Tetrachromacy

The four pigments in a bird's cones (in this example, estrildid finches) extend the range of color vision into the ultraviolet.

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.

Most birds are tetrachromats. Tetrachromacy is also suspected among several species of fish, amphibians, reptiles, arachnids and insects.

Physiology

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.

Examples

Fish

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

Birds

Some species of birds such as the Zebra Finch and the Columbidae use 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. A typical bird eye will respond to wavelengths from about 300 to 750 nm. In terms of frequency, this corresponds to a band in the vicinity of 400–1000 THz.

Insects

Foraging insects have the ability to see wavelengths that flowers reflect (ranging from 300 nm to 700 nm). Pollination being a mutualistic relationship, foraging insects and plants have coevolved, both increasing wavelength range: in perception (pollinators), in reflection and variation (flower colors). Directional selection has led plants to display increasingly diverse amounts of color variations extending into the ultraviolet color scale, thus attracting higher levels of pollinators. Some pollinators may use tetrachromatic color vision to increase and maintain a higher foraging success rate over their trichromatic competitors.

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.

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.

See also

• RGBY
• Monochromacy
• Dichromacy
• Trichromacy
• Pentachromacy
• Evolution of color vision
• Somatosensory amplification
• Supertaster

External links

• Timothy H. "What Birds See" Scientific American July 2006. An article about the tetrachromatic vision of birds
• 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.
• Holba, Á.; Lukács, B. "On tetrachromacy"
• 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.)
• Colors - The Perfect Yellow By Radiolab, 21 May 2012 (Explores tetrachromacy in humans)
• The dimensionality of color vision in carriers of anomalous trichromacy--Gabriele Jordan et al--Journal of Vision August 12, 2010: