Field Of Binocular And Monocular Physiology Notes

Field Of Vision Definition

The part of the external world seen by one eye when it is fixed in one direction is called the field of vision or the visual field of that eye. According to Traquir, the visual field is described as an “island of vision surrounded by a sea of blindness”.

Binocular And Monocular Vision

1. Binocular Vision:

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

Read And Learn More: Medical Physiology Notes

2. Monocular Vision:

• It is the vision in which each eye is used separately. In some animals like dogs, rabbits, and horses, the eyeballs are present at the sides of the head.
• So, the visual fields of both eyes overlap to a very small extent. Because of this, a different portion of the external world is seen by each eye.

Divisions Of Visual Field

The visual field of the human eye has an angle of 160° in the horizontal meridian and 135° in the vertical meridian. The visual field is divided into four parts:

• Temporal field
• Nasal field
• Upper field
• Lower field.

1. Temporal And Nasal Fields:

• The visual field of each eye is divided into two unequal parts, namely, the 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 the visual axis with the object.
• The temporal part of the visual field extends up to about 100°, but the nasal part extends only up to 60° because it is restricted by the nose.

2. Upper And Lower Fields:

• The 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. The extent of the upper field is about 60° as it is restricted by the upper eyelid and orbital margin.
• The extent of the lower field is about 75°. It is restricted by cheek. Thus, the visual field is restricted in all sides except in the temporal part.

Corresponding Retinal Points

• Corresponding retinal points are the area in the retina of both eyes on which the light rays from the object fails It occurs in the binocular vision. The two images developed on the retina of both eyes are fused into a single sensation. So, we see the objects with a single image.
• The single sensation is because of the ocular muscles, which direct the axes of the eyes in such v 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.

1. 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. The lesion in 3, 4, and 6 cranial nerves, oculomotor nucleus, red nucleus, and cerebral peduncles also results in diplopia.

Experimental Diplopia:

Diplopia can be produced experimentally by the following methods:

1. Applying pressure from the outer side of one eye and thus displacing the eye from its normal position
2. By holding an object like a pen or pencil vertically in front of the face at about 5 cm from the root of the nose. It is not possible for the convergence of the eyeballs sufficiently, and the light rays from the object do not fall on the corresponding retinal points. And, diplopia occurs.

Blind Spot

• The blind spot is the small area of the retina where visual colors are absent. The optic disk in the retina does not have any visual receptors and, if the image of any object fails on the optic disk, the object cannot be seen, t-o this part of the retina is blind hence the name blind sport.
• 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 the blind spot. However, one can recognize blind spots by some experimental procedures.

Visual Field And Retina

The light rays from different halves of each visual field do not fall on the same halves of the retina. The light rays from the temporal part of the visual field of an eye fall on the nasal half of the retina of that eye. Similarly, the light rays from the nasal part of the visual field fall on the temporal half of the retina on the same side.

Mapping Of Visual Field

The shape and extent of the visual field is mapped out by means of an instrument called Goldmann perimeter and this technique is called perimetry. The visual field is also determined by Bjerrum (Tangent) screen or by the confrontation test. Humphrey field analyzer is also used to map visual fields, and it is more useful to test the central portion of visual fields.