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66 Theory and Practice of Squint and Orthoptics
approximation to their geometric locations so
that a receptor 5° temporal to the fovea in one
eye corresponds to a receptor 5° nasal to the
fovea in the other eye.
Images falling on corresponding points give
rise to a single mental impression to visual
direction that we call sensory fusion.
Corresponding retinal elements, therefore, have
a common visual direction and permit a single
visual impression. They do not permit double
vision. The mental image resulting from the
fusion of the two retinal images can be conceived
as the image or mental image received by a
cyclopean eye. This is an eye, presumably
located in the middle of the forehead, that sees
in visual space the fused images of the two
individual eyes (Fig. 4.4).12 Normal retinal
correspondence is thus the basis for normal
binocular single vision. Thus the existence of
corresponding retinal elements with their
common relative subjective visual directions is
the essence of binocular vision. It may be called
the law of sensory correspondence in analogy with
the law of motor correspondence.
If, during fixation of an object, one eye of the
observer would be passively turned with
forceps, the object would no longer be imaged
on corresponding retinal points. While the
images would fall on the fovea in the one eye, it
Fig. 4.4 Normal retinal correspondence. The visual direction
of f1 and f2 proceed from F the fovea of the imaginary
cyclopean eye. A and B represent the points from which the
visual direction belonging to a and a´, b and b´ would
proceed (From Bielschowsky12).
would be received somewhere on the peripheral
retina in the deviated eye. Since these two areas
are non-corresponding and have different
spatial values, diplopia would be present (i.e.
the object would be seen double).
The oneness of the directional sensory
responses originating in each eye is impressively
demonstrated by means of afterimages. If one
creates an afterimage on the retina of one eye, it
will appear in the binocular field of view in the
common visual direction of the stimulated
retinal area and in its nonstimulated partner in
the other eye. It is difficult, indeed almost
impossible, for the observer to judge which eye
carries the afterimages. It will continue to be
seen and localized in the same direction,
whether the eyes are open or closed or whether
the stimulated eye is closed and the other eye
held open.
If a horizontal afterimage is formed in one eye
by a strong horizontal light stimulus, leaving the
fovea unstimulated, and if a similar vertical
afterimage is created in the other eye, the
resulting visual percept is an afterimage in the
form of a cross with a gap in its center
(Fig. 4.5). The gap is seen because of the lack of
stimulation in the foveae. The center of the
horizontal and vertical afterimages is
consequently a single spot localized in the
principal common visual direction. The
horizontal and vertical legs of the afterimages
are oriented accordingly. It is of great
importance to understand clearly that the
appearance of the afterimage cross is independent
of the position of the eyes. Once a lasting stimulus,
such as an afterimage, has been imparted, its
localization in subjective space depends solely
on the visual direction of the retinal elements
involved. One may topically anesthetize one eye
and move it passively with a forceps or push it
in any direction with one's finger-the cross
remains a cross. No change in the relative
localization of the vertical and horizontal
afterimage will occur. The use of afterimages has
an important place in the diagnosis of
anomalous retinal correspondence. The
principles underlying afterimage testing must
be fully understood to guard against gross errors
in interpretation.
Binocular Vision
A
Fig. 4.6 Diagrammatic representation of the theoretical
horopter (Vieth-Muller horopter circle). Points F, A and B fall
on geometrically corresponding retinal points f1 and f2, a
and a2 and b1 and b2, respectively.
67
1
B
Fig. 4.5 (A) Afterimages produced in the right and left eye,
respectively. The fovea is represented by the break in the
afterimage. (B) The combined binocular afterimage forms a
cross. The two gaps appear single.
HOROPTER
The term horopter, which literally means the
horizon of vision, was introduced by
Aguilonius.13 As we know, when, in normal
binocular vision, both eyes fixate the same object
point, this point will be imaged on
corresponding retinal elements of the two
eyes—the foveae. At the same time, there will
be other object points in space besides the
fixation point that also will be imaged on
corresponding retinal elements of the two eyes.
