Tetrachromacy is the condition of possessing four independent channels for conveying color information, or possessing four different types of cone cells in the eye. Organisms with tetrachromacy are called tetrachromats.
In tetrachromatic organisms, the sensory color space is four-dimensional, meaning that to match the sensory effect of arbitrarily chosen spectra of light within their visible spectrum requires mixtures of at least four different primary colors. As with the trichromacy normal in humans, the gamut of colors that can be made with these primaries will not cover all possible colors.
The normal explanation of tetrachromacy is that the organism's retina contains four types of higher-intensity light receptors (called cone cells in vertebrates as opposed to rod cells which are lower intensity light receptors) with different absorption spectra. This means the animal may see wavelengths beyond those of a typical human being's eyesight, and may be able to distinguish colors that to a human are identical.
Humans and closely related primates normally have three types of cone cells and are therefore trichromats (animals with three different cones). However, at low light intensities the rod cells may contribute to color vision, giving a small region of tetrachromacy in the color space.. In humans two cone cell pigment genes are located on the sex X chromosome, the classical type 2 opsin genes OPN1MW and OPN1MW2. It has been suggested that as women have two different X chromosomes in their cells, some of them could be carrying some variant cone cell pigments, thereby being born as full tetrachromats and having four different simultaneously functioning kinds of cone cells, each type with a specific pattern of responsiveness to different wave lengths of light in the range of the visible spectrum. One study suggested that 2–3% of the world's women might have the kind of fourth cone that lies between the standard red and green cones, giving, theoretically, a significant increase in color differentiation. Another study suggests that as many as 50% of women and 8% of men may have four photopigments.
Further studies will need to be conducted to verify tetrachromacy in humans. Two possible tetrachromats have been identified: "Mrs. M," an English social worker, was located in a study conducted in 1993, and an unidentified female physician near Newcastle, England, was identified in a study reported in 2006. Variation in cone pigment genes is widespread in most human populations, but the most prevalent and pronounced tetrachromacy would derive from female carriers of major red-green pigment anomalies, usually classed as forms of "color blindness" (protanomaly or deuteranomaly). The biological basis for this phenomenon is X-inactivation.
It is possible that some humans could have four rather than three color receptors. Preliminary visual processing occurs within the nerves of the eye. It is not known how these nerves would respond to a new color channel, if they could handle it separately or would just lump it in with an existing channel. Visual information leaves the eye by way of the optic nerve. It is not known if the optic nerve has the spare capacity to handle a new color channel. A variety of final image processing takes place in the brain. It is not known how the various areas of the brain would respond if presented with a new color channel.
Notably, mice, which normally have only two cone pigments, can be engineered to express a third cone pigment, and appear to demonstrate increased chromatic discrimination, arguing against some of these obstacles; however, the original publication's claims about plasticity in the optic nerve have also been disputed.
People with four photopigments were shown to have increased chromatic discrimination in comparison to trichromats.