Добавил:
Sekretar
kiopkiopkiop18@yandex.ru
t.me/Prokururor I Вовсе не секретарь, но почту проверяю
Опубликованный материал нарушает ваши авторские права? Сообщите нам.
Вуз:
Предмет:
Файл:Ординатура / Хирургия / Библиотека им академика М.И. Перельмана / Книга_5507_Библиотеки_им_академика_М_И_Перельмана.pdf
76 Theory and Practice of Squint and Orthoptics
Coarse stereopsis is a much less specific process
that can operate on visual images quite
dissimilar in form, luminance and contrast and
separated by several degrees in spatial positionup to as much as 7 to 10 degrees of retinal image
disparity. Since coarse stereopsis acts beyond the
range of retinal rivalry, dissimilar images can
be simultaneously perceived. For coarse fusion
(single vision), however, the images must again
be similar, but the disparity limits over which
fusion can take place (Panum’s area of up to
about 7 degrees) are much larger than they are
for fine stereopsis. Coarse single vision may
occur in the absence of fine fusion, but fine single
vision is always accompanied by coarse fusion.
STEREOPSIS AND FUSION
Fusion is not absolutely required for stereopsis.
Stereopsis is independent of fusion, but it is
necessary that similar parts of stereograms be
fused (stimuli reaching corresponding elements),
since these form the place of reference.
The presence of sensory fusion, that is ability
to unify images falling on corresponding retinal
areas, in itself does not guarantee the presence
of stereopsis. A patient may fuse similar targets
but have no stereopsis. Such patients suppress
selectively the disparately imaged elements of
a stereogram seen by one eye.
A certain degree of stereopsis may occur in
diplopia. Conversely, there may be fusion
without stereopsis. Some people are capable of
motor and sensory fusion but have no
perceptual response to disparate stimuli and,
therefore, have no stereopsis. Peripheral
stereopsis may exist where central stereopsis
does not. Our methods primarily test central
retinal areas. There are persons who show
regional suppression but who have peripheral
stereopsis.
Experiments of Burian10 and others have
shown that a sense of depth still occurs under
certain conditions even though the disparity of
the receptors stimulated is such that they lie
outside of Panum’s area and, therefore, are not
seen single, but double. Figure 4.14 shows the
regions, as determined by Ogle,46 in which there
is single vision without depth (determined as the
horopter), single vision with stereoscopic depth
Fig. 4.14 The regions of obligatory and qualitative
stereoscopic perception of depth about the point of fixation.
From Angle.
46
(determined as Panum’s area), and finally,
outside the Panum’s area a crude sense of depth
and diplopia. As Burian10 points out, it is not
strictly correct to call stereopsis third-degree
fusion. It is more than that. It is a sense sui genesis.
The degree of responsiveness to disparate
stimulation is also related to visual acuity. All
other things being equal, the better the visual
acuity, the better the stereoscopic acuity. Good
visual acuity, however, does not guarantee
stereoscopic acuity. Reduced visual acuity, such
as that with monocular amblyopia, does not
preclude stereopsis. While fusion and acuity are
important factors in stereopsis, there is no linear
relationship between them. It is important to
realize that a shift of visual directions takes place
in stereopsis. In other words, objects viewed by
stereopsis produce a visual impression and a
visual direction unlike the visual direction
initiated by either eye alone.
STEREOSCOPIC ACUITY
Stereopsis is one of the finest spatial discriminations of which the human visual system is
capable. There is minimal disparity beyond
Binocular Vision
77
which no stereoscopic effect is produced. This
limiting disparity characterizes a person’s
stereoscopic acuity.
Stereoacuity of as low as 2 to 7 sec of arc have
been found. But a threshold of 15 to 30 sec
obtained in clinical tests may be regarded as
excellent. There are no standardized clinical
stereoscopic acuity tests comparable to visual
acuity tests.
Under normal conditions, most observers
with no ocular abnormalities can discriminate
a depth difference between two images with a
relative disparity of only about 10 arc sec
(0.0028°). For the closest fixation of 10 inches
away, the best stereoscopic threshold
corresponds to the appreciation of a depth of
one thousandth of an inch.
25
Beyond 600 metres, there is no true stereopsis.
