Tetrachromacy

It has been theorized that females who carry recessive opsin alleles 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, so if the carrier expresses both opsin alleles, they may exhibit tetrachromacy.

In humans, two cone cell pigment genes are present on the X chromosome: 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.[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, giving, theoretically, 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]

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 or deuteranomaly). The biological basis for this phenomenon is X-inactivation of heterozygotic alleles for retinal pigment genes, which is the same mechanism that gives the majority of female New 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, that is, whether they could handle it separately or just combine it in with an existing channel. 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 the spare capacity to handle or respond if presented with a new color channel.

Tetrachromacy may also enhance vision in dim lighting, or in looking at a screen.[14]

Despite being trichromats, humans can experience slight tetrachromacy at low light intensities, using their mesopic vision. In mesopic vision, both cone cells 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.[15] Human rod cell sensitivity is greatest at 500 nm (bluish-green) wavelength, which is significantly different from the peak spectral 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 the lens of the eye blocks most light in the wavelength range of 300–400 nm; shorter wavelengths are blocked by the cornea.[16] The photoreceptor cells of the retina are sensitive to near ultraviolet light, and people lacking a lens (a condition known as aphakia) 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 in near-UV wavelengths that would enable tetrachromacy in the case of aphakia.[18] However, there is no peer-reviewed evidence supporting this claim.

Fish, specifically teleosts, are typically tetrachromats.[3] Exceptions include:

• Sharks and rays – range from monochromacy to trichromacy[3]
• Deep-sea fish – often rod monochromats
• Cichlid – arguably pentachromacy or higher[3]

Some species of birds, such as the zebra finch and the Columbidae, use the ultraviolet wavelength 300–400 nm specific to tetrachromatic color vision as a tool during mate selection and foraging.[19] When selecting for mates, ultraviolet plumage and skin coloration show a high level of selection.[20] 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.[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]

Many diurnal birds of prey such as eagles, falcons and hawks have limited ability to perceive ultraviolet light, as this contributes heavily to chromatic aberration which reduces visual acuity.[24]