View source for Trichromacy

{{redirect|Trichromat|the chemical ion species|Trichromate}}
{{short description|Possessing of three independent channels for conveying color information}}
{{Use American English|date=March 2021}}
{{Use dmy dates|date=March 2021}}
[[File:Tricolour television close up.jpg|thumb|310px|Close-up of a trichromatic in-line [[shadow mask]] [[CRT display]], which creates most visible colors through combinations and different levels of the three [[primary color]]s: red, green and blue]]

'''Trichromacy''' or '''trichromatism''' is the possession of three independent channels for conveying [[color]] information, derived from the three different types of [[cone cell]]s in the [[eye]].<ref>{{Cite web |url=http://tigger.uic.edu/~hilbert/Glossary.html |title=Color Glossary<!-- Bot generated title --> |access-date=8 November 2006 |archive-date=4 October 2015 |archive-url=https://web.archive.org/web/20151004164334/http://tigger.uic.edu/~hilbert/Glossary.html |url-status=dead }}</ref> Organisms with trichromacy are called trichromats.

The normal explanation of trichromacy is that the organism's [[retina]] contains three types of color receptors (called [[cone cell]]s in [[vertebrate]]s) with different [[Absorption spectroscopy|absorption spectra]]. In actuality, the number of such receptor types may be greater than three, since different types may be active at different light intensities. In vertebrates with three types of cone cells, at low light intensities the [[rod cell]]s may contribute to [[color vision]].

==Humans and other animals that are trichromats==
[[Human]]s and some other [[mammal]]s have evolved trichromacy based partly on [[pigment]]s inherited from early vertebrates. In fish and birds, for example, [[Tetrachromacy|four pigments]] are used for vision. These extra cone receptor visual pigments detect energy of other [[wavelength]]s, sometimes including [[ultraviolet]]. Eventually two of these pigments were lost (in [[placental mammals]]) and another was gained, resulting in trichromacy among some [[primate]]s.<ref name=Arrese>{{cite journal|last=Arrese|first=Catherine|author2=Thomas, Nathan |author3=Beazley, Lyn |author4= Shand, Julia |title=Trichromacy in Australian Marsupials|journal=Current Biology|year=2002|volume=12|issue=8|pages=657–660|pmid=11967153|doi=10.1016/S0960-9822(02)00772-8|s2cid=14604695|doi-access=free|bibcode=2002CBio...12..657A }}</ref> Humans and [[Catarrhini|closely related primates]] are usually trichromats, as are some of the females of most species of [[New World monkey]]s, and both male and female [[howler monkey]]s.<ref name=Rowe2002 />

Recent research suggests that trichromacy may also be quite general among [[marsupial]]s.<ref>{{cite journal | last1 = Arrese | first1 = CA | last2 = Oddy | first2 = AY | last3 = Runham | first3 = PB | last4 = Hart | first4 = NS | last5 = Shand | first5 = J | last6 = Hunt | first6 = DM | year = 2005 | title = Cone topography and spectral sensitivity in two potentially trichromatic marsupials, the quokka (''Setonix brachyurus'') and quenda (''Isoodon obesulus'') | doi = 10.1098/rspb.2004.3009 | pmid = 15888411 | journal = Proceedings of the Royal Society of London B | volume = 272 | issue = 1595| pages = 791–796 | pmc = 1599861 }}</ref> A study conducted regarding trichromacy in [[Australia]]n marsupials suggests the medium wavelength sensitivity (MWS), cones of the [[honey possum]] (''Tarsipes rostratus'') and the [[fat-tailed dunnart]] (''Sminthopsis crassicaudata'') are features coming from the [[Heredity|inherited]] [[reptile|reptilian]] [[retina]]l arrangement. Another study used behavioral tests, genetic analyses, and immunohistochemistry and found trichromacy and ultraviolet vision in nocturnal sugar gliders (''Petaurus breviceps'').<ref>{{cite journal |last1=Nielsen |first1=Leah |last2=Beck |first2=Harald |last3=Oufiero |first3=Christopher |year=2025 |title=Trichromacy and ultraviolet vision in a nocturnal marsupial |doi=10.1038/s41598-025-92039-z |journal=Scientific Reports |volume=15 |issue=7585|pmc=11880520 }} </ref> The possibility of trichromacy in marsupials potentially has another [[evolution]]ary basis than that of [[primate]]s. Further [[biological]] and [[behavioural]] tests may verify if trichromacy is a common characteristic of marsupials.<ref name=Arrese/>

