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86 Theory and Practice of Squint and Orthoptics
found mainly in the projection of the central
69
area.
Orientation specificity and disparity specificity
of cortical neurons are also dependent on the
visual experience in the early postnatal period. It
has also been demonstrated experimentally.
70, 71
THEORIES OF BINOCULAR VISION
Theory of correspondence and disparity
At present, this is the most widely accepted
theory of binocular vision. Salient features of this
theory are as follows:
• Corresponding elements of retina form the
framework or zero system of binocular vision.
Simultaneous stimulation of the corresponding points by one object transmits single
visual impression with no depth quality.
• Simultaneous stimulation by two object points
that differ in character, results in binocular
rivalry.
• Diplopia occurs when disparate elements are
stimulated by one object.
• Binocular single vision with stereopsis
results, when the horizontal disparity remains
within the limits of Panum’s area.
Neurophysiologic basis of correspondence
theory
Psychophysical data collected from human
studies and neurophysiologic evidence collected
from animal experimental studies of various
researchers, Hubel and Wiesel being the
pioneers,
72-74
have corroborated the correspondence theory. Till date following neurophysiological evidences are available:
• Approximately, 80% neurons of striate cortex
are derived from each eye, 10% from the
contralateral eye and 10% from the ipsilateral
eye only. The two receptive fields of binocularly
driven cortical cells are found to have
corresponding location in the two retinae.
72-74
• Of the binocularly driven cortical neurons,
only 25% are stimulated equally well from
each eye, while the remaining 75% show
graded degrees of influence from the right or
left eye (disparity sensitive binocular cells).
72-74
• Stereopsis has been linked with horizontal
disparity sensitive binocularly driven cortical
neurons.
75
• It has also been demonstrated that the
distribution of cortical neurons (as mentioned
above) is easily upset when animals are reared
with experimental strabismus, anisometropia,
or from vision deprivation by lid suture. This
observation corroborates the fact that the
properties of neurons in visual cortex are
greatly influenced by the visual experience
during the first a few postnatal months.
Older theories of binocular vision
All the older theories of binocular vision have been
abandoned. However, these are mentioned in brief,
just to become familiar with the old concepts.
1. Alternation theory of binocular vision. This
theory states that sensory fusion is perceptual
unification of images perceived in corresponding
locations in the two retinae. It assumes that
corresponding retinal units are represented
separately in the brain but that each of every pair is
represented in consciousness by the same single
unit. This conscious unit would receive the stimulus
from only one retinal unit at a time, the other being
excluded.
binocular vision, particularly stereopsis.
2. Projection theory of binocular vision. This theory
is based on the concept that the visual stimuli are
exteriorized (projected to physical space) along the
lines of directions.79 This theory is not able to explain
even the fundamental observation such as
physiological diplopia and so abandoned.
3. Motor theory. This theory conceptualizes that the
spatial orientation is obtained from the sensation
derived from the movement of the head, conjugate
movements of eyes and convergence. The eyes are
made aware of their movements by muscle sense. It
is this awareness that produces spatial localization.
The sensations arising from the convergence effort
determine whether one object is nearer or farther
away than the others. This theory again fails to
explain many sensory aspects of the binocular vision
especially stereopsis.
4. Theory of isomorphism. This theory states that
there exists a strict point-to-point relationship
between retina and cortex and strict comformity or
isomorphism between the distribution of objects in
space and cortical events form the basis of spatial
orientation. Subjective visual directions as a property
of the retinal-elements do not exist and that retinal
correspondence cannot change.
interesting to point out that, there is no evidence for
78
This theory fails to explain many phenomena of
80
However, it is
76,77
the physiologic rigidity of the retinocortical
relationship or the convergence of the pathways on
which this theory is based.
DISTURBANCES IN THE DEVELOPMENT OF
BINOCULAR VISION
The time at which a lesion or defect in the visual
system occurs is a most important factor with
respect to the effect it will have on the disruption
of existing functions and the prevention of
further development. During the formative
years, a neural pathway or neural function is
maintained only through the stimulus of normal
use. If this is disrupted, the involved structures
will lose their functional capability. Naturally,
if the disruption occurs before a certain function
has become established, it will not be learned at
all. Thus, the age of the child at the onset of a
tropia is extremely significant in assessing the
prognosis and deciding on the management.
