Hypothetical form of color vision that uses four channels. Human vision is trichromatic; that is, we have receptors for red, green, and blue light. All of the colors that you see are combinations of these three. Tetrachromats, if they exist, would have perceptors for a fourth color of light.
The notion of tetrachromacy emerged from the study of genetics in the late 1940s. A number of scientists who study the biology of human vision are currently conducting experiments to determine whether tetrachromacy can actually exist, and if so, how it could be detected.
If tetrachromacy is a reality, it is hypothesized that there would have to be a fourth neural channel to transmit information from the fourth color detector to the brain. This fourth channel is not naturally occuring in humans, so it would represent an adaptation, and prove the flexibility of the nervous system in adapting to genetic quirks.
According to current theories, owing to a quirk of genetics (specifically the way that the the color detector genes are transmitted) only women are possible candidates for tetrachromacy. Tetrachromatic females would likely have male children with some form of color blindness.