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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.

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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.

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.[3]

Most birds are tetrachromats.[4]

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

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.[5]

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.[6] 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.[7] Another study suggests that as many as 50% of women and 8% of men may have four photopigments.[6]

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

Mice, which normally have only two cone pigments, can be engineered to express a third cone pigment, and appear to demonstrate increased chromatic discrimination,[9] arguing against some of these obstacles; however, the original publication's claims about plasticity in the optic nerve have also been disputed.[10] People with four photopigments have been shown to have increased chromatic discrimination in comparison to trichromats.[6] 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.[7][11]

Medical College of Wisconsin professor of cell biology Jay Neitz and his wife Maureen Neitz have been working with a species of monkey that has only two kinds of cones. The Neitzes have created a virus containing a gene for a photopigment that the monkeys do not have, and they'll inject this virus into their eyes. If some of the cones absorb the viral DNA, the monkeys should become receptive to light of different wavelengths. The monkeys would then have three kinds of cones. If their brains are able to process the new information, the monkeys might leap from dichromatic vision to trichromatic vision. If this experiment succeeds in monkeys, this can be tested on colorblind humans to give them full color vision. If that proves successful, the Neitzes hypothesize, you ought to be able to give people with normal sight a fourth cone, equipping us "tetrachromatic" vision. [12]

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..."[13]

See also

References

  1. ^ Figure data, uncorrected absorbance curve fits, from Hart NS, Partridge JC, Bennett ATD and Cuthill IC (2000) Visual pigments, cone oil droplets and ocular media in four species of estrildid finch. Journal of Comparative Physiology A186 (7-8): 681-694.
  2. ^ Robinson, J., Schmitt, E.A., Harosi, F.I., Reece, R.J., Dowling, J.E. 1993. Zebrafish ultraviolet visual pigment: absorption spectrum, sequence, and localization. Proc. Natl. Acad. Sci. U.S.A. 90, 6009–6012.
  3. ^ [1]
  4. ^ Wilkie, Susan E.; Vissers, Peter M. A. M.; Das, Debipriya; Degrip, Willem J.; Bowmaker, James K.; Hunt, David M. (1998). "The molecular basis for UV vision in birds: spectral characteristics, cDNA sequence and retinal localization of the UV-sensitive visual pigment of the budgerigar (Melopsittacus undulatus)" (PDF). Biochemical Journal 330: 541–47. PMID 9461554. http://www.pubmedcentral.nih.gov/picrender.fcgi?artid=1219171&blobtype=pdf. 
  5. ^ Hansjochem Autrum and Richard Jung (1973). Integrative Functions and Comparative Data. 7 (3). Springer-Verlag. p. 226. ISBN 9780387057699. http://books.google.com/books?id=mrgoAAAAYAAJ&q=tetrachromacy+rods+cones&dq=tetrachromacy+rods+cones&ei=B8n5SYr3O4zUkwTouMXQBA&pgis=1. 
  6. ^ a b c Jameson, K. A., Highnote, S. M., & Wasserman, L. M. (2001). "Richer color experience in observers with multiple photopigment opsin genes" (PDF). Psychonomic Bulletin and Review 8 (2): 244–261. PMID 11495112. http://www.klab.caltech.edu/cns186/papers/Jameson01.pdf. 
  7. ^ a b c Mark Roth (September 13, 2006]). "Some women may see 100,000,000 colors, thanks to their genes". Pittsburgh Post-Gazette. http://www.post-gazette.com/pg/06256/721190-114.stm. 
  8. ^ "You won't believe your eyes: The mysteries of sight revealed". The Independent. 7 March 2007. http://news.independent.co.uk/world/science_technology/article2336163.ece. 
  9. ^ http://www.sciencemag.org/cgi/content/full/315/5819/1723
  10. ^ http://www.sciencemag.org/cgi/content/full/318/5848/196b
  11. ^ "Color Vision:Almost Reason for Having Eyes" by Jay Neitz, Joseph Carroll, and Maureen Neitz Optics & Photonics News January 2001 1047-6938/01/01/0026/8- Optical Society of America
  12. ^ http://www.slate.com/id/2079371>
  13. ^ "Some Experiments on Color", Nature 111, 1871, in John William Strutt (Lord Rayleigh) (1899). Scientific Papers. University Press. http://books.google.com/books?id=KWMSAAAAIAAJ&pg=PA84&dq=date:0-1923+light+red+green+yellow-or-orange&as_brr=1#PPA85,M1. 

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