The earlier the deviation occurs, the less function
will have developed and the easier will be
disruption of existing functions. If it occurs
between 18 months and 2 years of age, the
prognosis for eventual bifoveal single vision is
poor, whereas if it occurs at a later age, normal
function may be regained with the adequate
treatment. Anomalies in the development of
binocular vision may be in the form of
suppression, amblyopia, abnormal retinal
correspondence.
Diplopia and confusion
Diplopia usually results from an acquired
misalignment of the visual axes that causes an
image to fall on the fovea of one eye and
simultaneously on a nonfoveal point in the other
eye. The object that falls on these noncorresponding points must be outside Panum’s
area to be seen double. In diplopia, the same
object is seen as having two different locations
in subjective space; the foveal image is always
clearer than the nonfoveal image. The
symptomatology of diplopia depends on the age
at onset, duration, and subjective awareness.
Visually immature children (less than about 6
or 7 years) rarely complain of diplopia. The
younger the child, the greater the ability to
suppress.
Binocular Vision
Horror fusion is an intractable diplopia in
87
which there is an absence of central suppression.
The angle of strabismus may be small or
variable. Horror fusionis may occur in a number
of clinical settings: for example, after fusion has
been disrupted for a prolonged period, after
head trauma, and rarely in long-standing squint.
The management of these patients can be
frustrating.
Confusion, like diplopia, is associated with
ocular misalignment; however, confusion is
very rare. Most adult patients with acquired
ocular misalignment see double, i.e. two of the
same image. Rarely, however, patients will
describe the simultaneous perception of two
different images superimposed on each other.
Because the eyes are misaligned, dissimilar
images fall on each corresponding fovea, and
this, in a rare patient, will cause confusion rather
than diplopia. In other words, objects that are
physically separated in objective space are
imaged on corresponding areas of the two
retinas and are, therefore, seen as having the
same location in subjective space.
Suppression
Suppression is an active cortical inhibition of the
vision of one eye. Generally, it is supposed that
the whole of one retinal function is extinguished
in consciousness, but this is not usually so, and
Burian10 believes it is not generally the rule.
Instead of total extinction, he considers that
selective suppression in which only certain
regions of one retina are suppressed take place
more often. An example of this is the occurrence
of suppression scotoma which can be
demonstrated in the foveal area of some children
with convergent strabismus. This scotoma
disappears, however, in this eye when it is made
to take up fixation alone, showing that the
scotoma is purely a functional one and not due
to any organic disease of the retina or visual
pathways. Burian10 believes that suppression
may be selective also with regard to a specific
retinal function; that is, the ability to resolve
contours may be defective momentarily.
The suppression which occurs under
conditions of strabismus, as just outlined, is
88 Theory and Practice of Squint and Orthoptics
facultative, i.e. occurs only under certain
conditions. If the strabismus is not alternating,
but is monocular with one eye remaining the
fixation eye and the other constantly deviating,
the suppression may become so constant and
so deep that it persists. Then when the usually
deviating eye is forced to take up fixation, the
inhibition of its fovea remains, and the vision
in this eye is defective. The suppression under
these conditions is no longer facultative, but
obligatory.81 This obligatory suppression is
called amblyopia.
Amblyopia
Amblyopia (functional amblyopia) by definition
refers to a partial loss of sight in one or both
eyes, in the absence of ophthalmoscopic and/
or other marked objective signs. It results from
psychical suppression of the retinal image. It
may be anisometropic, strabismic or due to
stimulus deprivation-amblyopia exanopsia (e.g.
in a child with congenital cataract, severe ptosis).
Amblyopia follows through a stage of suppression. As just stated, suppression is a process of
active inhibition, and at first it is probably
always facultative, i.e. occurs only when both
eyes whose visual axes are not in alignment are
being used simultaneously. However, the
degree to which facultative suppression can
produce obligatory suppression or amblyopia
probably depends upon the age of the child
when one recalls that an infant is not born with
fully developed eyes, anatomically or
functionally. If suppression is induced in a very
young infant, it probably can become obligatory
in a much shorter period of time than if it starts
in an older child. The amblyopia is therefore
deeper and less easily broken up than when it
begins later in life. This is in accord with clinical
experience. Children whose strabismus begins
early in life generally have more deep-seated
amblyopia, if the strabismus is monocular than
those whose squint begins later. Further, it is
said that, if suppression stops the development
of foveal function before it has matured, normal
vision never can be expected. It will be possible
to restore only the function of the retina to the
level to which it had developed before
suppression set in.