If the position of these other points is
determined theoretically, a circle will be found
passing through the fixation point and the
entrance pupils of the two eyes. This circle is
known as the theoretical or geometric horopter
(Vieth-Muller horopter).
14,15
Theoretically, any
point on it will stimulate corresponding retinal
elements in the two eyes, because every point
on the horopter will form an angle with the two
entrance pupils that is equal to the angle formed
by the fixation point and the two entrance pupils
(Fig. 4.6). It follows that there is a different
horopter for each fixation distance.
In other words, the horopter can be defined
as the sum total of points in physical space that
stimulate corresponding retinal elements of the
two eyes. It is a complex mathematical model
having all three dimensions. The longitudinal
horopter is that surface that determines
horizontally distributed object points in space.
The longitudinal horopter curve is a line formed
by the intersection of the visual plane and the
longitudinal horopter plane. The visual plane
is determined by the fixation point and the
centres of the two pupils.
16, 17
The horopter can be determined with an
instrument utilizing movable, vertically placed
wires set at a certain distance. While fixation is
maintained upon a central wire, those on either
side in the periphery are adjusted forward or
backward by the subject until they appear to lie
in the same visual plane. In determining the
horopter, the difference between subjective
visual space and physical space is again
13
68 Theory and Practice of Squint and Orthoptics
demonstrated. Subjectively, the movable wires
appear to lie in the same visual plane; however,
they actually lie on a curved surface. Thus, the
horopters found experimentally do not actually
coincide with the theoretical horopters. They are
called empirical horopters.
PHYSIOLOGIC DIPLOPIA
Since only object points which lie on the
horopter will be imaged on corresponding
retinal elements, all objects that are either nearer
or farther away than the horopter will be imaged
on disparate (i.e. non-corresponding) retinal
elements and, consequently, will be seen double.
Point a in Fig. 4.7A, located closer than the
horopter (point b), is imaged temporal to the
fovea in each eye. The right eye will see this
point as being on the left side of the fixation
point, while the left eye will see it to the right of
the fixation point. In other words, point a will
be seen double (a1 and a2). Since each eye
perceives it as being on the opposite side, this is
referred to as crossed diplopia. With respect to
the retina, point a shows temporal disparity.
Conversely point a, which is located beyond the
horopter (point b) in Fig. 4.7B; will be imaged
on the nasal retinae of both eyes and, therefore,
will also appear double. However, this time the
diplopia will be uncrossed—the right eye will
localize the object to the right of the fixation
point, while the left eye will place it to the left.
The retinal images will have nasal disparity.
Physiologic diplopia can be demonstrated by
holding two pencils vertically in front of the eyes
with one pencil about twice as far away as the
other. If either one of them is fixated, the other
will be seen double. If the more distant one is
fixated, crossed diplopia will be experienced. If
the closer one is fixated, the diplopia will be
uncrossed.
This demonstration will not be immediately
successful in all subjects, since one of the images
may be suppressed from conscious perception,
a process which also is physiologic, as is
explained later in this chapter.
In view of what has been said about the
horopter and physiologic diplopia, it should be
expected that one would have constant diplopia
during casual daily vision, with the exception of
the object of fixation and the few objects that
happen to be on the horopter for the momentary
fixation distance. Obviously, this is not the case;
and it is not primarily suppression that prevents
the occurrence of physiologic diplopia, but the
exclusiveness with which our attention is usually
directed to the fixated object. Although diplopia
is present, this rarely enters consciousness.
Another contributing factor is the low visual
acuity of the peripheral retina, which frequently
does not yield sufficient resolution of peripherally
located objects for the recognition of diplopia.
Fig. 4.7 Diagram illustrating physiological diplopia. Points
a and b which are not located on the horopter, are imaged
on non-corresponding retinal points and so physiological
diplopia either crossed (A) or uncrossed (B) is produced.
