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Tetrachromacy (from Greek ''tetra'', meaning "four" and ''chromo'', meaning "color") is the condition of possessing four independent channels for conveying
color Color (American English) or colour (British English) is the visual perceptual property deriving from the spectrum of light interacting with the photoreceptor cells of the eyes. Color categories and physical specifications of color are associa ...
information, or possessing four types of
cone cell Cone cells, or cones, are photoreceptor cells in the retinas of vertebrate eyes including the human eye. They respond differently to light of different wavelengths, and the combination of their responses is responsible for color vision. Cones ...
in the eye. Organisms with tetrachromacy are called tetrachromats. In tetrachromatic organisms, the sensory
color space A color space is a specific organization of colors. In combination with color profiling supported by various physical devices, it supports reproducible representations of colorwhether such representation entails an analog or a digital represen ...
is four-dimensional, meaning that matching the sensory effect of arbitrarily chosen spectra of light within their visible spectrum requires mixtures of at least four primary colors. Tetrachromacy is demonstrated among several species of birds, fishes,
amphibian Amphibians are four-limbed and ectothermic vertebrates of the class Amphibia. All living amphibians belong to the group Lissamphibia. They inhabit a wide variety of habitats, with most species living within terrestrial, fossorial, arborea ...
s, and reptiles. The common ancestor of all vertebrates was a tetrachromat, but mammals evolved dichromacy, due to the
nocturnal bottleneck The nocturnal bottleneck hypothesis is a hypothesis to explain several mammalian traits. In 1942, Gordon Lynn Walls described this concept which states that placental mammals were mainly or even exclusively nocturnal through most of their evolu ...
, losing two of their four cones. Trichromats can see approximately 100 million colour combinations, but a tetrachromat can see more than a billion color combinations.


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
spectral sensitivity Spectral sensitivity is the relative efficiency of detection, of light or other signal, as a function of the frequency or wavelength of the signal. In visual neuroscience, spectral sensitivity is used to describe the different characterist ...
. This means that the organism may see wavelengths beyond those of a typical human's vision, and may be able to distinguish between colors that, to a normal human, appear to be identical. Species with tetrachromatic color vision may have an unknown physiological advantage over rival species.


Humans

Apes Apes (collectively Hominoidea ) are a clade of Old World simians native to sub-Saharan Africa and Southeast Asia (though they were more widespread in Africa, most of Asia, and as well as Europe in prehistory), which together with its sister ...
(including
humans Humans (''Homo sapiens'') are the most abundant and widespread species of primate, characterized by bipedalism and exceptional cognitive skills due to a large and complex brain. This has enabled the development of advanced tools, culture, ...
) and
Old World monkeys Old World monkey is the common English name for a family of primates known taxonomically as the Cercopithecidae (). Twenty-four genera and 138 species are recognized, making it the largest primate family. Old World monkey genera include babo ...
normally have three types of cone cell and are therefore trichromats. However, human tetrachromacy may be possible in some situations. Tetrachromacy requires that there be 4 independent photoreceptor cell classes with different
spectral sensitivity Spectral sensitivity is the relative efficiency of detection, of light or other signal, as a function of the frequency or wavelength of the signal. In visual neuroscience, spectral sensitivity is used to describe the different characterist ...
. However, there must also be the appropriate post-receptoral mechanism to compare the signals from the four classes of receptors. According to the
opponent process The opponent process is a color theory that states that the human visual system interprets information about color by processing signals from photoreceptor cells in an antagonistic manner. The opponent-process theory suggests that there are thr ...
theory, humans have three opponent channels, which grant trichromacy. Whether a fourth opponent channel is available to facilitate tetrachromacy is unclear. Mice, which normally have only two cone pigments (and therefore two opponent channels), have been engineered to express a third cone pigment, and appear to demonstrate increased chromatic discrimination, possibly indicating trichromacy and suggesting they were able to create or re-enable a third opponent channel. This would support the theory that humans should be able to utilize a fourth opponent channel for tetrachromatic vision. However, the original publication's claims about plasticity in the optic nerve have also been disputed.