Most of the evidence suggests that the site of
interference in amblyopia is a block in the cortex
and not a retinal activity. Most of the functions
of the macula are intact in the presence of
amblyopia, for example, dark adaptation and
colour vision, but that form vision alone is
affected.
82,83
In patients with amblyopia, the
absolute threshold was found to be normal, both
foveally and peripherally in cones and rods and
in light-adaptation and dark-adaptation. The
entire apparatus of light perception was found
to be normal in these patients. The capacity for
fixating and localizing illuminated points and
areas on the central and peripheral retinae was
also found to be normal. The capacity for
discrimination of pattern fell as low as 2/200 or
2/400, without any loss of sensitivity to light.
This shows that the apparatus for form vision is
to some degree distinct from that involved in
simple light perception.
Therefore, strabismic amblyopia, according to
most authors, probably consists of cortical
inhibition of the higher cortical function of
pattern vision, without notable impairment of
the lower cortical functions of simple light
perception and spatial localization. Not all
authors are satisfied with this concept that
amblyopia is a selective inhibition of the form
sense, as such, while all the other functions of
the retina remain intact. The flicker fusion
threshold of the foveal area of patients with
amblyopia has been found to be considerably
depressed as compared with values obtained
from the surrounding retina and with the values
obtained from the nonamblyopic eye in the same
patients. However, the true nature of amblyopia
is probably not known entirely. In addition to
decreased visual acuity, there is some evidence
which points to concomitant weakening of the
power of central fixation. It is reported that only
20% of the subjects with amblyopia fixate along
the central foveal axis of the poor eye when the
good eye is occluded.84 The shift from the true
foveal axis to some outlying area increases with
the increased depth of the amblyopia.
Abnormal retinal correspondence
Abnormal retinal correspondence (ARC) is an
active cortical adjustment in the directional
Binocular Vision
89
values of the two eyes which occurs in a child
with early onset of squint (especially esotropia,
occurring before 2 years of age). In this
condition, the two foveae no longer have a
common visual direction, and the fovea of one
eye and a peripheral retinal element of the other
eye acquire a common visual direction. This
adaptation is brought about by an inherent
desire for some form of binocular vision and to
avoid diplopia and confusion that would
otherwise take place. ARC is more common in
esotropia than in exotropia. It is less common
in vertical deviations and in true alternating
squints with equal vision.
Types: ARC is of two types:
1. Harmonious ARC is present when the angle of
anomaly (difference between the objective and
subjective angle of the squint) equals the
objective angle of squint.
2. Unharmonious ARC is present when the angle
of anomaly is less than the objective angle of
deviation.
Advantages of ARC include:
• It gives the patient a form of binocular single
vision.
• It tends to stabilize the angle of the deviation.
• The patient has a better visual judgement
because he may have some binocular
appreciation of depth.
Disadvantages of ARC are:
• Once developed, it is extremely difficult to
establish normal correspondence.
• Postoperatively, the angle of deviation may
increase sometimes.
BINOCULAR VISION TESTS
TESTS FOR SIMULTANEOUS MACULAR PERCEPTION
See page 61 and Fig. 4.1A.
in a variety of different ways. Most tests use
simple geometrical shapes as test objects
presented against a random patterned
background. It seems to be unavoidable that
stereo tests produce monocular clues of depth
to some degree; precautions may need to be
taken in the application of the tests to ensure
that these clues are minimised.
• For young children, the Lang test and the Frisby
screening test are designed to produce
behavioural response. With these, the child
attempts to reach out and grab the object.
These tests may provide a result in some
children as young as 6–12 months.
• In older children, a variety of tests are available.