Most other mammals are currently thought to be [[dichromat]]s, with only two types of cone (though limited trichromacy is possible at low light levels where the rods and cones are both active).<ref>{{Cite journal|last1=Jacobs|first1=Gerald H.|last2=Nathans|first2=Jeremy|date=2009|title=The Evolution of Primate Color Vision|url=https://www.jstor.org/stable/26001303|journal=Scientific American|volume=300|issue=4|pages=56–63|doi=10.1038/scientificamerican0409-56 |doi-broken-date=2 November 2024 |jstor=26001303 |pmid=19363921 |bibcode=2009SciAm.300d..56J |issn=0036-8733|url-access=subscription}}</ref> Most studies of carnivores, as of other mammals, reveal [[dichromacy]]; examples include the domestic [[dog]], the [[ferret]], and the [[spotted hyena]].<ref>{{cite journal | last1 = Calderone | first1 = JB | last2 = Jacobs | first2 = GH | year = 2003 | title = Spectral properties and retinal distribution of ferret cones | url =https://escholarship.org/content/qt2bm9v2td/qt2bm9v2td.pdf?t=lnq0vu | journal = Visual Neuroscience | volume = 20 | issue = 1| pages = 11–17 | doi=10.1017/s0952523803201024| pmid = 12699079 | s2cid = 10415194 }}</ref><ref>{{cite journal | last1 = Calderone | first1 = JB | last2 = Reese | first2 = BE | last3 = Jacobs | first3 = GH | year = 2003 | title = Topography of photoreceptors and retinal ganglion cells in the spotted hyena (''Crocuta crocuta'') | journal = Brain, Behavior and Evolution | volume = 62 | issue = 4| pages = 182–192 | doi=10.1159/000073270| pmid = 14573992 | s2cid = 9167855 }}</ref>  Some species of [[insect]]s (such as [[honeybee]]s) are also trichromats, being sensitive to [[ultraviolet]], blue and green instead of blue, green and red.<ref name=Rowe2002>{{cite journal | last1 = Rowe | first1 = Michael H | year = 2002 | title = Trichromatic color vision in primates | url = http://physiologyonline.physiology.org/cgi/content/full/17/3/93 | journal = News in Physiological Sciences | volume = 17 | issue = 3| pages = 93–98 | doi = 10.1152/nips.01376.2001 | pmid = 12021378 | s2cid = 15241669 | url-access = subscription }}</ref>

Research indicates that trichromacy allows animals to distinguish brightly colored fruit and young leaves from other vegetation that is not beneficial to their survival.<ref>{{cite journal |vauthors=Sharpe LT, de Luca E, Hansen T, Jägle H, Gegenfurtner KR |year=2006 |title=Advantages and disadvantages of human dichromacy |journal=Journal of Vision |volume=6 |issue=3 |doi=10.1167/6.3.3 |pmid=16643091 |pages=213–223|doi-access=free }}</ref> Another theory is that detecting skin [[flushing (physiology)|flushing]] and thereby mood may have influenced the development of primate trichromate vision. The color red also has other effects on primate and human behavior as discussed in the [[color psychology]] article.<ref name=AEP>Diana Widermann, Robert A. Barton, and Russel A. Hill. Evolutionary perspectives on sport and competition. In {{Cite book | last1 = Roberts | first1 = S. C. | editor1-last = Roberts | doi = 10.1093/acprof:oso/9780199586073.001.0001 | editor1-first = S. Craig | title = Applied Evolutionary Psychology | year = 2011 | publisher = Oxford University Press| isbn = 9780199586073 }}</ref>

