

and the VEP have yielded widely disparate results. Left-

handed subjects had greater VEPs from the right hemisphere

when stimulated than from the left when stimulated. Right-

handed subjects showed no clear pattern (Eason, Groves,

and Bonelli, 1967a; Eason et al., 1967b; Pfefferbaum &

Buchsbaum, 1971; Gott & Boyarsky, 1972). Left-eye dominant

subjects yielded larger VEPs than right-eyed subjects.

Handedness did not correlate to the VEP (Culver et al.,

1970). Handedness correlated to the VEP and sighting

dominance, but sighting dominance showed no relation to










VEP (Klemm et al., 1980). Sighting dominance correlated

to the VEP, but no clear pattern was seen for handedness

(Seyal et al., 1981).


A Behavioral Measure of Sensory Dominance

The VEP data generally parallel the psychophysical

data in regards to binocular rivalry, fusion, suppression,

and facilitation. Binocular facilitation, fusion, and

stereopsis can be seen in the VEP as the increase in the

amplitude of the waveform. Binocular rivalry and

suppression generally result in a decrease in amplitude.

The effects of stimuli characteristics as they affect

suppression have been detected in the VEP. Cerebral

dominance and ocular dominance have been examined with the

VEP with dramatically conflicting and inconclusive results.

Ocular dominance has been historically measured with a

sighting dominance measure. Sighting dominance is not a

purely visual task and any dominance it exhibits is likely

to be strongly influenced by the motor aspects of the

process. Sensory dominance, on the other hand, seems to be

entirely a cortical process and is related to the

mechanisms of fusion, suppression and facilitation.

Sensory dominance per se is evaluated infrequently and

little is known about it as an individual phenomenon.

Two studies have examined sensory dominance and their

findings offer an alternative conceptualization of this

dominance measure.










In an investigation of the monocular contribution

to binocular vision, Perry-and Childers (1972) found that

sensory dominance was distributed along a dimension

ranging from strong left eye preference to strong right

eye preference. A sample of seven subjects with normal

vision were presented dichoptic stimuli and asked to

report what they saw. Each subject was presented pairs

of letters, pairs of numbers, letter-numbers, or letter-

patterns and then they simply reported what they saw. The

results of the verbal reports indicated that the sensory

dominance demonstrated by these subjects was not an all-

or-none process, but that each eye dominated the percept

a certain percentage of the time. The majority of their

subjects demonstrated only a slight preference for one

eye or the other to dichoptic stimuli, although several

subjects showed a strong preference for one eye. The

percentage measure of this study clearly demonstrates a

continuum of dominance in a normal population and suggests

that a dichotomous left-right dominant measure might

obscure the variation between individuals. This type of

measure is certainly consistent with the variability of

other data and the failure of consistent findings on the

effects of ocular dominance.

Perry and Childers (1972) collected VEP data to the

dichoptic stimulation in hopes of obtaining an electro-

physiological measure of the sensory dominance established

behaviorally, but these data were not reported. The authors









described a methodological limitation which confounded the

VEP results and made interpretation of the electro-

physiological data impossible. Stable waveforms were

acquired for each subject, but evidently averaging across

the combinations of letter pairs, number pairs, letter-

numbers, and letter-patterns neutralized any differences

which may have correlated with the dominance measure.

The important contribution of this study was not diminished

by the lack of VEP data. The study demonstrated that

sensory dominance could be measured as a percentage of

dominance for each eye. The limited number of subjects

tested yielded data along this entire dimension. The

behavioral measure of sensory dominance of this experiment

was essentially replicated by Ondercin, Perry, and Childers

(1973) with similar results.

With a sample of 56 subjects with normal vision, the

results of dichoptic stimulation with letter pairs

corroborated the previous findings. When sensory dominance

was measured as a percentage, the results were normally

distributed about a mean representing only a slight

preference for one eye or the other. There were subjects

who demonstrated a distinct preference for one eye or the

other and these subjects represented the extreme of the

distribution. Representing sensory dominance along this

continuous dimension is an important departure from the

traditional dichotomous measure of left or right dominance.

The continuous measure provides a range of measurements









which 1) appear compatible and valid for the data

subjects are providing; and 2) may be more efficacious

when used in a comparison against other measures of the

same process. As the data indicate, most subjects fell

in the center of the distribution of sensory dominance.

If a dichotomizing measure was utilized, a very small

measurement error could shift the resulting designation

from one eye to the other, and more importantly, the

intermediate levels of sensory dominance which the majority

of subjects demonstrate would be lost.

Additional evidence for the appropriateness of a

continuous measure of sensory dominance comes from

electrophysiological data recorded from the visual center

of cats and monkeys where discrete left-right measures of

ocular dominance were uncommon (Hubel & Wiesel, 1977,

1979).


A Neurophysiological Model of Suppression

Single cell recordings of cortical neurons in area 17

of the cat and monkey have revealed that there are neurons

in this region which are binocular in nature; that is, they

respond to stimulation of either eye, and further, these

cells respond differentially to stimulation of each eye

(Barlow, Blakemore, & Pettigrew, 1967; Hubel & Wiesel,

1963, 1968, 1977). The binocular neurons described can be

influenced by stimulation of corresponding points of either

eye, but the response of the neuron is not equal for each










eye. One eye excites the cell quite easily while exact

stimulation of the other eye innervates the cell to a

lesser degree. The range of differential responsiveness

of the binocular neurons studied essentially describes a

normal distribution of eye dominance. Some of these

cells demonstrate only a weak response to one eye while

responding maximally to the other. Other binocular cells

show little preference and respond equally to stimulation

of either eye.