At this distance, monocular clues take over for
the perception of depth. This allows stereoscopic
discrimination of depth in some types of clouds.
These limits provide useful stereopsis over an
extensive range of environmental conditions.
Stereoacuity is excellent at the fovea, but
decreases from the centre to the periphery of
retina. Stereopsis rapidly becomes very poor
beyond about 20° eccentricity, or outside the
circle passing through the two blind spots in the
binocular visual fields.
47
DEPTH PERCEPTION
Depth perception is the perception of distances
of objects from each other or from the observer.
It is somewhat ambiguous term since it is also
used to imply the visual perception of threedimensional space.
Using it in its first connotation, it is
independent of the appreciation of three
dimensions and does not require binocular
vision. A monocular observer is quite capable
of judging distances and of obtaining an
impression of spatial order. Little is known
about the quality and mechanism of absolute
depth perception where a judgement cannot be
based on any additional clues, as, for instance,
when estimating the distance of a point light
source in an absolutely dark room. In a normal
visual environment, several factors contribute
to the perception of depth: stereopsis, parallactic
movements, interposition of objects, size in
which an object is seen, distance of an object
from the horizon, perspective, shadows, and
aerial perspective.
I. STEREOPSIS
As discussed earlier, stereopsis is the relative
localization of visual objects in depth which can
occur only under conditions of binocular vision.
As described in detail (see pages 73–77),
stereopsis is based on a physiologic process
derived from the organization of sensory visual
system. Stereopsis is apparently innate, not
acquired through experience and is unequivocal
and inescapable.
II. NON-STEREOSCOPIC CUES TO THE PERCEPTION
OF DEPTH UNDER BINOCULAR CONDITIONS
Under binocular conditions, retinal disparity
gives a clue to depth without stereopsis. When
there is temporal retinal disparity, an awareness
of nearness occurs. When there is nasal retinal
disparity, an awareness of distance occurs.
III. MONOCULAR CUES (NON-STEREOSCOPIC CUES
TO SPATIAL ORIENTATION)
Stereopsis is restricted to relatively short visual
distances and is not the only means we have for
spatial orientation. There is a set of monocular
or experimental clues that play an important role
in our estimation of the relative distance of
visual objects. They are the result of experience
and are equivocal. Monocular cues include the
following:
1. Parallactic movements
Next to stereopsis, parallactic movements are
most important in the perception of depth. They
are the apparent movements made by objects
when the observer moves his head. A slight shift
of the head in any direction while fixation is
maintained on the same point results in a change
of the relative position of all objects in the
direction of gaze. Objects beyond the fixation
point appear to move in the same direction as
the observer’s head, whereas objects closer than
the fixation point appear to move in the opposite
direction. Greater the displacement of an object
78 Theory and Practice of Squint and Orthoptics
relative to the fixation point the more distant
the object is from the fixation point.
2. Linear perspective
Object points having a constant size appear to
subtend smaller and smaller angles as they
recede from the subject, e.g. railroad tracks
which are in fact parallel seem to approach each
other in the distance. This is an example of the
perspective in which the three-dimensional
object space imaged geometrically in two
dimensions on the retina creates an impression
of depth. The geometric reproduction in two
dimensions is not only of theoretical interest
with respect to the translation of such twodimensional retinal stimuli into a threedimensional perception, but is also of great
importance in creating the feeling of depth in
drawings and paintings. Its most prominent
feature is the fact that lines which are parallel
in object space and at an angle with the picture
plane seem to be directed to one point, the
vanishing point (Fig. 4.15).
3. Overlay of contours
By superimposing or interposing the contour of
configuration, one can find distance clues.
Interposition of objects gives an absolute clue
as to their relative position, since the outline of
one object interrupts or hides the contour of
another object behind it (Fig. 4.16).
4. Size
The size in which an object is seen is another
important factor in the judgement of distances,
Fig. 4.15 Drawing demonstrating the affect of linear
perspective (vanishing point).
Fig. 4.16 Effect of overlay of contours; (A and D) depth is
achieved by interposing one object in front of the other;
(B, C and E) the same forms are drawn to avoid a clue of
depth.
provided the actual size is known to the
observer. Objects encountered in daily life, such
as people and cars, allow a definite judgement
of their distance. If the actual size of an object is
unknown, however, and a comparison with a
known object is impossible, marked errors occur.
