continue vision physiology (2)

VISUAL PERCEPTION

It is a complex integration of light sense, form sense, sense of contrast and colour sense. The receptive field organization of the retina and cortex are used to encode this information about a visual image.

1. The light sense
It is awareness of the light. The minimum brightness required to evoke a sensation of light is called the light minimum. It should be measured when the eye is dark adapted for at least 20-30 minutes. The human eye in its ordinary use throughout the day is capable of functioning normally over an exceedingly wide range of illumination by a
highly complex phenomenon termed as the visual adaptation. The process of visual adaptation
primarily involves :

• Dark adaptation (adjustment in dim illumination),and
• Light adaptation (adjustment to bright illumination). 
  
  

Dark adaptation
It is the ability of the eye to adapt itself to decreasing illumination. When one goes from bright sunshine into a dimly-lit room, one cannot perceive the objects in the room until some time has elapsed. During this period, eye is adapting to low illumination. The time taken to see in dim illumination is called ‘dark adaptation time’.

The rods are much more sensitive to low illumination than the cones. Therefore, rods are used more in dim light (scotopic vision) and cones in bright light (photopic vision).
 
Dark adaptation curve  plotted with  illumination of test object in vertical axis and duration
of dark adaptation along the horizontal axis shows that visual threshold falls progressively in the darkened room for about half an hour until a relative constant value is reached. Further, the dark adaptation curve consists of two parts: the initial small curve represents the adaptation of cones and the remainder of the curve represents the adaptation of rods.
When fully dark adapted, the retina is about one lakh times more sensitive to light than when bleached.
Delayed dark adaptation occurs in diseases of rods e.g., retinitis pigmentosa and vitamin A deficiency.

Light adaptation
When one passes suddenly from a dim to a brightly lighted environment, the light seems intensely and even uncomfortably bright until the eyes adapt to the increased illumination and the visual threshold rises. The process by means of which retina adapts itself to bright light is called light adaptation. Unlike dark adaptation, the process of light adaptation is very quick and occurs over a period of 5 minutes. Strictly speaking, light adaptation is merely the
disappearance of dark adaptation.

The rest of visual PERCEPTION visit these links plz

  2. The form sense

  3. Sense of contrast

  4. Colour sense