Note. Various tests employed to test stereopsis
must incorporate two essential features:
• The two eyes must be dissociated; that is, each
eye must be presented with a separate field
of view, and
• Each of the two fields or targets must contain
elements imaged on corresponding retinal
areas. The commonly employed tests are as
follows:
Tests employed to check stereopsis can be
grouped as below:
• Synaptophore or stereoscope tests
• Vectograph tests
• Random dot stereogram tests
• Simple motor task tests based on stereopsis
I. Synoptophore or stereoscope tests
See pages 62, 144 and Fig. 4.1C.
II. Vectograph tests
A vectograph consists of polaroid material on
which the two targets are imprinted so that each
target is polarized at 90° with respect to the
other. The vectograph dissociates the eyes
optically. With the use of properly oriented
polaroid spectacles, each target is seen
separately with the two eyes.
TESTS FOR FUSION
See page 62 and Fig. 4.1B.
TESTS FOR STEREOPSIS
Stereopsis tests are available in a variety of
designs and produce a three-dimensional object
Titmus stereo test
The Titmus stereo test utilizes the principle of
vectograph. This is perhaps the most familiar
stereo test.
The three-dimensional polaroid vectograph
which constitutes the Titmus test is basically
90 Theory and Practice of Squint and Orthoptics
made up of two plates in the form of a booklet
(Fig. 4.19). To perform the test, the plates are
reviewed with polaroid glasses. The Titmus
stereo test consists of three parts:
1. The fly test. The right side of the test booklet
contains a large housefly to test gross stereopsis
(threshold 3000 sec of arc). It is especially useful
in young children. The subject is asked to pick
up one of the wings of the fly. If the subject sees
stereoscopically, he will reach above the plate.
In the absence of gross stereopsis, the fly will
appear as an ordinary flat (Fig. 4.20).
2. The animal test. It is performed, if the gross
stereopsis is present. This test consists of three
rows of five animals each; one animal from each
row is imaged disparately (thresholds 10, 200
and 400 sec of arc, respectively) (Fig. 4.19). And,
in each row, one of the animals correspondingly
imaged in two eyes is printed heavily black
(serves as a misleading clue). The subject is
asked which one of the animals stands out. A
subject without stereopsis will name the animal
printed heavily (misleading clue); while in the
presence of stereopsis he will name the
disparately imaged animal.
3. The circles test. It consists of nine squares,
each containing four circles arranged in the form
of a lozenge (Fig. 4.19). Only one of the circles
in each square is disparately imaged at random
with threshold ranging from 800 to 40 sec of arc.
If the subject has passed other two tests, he is
asked to ‘push-down’ the circle that stands out,
beginning with the first set. When he makes
mistakes or finds no circle to push down, the
limit of his stereopsis is presumably reached.
Circle no. 5, equivalent to 100 sec of arc is
considered to be lowest limit of fine central
stereoacuity and is designated as the lowest limit
of good stereoacuity.
Advantages. The Titmus test is simple and
85
easy to perform and so is most widely used.
Disadvantages
1. Some of the circles of the Titmus test are
selected by even stereoblind observers, because
they look ‘different’ and not because they are
seen stereoscopically.
86
2. With the exception of the fine stereoacuity
circles 5 to 9, this test often is unreliable in
differentiating patients with amblyopia and
heterotropia from those with normal vision.
III. Random dot stereogram tests
87
The random dot stereogram tests are devoid of
monocular clues and the patients cannot guess
what the stereo figure is and where it is located
on the test plate. So, this test provides truer
Fig. 4.19 The Titmus stereo test.
Fig. 4.20 Titmus test using fly for gross stereopsis: (A) No stereopsis; (B) Stereopsis present.
measurement of stereopsis than the Titmus
88
test.
1. Random dot E-test (RDT). This test consists
of three cards to be viewed with polaroid
spectacles (Fig. 4.21).89 One card is a bas relief
model of the stereo test figure and is used to
show the patient for what he should look (A).
The second card contains the ‘E’ stereo figure
with a random dot background (B). The third
card is a stereoblank with an identical random
dot background (C).
To perform this test (after showing the bas
relief model), the two test cards are held 50 cm
in front of the patient, who is asked to indicate
which card contains the letter ‘E’. The patient
gives a ‘pass’ or ‘fail’ response. The stereoacuity,
when present, can be quantitated by increasing
the testing distance from the patient.