This may be the case, for example, when looking
at a bird against the sky. If its real size is
unknown to the observer, then distance
judgement is merely a guess. The difficulty is
particularly obvious at night if an attempt is
made to estimate the distance of a light source
of unknown size.
5. Distance from horizon
The distance of an object from the horizon or its
relative height in the visual field gives an
important clue to distance. Since the eyes of an
Fig. 4.17 Drawing showing effect of distance from horizon
in depth perception. Objects that are close to the horizon
are perceived as being farther from the observer.
observer are at a certain height above the
ground, it appears as if the ground is rising
toward the horizon. Thus, objects more distant
from the observer will be higher in the visual
field (Fig. 4.17).
6. Distribution of highlights, shadows, shades
and light
Highlights and shadows provide a very
important monocular clue for perception of
depth. Since, sunlight comes from above, we
have learned that the position of shadow is
helpful in judging the raised and depressed area,
that is the relative depth in objects. When there
Binocular Vision
79
is a shadow on the lower portion of an object,
the object appears to stick out in space. When
the shadow is above, the object appears to curve
in.
A classic example of this is the picture of a pit
in a wall with sunlight falling from above at a
small angle against the wall. The upper portion
of the pit will be filled with a dark shadow while
the lower part is brightly illuminated. Turning
the picture upside down, one no longer sees a
pit but instead a distinct hump, the upper
portion of which stands out brightly while its
lower part casts a shadow (Fig. 4.18).
7. Aerial perspective
The influence of the atmosphere on contrast
conditions and colours of more distant objects
is referred to as aerial perspective. Objects
appear less clear and acquire a more bluish tinge
as the viewing distance increases. The colour
change towards blue is particularly obvious, if
one views the shadows of a mountain chain
from some distance.
IV. INFLUENCE OF ACCOMMODATION AND
CONVERGENCE ON DEPTH PERCEPTION
A given object at a given distance is imagined
in a certain size on the retina. It would appear
natural that the size of the retinal image
determines the size in which the object is seen
Fig. 4.18 Drawing showing effect of highlight and shadows on depth perception: (A) Upright picture; (B) Same picture
turned upside down.
80 Theory and Practice of Squint and Orthoptics
subjectively. This, however, is not the case. There
is no doubt that the size in which an object is
perceived can be changed mentally. The
apparent increase in the size of an object that is
brought closer to the eye does not correspond
to the increase that might be expected on the
basis of the enlargement of its retinal image. This
effect has been attributed to the influence of
accommodation and/or convergence, and in
turn it was postulated that these functions
contribute a clue for depth perception. There is
no proof of this. However, experimental
evidence points in the other direction that after
a judgement of distance has been made, this then
influences the apparent size of the object.
CONCLUSION
The impression of three-dimensionality
imparted by these clues is a judgement, an
interpretation. So false judgements are possible.
Also these depend on past experience.
Monocular clues and binocular clues work
hand in hand, one enhancing the effect of the
other, but this is not always true. If one
introduces into a stereogram confusing clues, i.e.
monocular clues that conflict with stereoscopic
clues, false observations can be made.
Some people are more responsive to
stereoscopic clues, whereas others respond more
readily to monocular clues.
Thus humans have two sets of clues for their
orientation in space (depth perception).
Monocular clues depend upon past experience
and clues provided by fusion by disparate
retinal images afford the direct perception of this
relation on the basis of intrinsic physiologic
arrangements.
fixation whether the subject or the object moves.
It allows the field of vision to become the field
of fixation.
Sensory system is the feedback loop, which
controls this motor alignment.
Processes of normal retinal correspondence
allow the fusion of two physical retinal images
into a single mental impression. When this
fusion occurs, a new quality of the subjective
visual process takes place and the perception of
depth by parallax (stereopsis) results. The
correspondence mechanism only allows for the
relative localization of one object to another in
physical space, and stereopsis represents the
latitude that retinal correspondence allows and
in which diplopia does not occur. The image
perceived in depth is seen neither in the specific
visual direction dictated by the right eye nor by
the left eye but a visual direction corresponding
to non-stimulated elements of the two eyes.
