Visual Perception

Visual perception is the ability to perceive our surroundings when light falls on the retina. These are 4types:

1) Light Sense

2) Form Sense

3) Sense of Contrast

4) Colour Sense

Light Sense: It is the ability to perceive variations in the degree of light. This is assessed in 2ways:

A) Light Minimum

B) Light Difference or Differential Light Threshold

A) Light Minimum: Light minimum is the minimum amount of light energy which, when projected on the retina, causes a visual sensation. 

It varies greatly with the degree of adaptation of the eye at the time of observation. When the eye is dark-adapted(adaptation in dim light), the light minimum is very small as compared to when it is observed in a light-adapted(adaptation in bright light) eye.

• Dark adaptation is delayed by prolonged exposure to bright light.
• Delayed dark-adaptation occurs in Retinitis Pigmentosa(degenerative disease of Retina) & in case of vitamin-A deficiency 

B) Differential Light Threshold: The smallest difference in light intensity that can be appreciated.

Form Sense: The ability to discriminate between the shapes of the objects. This can be assessed by Visual Acuity(resolving power of the eye)

Sense of Contrast: It is the ability to perceive slight changes in luminance between regions which are not separated by definite borders. 

Contrast Sensitivity is reduced in many ocular diseases such as Glaucoma, Macular disease, Cataract, Refractive errors etc. 

In case of poor contrast sensitivity, the person may face the following problems:

• Difficult to see facial expression
• Difficulty in navigating steps of the staircase.
• Problem in night driving

Colour Sense: It is the ability to distinguish among different colours. The appreciation of colours occur only in photopic vision(bright light vision).