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.
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 appear to be identical.
Species with tetrachromatic color vision have a small physiological advantage over rival species.
Some species of birds such as the Zebra Finch and the Columbidae utilize the ultraviolet wavelength (300–400 nm) specific to tetrachromatic color vision as a tool during mate selection and foraging. When selecting for mates, ultraviolet plumage and skin coloration show a high level of selection.
Floral colors were categorized into two main wavelengths of light including 360–520 nm and 400–500 nm. Flowers now reflect four broad domains of wavelength including 300–400 nm, 400–500 nm, 500–600 nm, and 600–700 nm. These wavelengths represent the colors ultraviolet (UV), blue, green, and red respectively in the color spectrum. Flowers utilize these wavelengths to differentiate color patterns within species. It has been determined that these differences in color patterns are used for behavioral attractions in pollinator insects increasing survival. Many trichromatic pollinators such as honeybees use ultraviolet, blue, and green wavelengths. As the color space within insects has become more and more filled, the increase in the wavelengths flowers reflect has increased as a result. Flowers which reflect certain wavelengths such as UV and red wavelengths attract a greater number of pollinators. Pollination is a mutualistic relationship between plants and pollinators leading to an extremely high level of competition. This competition has led to a coevolution between plants and foraging insects increasing the color variation in both orders leading to directional selection.
Foraging insects have the ability to see all four color wavelengths. Plants display increasingly diverse amounts of color variation extending into the ultraviolet color scale. Plants that display higher levels of color will in return attract higher levels of pollinators. Pollinators which maintain a wider range of color can use tetrachromatic color vision to increase and maintain a higher foraging success rate over their trichromatic competitors. Within tetrachromatic insects, background displays play a large role in the view of flower color variation. Flowers which display pure color hues are easily distinguished by the pollinating insect. When a pollinator encounters a flower, the insect can distinguish the flower from the background based on reflectance within the petals. This use of reflectance then draws the insect in directing it to the center reproductive organs of the plant.
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 possibly 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 discovered in a study reported in 2006. Neither case has been fully verified.
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 of heterozygotic alleles for retinal pigment genes, which is the same mechanism that gives the majority of female new-world monkeys trichromatic vision.
In humans, preliminary visual processing occurs within the neurons of the retina. It is not known how these nerves would respond to a new color channel, that is, whether they could handle it separately or just lump it in with an existing channel. Visual information leaves the eye by way of the optic nerve; it is not known whether 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.
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 have been shown to have increased chromatic discrimination in comparison to trichromats.