In addition to the finding of binocular neurons in

the visual cortex of cats and monkeys, ocular dominance

columns which contain neurons responsive primarily to one

eye or the other have been reported (Wiesel, Hubel, &

Lam, 1974; LeVay, Hubel & Wiesel, 1975; Hubel & Wiesel,

1977). These researchers have reported that the visual

cortex is functionally organized into alternating columns

of neurons which demonstrate a preference for one eye or

the other. The ocular dominance columns extend perpen-

dicular from the surface of the cortex down through all

layers of area 17 to the white matter of the brain. Each

column is comprised of monucular neurons of one eye

preference and binocular neurons with a primary preference

for that same eye. The nature of these ocular dominance

columns is such that the division between left and right

eye dominance is not a sharp delineation. The center of

each functional column is occupied primarily by monocular

neurons and by binocular neurons with a strong preference









for the eye of that particular column. The neurons in

either lateral direction show a progressive change to a

mixed eye preference with the majority of the neurons on

the border of the dominance columns being equally

responsive to either eye. There are fewer monocular

neurons located at these borders.

Any relationship between sensory dominance as

measured behaviorally during dichoptic stimulation and

ocular dominance columns and their constituent neurons is

certainly not direct. Hubel and Wiesel are careful to

issue a caveat and reminder to their readers that the

visual cortex is in no sense the end of the visual pathway.

It is just one stage, a very early one in terms of the

processing of visual information. It is an apparent system

of hierarchies and building blocks. Looking at the visual

nervous system as a system of functional hierarchies, it

is even more important that the initial processing of

visual information has a significant degree of variability

intrinsic in its functioning. Variability may be evident

throughout the various stages of visual functioning and

it is certainly apparent in the final percept itself.

During the classical binocular rivalry situation, the

ultimate percept seems to fluctuate capriciously, albeit,

according to factors already outlined. These fluctuations

occur within boundaries partly established or defined by

the initial stage of processing, the visual cortex.









If we examine the data of Perry and Childers (1972)

and Ondercin et al. (1973) we will see that in any block

of trials, one eye, the other eye, or neither eye will be

preferred. The subject will have reported the stimulus of

the right eye or the left eye more frequently or an equal

number for both within a single block. It is only when the

entire session is tabulated that a percentage measure is

acquired and assigned. The domain of this percentage

measure could be influenced at the initial stage of visual

processing. The variability of the final product coincides

with the variability of the initial building blocks of the

visual system. What we do not see in the data is an

apparent replication of visual processing after each trial

and an identical mechanistic response. The exact same

stimuli may result in a different percept on two independent

trials. Because of this, the continuous measure of sensory

dominance as a percentage of right or left eye preference

seems more appropriate as a useful tool in evaluating

visual functioning. This measure is consistent with the

apparent physiological functioning of the visual system.

The use of a percentage measure for sensory dominance

is really more than just appropriate if we examine the

neurophysiological data and integrate this, with some

speculation, with the results of evoked potential research.

The amplitude of the VEP to binocular stimulation

versus monocular stimulation demonstrates an additive

process. Binocular facilitation has been reported to range










from 25%-100% increase over the amplitude of the monocular

VEP (Perry et al., 1968; Ciganek, 1970; Harter et al.,

1973; Srebro, 1978; Fiorentini et al., 1978; Hoeppner,

1980; Trick et al., 1982). These results were initially

unexpected since the energy input is doubled during

binocular stimulation (Gouras, Armington, Kropfl, &

Gunkel, 1964; Shipley, Jones, & Fry, 1966; Perry et al.,

1968). Neurophysiological research offers data which

could be used to partially explain the varying results of

binocular facilitation.

Since the visual system appears to be comprised of

not only monocular cells, but binocular cells with varying

degrees of eye preference and cells which respond to

different combinations of disparities, binocular stimulation

will not necessarily innervate twice the number of cells

over monocular stimulation. Simple binocular presentation

of the same stimulus to each eye may only involve the

monocular neurons for both eyes and the binocular neurons

they connect with. This would result in the 25%-40%

binocular facilitation seen under these conditions (Perry

et al., 1968; Srebro, 1978; Trick et al., 1982).