BINOCULAR FUSION
When images of an object fall on corresponding
retinal points, in the normal subject, they seem
to be fused into a single mental impression.
Binocular Vision
69
Sensory fusion, i.e. single vision is the hallmark of
retinal correspondence. Put otherwise, the
stimulus to sensory fusion is the excitation of
corresponding retinal elements. It should be
distinguished from motor fusion, which refers
to the ability to align the eyes in such a manner
that sensory fusion can be maintained. The
stimulus for these fusional eye movements is
retinal disparity. Unlike sensory fusion, the
motor fusion is exclusive function of the extrafoveal retinal periphery. No stimulus for motor
fusion exists when the images of a fixated visual
object fall on the fovea of each eye.
Under normal conditions, sensory fusion
occurs when corresponding retinal elements of
the two eyes are stimulated by images from the
same object. Thus for sensory fusion to occur,
the images not only must be located on
corresponding retinal areas but also must be
sufficiently similar in size, brightness, and
sharpness. Unequal images are a severe sensory
obstacle to fusion. Obstacles to fusion may
become important factors in the etiology of
strabismus. However, fusion will also take place
when the stimuli in the two eyes are not identical
but only similar. The dissimilarity may be one
of form, size, colour, luminosity, distinctness or
contrast. To a limited extent, it is also possible
to fuse signals from non-corresponding points
(see discussion on fixation disparity below).
SENSORY FUSION
Sensory fusion, also known as binocular fusion,
thus, can be defined as the ability of the brain to
combine the slightly disparate images from each
eye into a single, unified visual perception. This
process is crucial for achieving and maintaining
binocular vision, which allows for depth
perception and a three-dimensional perception
of the world.
The anatomic basis which allows sensory fusion
to occur is the course of the visual nerve fibres.
By crossing in the chiasma, nerve fibres from
the nasal retina are brought to the same side as
their counterparts from corresponding points of
the temporal retina of the other eye. While
ascending in the visual pathway, the fibres from
corresponding retinal points converge until
finally they are adjacent to each other. They
terminate in the same cortical cell complexes.
But for our knowledge of the anatomic
arrangement, little definite information exists on
the physiology of sensory fusion. It is probable
that fusion occurs in the cortex. It is not certain,
however, whether the two signals transmitted
separately from two corresponding points are
synthesized so that they simultaneously
contribute to the resulting single perception, or
whether the two signals are mutually exclusive
and are utilized alternately to be combine into
the resulting single constant perception so that
first one eye and then the other contributes its
stimulus to a given point of the whole.
MOTOR FUSION
• As mentioned above, the motor fusion is a
corrective vergence movement in response to
image disparity.
• Motor fusion adjusts eye position to maintain
sensory fusion.
• As a fixation target approaches the observer,
the retinal images move temporally from each
fovea if the eye remains in an unchanged
position.
• To prevent diplopia, the crossed image
disparity induces both eyes to converge (turn
inwards) and maintain the image focused on
the foveae.
• A similar divergent movement occurs as
objects move from near to far.
• Fusional reserve indicates the level at which
motor fusion breaks down, usually causing
diplopia. It can be measured by adding prism
bars (base in or out) until fusion is lost
(Table 4.1).
• Torsional fusional vergences also exist for up
to 6-10° of torsional image disparity.
At birth, coordinated conjugate eye movements
are absent, visual perception is poor, and fusion
is not established as a stable binocular function.
If development proceeds normally, fusion
evolves later as a conditioned reflex. If there is
a significant abnormality in the optical
performance of the eyes, in the function of the
nerve elements that are concerned with the
transmission or perception of visual stimuli, or
in the motor cooperation of the two eyes, fusion
70 Theory and Practice of Squint and Orthoptics
Table 4.1 Normal fusional amplitudes
Testing distance (m) Convergence Divergence Vertical
(prism diopters, ) (prism diopters, ) (prism diopters, )
6 (distance) 16 6 5–6
0.25 (near) 32 16 3–4
may never develop normally. It is conceivable
that there may be individuals in whom a basic
ability for fusion does not exist.