Tetrachromacy in carriers of CVD

It has been theorized that females who carry
recessive In genetics, dominance is the phenomenon of one variant (allele) of a gene on a chromosome masking or overriding the effect of a different variant of the same gene on the other copy of the chromosome. The first variant is termed dominant and t ...
opsin Animal opsins are G-protein-coupled receptors and a group of proteins made light-sensitive via a chromophore, typically retinal. When bound to retinal, opsins become Retinylidene proteins, but are usually still called opsins regardless. Most p ...
allele An allele (, ; ; modern formation from Greek ἄλλος ''állos'', "other") is a variation of the same sequence of nucleotides at the same place on a long DNA molecule, as described in leading textbooks on genetics and evolution. ::"The chro ...
s that can cause color vision deficiency (CVD) could possess tetrachromacy. Female carriers of anomalous trichromacy (mild color blindness) possess heterozygous alleles of the genes that encode the L-opsin or M-opsin. These alleles often have a different
spectral sensitivity Spectral sensitivity is the relative efficiency of detection, of light or other signal, as a function of the frequency or wavelength of the signal. In visual neuroscience, spectral sensitivity is used to describe the different characterist ...
, so if the carrier expresses both opsin alleles, they may exhibit tetrachromacy. In humans, two
cone cell Cone cells, or cones, are photoreceptor cells in the retinas of vertebrate eyes including the human eye. They respond differently to light of different wavelengths, and the combination of their responses is responsible for color vision. Cones ...
pigment genes are present on the
X chromosome The X chromosome is one of the two sex-determining chromosomes ( allosomes) in many organisms, including mammals (the other is the Y chromosome), and is found in both males and females. It is a part of the XY sex-determination system and XO sex ...
: the classical type 2 opsin gene OPN1MW. People with two X chromosomes could possess multiple cone cell pigments, perhaps born as full tetrachromats who have four simultaneously functioning kinds of cone cell, each type with a specific pattern of responsiveness to different wavelengths of light in the range of the visible spectrum. One study suggested that 15% of the world's women might have the type of fourth cone whose sensitivity peak is 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 and corresponding increased chromatic discrimination compared to trichromats. In 2010, after twenty years' study of women with four types of cones (non-functional tetrachromats), neuroscientist Gabriele Jordan identified a woman (subject ''cDa29'') who could detect a greater variety of colors than trichromats could, corresponding with a functional tetrachromat (or true tetrachromat). Variation in cone pigment genes is wide-spread 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 Color blindness or color vision deficiency (CVD) is the decreased ability to see color or differences in color. It can impair tasks such as selecting ripe fruit, choosing clothing, and reading traffic lights. Color blindness may make some aca ...
" ( protanomaly or deuteranomaly). The biological basis for this phenomenon is X-inactivation of heterozygotic
allele An allele (, ; ; modern formation from Greek ἄλλος ''állos'', "other") is a variation of the same sequence of nucleotides at the same place on a long DNA molecule, as described in leading textbooks on genetics and evolution. ::"The chro ...
s 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 in 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 combine 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. Tetrachromacy may also enhance vision in dim lighting, or in looking at a screen.


Conditional tetrachromacy

Despite being trichromats, humans can experience slight tetrachromacy at low light intensities, using their
mesopic vision Mesopic vision, sometimes also called twilight vision, is a combination of photopic and scotopic vision under low-light (but not necessarily dark) conditions. Mesopic levels range approximately from 0.01 to 3.0  cd/m2 in luminance. Most nig ...
. In mesopic vision, both
cone cell Cone cells, or cones, are photoreceptor cells in the retinas of vertebrate eyes including the human eye. They respond differently to light of different wavelengths, and the combination of their responses is responsible for color vision. Cones ...
s and rod cells are active. While rods typically don't contribute to color vision, they may in these specific light conditions, giving a small region of tetrachromacy in the color space. Human rod cell sensitivity is greatest at 500 nm (bluish-green) wavelength, which is significantly different from the peak
spectral sensitivity Spectral sensitivity is the relative efficiency of detection, of light or other signal, as a function of the frequency or wavelength of the signal. In visual neuroscience, spectral sensitivity is used to describe the different characterist ...
of the cones (typically 420, 530 and 560 nm).


