THE VISUAL EVOKED POTENTIAL: CORTICAL CORRELATES OF
SENSORY OCULAR DOMINANCE
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
I would like to express heartfelt appreciation and
gratitude to my wife, Anne.
TABLE OF CONTENTS
ABSTRACT............................ .. .. ............. iv
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
A Neurophysiological Model of
The Present Study.......................... 45
II METHOD............ ... ....................... 46
Subjects..... ... ......................... .. 46
Preliminary Procedures..................... 46
VEP Collection............................ 47
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
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
Gregory Hugh Nelson
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
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.
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
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, &
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
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
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
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
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
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
Figure 2. Dichoptic stimuli presented to establish binocular
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
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
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
100 200 300 400
Figure 3. Representative Visual Evoked Potential.
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
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
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
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
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
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,
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
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
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.
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.
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
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.
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
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
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 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.
Figure 4. Representation of seven-segment alpha-numeric LED.
The following instructions were read to each
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
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
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.
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.
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,
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
This percentage measure, the percent of amplitude
reduction from binocular to dichoptic stimulus conditions,
a. II .-- I I I ---I
100 200 300 400 500
Figure 6. a. VEP to binocular stimulation, b. VEP to
each of eight letter pairs. All VEPs from the same
t00 200 300 400 500
Figure 6. a. VEP to binocular stimulation. b. VEP to
each of eight letter pairs. All VEPs from the same
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
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
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
Figure 7. Sensory dominance score compared to percent of
amplitude reduction for all subjects.
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.
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
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
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
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
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
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
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
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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
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(Callaway, 1975). An additional alternative may be to
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DATA FOR EACH LETTER PAIR FOR ONE SUBJECT
Sensory Dominance Data
No. Letters Reported Correctly*
Left Eye Right Eye
*A total of 480 possible correct for each eye.
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
1 1 I I
1 I 1 1 1
40 45 50 55 60 E
Percent of Reduction
5 70 75
Sensory Dominance Score Plotted Against % Reduction.
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,
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
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.
Dean, College of Health Related
Dean for Graduate Studies and
UNIVERSITY OF FLORIDA
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