Color vision

Practical medical physiology colour vision
85
13- Color vision
Introduction:
There are two types of photoreceptors in the human retina, rods and cones. Rods
are more sensitive to light than the cones. However, they are not sensitive to color.
Consequently, rods are responsible for vision at low light levels. Cones on the other
hand mediate color vision. There are three types of cones in the eye; red, green &
blue. Human eye can detect almost all gradations of colors when only red, green,
and blue monochromatic lights are appropriately mixed in different combinations.
Normal color vision is called trichromacy.
The color blindness is an inherited sex-linked recessive condition which occurs in 8%
of male Caucasian population and 0.5 % of female. These abnormalities are due to
abnormal gene on the X chromosome. Cones either completely absent or defective
and the severity of the condition ranges from mild to severe. The condition affects
certain daily activities like workplace, driving or preparing food.
Other causes of color blindness:
Diseases like diabetes mellitus, multiple sclerosis, ageing or medications.
Types of colour blindness by clinical appearance:
I- Total color blindness (rare).
II- Partial color blindness:
1. Red-Green color blindness is the most common one. Patient mainly confuses red
with green and blue with purple.
2. Blue-Yellow color blindness. Patient confuses blue with green & yellow with
violet.
Treatment of color blindness:
-No treatment but filters or contact lenses can be used to enhance brightness of
colors at work.
-Research is underway for genetic therapy.Practical medical physiology colour vision
85
Objective:
To differentiate between subjects with normal and abnormal color vision.
Materials:
1- Subject.
2- Ishihara plates (Fig. 1).
Figure 13-1 Ishihara plate.
Procedure:
The subject is asked to read the figures in Ishihara plate.