Blocked tetrachromacy

Although many birds are tetrachromats with a fourth color in the ultraviolet, humans cannot see ultraviolet light directly because the
lens of the eye The lens, or crystalline lens, is a transparent biconvex structure in the eye that, along with the cornea, helps to refract light to be focused on the retina. By changing shape, it functions to change the focal length of the eye so that it ca ...
blocks most light in the wavelength range of 300–400 nm; shorter wavelengths are blocked by the
cornea The cornea is the transparent front part of the eye that covers the iris, pupil, and anterior chamber. Along with the anterior chamber and lens, the cornea refracts light, accounting for approximately two-thirds of the eye's total optical powe ...
. The photoreceptor cells of the retina are sensitive to near ultraviolet light, and people lacking a lens (a condition known as
aphakia Aphakia is the absence of the lens of the eye, due to surgical removal, such as in cataract surgery, a perforating wound or ulcer, or congenital anomaly. It causes a loss of accommodation, high degree of farsightedness (hyperopia), and a deep an ...
) see near ultraviolet light (down to 300 nm) as whitish blue, or for some wavelengths, whitish violet, probably because all three types of cones are roughly equally sensitive to ultraviolet light (with blue cone cells slightly more sensitive). While an extended visible range does not denote tetrachromacy, some believe that visual pigments are available with sensitivity in near-UV wavelengths that would enable tetrachromacy in the case of
aphakia Aphakia is the absence of the lens of the eye, due to surgical removal, such as in cataract surgery, a perforating wound or ulcer, or congenital anomaly. It causes a loss of accommodation, high degree of farsightedness (hyperopia), and a deep an ...
. However, there is no peer-reviewed evidence supporting this claim.


Other animals


Fish

Fish, specifically
teleosts Teleostei (; Greek ''teleios'' "complete" + ''osteon'' "bone"), members of which are known as teleosts ), is, by far, the largest infraclass in the class Actinopterygii, the ray-finned fishes, containing 96% of all extant species of fish. Teleo ...
, are typically tetrachromats. Exceptions include: * Sharks and Rays – range from monochromacy to trichromacy * Deep-sea fish – often rod monochromats *
Cichlid Cichlids are fish from the family Cichlidae in the order Cichliformes. Cichlids were traditionally classed in a suborder, the Labroidei, along with the wrasses ( Labridae), in the order Perciformes, but molecular studies have contradicted this ...
– arguably pentachromacy or higher


Birds

Some species of birds, such as the zebra finch and the
Columbidae Columbidae () is a bird family consisting of doves and pigeons. It is the only family in the order Columbiformes. These are stout-bodied birds with short necks and short slender bills that in some species feature fleshy ceres. They primarily ...
, use the ultraviolet wavelength 300–400 nm specific to tetrachromatic color vision as a tool during
mate selection Mate choice is one of the primary mechanisms under which evolution can occur. It is characterized by a "selective response by animals to particular stimuli" which can be observed as behavior.Bateson, Paul Patrick Gordon. "Mate Choice." Mate Choic ...
and foraging. When selecting for mates, ultraviolet
plumage Plumage ( "feather") is a layer of feathers that covers a bird and the pattern, colour, and arrangement of those feathers. The pattern and colours of plumage differ between species and subspecies and may vary with age classes. Within species, ...
and skin coloration show a high level of selection. A typical bird eye responds to wavelengths from about 300 to 700 nm. In terms of frequency, this corresponds to a band in the vicinity of 430–1000 THz. Most birds have retinas with four spectral types of cone cell that are believed to mediate tetrachromatic color vision. Bird color vision is further improved by filtering by pigmented oil droplets in the photoreceptors. The oil droplets filter incident light before it reaches the visual pigment in the outer segments of the photoreceptors. The four cone types, and the specialization of pigmented oil droplets, give birds better color vision than that of humans. However, more recent research has suggested that tetrachromacy in birds only provides birds with a larger visual spectrum than that in humans (humans cannot see ultraviolet light, 300-400 nm), while the spectral resolution (the "sensitivity" to nuances) is similar.


