Dichromacy

Dichromacy is the state of having two types of functioning color receptors, called cone cells, in the eyes. Organisms with dichromacy are called dichromats. Dichromats can match any color they see with a mixture of no more than two pure spectral lights. By comparison, trichromats can perceive colors made of up to three pure spectral lights, and tetrachromats can perceive colors made of four.

Not to be confused with Heterochromia iridum.
"dichromat" redirects here. For the chemical ion species, see dichromate.

Dichromacy
SpecialtyOphthalmology

Dichromacy in humans is a color vision defect in which one of the three basic color mechanisms is absent or not functioning. It is hereditary and sex-linked, predominantly affecting males.[1] Dichromacy occurs when one of the cone pigments is missing and color is reduced to two dimensions.[2] The term is from di meaning "two" and chroma meaning "color".

Classification

There are various kinds of color blindness:

  • Protanopia is a severe form of red-green color blindness, in which there is impairment in perception of very long wavelengths, such as reds. To these individuals, reds are perceived as beige or grey and greens tend to look beige or grey like reds. It is also the most common type of dichromacy today. This problem occurs because patients do not have the red cone cells in the retina.[3] Protanomaly is a less severe version.
  • Deuteranopia consists of an impairment in perceiving medium wavelengths, such as greens. Deuteranomaly is a less severe form of deuteranopia. Those with deuteranomaly cannot see reds and greens like those without this condition; however, they can still distinguish them in most cases. It is very similar to protanopia. In this form, patients do not have green cone cells in the retina, which makes it hard to see the green color.[3]
  • A rarer form of color blindness is tritanopia, where there exists an inability to perceive short wavelengths, such as blues. Sufferers have trouble distinguishing between yellow and blue. They tend to confuse greens and blues, and yellow can appear pink. This is the rarest of all dichromacy, and occurs in around 1 in 100,000 people. Patients do not have the blue cone cells in the retina.

Diagnosis

The three determining elements of a dichromatic opponent-colour space are the missing colour, the null-luminance plane, and the null-chrominance plane.[4] The description of the phenomena itself does not indicate the colour that is impaired to the dichromat, however, it does provide enough information to identify the fundamental colour space, the colours that are seen by the dichromat. This is based on testing both the null-chrominance plane and null-luminance plane which intersect on the missing colour. The cones excited to a corresponding colour in the colour space are visible to the dichromat and those that are not excited are the missing colours.[5]

Color detecting abilities of dichromats

According to colour vision researchers at the Medical College of Wisconsin (including Jay Neitz), each of the three standard colour-detecting cones in the retina of trichromats – blue, green and red – can pick up about 100 different gradations of colour. If each detector is independent of the others, simple exponentiation gives a total number of colours discernible by an average human as their product, or about 1 million;[6] nevertheless, other researchers have put the number at upwards of 2.3 million.[7] Exponentiation suggests that a dichromat (such as a human with red-green color blindness) would be able to distinguish about 10,000 different colours,[8] but no such calculation has been verified by psychophysical testing.

Furthermore, dichromats have a significantly higher threshold than trichromats for coloured stimuli flickering at low (1 Hz) frequencies. At higher (10 or 16 Hz) frequencies, dichromats perform as well as or better than trichromats.[9][10] This means such animals would still observe the flicker instead of a temporally fused visual percept as is the case in human movie watching at a high enough frame rate.

Other animals

It is more informative to use situations where less than the total visual system is operating when studying about vision. For example, a system by which cones are the sole visual receptors could be used. This is rare in humans but certain animals possess this trait and this proves useful in understanding the concept of dichromacy.[11]

While their Triassic ancestors were trichromatic,[7] placental mammals are as a rule dichromatic;[12] the ability to see long wavelengths (and thus separate green and red) was lost in the ancestor of placental mammals, though it is believed to have been retained in marsupials, where trichromatic vision is widespread.[13] Recent genetic and behavioral evidence suggests the South American marsupial Didelphis albiventris is dichromatic, with only two classes of cone opsins having been found within the genus Didelphis.[14] Dichromatic vision may improve an animal's ability to distinguish colours in dim light;[15] the typically nocturnal nature of mammals, therefore, may have led to the evolution of dichromacy as the basal mode of vision in placental animals.[16]

The exceptions to dichromatic vision in placental mammals are primates closely related to humans, which are usually trichromats, and sea mammals (both pinnipeds and cetaceans) which are cone monochromats.[17] New World Monkeys are a partial exception: in most species, males are dichromats, and about 60% of females are trichromats, but the owl monkeys are cone monochromats, and both sexes of howler monkeys are trichromats.[18][19][20][21]