Binocular facilitation as measured by an increase in

VEP amplitude is greater when binocular stimulation results

in the perception of depth than when no depth is perceived

(Fiorentini & Maffei, 1970; Harter, 1977). The amount of

facilitation recorded during depth perception seemed

proportional to the degree of depth perceived. These data









correspond to neurophysiological findings of single

cortical neurons in cats and monkeys which were responsive

only to simultaneous stimulation of both eyes, but with

some degree of disparity between stimulated elements

(Nikara, Bishop, & Pettigrew, 1968; Pettigrew, Nikara, &

Bishop, 1968; Hubel & Wiesel, 1970, 1973, 1977; Poggio &

Fischer, 1977; Fischer & Kruger, 1978; von der Heydt,

Adorjani, Hanny, & Baumgartner, 1978). These reports

describe individual neurons which seem "tuned" to certain

disparities and which are unresponsive to all other

stimulation. If, during stimulation which produces the

perception of depth, these cells are innervated in addition

to the monocular and binocular cells which are already

responding, the subsequent increase in the neuronal

population responding to disparate stimuli may give rise

to increased electrical activity which is detectable in

the VEP. Additional support for this possible relationship

is offered by the finding that binocular facilitation is

greater to horizontal disparities than to vertical

disparities (Regan & Spekreijse, 1970; Apkarian et al.,

1981) and reports that the disparity neurons identified

in the visual cortex were stimulated predominantly by

horizontal disparities (Hubel & Wiesel, 1970, 1977; von

der Heydt et al., 1978).

In contrast to binocular facilitation which is

evidenced electrophysiologically by enhanced amplitude

of the VEP, suppression during binocular stimulation is










accompanied by an attenuation of the binocular VEP amplitude.

Suppression during binocular rivalry can reduce the

amplitude of the binocular VEP to levels no greater than

obtained with monocular stimulation. Suppression results

when there is sufficient disparity in the stimulation of

corresponding elements of the left and right retinae.

Luminance intensity, contrast, spatial frequency, and

contour orientation are important factors influencing

suppression. Given what we know of the architecture of

the visual cortex of cats and monkeys, it is possible that

the process of suppression is initiated in the visual

cortex. Unequal input into two adjacent ocular dominance

columns may allow the inhibitory collaterals of the

"stronger" channel to attenuate the output of the less

stimulated channel.

Creutzfeldt, Kuhnt, and Benevento (1974b) and

Creutzfeldt, Innocenti, & Brooks (1974a) suggested that

the majority of the connections between orientation

columns in the visual cortex (described by Hubel & Wiesel,

1962, 1963, 1968) are inhibitory in nature. Inhibition

of adjacent orientation columns was speculated upon by

Abadi (1976) in explaining the suppression of similarly

oriented grids. Collateral connections are not likely to

be limited solely within ocular dominance columns or

within orientation columns. It is probable that

collaterals from neurons in orientation columns connect

with the adjacent eye dominance columns and exercise some










level of influence there. If the right and left eyes

were not stimulated equally, it is possible that the

innervated neurons of one eye-dominance column would have

an inhibitory effect on the neurons of the adjacent

dominance column. These neurons would not have been

stimulated in an equal fashion and the inhibitory effect

would be more significant than when adjacent cells have

been innervated equally. The summed consequence will be

reduced output of neural activity. If these effects are

mirrored at progressively higher levels of processing, the

total reduction of electrical activity would be significant

and detectable in the VEP.

Working with this rationale, the reduction in the

amplitude of the VEP during suppression could result from

the inhibition of adjacent ocular dominance columns. The

amount of cortical inhibition may be reflected in a

corresponding decrease in the amplitude of the VEP. The

extent of cortical inhibition may also be reflected in a

behavioral response to dichoptic stimulation, that is, a

percentage measure of sensory dominance. If the percentage

measure of sensory dominance and the reduction of the

amplitude of the evoked potential during suppression are

two manifestations of the same process, then these two

measures should demonstrate a correlation with one another.









The Present Study

The present study utilized a methodology similar to

that of Perry and Childers (1972) and Ondercin et al.

(1973) for collecting a percentage measure of sensory

dominance to dichoptic stimuli. Subjects verbally reported

their perception of dichoptically presented pairs of

letters. A procedure was devised which allowed acquisition

of discrete VEPs to each letter-pair combination. This

process reduced the number of variables contributing to

the VEP and gave repeated measures of dichoptic stimulation

within each subject. The study was specifically designed

to test the relationship between behavioral measure of

sensory dominance and the amplitude of the VEP which was

acquired during the same dichoptic stimulation. Stated

differently, the study tested the relationship between the

behavioral measure of cortical inhibition and the

electrophysiological measure of cortical inhibition.

There is evidence which suggests that the amplitude

of the VEP should vary in a direct relation to the extent

of suppression experienced by one of the eyes.













CHAPTER II
METHOD


Subjects

From a pool of 25 volunteer subjects, 15 subjects

with 20/20 visual acuity were retained. The subject

pool was a mixture of undergraduate, graduate, and

nonstudent individuals. The results reported in this

paper are from ten male and five female volunteer subjects

with normal vision and 20/20 acuity in each eye. The

ages of these subjects ranged from 18 to 31 with a mean

age of 22.26 years.


Preliminary Procedures

A Bausch and Lomb Ortho-Rater was used to evaluate

the visual acuity of each subject. To continue in the

experiment subjects were required to have 20/20 uncorrected

acuity in each eye. Subjects selected to continue in the

experiment were then tested for sighting dominance and

handedness. Sighting dominance was assessed with three

tests:

1. Sighting tests--subject was instructed to hold

a 4 x 28 cm tube with both hands and view a target. Four

administrations and four targets were used.

2. Hole tests--subject was instructed to hold a 30

cm square black card with both hands and view a target









through a 1.5 cm hold in the card. Four administrations

and four targets were used.

