Retinal points

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.