Binocular and Monocular Vision

Part of the external world seen by one eye, when it is fixed in one direction is called field of vision or visual field of that eye. According to Traquair, the visual field is described as ‘island of vision, surrounded by a sea of blindness.

Binocular and Monocular Vision


BINOCULAR AND MONOCULAR VISION

BINOCULAR VISION

Binocular vision is the vision in which both the eyes are used together, so that a portion of external world is seen by the eyes together. In human and some animals, eyeballs are placed in front of the head. So, the visual fields of both the eyes overlap. Because of this, a portion of the external world is seen by both the eyes.

MONOCULAR VISION

Monocular vision is the vision in which each eye is used separately. In some animals like dog, rabbit and

horse, the eyeballs are present at the sides of head. So, the visual fields of both eyes overlap to a very

small extent. Because of this, different portion of the external world is seen by each eye.

DIVISIONS OF VISUAL FIELD

Visual field of the human eye has an angle of 160° in horizontal meridian and 135° in vertical meridian.

Visual filed is divided into four parts:

1. Temporal field

2. Nasal field

3. Upper field

4. Lower field.

TEMPORAL AND NASAL FIELDS

Visual field of each eye is divided into two unequal parts, namely outer or temporal field and the inner

or nasal field, by a vertical line passing through the fixation point. The fixation point is the

meeting point of visual axis with the object. Temporal part of visual field extends up to about

100° but the nasal part extends only up to 60°, because it is restricted by nose.

UPPER AND LOWER FIELDS

Visual field of each eye is also divided into an upper field and a lower field by a horizontal line passing

through the fixation point. Extent of the upper field is about 60°, as it is restricted by upper eyelid and

orbital margin. Extent of lower field is about 75°.

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. restricted by cheek. Thus, the visual field is restricted

in all the sides, except in the temporal part.

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 developed 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

restricted by cheek. Thus, the visual field is restricted in all the sides, except in the temporal part.

 

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