Color Vision

 

Color Vision:

            Our ability to see colours is possible because of the functioning of the cones. It is estimated that there are six million cones and that they are found concentrated in the central part of the retina. The colour or hue that we see, depends on the wavelength of the light rays. Each cone transmits an impulse to the brain, thus, producing a high degree of precision. There are two main theories of colour vision; the trichromatic theory and the opponent process theory.

The trichromatic theory:

            The trichromatic theory (also known as the Young-Helmholtz Trichromatic Theory) is a theory of color and how humans perceive color. According to this theory, the human retina contains three different receptors for colour (meaning each one is most sensitive to one color): one is most sensitive to red, one is most sensitive to green, and one is most sensitive to blue. These color receptors combine the colors to produce the perception of virtually any color. You notice that there are no receptors specific to orange, but by stimulating the right cones in the right way, orange color is produced.

The opponent process theory:

The opponent process theory suggests that the way humans perceive colors is controlled by three opposing systems. We need four unique colors to characterize perception of color: blue, yellow, red, and green. According to this theory, there are three opposing channels in our vision. They are:

·         blue versus yellow

·         red versus green

·         black versus white

We perceive a hue based on up to two colors at a time, but we can only detect one of the opposing colors at a time. The opponent process theory proposes that one member of the color pair suppresses the other color. For example, we do see yellowish-greens and reddish-yellows, but we never see reddish-green or yellowish-blue color hues.

Color-Blindness:

An important experience in human visual sensations is the inability of some people to respond to colours. There are some people who are not able to differentiate any type of colour. There are also others who cannot respond to red and green but can see other colours. There are still others who cannot see blue and yellow, though this is rare.

The evolutionary theory of Ladd Franklin:

This theory proposed by Christina Ladd Franklin is evolutionary in its approach. According to this theory, at the earliest stage of development, the human individual does not respond to colour but responds only to form and outline. This is because only the rods are fully developed at this stage. At the next stage the child is able to respond to yellow and blue and at a still later stage to red and green. Thus, the ability to experience different colours is evolutionary in nature. This theory is able to explain different facts of colour vision and also the phenomenon of colour blindness. It has been found that colour blind people who are unable to see yellow are also unable to see blue.

Sensory Adaption:

            A major characteristic of sensory experience is sensory adaptation. It was pointed out that as one continues to look at a red object, the object tends to appear less and less red. This is because there is a decrease in sensitivity of the receptors and consequently there is an increase in the threshold or limen. A very interesting experience in this process of adaptation is the shift from day vision to night vision. For example, when we enter a cinema-hall which is dark inside we are unable to see anything for some time. Similarly, when we enter a brightly lit place, we experience some difficulty in seeing things properly. This is because such experiences involve a shift from rod vision to cone vision or vice versa. This shift naturally takes some time. The phenomenon of sensory adaptation is a process of adaptation from one type of visual reaction to another type.