3. Miles ABC test--subjects covered his/her face

with a truncated cardboard cone which must be squeezed

to be placed up to the face and look through. Four

demonstrations and four targets were used (Miles, 1929).

Sighting dominance was designated left or right

according to the eye preference during these tasks. If

the eye preference changed between the tests or between

administrations, the eye used most often was designated

as dominant. No subject demonstrated exactly equal eye

preference on these tests.

Handedness was assessed by the subject's performance

on writing his/her own name, simulated brushing of his/her

teeth, and their own report of their handedness. None of

the subjects used in this study was ambidextrous.


VEP Collection

Silver-chloride cup electrodes were applied to the

scalp with Beck electrode paste as location 0z of the

International 10-20 electrode system (Jasper, 1958).

The electrode was referenced to linked ear lobes for a

monopolar configuration. Impedances of 3K ohms or less

were established as measured with an IMA Electronics

impedance tester.

Each subject was seated in an adjustable ophthalmic

chair in an electrically shielded and light proof room









manufactured by ACE. A ventilation fan which was an

integral part of the experimental room provided background

masking for possible noise created in the laboratory during

testing. Solid state dc powered differential amplifiers,

especially designed for VEP recording were located inside

the shielded room near the subject enabling the use of

very short electrode leads (Harwood, 1971). Following

amplification, electrical activity was filtered (1.0 to

50 Hz, Krohn-hite 330 Br) and simultaneously routed to a

Sanborn 7000 FM tape recorder and to a laboratory computer

(NICOLET MED 80). Frequency response of the complete

recording system is relatively flat from 2.0 to 30Hz. A

5 microvolt calibration signal was processed by the system

preceding each subject and was used to standardize the

data.

The laboratory computer was programmed to acquire

500 msec of EEG immediately subsequent to the presentation

of the stimuli. A computer generated pulse simultaneously

enabled the alpha-numeric displays and triggered signal

averaging. The resultant VEPs are an average of 60

samples of 500 msec of EEG.


Stimuli

Stimuli for this study were letters. The letters

were presented binocularly for Procedure I and were

presented dichoptically for Procedure II. The same letters

were used for both procedures. The letters were presented









via a Clement Clark synoptophore modified so as to

accommodate two seven segment alpha-numeric LED displays

(red in color). As seen by the subject, the display

subtended a visual angle of 2.50 vertically and 1.750

horizontally. A fixation point on each display facilitated

fusion of the left and right stimuli. Figure 4 shows the

design of the alpha-numeric display. Duration of the

stimuli was 50 msec for each presentation with an inter-

stimulus interval of 2.5 sec. This interval allowed

sufficient time for the subject to report what was seen.

The collection of data was divided into two separate

procedures. During the first procedure, the subjects

were stimulated binocularly (the same letter to each eye).

Procedure II stimulated the subjects dichoptically (a

different letter to each eye). Procedure I established

the standard or comparison VEP amplitude while Procedure

II established the sensory dominance measure (the

subjects' report of what they saw) and the corresponding

VEP amplitude under dichoptic conditions.
















































0i


Figure 4. Representation of seven-segment alpha-numeric LED.










Procedure I

The following instructions were read to each

subject:

I am going to present letters to you and I
want you to identify them as you see them.
They will be illuminated for only a short
period of time, but there will be sufficient
time between presentations for you to report
what you have seen. There are no right or
wrong answers, I merely want you to tell me
what you see. If you cannot make out what
the letter is, you may guess, or you may
say you do not know. There will be a two-
way communication at all times and I will
instruct you as we change procedures.

Each letter which appeared later in a pair of

letters was presented binocularly during this procedure.

Nine different letters were presented 60 times each.

Random presentation of the letters was controlled by the

computer. A short sequence of letters was presented to

the subjects to familiarize them with the procedure.

The letters of this practice set were not contained

within the experimental set. This procedure resulted in

a behavioral response to binocularly presented letters

and a VEP for each letter presented. These data were

compared to the measures of dichoptic presentation of

Procedure II.










Procedure II

Eight pairs of letters were presented dichoptically

to each subject. Each letter pair was presented 60

times with the letter-eye combination being reversed

for one-half (30) of the presentations. The order of

presentation was randomized by the computer. This

procedure resulted in behavioral data which formed the

index of sensory dominance and yielded a VEP for each

letter pair.

The experimental session for each subject was

completed in approximately 1 hour 15 minutes. Procedure

I always preceded Procedure II. The subjects' reports

of what letters were seen were recorded on protocol

sheets and later tabulated into the sensory dominance

measure for each subject. VEP amplitudes were calculated

and printed by the laboratory computer. Statistical

analyses reported in the results were accomplished

utilizing the facilities of the Northeast Regional Data

Center of the State University System of Florida, located

on the campus of the University of Florida in Gainesville,

Florida. The data were analyzed using the Biomedical

Computer Programs, P-Series, University of California,

Los Angeles, 1981.













CHAPTER III
RESULTS


Behavioral Data

Each subject's report of the letters perceived when

the letters were presented binocularly was essentially

100% correct. This performance was reduced dramatically

under the dichoptic viewing conditions. Under conditions

of disparate input to the two eyes, accuracy fell to a

range of 11-79% correct. Eye dominance scores were

calculated from the dichoptic data according to the

formula R-L/R+L, where the numerator is the difference

between the total number of correct responses from the

right eye minus the total number of correct responses

from the left eye and the denominator is the sum of the

number of correct responses for both eyes. This yielded

data with a possible range of -1.0 (completely left eye

dominant) to +1.0 (completely right eye dominant). Figure

5 gives the distribution of the eye dominance scores for

the 15 subjects.