Mental process of absolute localization is
essentially a compromise that not only requires
decoding and analysis of retinal information,
supplied by the correspondence mechanism, but
that is also modified by other information from
the efferent and afferent motor monitoring
systems. This is best demonstrated by the patient
with paralytic strabismus with normal retinal
correspondence. The integration of the motor
and sensory mechanisms in binocular vision
represents a complex process, as will be seen in
pathologic conditions affecting either one or
both processes. The child with strabismus may
show sensory adaptation to a motor anomaly
or, may adapt to a sensory anomaly by changing
motor response.
INTEGRATION OF THE MOTOR AND
SENSORY SYSTEM INTO BINOCULAR
VISION
While it is convenient from a teaching point of
view to separate the motor and sensory
mechanisms of binocular vision, they are
absolutely integrated in the visual process that
we call binocular vision.
Motor system aligns the foveae of the two eyes
on the object of regard and maintains this
DEVELOPMENT OF BINOCULAR VISION
After having a workable knowledge about the
psychophysical and sensory aspects of the
binocular vision, it will be easier to understand
development of this complex phenomenon. It
is unequivocal that basic visual functions are
innate and, therefore, present at birth. But their
coordination, maturation and refinement take
place during early postnatal period. Therefore,
any obstacle during this period may cause
Binocular Vision
81
abnormalities in the development of normal
binocular vision. The subject matter on
development of binocular vision includes:
• Prerequisites for development of binocular
vision
• Eye at birth and normal postnatal development
• Maturation of binocular functions
• Neurophysiology of development
• Theories of development
PREREQUISITES FOR DEVELOPMENT OF
BINOCULAR VISION
In order that simultaneous binocular vision be
obtained, following conditions must be
present:
53
1. Fixation. There must be proper fixation with
each eye. The muscles controlling the
movements of each eye must function normally
and turn both eyes in such a manner that the
object of regard is fixated by corresponding
retinae, i.e. the two foveae.
2. Visual fields of the two eyes must overlap to
a large extent.
3. Image formed on each retina must be
approximately similar, i.e. should be of the same
size, shape, colour and intensity. This necessitates
approximate equality in the optical apparatus
of each eye.
4. Common visual direction must be there of two
eyes, i.e. the retinae must possess physiologically corresponding points.
5. Reflex activities which produce fusional
movements are very important. The eyes must
be co-ordinated by this complex mechanism at
all times so that retinal receptors which have a
common visual direction will receive the same
image at all times.
EYE AT BIRTH AND NORMAL
POSTNATAL DEVELOPMENT
The anatomic development of the orbits and
eyes is not completed at birth.
Orbits continue to change their size and shape
throughout childhood into the teenage years,
and the angle which their axes form with one
another decreases from 50° at birth to 45° in later
life. Thus, in adults, the medial wall of the two
orbits are parallel to each other while the lateral
wall diverge at 90°. At the same time, the
interpupillary distance increases from an
average of 45 mm at birth to about 58 to 66 mm
in adulthood. Accordingly, the convergence
required to fixate an object at 33 cm distance
increases from about 13 PD to about 19 PD.
Angle lambda () which is positive and may be
as large as 10°, reduces to at least one-half of
that amount during postnatal development.
Eyeballs also continue to grow. Growth of the
globe is indicated by:
• Change in length from 17.5 mm at birth to
24 mm in adulthood.
• Meridional fibres of the ciliary muscle are
completely formed at birth, but the circular
portion continues to develop during the first
one or two years of life.
• Gross appearance of foveal cones changes during
the first a few months of life. Their fine
structure, as revealed by electron microscopy,
seems to be well developed.
• Extraocular muscles, although not of ultimate
size, are fully functioning at birth.
• Postural reflexes, which are innate and
unconditioned, are also functioning fully.
Bifoveal fixation does not exist at birth since
binocular movements are not coordinated. The
two eyes move more or less independent of each
other. Fixation, therefore, is at first only
monocular. The principal visual direction of the
central fovea and the visual directions of the
other retinal points relative to the principal
direction are innate. Therefore, monocular
fixation is present at birth, but it is poor.
Furthermore, conscious fixation requires
awareness of the presence of an object and then
enough interest in the object to occupy an
individual’s attention for a certain length of
time. The degree of mental activity necessary
for this exceeds the capability of the newborn
infant, and this is another reason why fixation
is only rudimentary in the beginning.
