Tetrachromacy

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The four pigments in a bird's cones extend the range of colour vision into the ultraviolet.^[1]

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.

[edit] Description

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.

[edit] Animals that are tetrachromats

The zebrafish (Danio rerio) is an example of a tetrachromat, containing cone cells sensitive for red, green, blue, and ultraviolet light.^[2]

The mantis shrimp (Gonodactylus smithii) sees at least four basic colours.[1]

Most birds are tetrachromats.^[3]

Tetrachromacy is also expected to occur in several species of fish, amphibians, reptiles, arachnids and insects.^{[citation needed]}

[edit] Possibility of human tetrachromats

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.^[4]

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.^[5] 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.^[6] Another study suggests that as many as 50% of women and 8% of men may have four photopigments.^[5]

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,^[7] and an unidentified female physician near Newcastle, England, was discovered in a study reported in 2006.^[6] 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 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.[2]

Mice, which normally have only two cone pigments, can be engineered to express a third cone pigment, and appear to demonstrate increased chromatic discrimination,^[8] arguing against some of these obstacles; however, the original publication's claims about plasticity in the optic nerve have also been disputed.^[9] People with four photopigments have been shown to have increased chromatic discrimination in comparison to trichromats.^[5] Each of the three cone types in a trichromatic human retina can pick up about 100 different gradations of color, and the brain can combine those variations so that the average human can distinguish about 1 million different colors; a true human tetrachromat would have another type of cone, and its 100 shades theoretically would allow them to see 100 million different colors.^[6][10]

[edit] Historical remarks

According to Lord Rayleigh in 1871, "Sir John Herschel even thinks that our inability to resolve yellow leaves it doubtful whether our vision is trichromatic or tetrachromatic..."^[11]

[edit] See also

• Evolution of color vision
• Somatosensory Amplification
• Supertaster

[edit] References

[edit] External links

• Goldsmith, Timothy H. "What Birds See" Scientific American July 2006—Article about the tetrachromatic vision of birds: "What Birds See" in PDF format:
• Thompson, Evan (2000). "Comparative color vision: Quality space and visual ecology." In Steven Davis (Ed.), Color Perception: Philosophical, Psychological, Artistic and Computational Perspectives, pp. 163-186. Oxford: Oxford University Press. http://www.yorku.ca/evant/ETVancouvercolour.pdf
• Holba, Á.; Lukács, B. "On tetrachromacy." http://www.rmki.kfki.hu/~lukacs/TETRACH.htm
• Looking for Madam Tetrachromat By Glenn Zorpette. Red Herring magazine, 1 November 2000
• Ultraviolet vision
• The Human is a blocked tetrachromat A review of the spectral sensitivity of the human visual system. (Suggests that the human lens is responsible for blocking the ultraviolet frequencies, that we already have a UV sensor in the retina ready and waiting, and if the UV wasn't blocked, we'd all be tetrachromats.)