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See also

References
  1. Cassin, B. and Solomon, S. Dictionary of Eye Terminology. Gainesville, Florida: Triad Publishing Company, 1990.
  2. "Guidelines: Colour Blindness." Tiresias.org. Retrieved 29 September 2006.
  3. Hanggi, Evelyn B.; Ingersoll, Jerry F.; Waggoner, Terrace L. (2007). "Color vision in horses (Equus caballus): Deficiencies identified using a pseudoisochromatic plate test". Journal of Comparative Psychology. 121 (1): 65–72. doi:10.1037/0735-7036.121.1.65. ISSN 1939-2087. PMID 17324076.
  4. Scheibner, H.; Cleveland, S. (1998). "Dichromacy characterized by chrominance planes". Vision Research. 38 (21): 3403–3407. doi:10.1016/s0042-6989(97)00373-8. PMID 9893856.
  5. Scheibner, H.; Cleveland, S. (1997). "Dichromacy characterized by chrominance planes". Vision Research. 38 (1): 3403–3407. doi:10.1016/s0042-6989(97)00373-8. PMID 9893856.
  6. Mark Roth (13 September 2006). "Some women who are tetrachromats may see 100,000,000 colors, thanks to their genes". Pittsburgh Post-Gazette.
  7. Jacobs, G. H. (2009). "Evolution of colour vision in mammals". Philosophical Transactions of the Royal Society B. 364 (1531): 2957–67. doi:10.1098/rstb.2009.0039. PMC 2781854. PMID 19720656.
  8. "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
  9. Sharpe Lindsay, T.; de Luca, Emanuela; Thorsten, Hansen; Gegenfurtner Karl, R. (2006). "Advantages and disadvantages of human dichromacy". Journal of Vision. 6 (3): 213–23. doi:10.1167/6.3.3. PMID 16643091.
  10. Bayer Florian, S.; Vivian Paulun, C.; David, Weiss; Gegenfurtner Karl, R. (2015). "A tetrachromatic display for the spatiotemporal control of rod and cone stimulation". Journal of Vision. 15 (11): 15. doi:10.1167/15.11.15. PMID 26305863.
  11. Jacobs, G. H.; Yolton, R. L. (1969). "Dichromacy in a ground squirrel". Letters to Nature. 223 (5204): 414–415. Bibcode:1969Natur.223..414J. doi:10.1038/223414a0. PMID 5823276.
  12. Bowmaker, JK (1998). "Evolution of colour vision in vertebrates". Eye (London, England). 12 ( Pt 3b) (3): 541–7. doi:10.1038/eye.1998.143. PMID 9775215.
  13. Arrese, C. A.; Oddy, A. Y.; Runham, P. B.; Hart, N. S.; Shand, J.; Hunt, D. M.; Beazley, L. D. (2005). "Cone topography and spectral sensitivity in two potentially trichromatic marsupials, the quokka (Setonix brachyurus) and quenda (Isoodon obesulus)". Proceedings of the Royal Society B. 272 (1565): 791–796. doi:10.1098/rspb.2004.3009. PMC 1599861. PMID 15888411.
  14. Gutierrez, E.A.; Pegoraro, B.M.; Magalhães-Castro, B.; Pessoa, V.F. (2011). "Behavioural evidence of dichromacy in a species of South American marsupial". Animal Behaviour. 81 (5): 1049–1054. doi:10.1016/j.anbehav.2011.02.012.
  15. Vorobyev, M. (2006). "Evolution of colour vision: The story of lost visual pigments". Perception. ECVP Abstract Supplement. 35. Archived from the original on 6 October 2014. Retrieved 1 February 2013.
  16. Neitz, GH; Neitz, M; Neitz, J (1996). "Mutations in S-cone pigment genes and the absence of colour vision in two species of nocturnal primate" (PDF). Proceedings of the Royal Society B. 263 (1371): 705–10. Bibcode:1996RSPSB.263..705J. doi:10.1098/rspb.1996.0105. PMID 8763792. Archived from the original (PDF) on 31 May 2013. Retrieved 19 January 2013.
  17. Vorobyev, M (July 2004). "Ecology and evolution of primate colour vision" (PDF). Clinical & Experimental Optometry. 87 (4–5): 230–8. doi:10.1111/j.1444-0938.2004.tb05053.x. PMID 15312027. Retrieved 7 January 2013.
  18. Jacobs, G. H.; Deegan, J. F. (2001). "Photopigments and colour vision in New World monkeys from the family Atelidae". Proceedings of the Royal Society B. 268 (1468): 695–702. doi:10.1098/rspb.2000.1421. PMC 1088658. PMID 11321057.
  19. Jacobs, G. H.; Deegan, J. F.; Neitz; Neitz, J.; Crognale, M. A. (1993). "Photopigments and colour vision in the nocturnal monkey, Aotus". Vision Research. 33 (13): 1773–1783. doi:10.1016/0042-6989(93)90168-V. PMID 8266633.
  20. Mollon, J. D.; Bowmaker, J. K.; Jacobs, G. H. (1984). "Variations of colour vision in a New World primate can be explained by polymorphism of retinal photopigments". Proceedings of the Royal Society B. 222 (1228): 373–399. Bibcode:1984RSPSB.222..373M. doi:10.1098/rspb.1984.0071. PMID 6149558.
  21. Sternberg, Robert J. (2006) Cognitive Psychology. 4th Ed. Thomson Wadsworth.

Sources
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