At about 2 to 3 weeks of age, the infant begins
to make movements of regard, turning his eyes
to fixate an object. At 4 to 5 weeks of age, he can
sustain monocular fixation of large near objects.
Only after the age of 3 months the fixation
become conscious rather than reflexive.
82 Theory and Practice of Squint and Orthoptics
Although the two eyes move independently at
birth, aimless conjugate movements may occur.
At about 6 weeks of age, fixation alternates
rapidly between the two eyes, and a short time
later the child begins to fixate binocularly and
to perform conjugate pursuit movements,
following persons or large near objects. Initially,
the 3e pursuit movements are saccadic in
character but become smooth and gliding
between 3 and 5 months of age.
Disjugate vergence movements develop after
conjugate movements. Convergence is demonstrable after 3 months of age and is stable at about
6 months.
Fusional movements occur at the same time, so
that following an interruption of fusion, such
as by introducing a prism before one eye, a
corrective movement occurs to reestablish
bifoveal fixation. Fusional movements are firmly
established by the age of 1 year.
Visual acuity and fixation has always been
considered to be very poor at birth. However,
psychophysical and electrophysiologic research
during recent years has established that visual
acuity in infants develops much more rapidly
than once thought, and that an infant’s visual
capacities are surprisingly rather well established
shortly after birth, and that adult levels are
reached at approximately 2 to 3 years of age.
This, of course, does not take into account the
state of development of the interpretative
mental functions but refers strictly to the
resolving power of the visual system.
Visual development and estimated visual acuity
At birth, the fovea as well as lateral geniculate
nucleus (LGN) is not developed, so an infant
has peripheral vision only and can see to the
sides but cannot focus the eyes straight ahead
or smile back at the parents because of blurred
central vision.
54,55
Visual acuity at birth is about
6/240 (Table 4.2).
At about 6 weeks, the child fixates peripherally which alternates rapidly. Vision is
about 6/60 (Table 4.2).
By 2 months of age, LGN and its connections
to the visual cortex in occipital lobe develop and
the infant begins to follow moving objects.
55
By 3 months of age, the fovea is fully
developed. The fixation becomes central
conscious rather than reflexive.56 The child starts
to focus on the parent’s face, toys and responds.
The infant responds better to high contrast
images, bold colors and bright objects.
Therefore, the parents should be advised to
paint the baby’s room in bright colours, decorate
with contrasting shapes, handing brightly
coloured toys over the crib, adding new
furnishing to the room to retain interest,
changing the direction of the crib frequently so
that baby can see new furnishings. Even at night
dim light should be kept on to help stimulating
the baby’s vision.
At 6 months of age, vision improves to 6/12
from 6/240 at birth (Table 4.2) as tested by
visual evoked potential (VEP). However, on
testing with optokinetic nystagmus (OKN)
and preferential looking (PL) the visual acuity
is reported about 6/30.
57,58
By the age of 6
months the hand–eye coordination also
develops and the infant can now locate toys
and tries to grasp them. So 6 months is the
time when a detailed ophthalmological
examination should be conducted for all
infants to rule out gross refractive errors
amblyopia, strabismus and a definitive
treatment is started.
By 1 year of age, most children become
emmetropic. At birth, all of the infants are born
hypermetropic due to smaller axial length of
eyeball.59 Stimulation of retina by light results
in growth of eyeball and resultant emmetropia
by the age of 1 year.60 The optic nerve is fully
myelinated, the visual pathway is matured and
the vision is about 6/6 to 6/9 by the age of
1 year.