Pentachromacy and higher

The
dimensionality In physics and mathematics, the dimension of a mathematical space (or object) is informally defined as the minimum number of coordinates needed to specify any point within it. Thus, a line has a dimension of one (1D) because only one coordin ...
of color vision has no upper bound, but vertebrates with color vision higher than tetrachromacy are rare. The next level is ''Pentachromacy'', which is five-dimensional color vision requiring at least 5 different classes of photoreceptor as well as 5 independent
channels Channel, channels, channeling, etc., may refer to: Geography * Channel (geography), in physical geography, a landform consisting of the outline (banks) of the path of a narrow body of water. Australia * Channel Country, region of outback Austral ...
of
color Color (American English) or colour (British English) is the visual perceptual property deriving from the spectrum of light interacting with the photoreceptor cells of the eyes. Color categories and physical specifications of color are associa ...
information through the primary visual system. A female that is heterozygous for both the LWS and MWS opsins (and therefore a carrier for both protanomaly and deuteranomaly) would express five
opsin Animal opsins are G-protein-coupled receptors and a group of proteins made light-sensitive via a chromophore, typically retinal. When bound to retinal, opsins become Retinylidene proteins, but are usually still called opsins regardless. Most p ...
s of different
spectral sensitivity Spectral sensitivity is the relative efficiency of detection, of light or other signal, as a function of the frequency or wavelength of the signal. In visual neuroscience, spectral sensitivity is used to describe the different characterist ...
. However, for her to be a true (strong) pentachromat, these opsins would need to be segregated into different photoreceptor cells and she would need to have the appropriate post-receptoral mechanisms to handle 5
opponent process The opponent process is a color theory that states that the human visual system interprets information about color by processing signals from photoreceptor cells in an antagonistic manner. The opponent-process theory suggests that there are thr ...
channels, which is contentious. Some birds (notably
pigeon Columbidae () is a bird family consisting of doves and pigeons. It is the only family in the order Columbiformes. These are stout-bodied birds with short necks and short slender bills that in some species feature fleshy ceres. They primarily ...
s) have five or more kinds of color receptors in their retinae, and are therefore believed to be pentachromats, though psychophysical evidence of functional pentachromacy is lacking. Research also indicates that some
lamprey Lampreys (sometimes inaccurately called lamprey eels) are an ancient extant lineage of jawless fish of the order Petromyzontiformes , placed in the superclass Cyclostomata. The adult lamprey may be characterized by a toothed, funnel-like ...
s, members of the
Petromyzontiformes Lampreys (sometimes inaccurately called lamprey eels) are an ancient extant lineage of jawless fish of the order Petromyzontiformes , placed in the superclass Cyclostomata. The adult lamprey may be characterized by a toothed, funnel-like s ...
, may be pentachromats. Invertebrates can have large numbers of different opsin classes, including 15 opsins in bluebottle butterflies or 33 in Mantis Shrimp. However, it has not been shown that color vision in these invertebrates is of a dimension commensurate with the number of opsins.


See also

* Dimensionality of color vision * Monochromacy * Dichromacy *
Trichromacy Trichromacy or trichromatism is the possessing of three independent channels for conveying color information, derived from the three different types of cone cells in the eye. Organisms with trichromacy are called trichromats. The normal expl ...
*
Evolution of color vision Color vision, a proximate adaptation of the vision sensory modality, allows for the discrimination of light based on its wavelength components. Improved detection sensitivity The evolutionary process of switching from a single photopigment to t ...
* Infrared vision * RG color space * RGBY *
Somatosensory amplification Somatosensory amplification (SSA) is a tendency to perceive normal somatic and visceral sensations as being relatively intense, disturbing and noxious. It is a common feature of hypochondriasis and is commonly found with fibromyalgia, major depress ...
*
Supertaster A supertaster is a person whose sense of taste is of far greater intensity than the average person. Some studies also show that an increased sensitivity to bitter tastes may be a cause of selective eating. History The term originated with ...


References


External links


Goldsmith, 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.

By Glenn Zorpette. ''Red Herring'' magazine, 1 November 2000 *

University of Bristol The University of Bristol is a Red brick university, red brick Russell Group research university in Bristol, England. It received its royal charter in 1909, although it can trace its roots to a Society of Merchant Venturers, Merchant Venturers' sc ...
School of Biological Sciences. March 20, 2009.
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:


Ágnes Holba & B. Lukács {{Color topics Color vision