VEP Measures

VEPs were obtained for all binocular and dichoptic

letter presentations for all subjects. VEP amplitude,

defined as the measured difference between the largest

positive peak and the largest negative peak of the waveform,































0

3
4-




*2
0

a






II I I

-1 -.5 0 .5







Figure 5. Distribution of sensory ocular dominance scores
for fifteen subjects (R-L/R+L).









was used as the primary electrophysiological measure

for this study.

The VEP to dichoptic stimulus conditions was

reduced in amplitude in relation to binocular stimulus

conditions. This effect was evident for all subjects and

all letter pairs. The average reduction between these

two stimulus conditions was 44.92% of the binocular

amplitude with a range of 25-75% across all subjects and

all letter pairs. A representative VEP to binocular

stimulation is presented in Figure 6. Below it in the

figure are the VEPs to dichoptic stimulation for each of

the eight letter pairs for the same subject. The decrease

in the overall amplitude from the binocular stimulus

condition to dichoptic conditions is clear from a visual

examination of the waveforms.


VEP/Sensory Dominance Correlation

The VEP for each letter pair during dichoptic

stimulus conditions was compared to the binocular VEP

amplitude and a percentage of amplitude reduction across

these stimulus conditions (Procedures I & II) was calculated.

This percentage measure has the advantage of losing the

absolute value unique to a subject or trial and, hence,

can be validly used in comparisons which cross experimental

variables.

This percentage measure, the percent of amplitude

reduction from binocular to dichoptic stimulus conditions,


















Binocula


a. T__
a. II .-- I I I ---I
100 200 300 400 500













































msec



Figure 6. a. VEP to binocular stimulation, b. VEP to
each of eight letter pairs. All VEPs from the same
subject.
4


5






6














t00 200 300 400 500
msec



Figure 6. a. VEP to binocular stimulation. b. VEP to
each of eight letter pairs. All VEPs from the same
subject.









was compared to the sensory dominance measures with

regression analysis. Analyses were performed across

letter pairs within each subject, across subjects within

each letter pair, and across all subjects and all letter

pairs. The results revealed no systematic relationship

between reduction in amplitude and the behavioral measure

of sensory dominance. Table 1 gives the sensory dominance

scores (R-L/R+L), percent of amplitude reduction, and

correlation coefficients for each subject. Figure 7 is

a comparison of sensory dominance scores to percent of

amplitude reduction for all subjects.


Gender, Handedness, and Sighting Dominance

Table 1 also gives the gender, handedness, and

sighting dominance data. There tended to be a positive

correlation between handedness and sighting dominance, but

with only one left-hander in the sample, statistical

sensitivity is questionable. There was no relationship

between sex and sighting dominance. The VEP measures and

sensory dominance measures were evaluated by gender,

handedness, and sighting dominance with analysis of

variance. Handedness and sighting dominance showed no

significant effects across these variables, but there was

a surprising effect of gender on the percentage of

reduction of the amplitude of the VEP. Male subjects

demonstrated a greater percentage reduction of the VEP

from the binocular to the dichoptic stimulus conditions

(F=4.2578, p=.0414, male=47.1%, females=39.4%).











Table 1. Sighting Dominance score (R-L/R+L), average
percent of reduction, coefficient of correlation, gender,
hand, sighting dominance, sensory dominance for each
subject.


R-L/R+L %


Average
Reduction


r
value


Gender


Hand


Sighting
Dominance


Sensory
Dominance


1.0 75.4 -.3475 Male Right Right Right

.504 45.9 -.3890 Male Left Left Left

.471' 29.3 .8298 Male Right Right Right

.459 61.2 .1300 Male Right Right Right

.398 49.8 -.0809 Male Right Right Right

.274 33.6 .2209 Female Right Right Right

.230 30.8 -.0286 Male Right Right Right

.067 31.9 -.4859 Female Right Right None

.031 72.1 -.2567 Male Right Left None

.023 42.2 .2334 Female Right Right None

-.103 50.6 -.2224 Male Right Right None

-.207 39.4 .4228 Female Right Right Left

-.375 38.6 -.3259 Male Right Left Left

-.483 44.9 .0974 Female Right Left Left


Male Right


Left


Left


-.633


28.2


.3427


















N=15
COR=.2300







*


I I I I I
25 30 35 40 45


I I I I I I I
50 55 60 65 70 75 80


Percent of Reduction

absolute value

Figure 7. Sensory dominance score compared to percent of
amplitude reduction for all subjects.


1.0-


.9 -


.8-


.7-









Sighting Dominance / Sensory Dominance

Table 1 also gives the sensory dominance results for

all subjects. Nine subjects were in concordance between

these measures while only two were in disagreement.

Three right and one left sighting dominant subjects had

sensory dominance scores indicating no preferred eye.

The criterion for defining nondominant was an absolute

sensory dominance score .20. This dichotomizing of the

sensory dominance score was performed solely for the

purpose of comparing this measure of ocular dominance

to the more frequently reported sighting dominance.

