CORRESPONDING RETINAL POINTS
Corresponding retinal points are the area in retina of both eyes, on which the light rays from the object falls. It occurs in the binocular vision. The two images on retina of both eyes are fused into a single sensation. So, we see the objects with single image. The single sensation is because of the ocular muscles, which direct the axes of the eyes in such a way that the light rays from the object fall upon the corresponding points of both retinas. If the light rays do not fall on the corresponding retinal points, diplopia occurs.
DIPLOPIA
Diplopia means double vision. While looking at an object, if the eyeballs are directed in such a way that the light rays from the object do not fall upon the corresponding point on the retina of both eyes, a double vision occurs, i.e. one single object is seen as two.
Causes of Diplopia
1.
Permanent diplopia occurs during paralysis or weakness of ocular muscles. It
occurs in myasthenia
gravis
also.
2. In
alcoholic intoxication, the imbalanced actions of ocular muscles produce
temporary diplopia
3. Lesions
in III, IV and VI cranial nerves, oculomotor nucleus, red nucleus and cerebral
peduncles also
results in
diplopia.
Experimental Diplopia
Diplopia
can be produced experimentally, by the follow ing methods:
1.
Applying pressure from outer side of one eye and thus displacing the eye from
its normal position
2. By
holding an object like pen or pencil vertically in front of face, at about 5 cm
from the root of
nose. It
is not possible for the convergence of the eyeballs sufficiently. The light
rays from the object
do not
fall on the corresponding retinal points and diplopia occurs.
BLIND SPOT
Blind spot
is the small area of retina where visual receptors are absent. The optic disk
in the retina
does not
have any visual receptors and if the image of any object falls on the optic
disk, the object cannot
be seen.
So this part of the retina is blind hence the name blind spot. Normally, the
darkness in the visual field due to the blind spot does not cause any inconvenience
because, the fixation of each eye is at different angles. Even when one eye is
closed or blind, the person is not aware of blind spot. However, one can
recognize blind spot by some experimental procedures.
VISUAL FIELD AND RETINA
Light rays
from different halves of each visual field do not fall on the same halves of
the retina. Light rays
from
temporal part of visual field of an eye fall on the nasal half of retina of
that eye. Similarly, the light rays from nasal part of visual field fall on the
temporal half of retina of the same side.
MAPPING OF VISUAL FIELD
The shape
and extent of visual field is mapped out by means of an instrument called Goldmann
perimeter and this technique is called perimetry.
Visual field is also determined by Bjerrum
(Tangent) screen or by confrontation test. Humphrey field analyzer is also used to map visual field and it is more useful to test the
central portion of visual fields.
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