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Tetrachromacy
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From Wikipedia, the free encyclopedia
Type of color vision with four types of cone cells
"Tetrachromat" redirects here. For the chemical ion species, see Tetrachromate.
The four pigments in a bird's cone cells (in this example, estrildid finches) extend the range of color vision into the ultraviolet.[1]
Tetrachromacy (from Ancient Greek tetra, meaning "four" and chroma, meaning "color") is the condition of possessing four independent channels for conveying color information, or possessing four types of cone cell in the eye. Organisms with tetrachromacy are called tetrachromats.
In tetrachromatic organisms, the sensory color space is four-dimensional, meaning that matching the sensory effect of arbitrarily chosen spectra of light within their visible spectrum requires mixtures of at least four primary colors.
Tetrachromacy is demonstrated among several species of birds,[2] fish,[3] and reptiles.[3] The common ancestor of all vertebrates was a tetrachromat, but a common ancestor of mammals lost two of its four kinds of cone cell, evolving dichromacy, a loss ascribed to the conjectured nocturnal bottleneck. Some primates (including the ancestor of humans) then later re-evolved a third cone.[4]
Physiology<br>[edit]
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 spectral sensitivity. This means that the organism may see wavelengths beyond those of a typical human's vision, and may be able to distinguish between colors that, to a non-tetrachromat human, appear to be identical. Species with tetrachromatic color vision may have an unknown physiological advantage over rival species.[5]
Humans<br>[edit]
Normalized responsivity spectra of human cone cells, S, M, and L types.<br>Apes (including humans) and Old World monkeys normally have only three types of cone cell, and are therefore trichromats. However, human tetrachromacy is suspected to exist in a small percentage of the population. Trichromats have three types of cone cells, each type being sensitive to a corresponding portion of the spectrum as shown in the diagram. But at least one woman has been implied to be a tetrachromat.[6] More precisely, she had an additional cone type L′, intermediate between M and L in its responsivity, and showed 3 dimensional (M, L′, and L components) color discrimination for wavelengths 546–670 nm (to which the fourth type, S, is insensitive).
Tetrachromacy requires that there be four independent photoreceptor cell classes with different spectral sensitivity. However, there must also be the appropriate post-receptoral mechanism to compare the signals from the four classes of receptors. According to the opponent process theory, humans have three opponent channels, which give trichromacy. It is unclear whether having available a fourth opponent channel is sufficient for tetrachromacy.[citation needed]
Mice, which normally have only two cone pigments (and therefore two opponent channels), have been engineered to express a third cone pigment, and appear to demonstrate increased chromatic discrimination,[7]<br>possibly indicating trichromacy, and suggesting they were able to create or re-enable a third opponent channel. This would support the theory that humans should be able to utilize a fourth opponent channel for tetrachromatic vision. However, the original publication's claims about plasticity in the optic nerve have also been disputed.[8]
Tetrachromacy in carriers of CVD<br>[edit]
It has been theorized that females who carry recessive opsin alleles that can cause color vision deficiency (CVD, a.k.a. color blindness) could possess tetrachromacy. Female carriers of anomalous trichromacy (mild color blindness) possess heterozygous alleles of the genes that encode the L-opsin or M-opsin. These alleles often have a different spectral sensitivity, so if the carrier expresses both opsin alleles, they may exhibit tetrachromacy.
In humans, two cone cell pigment genes are present on the X chromosome: the classical type 2 opsin genes OPN1MW and OPN1MW2. People with two X chromosomes could possess multiple cone cell pigments, perhaps born as full tetrachromats who have four simultaneously-functioning kinds of cone cell, each type with a specific pattern of responsiveness to different wavelengths of light in the range of the visible spectrum.[9]<br>One study suggested that 15% of the world's women might have the type of fourth cone whose sensitivity peak is between the standard red and green cones, theoretically...