Age by which 6/6 vision is achieved varies
depending upon the test used as below:
61
• Optokinetic nystagmus (OKN): 24–36 months
• Preferential looking test (PLT): 24–36 months
• Visual evoked potential (VEP): 6–12 months
MATURATION OF BINOCULAR FUNCTIONS
Development of fusion
Although basic visual functions are innate and,
therefore, present at birth, other complex
Binocular Vision
Table 4.2 Visual development and estimated visual acuity from birth to 6 years of age
Age Refractive Normal visual Estimated visual acuity
status development Optokinetic- Preferential Visual evoked
nystagmus (OKN) looking potential
(Keelar PL
cards)
At birth Hypermetropia Pupillary light reflex —————— 6/300 6/240
present, fovea and
lateral geniculate body
not developed, no
central fixation
1 month Hypermetropia 6/120 6/200–6/90 6/120
2 months Hypermetropia Follow moving objects 6/60 6/90–6/60 6/60
3 months Hypermetropia Fovea developed, 6/36 6/90–6/60 6/36
central fixation developed
6 months Hypermetropia Stereopsis developed 6/30 6/36–6/30 6/6–6/12
up to 600 sec of arc
(Frisby)
1 year Emmetropia Optic nerve myelination, 6/18 6/24 6/6–6/9
complete visual differen-
tiation of objects
developed, stereopsis
up to 210–170 sec of
arc (Frisby)
18 months Emmetropia Visual acuty at adult 6/12 6/18 6/6
level on paediatric acuity
card, stereopsis
developed up to 170–150
sec of arc (Frisby)
24 months Emmetropia Stereopsis developed 6/9 6/12 6/6
up to 100–85 sec of arc
(Frisby)
36 months Emmetropia Contrast sensitivity 6/6 6/6 6/6
fully developed,
stereopsis developed
up to 85–55 sec of
arc (Frisby)
Up to Emmetropia Stereopsis developed 6/6 6/6 6/6
5 years by 30–20 sec of
arc (Frisby)
Up to Emmetropia Stereopsis developed 6/6 6/6 6/6
6 years up to 10–5 sec of
arc (Frisby)
83
functions of binocular cooperation have to be
learned before normal bifoveal single vision
eventually is firmly established.
At birth, the eyes are not associated with each
other, but act as two independent sense organs.
The mechanisms necessary for binocular single
84 Theory and Practice of Squint and Orthoptics
vision are not completely developed. The
foveas are not formed until the third month of
life. As they develop, the stimulus to associate
these areas is provided. By trial and error, the
child learns that, when the image of an object
is brought onto the two foveas simultaneously,
the image is most detailed. For this reason
alone, the visual axes are oriented in such a way
that each fovea is directed at the object of
regard.
Once this has become an established habit, the
relative space perceptions of the child begin to
take form. Objects to the right of fixation send
images to retinal areas in the two eyes which
have a common visual direction, i.e. to the right
of fixation. The crossfiring of various sensory
phenomena, such as touch with vision,
eventually leads to an accurate determination
of the child’s space. An object seen so many
degrees to the right of fixation is eventually
interpreted in its correct position, and this is
rewarded by checking accurately with the
experiences of touch. Hence, by trial and error,
and with tactile sensations the eyes become
accurately associated with one another until
gradually a normal child develops perception
of space.
If the eyes are never allowed to become
associated, as for example by a failure of
development of one fovea or because of
paralysis of an ocular muscle so that the two
foveas cannot always be focussed together on
the object of regard, the child never acquires
binocular single vision and never learns to fuse
the two images into one. Vision under these
circumstances is always monocular and
generally alternating; first one eye is used and
then the other. The image of the eye which is
not fixating the object is suppressed.
Development of stereopsis
Anatomic and physiologic factors necessary
for depth perception in the human being are
either present at birth or develop shortly after
birth so that, if the eyes are normal and the
neuromuscular mechanism for moving the eyes
is normal, depth perception will follow
automatically.
Stereopsis seems to be in a class by itself
because it is claimed by some that there are
persons with entirely normal eyes and
neuromuscular apparatus who do not have
depth perception by parallax when all other
clues to depth are eliminated. As Ogle46 has
stated, "Stereopsis is a sensory phenomenon in
its own right, with its own physiological
mechanisms. It seems to be an all-or-none
phenomenon, in that in a given person it is either
present or not present. Training does not seem
to develop stereopsis as such, but training may
increase one’s ability to discriminate depth
differences just as the visual acuity may be
slightly improved by training".
Age norms for stereoacuity development and
the tests used are depicted in Table 4.3.
NEUROPHYSIOLOGY OF DEVELOPMENT
The M and P cell neurophysiology. Neurophysiological animal studies have identified two
specific pathways used to process visual
information. These two pathways arise from
different populations of retinal ganglion cells.