PANUM’S AREA
As stated previously that only object points
which lie on the horopter and which stimulate
corresponding retinal points in the two eyes are
seen as single is not quite exact and must be
qualified. When a horopter is determined
experimentally, it is found that an object may
be located a certain distance in front of or behind
it without producing diplopia. The field in front
of and behind the horopter, in which the
expected diplopia does not occur is known as
Panum’s fusional space.
Panum’s fusional space is smallest at the
fixation point (Fig. 4.8). Toward the periphery,
it gains increasingly in depth, so that objects
located peripherally may be farther from the
horopter without producing diplopia than
objects located more centrally. Any point not on
the horopter is not imaged on corresponding
retinal areas but will stimulate disparate retinal
areas. If, despite the disparity, no diplopia
occurs, fusion of signals from non-corresponding points must have taken place. Thus, a
signal from a given retinal point not only is fused
with the signal from its corresponding point in
the other eye, but also may be fused with signals
from a limited area surrounding that
corresponding point. Such a retinal Panum’s
area corresponds in size to the depth of Panum’s
fusional space. Since the latter is small in the
vicinity of the fixation point, Panum’s areas in
the fovea are also small, actually in the order of
5’ of arc. Away from the fixation point, Panum’s
fusional space enlarges (i.e. the size of Panum’s
areas in the peripheral retina increases). Since
retinal disparity in the horizontal meridian can
be overcome to a larger extent than disparity in
the vertical meridian, Panum’s areas have the
shape of an oval with the longer axis horizontal.
If the fixation distance is more than 20 m, objects
lying behind the horopter will always be seen
as single, since the disparity of their images is
always smaller than Panum’s areas.
Fig. 4.8 Diagram of Panum's fusional space.
FIXATION DISPARITY
Under laboratory conditions, it can be
demonstrated that, during binocular fixation,
the point of fixation is rarely ever imaged exactly
on corresponding points of the two foveae but
that the primary line of sight of one eye misses
the fixation point very slightly, being either
under-converged or over-converged (Fig. 4.9).
This phenomenon is called fixation disparity. It
does not give rise to diplopia, because the
disparity with which the fixation point is imaged
on the two retinae is less than the size of
Panum’s area.
Thus, in fixation disparity, one eye will fixate
the object directly with the central fovea, while
the other eye will fixate slightly eccentrically.
Fig. 4.9 Diagram illustrating fixation disparity.
However, the horopter simply passes through
the actual point of fixation rather than through
the fixation stimulus. Fixation disparity may
arise from a small foveal scotoma or from an
oculomotor imbalance which is tending to pull
the eyes away from the appropriate angle of
convergence. An example of a horopter showing
fixation disparity was obtained by Olge18 on his
own eyes.
DICHOPTIC STIMULATION
Dichoptic stimulation simply refers to a different
stimulation in the two eyes, which occurs when
binocular stimuli fall on non-corresponding
points on the two retinae. There are five classes
of percepts which are obtained, depending upon
the degree of non-correspondence between the
stimuli:
1. Depth with fusion;
2. Depth with diplopia;
3. Diplopia without depth;
4. Binocular rivalry and suppression; and
5. Binocular lustre.
DEPTH WITH FUSION AND DEPTH WITH DIPLOPIA
Strictly speaking, these are not generally
referred to as dichoptic stimuli, since the two
retinal patterns are sufficiently similar to be
Binocular Vision
71
combined into a unified impression (particularly
for fused stereopsis). Therefore, these will be
dealt in the section of stereopsis.
