Trichromacy ortrichromatism is the possession of three independent channels for conveyingcolor information, derived from the three different types ofcone cells in theeye.[1] Organisms with trichromacy are called trichromats.
The normal explanation of trichromacy is that the organism'sretina contains three types of color receptors (calledcone cells invertebrates) with differentabsorption 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 therod cells may contribute tocolor vision.
Humans and some othermammals have evolved trichromacy based partly onpigments inherited from early vertebrates. In fish and birds, for example,four pigments are used for vision. These extra cone receptor visual pigments detect energy of otherwavelengths, sometimes includingultraviolet. Eventually two of these pigments were lost (inplacental mammals) and another was gained, resulting in trichromacy among someprimates.[2] Humans andclosely related primates are usually trichromats, as are some of the females of most species ofNew World monkeys, and both male and femalehowler monkeys.[3]
Recent research suggests that trichromacy may also be quite general amongmarsupials.[4] A study conducted regarding trichromacy inAustralian marsupials suggests the medium wavelength sensitivity (MWS), cones of thehoney possum (Tarsipes rostratus) and thefat-tailed dunnart (Sminthopsis crassicaudata) are features coming from theinheritedreptilianretinal arrangement. Another study used behavioral tests, genetic analyses, and immunohistochemistry and found trichromacy and ultraviolet vision in nocturnal sugar gliders (Petaurus breviceps).[5] The possibility of trichromacy in marsupials potentially has anotherevolutionary basis than that ofprimates. Furtherbiological andbehavioural tests may verify if trichromacy is a common characteristic of marsupials.[2]
Most other mammals are currently thought to bedichromats, with only two types of cone (though limited trichromacy is possible at low light levels where the rods and cones are both active).[6] Most studies of carnivores, as of other mammals, revealdichromacy; examples include the domesticdog, theferret, and thespotted hyena.[7][8] Some species ofinsects (such ashoneybees) are also trichromats, being sensitive toultraviolet, blue and green instead of blue, green and red.[3]
Research indicates that trichromacy allows animals to distinguish brightly colored fruit and young leaves from other vegetation that is not beneficial to their survival.[9] Another theory is that detecting skinflushing 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 thecolor psychology article.[10]
Primates are the only known placental mammalian trichromats.[11][failed verification] Their eyes include three different kinds of cones, each containing a differentphotopigment (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.[12] S cones make up 5–10% of the cones and form a regular mosaic. Specialbipolar andganglion cells pass those signals from S cones and there is evidence that they have a separate signal pathway through thethalamus to thevisual 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[13] did find that L cones and M cones are randomly distributed and are in equal numbers.[14]
Trichromatic color vision is the ability of humans and some other animals to see differentcolors, mediated by interactions among three types of color-sensingcone cells. Thetrichromatic color theory began in the 18th century, whenThomas Young proposed that color vision was a result of three differentphotoreceptor cells. From the middle of the 19th century, in hisTreatise on Physiological Optics,[15][16]Hermann von Helmholtz laterexpanded 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 byGunnar Svaetichin (1956).[17]
Each of the three types of cones in theretina of theeye contains a different type ofphotosensitive pigment, which is composed of atransmembrane protein calledopsin and a light-sensitive molecule called11-cis retinal. Each different pigment is especially sensitive to a certainwavelength oflight (that is, the pigment is most likely to produce acellular response when it is hit by aphoton 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 nm), medium (530 nm), and short (420 nm) wavelengths respectively.[18][19]
Since the likelihood of response of a given cone varies not only with thewavelength of the light that hits it but also with itsintensity, thebrain 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.[20]
{{cite journal}}: CS1 maint: DOI inactive as of July 2025 (link)