Tetrachromacy (fromAncient Greektetra, meaning "four" andchroma, meaning "color") is the condition of possessing four independent channels for conveyingcolor information, or possessing four types ofcone cell in theeye. Organisms with tetrachromacy are called tetrachromats.
In tetrachromatic organisms, the sensorycolor space is four-dimensional, meaning that matching the sensory effect of arbitrarily chosen spectra of light within theirvisible spectrum requires mixtures of at least fourprimary colors.
Tetrachromacy is demonstrated among several species ofbirds,[2]fishes,[3] andreptiles.[3] The common ancestor of allvertebrates was a tetrachromat, but a common ancestor ofmammals lost two of its four kinds of cone cell, evolvingdichromacy, a loss ascribed to the conjecturednocturnal bottleneck. Someprimates then later evolved a third cone.[4]
The normal explanation of tetrachromacy is that the organism'sretina contains four types of higher-intensity light receptors (called cone cells in vertebrates as opposed torod cells, which are lower-intensity light receptors) with differentspectral sensitivity. 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 beidentical. Species with tetrachromatic color vision may have an unknown physiological advantage over rival species.[5]
Apes (includinghumans) andOld World monkeys normally have only three types of cone cell, and are thereforetrichromats. However, human tetrachromacy is suspected to exist in a small percentage of the population. Trichromats have three types of cone cells, each type being sensitive to a corresponding portion of the spectrum as shown in the diagram. But at least one woman has been implied to be a tetrachromat.[6] More precisely, she had an additional cone typeL′, intermediate betweenM andL in its responsivity, and showed 3 dimensional (M,L′, andL components) color discrimination for wavelengths 546–670 nm (to which the fourth type,S, is insensitive).
Tetrachromacy requires that there be four independentphotoreceptor cell classes with differentspectral sensitivity. However, there must also be the appropriate post-receptoral mechanism to compare the signals from the four classes of receptors. According to theopponent process theory, humans have three opponent channels, which give trichromacy. It is unclear whether having available a fourth opponent channel is sufficient for tetrachromacy.[citation needed]
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,[7]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.[8]
It has been theorized that females who carryrecessiveopsinalleles that can causecolor vision deficiency (CVD) could possess tetrachromacy. Femalecarriers of anomalous trichromacy (mild color blindness) possessheterozygous alleles of the genes that encode theL-opsin orM-opsin. These alleles often have a differentspectral sensitivity, so if the carrier expresses both opsin alleles, they may exhibit tetrachromacy.
In humans, twocone cell pigment genes are present on theX chromosome: Theclassical type 2 opsin geneOPN1MW andOPN1MW2. 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.[9]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, theoretically giving a significant increase in color differentiation.[10]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.[11]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 or "true" tetrachromat.[6][12]Specifically, she has been shown to be a trichromat in the range 546–670 nm where people with normal vision are essentially dichromats due to negligible response ofS cones to those wavelengths. Thus, ifS cones of 'cDa29' provide independent color perception dimension as they normally do, that would confirm her being a tetrachromat when the whole spectrum is considered.
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 ordeuteranomaly). The biological basis for this phenomenon isX-inactivation of heterozygotic alleles for retinal pigment genes, which is the same mechanism that gives the majority of femaleNew World monkeys trichromatic vision.[13]
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: Whether they would handle it separately, or just combine it with one of the existing channels. Similarly, visual information leaves the eye by way of the optic nerve, and a variety of final image processing takes place in the brain; it is not known whether the optic nerve or the areas of the brain have any capacity to effectively respond if presented with a stimulus from anew color signal.
Tetrachromacy may also enhance vision in dim lighting,[14] or in looking at a screen.[14][failed verification]
Despite being trichromats, humans can experience slight tetrachromacy at lowlight intensities, using theirmesopic vision. In mesopic vision, bothcone cells androd cells are active. While rods typically do not contribute to color vision, in these specific light conditions, they may give a small region of tetrachromacy in the color space.[15]Human rod cell sensitivity is greatest at 500 nm (bluish-green) wavelength, which is significantly different from the peakspectral sensitivity of the cones (typically 420, 530, and 560 nm).
Although many birds are tetrachromats with a fourth color in the ultraviolet, humans cannot see ultraviolet light directly because thelens of the eye blocks most light in the wavelength range of 300–400 nm; shorter wavelengths are blocked by thecornea.[16]Thephotoreceptor cells of theretina are sensitive tonear ultraviolet light, and people lacking a lens (a condition known asaphakia) 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).[17]
While an extended visible range does not denote tetrachromacy, some believe that visual pigments are available with sensitivity innear-UV wavelengths that would enable tetrachromacy in the case ofaphakia.[18]However, there is no peer-reviewed evidence supporting this claim.
Fish, specificallyteleosts, are typically tetrachromats.[3] Exceptions include:
Some species of birds, such as thezebra finch and theColumbidae, use the ultraviolet wavelength 300–400 nm specific to tetrachromatic color vision as a tool duringmate selection andforaging.[19] When selecting for mates, ultravioletplumage and skin coloration show a high level of selection.[20] A typical bird eye responds to wavelengths from about 300–700 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.[21][22] 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.[23]
Some birds such ascorvids,Old andNew World flycatchers,[24] as well as most diurnalraptors,[25][26] have little ability to see UV light, with the fourth cone type instead peaking in theviolet range. It is believed that UV vision in raptors isselected against because short-wavelengthUVA light contributes highly tochromatic aberration, reducing visual acuity which raptorial birds rely on for hunting.[25]
Thedimensionality of color vision has no upper bound, butvertebrates with color vision greater than tetrachromacy are rare. The next level ispentachromacy, which is five-dimensional color vision requiring at least 5 different classes ofphotoreceptor as well as 5 independentchannels ofcolor information through the primary visual system.
A female that is heterozygous for both theLWS andMWS opsins (and therefore a carrier for bothprotanomaly anddeuteranomaly) would express fiveopsins of differentspectral sensitivity. However, for her to be a true (strong) pentachromat, these opsins would need to be segregated into differentphotoreceptor cells and she would need to have the appropriate post-receptoral mechanisms to handle 5opponent process channels, which is contentious.[citation needed]
Somebirds (notablypigeons) have five or more kinds of color receptors in their retinae, and are therefore believed to be pentachromats, thoughpsychophysical evidence of functional pentachromacy is lacking.[27] Research also indicates that somelampreys, members of thePetromyzontiformes, may be pentachromats.[28]
Invertebrates can have large numbers of different opsin classes, including 15 opsins inbluebottle butterflies[29] or 33 inmantis shrimp.[30] However, it has not been shown that color vision in these invertebrates is of adimension commensurate with the number of opsins.
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