DIPLOPIA WITHOUT DEPTH
When the degree of non-correspondence is such
that no fusion occurs and diplopia results
without depth perception. Depending upon the
location of non-corresponding points
stimulated, the diplopia may be homonymous
or uncrossed (Fig. 4.7B) and heteronymous or
crossed (Fig. 4.7A). Sensory adaptations which
may occur to compensate for the annoying
diplopia include: suppression, amblyopia, and
abnormal retinal correspondence. Motor
mechanisms which compensate for diplopia are
an abnormal head posture and certain
permanent changes in the extraocular muscles
such as inhibitional palsy, overaction and
contractures.
RETINAL RIVALRY AND SUPPRESSION
When dissimilar contours are presented to
corresponding retinal areas (having same visual
direction), fusion becomes impossible, since two
dissimilar objects localized in the same place
give rise to a conflict and confusion. The conflict
which results when contradictory and
incompatible signals are transmitted from
corresponding points to the visual cortex is
termed retinal rivalry. The phenomenon of
retinal rivalry, also termed as binocular rivalry,
must be clearly distinguished from local adaptation, or Troxer’s phenomenon.24 Binocular
rivalry may also be produced by uniform
surfaces of different colour (colour rivalry) and
unequal luminances of the two targets. Many
combinations of contours, colours and
luminances have been studied exhaustively
since the days of Panum,25 Fechner,
Helmholtz27 and Hering.
Retinal rivalry is a very normal event, since
28
corresponding areas are consistently exposed to
dissimilar images from objects located outside
Panum’s fusional space.
The visual system responds to retinal rivalry
by rejecting the signals transmitted from a given
area of one retina in favour of those coming from
26
72 Theory and Practice of Squint and Orthoptics
the corresponding parts of the other retina.
Thus, only information from one retina becomes
a visual perception while the information
simultaneously transmitted from corresponding
parts of the other retina is not used, so that the
involved area may be considered temporarily
blind. This reaction to the influx of conflicting
information is called suppression.
Suppression is an innate, involuntary process.
Under certain conditions, all the signals from
one retina may be suppressed so that no
information from that eye reaches the threshold
of conscious perception.
29-33
Usually, however,
responding to the demand of the momentary
conditions, suppression is restricted to limited
areas in both retinae so that those portions of
the subjective visual field in which fusion cannot
occur may be composed, like a mosaic, of pieces
of information from only one or the other retina
in constant variation (Fig. 4.10).
Although an eye may be suppressed for long
periods, suppression basically is a transient
phenomenon, present only momentarily when
needed. Because it is a physiologic function that
does not have to be learned but is readily
available, it frequently becomes the firstline of
defence against pathologic interruption or
Fig. 4.10 Retinal rivalry produced by dissimilar contours:
(A) pattern viewed by left eye; (B) Pattern viewed by right
eye; (C) binocular impression (mosaic of pieces). After
Panum.25
embarrassment of bifoveal single vision. For
example, a patient who has a marked refractive
error in only one eye may show suppression of
that eye to eliminate the disturbing effect of the
blurred image. In patients with strabismus, a
condition in which objects are not imaged on
normally corresponding retinal areas, suppression is a frequent response to solve the problem
of incompatible information transmitted from
corresponding areas of the two eyes. It has been
shown experimentally that retinal rivalry and
the suppression resulting from it are influenced
by certain conditions. If non-fusable patterns are
presented to the two eyes, it is found that
perception from corresponding areas alternates
equally between the two eyes, if the objects are
of equal brightness, size and prominence. The
rate of alternation under such test conditions is
influenced by the brightness of the field, the size
of the target area, the distinctness of the targets,
and the location of the stimulated area in the
retina. If any of these factors is impaired equally
for both eyes, the rate of alternation will
decrease. If the visual conditions are improved,
it will increase. However, suppression will
always alternate equally between the two eyes.
In contrast, by changing the stimulus
conditions for only one eye, the rate of
alternation can be changed markedly in its
favour or disfavour.