Sensory dominance and VEP data for one subject,

the same subject whose VEPs are shown in Figure 6, are

provided in Appendix A.













CHAPTER IV
DISCUSSION


The distribution of sensory dominance scores obtained

in this study is similar to those reported by Ondercin

et al. (1973) and Perry and Childers (1972). The pattern

of few extreme scores and numerous central scores of

sensory dominance suggests that a significant amount of

information is lost when sensory dominance is measured as

a dichotomous variable. The decrease in the accuracy

of correctly detecting the stimuli from binocular to

dichoptic stimulus conditions for the present study was

also in agreement with the above two studies which used

this paradigm.

A decrease in amplitude of the VEP from the binocular

to the dichoptic stimulus condition was expected and is a

generally accepted electrophysiological correlate of

suppression (Harter, 1977). An unexpected result was the

gender difference in the percent of reduction of the

amplitude of the VEP. The VEPs of female subjects were

reduced by a smaller percentage than the VEPs of male

subjects. The literature indicates that differences in

the VEP amplitude can be seen across the sexes, specifically,

females giving slightly larger waveforms with shorter

latencies (Rodin, Grissell, Gudobba, & Zachary, 1965;









Shagass & Schwartz, 1965; Perry & Childers, 1969; Shagass,

1972). An analysis of the amplitude data of this study

did reveal a significant difference in amplitude (p<.01)

between male and female subjects with dichoptic stimulation.

Females produced larger VEPs than males. No differences

in VEP amplitude were observed with binocular stimulation.

The majority of studies evaluating suppression and eye

dominance have not looked at gender differences. Studies

which used entirely male or entirely female samples have

reported somewhat conflicting results (Eason et al.,

1967a; Eason et al., 1967b; Culver et al., 1970). When

the data of this study were grouped by gender, there was

no significant difference in the amplitude reduction/

sensory dominance correlation for these two groups. No

effects were seen when the data were grouped by handedness

or by preferred eye for sighting.

The comparison of sighting dominance to sensory

dominance as measured in this study is made without a

historical data base. The important characteristic of

the sensory dominance score is that it does not dichotomize

the results. The measure is a continuous variable ranging

from completely left-eye-dominant to completely right-

eye-dominant. The data of this and the previous studies

show that few individuals actually obtain the extreme

scores. For comparative purposes only, a cut-off criterion

can be established to define dominant and nondominant

subjects. If a criterion of .20 is used to separate









subjects into dominant and nondominant groups, 9 subjects

are in agreement on eye preference measures, 2 subjects

have crossed preferences, and 4 subjects are classified

as nondominant. The theoretical relationship between

sighting dominance and sensory dominance is difficult to

define because the neurological mechanisms of each remain

unknown.

Coren and Kaplan (1973), in their factor analytic

study of tests measuring ocular dominance, reported

correlations among thirteen different measures of

dominance. The tests that they evaluated which would be

most similar to the sensory dominance scores of this

study are tests of form rivalry and color rivalry. These

tests did not significantly correlate with any of the

three specific tests of the present study which were used

to determine sighting dominance. If the sensory dominance

measure is indeed similar to form and color rivalry, the

evidence would not predict a strong relationship between

sensory dominance and sighting dominance.

Handedness tended to be correlated with sighting

dominance in this study, although, as mentioned, there

was only one left-handed subject in the sample. There

was a total of five left-eyed subjects. Generally,

handedness and eye preference agree (Walls, 1951: Coren

& Kaplan, 1973; Klemm et al., 1980). Discussion beyond

a correlational relationship of these two variables is

the topic of a completely different research area.









The primary results of this study do not support

the initial proposal of a systematic relationship between

the VEP amplitude and a behavioral measure of sensory

dominance. One explanation is that there simply is no

relationship between these two variables. It is possible

that amplitude, as defined in this study, is too gross a

measure to reveal subtle changes and subtle relationships

which may exist. An alternative would be to redefine

amplitude so as to make it a more narrow, and therefore

more sensitive, variable. The literature presents many

acceptable schemes for measuring the amplitude of the

waveform (Harter et al., 1977; Harter et al., 1980;

Apkarian et al., 1981; Trick et al., 1982). But a broader

question should be asked. What does a correlational

study such as this one yield even when the results are

significant? Correlational studies rarely can indicate

which variable influences the other, or whether either

variable is influencing the other one directly. In this

study, what is the nature of the relationship between

sensory dominance measures and VEP amplitude and how

direct or indirect is any influence? The data generated

by the subjects of this experiment help to form

speculation on this relationship.

As the behavioral and electrophysiological data of

this study are reviewed, there is a conspicuousness about

the variability contained within these measures. The

behavioral measure of sensory dominance is calculated









from the subject's own report of what he/she saw after

each presentation of a letter pair. The protocol sheets

for each subject show that while each presentation of a

letter pair was identical, the subject's responses could

be quite varied. For example, if the letter pair 'OA'

was presented, the subject's response could be 'O' or

'A', or possibly 'OA'. (Several subjects reported on a

few presentations. On these occasions, the subjects were

likely not fixated properly and the two eyes were not

aligned.) But some subjects also reported 'R', 'B',

and 'H', and sometimes could not discern a letter at all.