==Types of cones specifically found in primates==
Primates are the only known placental mammalian trichromats.<ref>
{{cite book
 | title = Perception of the visual environment
 | author = Ronald G. Boothe
 | publisher = Springer
 | year = 2002
 | isbn = 978-0-387-98790-3
 | page = 219
 | url = https://books.google.com/books?id=rCBuW7u6qhsC&pg=PA219
 }}</ref>
{{Failed verification|date=December 2014|reason=The reference contains the sentence "The primates are unique among mammals in having species that possess trichromatic color vision", which fails verification since it contradicts the rest of this article which states that marsupial mammals have trichromacy and the reference doesn't constrain its claim to placental mammals}}
Their eyes include three different kinds of cones, each containing a different [[photopigment]] ([[opsin]]). Their peak sensitivities lie in the blue (short-wavelength S cones), green (medium-wavelength M cones) and yellow-green (long-wavelength L cones) regions of the color spectrum.<ref>{{cite journal |last1=Schnapf |first1=J. L. |last2=Kraft |first2=T. W. |last3=Baylor |first3=D. A. |title=Spectral sensitivity of human cone photoreceptors |journal=Nature |date=January 1987 |volume=325 |issue=6103 |pages=439–441 |doi=10.1038/325439a0|pmid=3808045 |bibcode=1987Natur.325..439S |s2cid=11399054 }}</ref> S cones make up 5–10% of the cones and form a regular mosaic. Special [[Retina bipolar cell|bipolar]] and [[Retinal ganglion cell|ganglion]] cells pass those signals from S cones and there is evidence that they have a separate signal pathway through the [[thalamus]] to the [[visual cortex]] as well. On the other hand, the L and M cones are hard to distinguish by their shapes or other anatomical means – their opsins differ in only 15 out of 363 amino acids, so no one has yet succeeded in producing specific antibodies to them. But Mollon and Bowmaker<ref>{{cite journal |last1=Mollon |first1=J. D. |last2=Bowmaker |first2=J. K. |title=The spatial arrangement of cones in the primate fovea |journal=Nature |date=December 1992 |volume=360 |issue=6405 |pages=677–679 |doi=10.1038/360677a0|pmid=1465131 |bibcode=1992Natur.360..677M |s2cid=4234999 }}</ref> did find that L cones and M cones are randomly distributed and are in equal numbers.<ref>
{{cite journal
 |last=Wässle
 |first=Heinz
 |title=Colour vision: A patchwork of cones|journal=Nature
 |date=11 February 1999
 |volume=397
 |pages=473–475
 |doi=10.1038/17216|issue=6719
 |pmid=10028963
 |bibcode=1999Natur.397..473W
 |s2cid=4431471
 }}</ref>

==Mechanism of trichromatic color vision==

[[File:Cones SMJ2 E.svg|right|thumb|200px|Normalised [[responsivity]] spectra of human cone cells]]
{{metamerism_spectrum_example.svg}}

Trichromatic color vision is the ability of humans and some other animals to see different [[color]]s, mediated by interactions among three types of color-sensing [[cone cell]]s. The ''[[Young–Helmholtz theory|trichromatic color theory]]'' began in the 18th century, when [[Thomas Young (scientist)|Thomas Young]] proposed that color vision was a result of three different  [[photoreceptor cell]]s. From the middle of the 19th century, in his ''Treatise on Physiological Optics'',<ref>{{cite book |last1=von Helmholtz |first1=Hermann |title=Handbuch der Physiologischen Optik |date=1909 |publisher=Leopold Voss |location=Hamburg ; Leipzig |edition=3 |url=https://archive.org/details/b21287314/mode/2up |access-date=18 February 2020}}</ref><ref>{{cite book |last1=von Helmholtz |first1=Hermann |title=Treatise on Physiological Optics |date=2013 |publisher=Courier Corporation |isbn=978-0486174709 |url=https://books.google.com/books?id=cSjEAgAAQBAJ&q=Treatise+on+Physiological+Optics |access-date=18 February 2020}}</ref> [[Hermann von Helmholtz]] later [[Young–Helmholtz theory|expanded on Young's ideas]] using color-matching experiments which showed that people with normal vision needed three wavelengths to create the normal range of colors. Physiological evidence for trichromatic theory was later given by [[Gunnar Svaetichin]] (1956).<ref>{{cite journal |last=Svaetichin |first=G |year=1956 |title=Spectral response curves from single cones |journal=Acta Physiologica Scandinavica |volume=39 |issue=134 |pages=17–46 |pmid=13444020}}</ref>