Ganglion cell stimulation from a retinal image
results in simultaneous parallel processing
through these two different pathways. In the
lateral geniculate nucleus, the nuclei can be
divided into parvocelluar (P cells, or small cells)
and magnocellular (M cells, or large cells). In
the striate cortex, parvo- and magnorecipient
Table 4.3 Age norms for stereoacuity and the tests used
(measurements are all approximate)
Age Stereo (secs of arc)
Birth ——
One month ——
Three months ——
Six months 600 Frisby
Nine months 300 Frisby
One year 210 – 170 Frisby
18 months 170 – 150 Frisby
Two years 100 – 85 Frisby
Three years 85–55 Frisby
Four years 40–30 Frisby
Five years 30–20 Frisby
Six years 10–5 Frisby
(Adapted from A. Grounds by C. Rushen and L. Speedwell)
Binocular Vision
85
lamellae are segregated; however, there are
interconnecting pathways, so information
commingles. Parvocellular neurons are more
sensitive to colour, high spatial frequencies, fine
two-point discrimination, and fine stereopsis,
and they project to areas of the central visual
field and fovea. Magnocellular neurons, on the
other hand, are sensitive to direction, motion,
speed, flicker, gross binocular disparities, and
gross stereopsis. Magnocellular neurons project
to parafoveal and more peripheral retina.
Magnocellular neurons are used for determining
where, whereas parvocellular neurons examine
static objects and determine what. Even though
the two pathways are distinct, they overlap; and
both systems interact to process visual
information. From the striate cortex, information
from M cells goes predominantly to parietooccipital areas, while information from P cells
goes to temporo-occipital areas.
Basic receptive field organisation of neurons
and cortical architecture are present since birth,
although the retina and optic pathway are not
completely developed.
Properties of neurons in the visual cortex are
62
markedly influenced by visual experience during
the first a few postnatal months. Neuro-
physiologic studies have demonstrated there
specifically, these neural properties have been
determined to involve binocularity, orientation
specificity and disparity specificity.
Excitatory connections of receptive fields
63,64
located in both retinae or retinotopic projections
are largely present at birth. However, simultaneous occurrence of patterned visual input to
both eyes during the development period is
necessary to maintain their association.
Plasticity during maturation stage (i.e. in the
63,64
neonatal visual system) is present to a great
65, 66
deal.
This early plasticity seems to be vital
for the formation of cells with closely matched
receptive field properties in the two eyes, which
is a necessary condition to form the substrate
for stereoscopic vision. Patterned visual stimuli
during stage of plasticity of visual system seem
to act not only as a catalyst but also as directional
stimuli in the consolidation, maintenance and
refinement of the neuronal connections in the
visual cortex. A possible function of this
plasticity in neuronal properties during early
visual experience would be to allow the
opportunity to match the properties of
maximizing its capacity for analysis of the more
important components of its environment. Also,
the capacity to make modification in optimal
disparity and preferred orientation would
ensure that binocular cortical cells adapt similar
receptive field positions and preferred
orientations on the two retinae, which is a
primary requirement for the probable role of
these cells in binocular fusion and stereopsis.
Thus, any disruption of the normal developmental conditions, such as a congenital or early
strabismus or significantly subnormal vision in
one eye (e.f. due to congenital cataract), will
prevent the development of normal functional
interrelationship and the loss of binocularity.
Maldevelopment of M versus P pathways
secondary to strabismus or anisometropia and
a blurred retinal image is currrently being
studied. Pattern deprivation amblyopia, which
is a failure to develop fine two-point discrimination, is probably associated predominantly with
abnormal P cell development. M cell development
is also affected, especially if the retinal image
disparity is quite large. M-neuron maldevelopment occurs predominantly in case of
strabismus and may contribute to associated
motor abnormalities such as latent nystagmus
and asymmetrical horizontal smooth pursuit
often seen in patients with congenital or infantile
strabismus.
Postnatal morphological changes in the retina
and retinal projections relative to the presence
or absence of visual stimuli are well-established.
It has been observed that, in the lateral
geniculate nucleus, neuronal cell growth is
markedly reduced in the laminae with
connection to visually deprived eye as compared
with a nondeprived eye.
67, 68
The two types of
retinal ganglion cells and geniculate cells seem
to be differentially affected by visual deprivation. The large cells of the binocular segment are
much more affected than either the large cells
of the monocular segment or the small cells