These various perceptions are the direct output of a

"black box," the visual nervous system, which has had

identical input for each report. These characteristics

of the results may contain a certain amount of face value

which is being overlooked. What may be important here,

though, is that we are seeing what is, or what appears to

be, the variability inherent in the suppression mechanism.

Certainly the data from the binocular stimulus conditions

show that the visual system, when not undergoing

suppression, is capable of responding with 100% accuracy,

or zero variability. But when the visual system is

experiencing significant suppression, detection accuracy

drops and the variability of neuronal activity increases.

If we assume that a subject's responses to

dichoptically presented letters represent the variability

of the suppression mechanism, then it seems that there









could be a similar variability within the electro-

physiological responses which make up the VEP. The EEG

sample which corresponds to a perceived 'A' is likely to

differ from the EEG response corresponding to the

perceived '0' or 'H' or 'B'. Perhaps, then, there would

be a relationship between the variability within the

sensory dominance score and the variability within the

corresponding VEP.

Variability is of critical importance to the VEP.

The VEP is an arithmetical mean of a distribution of

electrical potentials, and like any distribution, the

variation of its constituent points is important to the

"validity" of the mean. As described earlier, the VEP

is an average of time-locked EEG in response to a stimulus.

Activity not related to the event will cancel or neutralize

itself, while EEG activity related to the event will summate.

Theoretically, then, the VEP collected without stimulation,

without an event, would summate to zero. Operationally,

though, this is not the case. Even though each researcher

strives to minimize extracerebral factors from influencing

the VEP, it is impossible to eliminate every possible

contaminant. Amplifiers, filters, and the general

integrity of the recording system all make their own

unique contribution to the signal. One only hopes to

hold all of these constant for each subject, trial, and

session. With this accomplished, VEP variability should

contain background EEG and true evoked potential









variation. In a discussion of VEP variability, Callaway

summarizes, "amplitude, latency, and variability are not

independent; and variability, rather than something to be

gotten rid of, may be a more fundamental factor than it

seems at first glance," (Callaway, 1975, p. 63). Regan

(1972, p. 250) echoed the warning of Dawson (1954) and

other earlier authors, "that the variabilities rejected by

averaging might in themselves be of physiological

significance."

There is some evidence that increased variability

of the VEP correlates with reduced amplitude of the VEP.

Research suggests that this relationship is mediated by

the particular measure of variability taken and the

experimental design (Callaway, 1975). In looking at the

data of the present study, if we assume that extracerebral

factors have been held constant, the remaining components

of variability are primarily the background EEG and the

true variability of the event response. Background EEG

can be roughly described as the electrical activity of

the working brain which is not related to the visual

cortical response. Cancelling out the background activity

is the essential purpose of signal averaging. The design

of the majority of VEP studies manipulates the stimuli

so as to keep the evoked potential as pure and unvaried as

possible. The design of the present study manipulates

the stimuli so as to produce suppression, and the results

indicate that suppression produces visual perception which









is significantly variable. In this case, then, it is

possibly the variability of the VEP, not the amplitude,

which is correlated with the behavioral measures of sensory

dominance.

The data from this study clearly show a decrease in

amplitude from the binocular to the dichoptic stimulus

conditions for each letter pair for each subject. This

decrease is expected and is generally accepted as an

electrophysiological correlate of suppression (Harter,

1977). Research data on the relationship between VEP

variability and amplitude is equivocal, but there continues

to be a persistent opinion that increased variability

reduces the amplitude of the VEP. Callaway (1975)

reported pilot data collected by a co-author which help

clarify this issue. Ten subjects were tested so as to

establish the following conditions: same subject, same

day; same subject, different days; different subjects,

different days. VEPs were acquired to four light

intensities, a variable which is known to affect VEP

amplitude (Perry & Childers, 1969). The results of this

work show that for the VEPs from the same subject/same

day tests, half showed a positive correlation and half

showed a negative correlation between variability and

amplitude. The same subject/different days tests

yielded positive correlations (no significance levels

given). An evaluation across the ten subjects showed

amplitude and variability correlating r=+.42, a









nonsignificant relationship. An important characteristic

of this project is the experimental manipulation of

amplitude with stimulus intensity. Variability was

evaluated in relation to the independent variable of

amplitude. In the present experiment, both amplitude

and variability could be considered dependent variables

and they could be correlated in some way.

The initial proposal of this paper hypothesized that

cortical inhibition of adjacent ocular dominance columns

might reduce the total amount of neuronal activity related

to the stimulus. Using overall amplitude of the VEP as

the measure of this process, the present data did not

support that proposal. The results would conform to an

alternative hypothesis.

Maintaining a model of inhibition of adjacent ocular

dominance columns, the varied verbal responses to dichoptic

stimulation imply a different underlying "neural circuitry,"

which is responsible for the variability of the responses.

It may be erroneous to think in terms of greater and lesser

neural activity, and much more accurate to address the

issue of varied neural activity. A stimulus (input) is

processed by the visual nervous system and a perception

is made (output). The same input can result in a variety

of output. This implies that the formation is processed

differently. We could say that the visual information

reaches the locus of visual perception via a different

pathway and that a determining characteristic of this









process is the variation of these pathways and not the

total neural units involved.

