Light Adaptation

LIGHT ADAPTATION

Rod adaptation

Second phase of the curve is slow. During this phase, there is a gradual decrease in the threshold and it is completed in 20 to 30 minutes.

Definition

Light adaptation is the process in which eyes get adapted to increased illumination. When a person enters a bright-lighted area from a dim-lighted area, he feels discomfort due to the dazzling effect of bright light. After sees the objects around him without any discomfort. It is the mere disappearance of dark adaptation. Maximum period for light adaptation is about 5 minutes.

LIGHT ADAPTATION

Causes of Light Adaptation

There are two causes of light adaptation:

1. Reduced sensitivity of rods

During light adaptation, the sensitivity of rods decreases. It is due to the breakdown of rhodopsin.

2. Constriction of pupil

Constriction of pupil reduces the quantity of light rays entering the eye.

NIGHT BLINDNESS

Definition

Night blindness is defined as the loss of vision when light in the environment becomes dim. It is otherwise called nyctalopia or defective dim light (scotopic) vision.

Causes of Night Blindness

Night blindness is due to the deficiency of vitamin A, which is essential for the function of rods.

Deficiency of vitamin A occurs because of following causes:

1. Diet containing less amount of vitamin A

2. Decreased absorption of vitamin A from intestine.

Vitamin A deficiency causes defective cone function. Prolonged deficiency leads to anatomical

changes in rods and cones and finally the degeneration of other retinal layers occurs. So, retinal function can be restored, only if treatment is given with vitamin A before the visual receptors start degenerating.

ELECTRICAL BASIS OF VISUAL PROCESS – ELECTRORETINOGRAM

DEFINITION

Electroretinogram (ERG) is the record of electrical activity in retina. When light rays stimulate the retina,

a characteristic sequence of potential changes occurs, which can be recorded in the form of ERG. This

diagnostic procedure is useful in determining retinal disorders such as cone dystrophy (degeneration of

cones) and retinitis pigmentosa (hyperactivity of the pigmented retinal epithelial cells, leading to damage of photoreceptors and blindness).

METHOD OF RECORDING ERG

Electroretinogram is recorded by using a galvanometer or a suitable recording device. Recording electrode is placed on the cornea of eye in its usual forward up looking position. Indifferent electrode is placed over any moist surface of body, like inside the mouth.

WAVES OF ELECTRORETINOGRAM

Electroretinogram has 4 waves namely ‘A’, ‘B’, ‘C’ and ‘D’ . ‘A’ is the only negative wave and other

three are positive waves. ‘A’, ‘B’ and ‘C’ waves occur when light stimulus falls on retina. ‘D’ wave occurs when light stimulus is stopped. ‘A’ and ‘B’ waves arise from rods and cones. ‘C’ wave arises from pigment epithelial layer and ‘D’ wave arises from inner nuclear layer.

 

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