2. TNO random dot test. The TNO random dot
stereo test is graded to provide retinal disparities
ranging from 15 to 480 sec of arc. It is based on
the same principle as ‘Random dot E-test’, but
has the advantage of eliciting quantitative
responses without changing the testing distance.
It consists of a booklet containing seven plates.
Each test plate consists of a stereogram in which
various shapes (squares, dots, crosses) have been
created by random dots in complementary
colours. The plates contain two types of figures,
the one which can be perceived when viewed
Binocular Vision
Fig. 4.21 Random dot E-test: (A) Bas relief model of stereo
test figure; (B) Random dot stereo figure seen without
polaroid glasses; (C) Random dot stereo figure as seen in
the polaroid glasses—with stereopsis present; (D and E)
Stereo-blank card with random dot background as seen
with (E) and without (D) polaroid glasses.
91
92 Theory and Practice of Squint and Orthoptics
binocularly with red green spectacles by normal
subject having stereopsis. The second set of
figures can be seen with and without the
spectacles even in the absence of stereopsis. The
first three stereograms of the test booklet are
used to establish the presence of gross stereopsis
quickly, while the remaining four plates allow
to quantitate the level of stereopsis. TNO test is
available in two versions one for adults and
another for children (Fig. 4.22).
3. Lang-test. This test consists of random dot
stereogram with panographic presentation.
90,91
The stereoscopic images of a car, star and a cat
(Fig. 4.23) embedded in random dots on the test
card are seen disparately by each eye through
the cylindrical lenses imprinted on the surface
lamination of the test (Fig. 4.24). Therefore,
polaroid glasses or red green spectacles are not
required in this test; so especially useful in
young children who refuse to wear glasses.
Fig. 4.22 TNO test plates I to VII.
Fig. 4.23 Stereoscopic images embedded in random dots
of the Lang test stereogram.
Binocular Vision
A
B
Fig. 4.25 Lang test I (A) and II (B).
93
squares in each plate contains a hidden circle
which is seen disparately. The disparity is
created by displacement of random shapes by
the thickness of the plate. So, this test also does
not require use of glasses. Thus, it is especially
useful for young children who refuse to accept
glasses.
Care must be taken to avoid monocular clues
through parallax movements when using the
test. The test can measure stereoacuity in the
range 600–15 seconds of arc. The Frisby test is
available in a screening version designed for
younger children and infants. It presents a three-
Fig. 4.24 Cylindrical gratings provide separate images for
each eye (From Lang).
89
Lang test is available in two forms:
• Lang I test, which measures stereopsis at 550,
600 and 1200 seconds of arc and (Fig. 4.25A);
• Lang II test, which is finer and measures at 200,
400 600 seconds of arc (Fig. 4.25B).
To perform this test, the test card is held at a
distance of 40 cm in front of the subject, who is
asked to name or point to the shapes on the test
card. The disparity of the car and star is 600 sec
and of the cat 1200 sec of arc.
92
4. Frisby test. In this test, stereogram consists of
three plastic cards each containing four squares
of small random shapes (Fig. 4.26). One of the
Fig. 4.26 The Frisby test.
94 Theory and Practice of Squint and Orthoptics
dimensional object field, together with a flat
image side by side in a preferential looking
format. In this case, a spontaneous pointing or
looking responses can be observed to establish
that stereopsis is present.
5. Stereoscopic contours induced optokinetic
nystagmus test
87-88
and Television random dot
stereo test95 have recently been suggested to test
stereopsis in infants. Such electronically
generated stereopsis tests may become more
relevant in view of the current emphasis on early
diagnosis and treatment of strabismus in infants.
IV. Simple motor task test based on stereopsis
The two-pencil test
It is very simple, primitive but an effective test
for detecting presence or absence of gross
stereopsis (threshold value 3000–5000 sec of
Fig. 4.27 Two-pencil test for stereopsis. For description
see text.
arc).92 Though known to even earlier ophthalmologists, it was popularized by Lang in 1975.
92
To perform this test, examiner holds a pencil
vertically in front of the patient, who is asked to
touch its upper tip with the tip of the pencil held
in his hand by one swift movement from above
(Fig. 4.27A). Patient having stereopsis passes the
test with both eyes open (Fig. 4.27B). Patients
fail the test with one eye closed or when both
eyes are open but stereopsis is absent (Fig. 4.267).
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