This hypothesis fits logically into a broader

theoretical model of how suppression may function. Drawing

from neurophysiological data, suppression could be

initiated in the primary visual cortex as a consequence

of inhibition of adjacent ocular dominance columns.

Suppression does not culminate at this level of processing,

though. There is much evidence that the suppressed

images are processed later in the visual system and in

other areas of the brain (Walker, 1978). The effects of

inhibition at the primary cortex level are amplified as

the signal passes along the hierarchy of cortical pro-

cessing. A small change at the primary cortex level may

have a more wide spread change at subsequent levels.

This suggests not so much a change in the amount of neural

activity, but a change in the characteristics of the neural

activity.

The concluding statement of this paper has to be

a recommendation for further research to evaluate the

relationship between the sensory dominance measure

described in this paper, and the variability contained

within the corresponding VEP. Several procedures are

currently employed to obtain VEP variability data

(Callaway, 1975). An additional alternative may be to

analyze the individual EEG responses to each dichoptic






71


presentation as a single unit of data. Bremner et al.

(1982) describe a procedure for this type of EEG analysis.












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APPENDIX A
DATA FOR EACH LETTER PAIR FOR ONE SUBJECT

Sensory Dominance Data


Letter Pair


OA

HE

SP

HS

EU

FC

PH

SE


TOTAL


No. Letters Reported Correctly*
Left Eye Right Eye

16 28


24

216


*A total of 480 possible correct for each eye.


R-L/R+L


.272

.782

.437

.688

.238

.153

.760

.333

.459











VEP AMPLITUDE DATA


Binocular VEP Dichoptic VEP
Letter Pair Ampl. Ampl.

OA 9.46 5.22

HE 12.03 4.60

SP 12.42 4.52

HS 13.09 4.64

EU 11.62 3.42

FC 9.08 3.40

PH 11.78 4.56

SE 12.67 5.03


% Reduction

44.82

61.76

63.60

64.55

70.56

62.55

61.29

60.29


























N= 8
COR=.1300


*OA


.HE
PH*
*HS


.SP

*SE


*EU


*FC


1 1 I I


1 I 1 1 1
40 45 50 55 60 E


Percent of Reduction


I I0
5 70 75


Sensory Dominance Score Plotted Against % Reduction.


.900-


.750-


.600-


.450-


.300-


.150-










BIOGRAPHICAL SKETCH


I, Gregory Hugh Nelson, was born in 1947 in Oceanside,

New York, the second of three sons. In 1952 my parents moved

their family from the suburbs of Long Island to a cattle ranch

in the grasslands of central Florida. I was one of 61 Florida

crackers to be graduated in 1965 from Lake Weir High, in

Summerfield, Florida. Upon my graduation, the U.S. Marine

Corps recruited me; at the age of 18 I was flying jet aircraft

and being paid to do it. I served a tour of combat duty in

Vietnam from 1968 to 1969, and in 1970 I was honorably dis-

charged and commenced my college education. In the spring of

1975 I celebrated both my graduation from the University of

Florida and my marriage to my wife, Anne. Upon acceptance

into the University of Florida's clinical psychology graduate

program, I received a USAF Health Professions Scholarship, and

thus began the long and arduous course toward my doctorate.

My internship was completed in 1979, and I served for the next

three years as staff clinical psychologist at Plattsburgh Air

Force Base Hospital, in the Adirondack Mountains of upstate

New York. My daughter, Caitlin, was born in April, 1981, and

in 1982 my wife and I decided to return home to pursue our

careers and raise our family. We are residents of Gainesville,

Florida.












I certify that I have read this study and that in
my opinion it conforms to acceptable standards of scholarly
presentation and is fully adequate, in scope and quality,
as a dissertation for the degree of Doctor of Philosophy.




Nathan W. Perry, Jr., Chairman
Professor of Clinical Psychology




I certify that I have read this study and that in
my opinion it conforms to acceptable standards of scholarly
presentation and is fully adequate, in scope and quality,
as a dissertation for the degree of Doctor of Philosophy.




Hugh C./bavis, Jr.
Professor of Clinical Psychology





I certify that I have read this study and that in
my opinion it conforms to acceptable standards of scholarly
presentation and is fully adequate, in scope and quality,
as a dissertation for the degree of Doctor of Philosophy.




Cynt ia D. Belar
Associate Professor of Clinical
Psychology











I certify that I have read this study and that in my
opinion it conforms to acceptable standards of scholarly
presentation and is fully adequate, in scope and quality,
as a dissertation for the degree of Doctor of Philosophy.




Charles M. Levy, Jr.
Professor of Psychology



I certify that I have read this study and that in my
opinion it conforms to acceptable standards of scholarly
presentation and is fully adequate, in scope and quality,
as a dissertation for the degree of Doctor of Philosophy.



/-' C/ _
Mark C. K. Yang
Associate P ofssrof Statistics



This dissertation was submitted to the Graduate Faculty of
the Department of Clinical Psychology in the College of
Health Related Professions and to the Graduate Council,
and was accepted as partial fulfillment of the requirements
for the degree of Doctor of Philosophy.


April, 1983

Dean, College of Health Related
Professions




Dean for Graduate Studies and
Research









































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