Retinal or binocular rivalry fluctuations are
similar in many respects to fluctuations of
attention, and are widely supposed to be under
voluntary control to some extent. Actually, a
number of studies have found that there is very
little voluntary control over which eye
dominates at any given time.34 The change of
dominance is not affected by eye blinks35 or by
variations in accommodation or pupil size.36 In
fact, the fluctuations in rivalry are well described
by a sequentially independent random variable
with no periodicities, as though the arrival of
each change in dominance had no effect on the
occurrence of subsequent changes.
37
The phenomenon of retinal rivalry has been
explained in neurophysiologic terms by the
presumption that separate channels are present
for the right and left eyes which compete for
access to the visual cortex. A third binocular
Binocular Vision
73
channel is activated only by fusible input.
38,39
Because of this competition and inhibition
elicited, only fragments of the image seen by
each eye are transmitted to the visual cortex in
the case of non-fusible binocular output. This
hypothesis looks logical but needs experimental
verification.
BINOCULAR LUSTRE
The lustrous appearance of surfaces like a waxed
surface, tabletop or a car body is essentially due
to binocular lustre. It results from the different
position of partially reflected objects in the surface
by virtue of the different position of the two eyes.
The partial reflection provides a fixation place at
which the partially reflected image usually has a
large disparity and hence areas of binocular
luminance difference. The lustrous region is not
localizable in depth, but it seems unitary and does
not fluctuate in the manner of binocular rivalry.
Binocular lustre was described by early authors
in visual science, such as Panum19 and
Helmholtz,21 as a kind of lustrous or shimmering
surface of indeterminate depth.
The detection of binocular lustre during static
and dynamic random dot stereogram testing is
even more rapid than the detection of depth
changes.
40-42
STEREOPSIS
Stereopsis is the visual appreciation of three
dimensions during binocular vision. It occurs
through fusion of signals from disparate retinal
elements.
As shown in Fig. 4.11, if one views two posts,
A and B, situated at slightly different distances,
with both eyes open, their separation in depth
is readily apparent to a normal person. This
perception of depth is maintained as long as
both eyes are used. If either eye is closed, the
effect of depth then vanishes. If the right eye is
closed, the appearance of the two posts is that
shown on the left hand side of Fig. 4.11, and, if
the left eye is closed, the appearance is that
shown on the right side of the figure. The only
difference between these two images is that the
posts seem to be farther apart in the
frontoparallel plane when seen with the left eye
Fig. 4.11 Disparateness of retinal images producing
stereopsis.
than when seen with the right. This difference
in lateral separation is due to the fact that the
receptors stimulated in the left eye are more
widely separated from each other than those in
the right retina (note distance between a and b
of two eyes in Fig. 4.11).
An object point which is located in front of
the fixation point but is still within the limits of
Panum’s fusional space will stimulate disparate
retinal elements. The disparity will be temporal,
each image being temporal to the point that
corresponds to the location of the image in the
other eye. Although both images are fused since
they are within the limits of Panum’s area, the
fact that they are temporarily desperate is not
lost in the resulting single perception but gives
the perception of its additional quality of
nearness relative to the fixation point.
Conversely, when an object is located behind
the horopter but still within the Panum’s
fusional space, nasal disparity will exist, adding
the quality of farness to the perception. Thus, it
can be seen from Fig. 4.12 that, when the images
do not fall on exactly corresponding retinal
74 Theory and Practice of Squint and Orthoptics
Fig. 4.12 Scheme showing the effect produced by
stimulation of slightly incongruous retinal points. a-a',
corresponding points; b-b' corresponding points; F-F', the
foveas.
points, an impression that the image is in front
or behind a given frontal plane is produced. In
the figure 4.12, F and F' are the foveae fixating a
point C, straight ahead. This point determines
the frontal plane, XX’. An object A, whose image
falls on a in the left eye produces an image at b’
in the right eye. b’ is not the corresponding
point, which is a’. Because of this, the impression
is that the point A lies nearer the eyes than the
frontal plane, XX’. Similarly, an object O, whose
image falls on b in the left eye produces its image
at a' in the right eye. The corresponding retinal
point of b is b’. The image of object O lies to the
nasal side of this point at a’ in the right eye. The
impression is gained, therefore, that point O lies
behind frontal plane XX’.