Each of the three types of cones in the [[retina]] of the [[human eye|eye]] contains a different type of [[photosensitive pigment]], which is composed of a [[transmembrane protein]] called [[opsin]] and a light-sensitive molecule called [[11-cis retinal]]. Each different pigment is especially sensitive to a certain [[wavelength]] of [[light]] (that is, the pigment is most likely to produce a [[cell (biology)|cellular]] response when it is hit by a [[photon]] with the specific wavelength to which that pigment is most sensitive).  The three types of cones are L, M, and S, which have pigments that respond best to light of long (especially 560&nbsp;nm), medium (530&nbsp;nm), and short (420&nbsp;nm) wavelengths respectively.<ref name="Kandel2000">{{cite book |vauthors=[[Eric R. Kandel|Kandel ER]], Schwartz JH, Jessell TM |title=Principles of Neural Science |edition=4th |pages=[https://archive.org/details/isbn_9780838577011/page/182 182–185] |publisher=McGraw-Hill |location=New York |year=2000 |isbn=978-0-8385-7701-1|title-link=Principles of Neural Science }}</ref><ref name="Jacobs2009">{{cite web |title=Color Vision: How Our Eyes Reflect Primate Evolution |vauthors=Jacobs GH, Nathans J |url=http://www.sciam.com/article.cfm?id=evolution-of-primate-color-vision |date=March 2009 |publisher=Scientific American}}</ref>

Since the likelihood of response of a given cone varies not only with the ''wavelength'' of the light that hits it but also with its ''intensity'', the [[brain]] would not be able to discriminate different colors if it had input from only one type of cone.  Thus, interaction between at least two types of cone is necessary to produce the ability to perceive color.  With at least two types of cones, the brain can compare the signals from each type and determine both the intensity and color of the light.  For example, moderate stimulation of a medium-wavelength cone cell could mean that it is being stimulated by very bright red (long-wavelength) light, or by not very intense yellowish-green light.  But very bright red light would produce a stronger response from L cones than from M cones, while not very intense yellowish light would produce a stronger response from M cones than from other cones.  Thus trichromatic color vision is accomplished by using combinations of cell responses.

It is estimated that the average human can distinguish up to ten million different colors.<ref>{{cite web|last=Leong|first=Jennifer|title=Number of Colors Distinguishable by the Human Eye|url=http://hypertextbook.com/facts/2006/JenniferLeong.shtml|publisher=hypertextbook|access-date=21 February 2013}}</ref>

==See also==
* [[Visual system]]
* [[Monochromacy]]
* [[Dichromacy]]
* [[Tetrachromacy]]
* [[Pentachromacy]]
* [[Mantis shrimp#Eyes|Mantis shrimp]] (dodecachromats)
* [[Evolution of color vision in primates]]
* [[Young–Helmholtz theory]]
* [[LMS color space]]

==References==
{{reflist|30em}}

==External links==
* [http://www.straightdope.com/classics/a2_004.html The Straight Dope]: "Are cats and dogs really color-blind? How do they know?"
* {{cite journal|last1=Svaetichin|first1=Gunnar|last2=MacNichol|first2=Edward F.|title=Retinal mechanisms for chromatic and achromatic vision.|journal=Annals of the New York Academy of Sciences|date=November 1958|volume=74|issue=2|pages=385–404|doi=10.1111/j.1749-6632.1958.tb39560.x|pmid=13627867|bibcode=1959NYASA..74..385S |s2cid=27130943}}
{{color vision}}

[[Category:Color vision]]
[[Category:Visual system]]