The special sense we call vision involves several factors, including perception of light, form, distance, depth, and color. In this regard, the internal structure of the eye, with emphasis on the three distinct layers that are observable in the eyeball’s wall, is mentioned here.
The outer layer, called sclera, is the dense, fibrous opaque white tissue that encloses the eyeball (excluding the portion covered by the cornea). The intermediate layer, called the choroid, is the vascular layer containing pigment and some of the vessels supplying the eye with blood. And the inner layer, known as the retina, is the tissue containing the specialized light-sensitive receptors called rods and cones. These rods and cones are linked by intermediary neurons and synapses with the optic nerve’s fibers.
It has been observed that there is no color discrimination on the perimeter of the retina where there are no cones. This gives credence to the idea that color perception depends upon the function of the cones. It has also been recognized that color discrimination is likewise not possible in dim light, when the cones play but a small role in vision. The cones are known, too, to be sensitive to white light of sufficient intensity. In bright light, the keenest vision – both chromatic and achromatic – is in the middle part of the retina where there are no rods.
For many years, the mechanism of color perception has puzzled physiology and psychology experts. The mechanism involves very basic neurological problems. Generally, a stronger stimulus of any type produces a more intense sensation, either because more nerve fibers are involved or more nerve impulses are initiated in each fiber.
However, perception of different qualities of sensation depends upon distinct sensory systems, such as the afferent nerves, receptors, fiber tracts in the central nervous system, and separate though tiny brain areas. This is obviously the case where broadly different senses are concerned – hearing, taste, or vision. Each of these senses has its specific peripheral nerves, sense organs, conduction courses, and cerebral areas.
Apparently, nerve fibers are not equipped to conduct different kinds of impulses. It is very unlikely, in vision, that a given neuron or receptor transmits one kind of impulse for red and another for green or blue. Discrete neural mechanisms are evidently necessary for each qualitative difference not only in vision but likewise in pitch discrimination in hearing and in differentiation of various taste qualities in the gustatory sense.
All the theories related to color perception start from the understanding that all colors (white included) are combinations of red, green, and blue. Any color, therefore, can be reproduced by an appropriate mixture of the correct shades of these three basic colors. Numerous studies have shown that there are at least three different kinds of cones; by some means, each of these cones is activated by one of the three basic colors.
Simultaneous though often unequal stimulation of two kinds of cones causes sensations of the intermediate colors. Perception of white results from equal stimulation of all three kinds. Inasmuch as the middle part of the retina is sensitive to all colors, it would be necessary that all three kinds of cones be represented in each minute portion of this part. The understanding that there are several kinds of cones, which correspond to the several main colors in the spectrum, stems from the knowledge that each of these several kinds of cones responds to light in a narrow range of wavelength.