Thus, it proves that in general nasal disparity
gives the impression of remoteness of an object,
and temporal disparity gives the impression of
nearness of an object.
Stereopsis is a unique cognition, a distinct
perceptional quality of its own. If one does not
have it, one cannot learn it even in the presence
of all requirements such as bifoveal fixation,
fusion, and good visual acuity. Although
stereopsis is the most important factor in
judging distances, especially at close range,
additional information about the absolute and
relative distances of objects is gained in several
other ways. While in bifoveal vision these other
means are only an addition to stereopsis, they
are the only source of information on which a
monocular observer can base his distance
judgement.
Worth classified fusion in three degrees. First-
degree fusion was the simultaneous macular
perception of two different non-fusable objects.
Second-degree fusion was characterized as the
ability of the motor system to align the eyes in
such a way that sensory fusion was possible.
Third-degree fusion was stereopsis. Worth’s
classification of third-degree fusion, however,
is not entirely correct since stereopsis is a
separate quality of the visual system, a sensory
responsiveness to disparate stimuli that can
occur, under certain conditions, without seconddegree fusion being present.
In a two-dimensional representation, certain
colours seem to stick out and others do not.
Colour stereopsis is related to the differential
refraction for various wavelengths of light. The
different blur circles can stimulate disparate
retinal points within Panum’s area and thus give
true stereopsis to a flat picture in which there is
no lateral or horizontal separation of the retinal
images.
PHYSIOLOGICAL BASIS OF STEREOPSIS
Stereopsis: A function of spatial disparity
Stereopsis arises when horizontally disparate
retinal elements are stimulated simultaneously.
The fusion of such disparate images results in a
single visual impression perceived in depth,
provided the fused image lies within Panum's
area of single binocular vision. Vertical
disparities produce no stereoscopic effect. A
solid object placed in the median plane of the
head produces unequal images in the two eyes
due to horizontal separation of the two eyes
(Fig. 4.13). The sensory fusion of the two
unequal retinal images results in a threedimensional percept.
A stereoscopic effect can also be produced by
two-dimensional pictures, some elements of
which are imaged on corresponding retinal
points to give the frame of reference for the
relative localization of other elements of figures
43
Binocular Vision
75
Fig. 4.13 A solid object placed in the midline of the head creates slightly different or disparate retinal images, the fusion
of which results in a three-dimensional sensation.
constructed to provide horizontally disparate
imagery. Such figures must be viewed separately
but binocularly in a stereoscope or some
haploscopic device.
recognition of a three-dimensional form. Julesz
refers to this additional mechanism as global
stereopsis, since the problem of ambiguities can
only be resolved on a global basis. Therefore, a
satisfactory theory for binocular depth
Local and global stereopsis
44,45
Julesz
studied stereopsis perception utilizing
perception must obviously take account of both
local and global stereopsis.
random stereogram and put forward the
concept of local and global stereopsis. The term
local stereopsis is used to denote stereopsis
elicited by the dot-by-dot or square-by-square
matching process that occurs between the right
and left stereogram.
When there is ambiguity as to which elements
in the two retinal images correspond to each
other, a global process is needed that evaluates
different possible sets of corresponding pairs
and selects one set of matched pairs that, by their
depth values, can provide the data for
Fine versus coarse stereopsis
Fine stereopsis is a highly specific pattern
matching process involving very local features
of the two retinal images and operating over a
relatively narrow range of spatial disparities
probably no more than about 0.5 degrees.
Dissimilar images cannot be simultaneously
perceived. If binocular vision is to occur, the two
retinal images must be closely similar, otherwise
there is retinal rivalry and suppression of one
or other of the antagonistic image features.