Tetrachromacy

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The four pigments in a bird's cone cells (in this example, estrildid finches) extend the range of color vision into the ultraviolet. BirdVisualPigmentSensitivity.svg
The four pigments in a bird's cone cells (in this example, estrildid finches) extend the range of color vision into the ultraviolet.

Tetrachromacy (from Greek tetra, meaning "four" and chroma, meaning "color") is the condition of possessing four independent channels for conveying color information, or possessing four types of cone cell in the eye. Organisms with tetrachromacy are called tetrachromats.

Contents

In tetrachromatic organisms, the sensory color space 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, [2] fishes, [3] and reptiles. [3] The common ancestor of all vertebrates was a tetrachromat, but a common ancestor of mammals lost two of its four kinds of cone cell, evolving dichromacy, a loss ascribed to the conjectured nocturnal bottleneck. Some primates then later evolved a third cone. [4]

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. 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. [5]

Humans

Normalized responsivity spectra of human cone cells, S, M, and L types. Cones SMJ2 E.svg
Normalized responsivity spectra of human cone cells, S, M, and L types.

Apes (including humans) and Old World monkeys normally have three types of cone cell and are therefore trichromats. 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 type L', intermediate between M and L in its responsivity, and showed 3-dimensional (M, L' and L components) color discrimination for wavelengths 546-670nm (to which the fourth type S is insensitive).

Tetrachromacy requires that there be four independent photoreceptor cell classes with different spectral sensitivity. 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 theory, humans have three opponent channels, which grant trichromacy. Whether a fourth opponent channel is available to facilitate tetrachromacy is unclear.[ 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]

Tetrachromacy in carriers of CVD

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 and OPN1MW2. 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] Specifically, she has been shown to be a trichromat in the range 546-670nm where people with normal vision are essentially dichromats due to negligeable response of S-cones to those wavelengths. Thus, if S-cones of cDa29 provide independent color perception dimension as they normally do, that would confirm her being a tetrachomat 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 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]

Conditional tetrachromacy

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 do not contribute to color vision, they may, in these specific light conditions, 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 peak spectral sensitivity 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 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.

Other animals

Goldfish have tetrachromacy. Katri.jpg
Goldfish have tetrachromacy.

Fish

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

Birds

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]

Pentachromacy and greater

The dimensionality of color vision has no upper bound, but vertebrates with color vision greater 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 of color 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 opsins of different spectral sensitivity. 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 channels, which is contentious. [ citation needed ]

Some birds (notably pigeons) 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. [25] Research also indicates that some lampreys, members of the Petromyzontiformes, may be pentachromats. [26]

Invertebrates can have large numbers of different opsin classes, including 15 opsins in bluebottle butterflies [27] or 33 in mantis shrimp. [28] However, it has not been shown that color vision in these invertebrates is of a dimension commensurate with the number of opsins.

See also

Related Research Articles

<span class="mw-page-title-main">Color</span> Visual perception of the light spectrum

Color or colour is the visual perception based on the electromagnetic spectrum. Though color is not an inherent property of matter, color perception is related to an object's light absorption, reflection, emission spectra and interference. For most humans, colors are perceived in the visible light spectrum with three types of cone cells (trichromacy). Other animals may have a different number of cone cell types or have eyes sensitive to different wavelength, such as bees that can distinguish ultraviolet, and thus have a different color sensitivity range. Animal perception of color originates from different light wavelength or spectral sensitivity in cone cell types, which is then processed by the brain.

<span class="mw-page-title-main">Visible spectrum</span> Portion of the electromagnetic spectrum that is visible to the human eye

The visible spectrum is the band of the electromagnetic spectrum that is visible to the human eye. Electromagnetic radiation in this range of wavelengths is called visible light. The optical spectrum is sometimes considered to be the same as the visible spectrum, but some authors define the term more broadly, to include the ultraviolet and infrared parts of the electromagnetic spectrum as well, known collectively as optical radiation.

<span class="mw-page-title-main">Color vision</span> Ability to perceive differences in light frequency

Color vision, a feature of visual perception, is an ability to perceive differences between light composed of different frequencies independently of light intensity.

<span class="mw-page-title-main">Photoreceptor cell</span> Type of neuroepithelial cell

A photoreceptor cell is a specialized type of neuroepithelial cell found in the retina that is capable of visual phototransduction. The great biological importance of photoreceptors is that they convert light into signals that can stimulate biological processes. To be more specific, photoreceptor proteins in the cell absorb photons, triggering a change in the cell's membrane potential.

<span class="mw-page-title-main">Trichromacy</span> Possessing of three independent channels for conveying color information

Trichromacy or trichromatism is the possession 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.

<span class="mw-page-title-main">Cone cell</span> Photoreceptor cells responsible for color vision made to function in bright light

Cone cells or cones are photoreceptor cells in the retinas of vertebrates' eyes. They respond differently to light of different wavelengths, and the combination of their responses is responsible for color vision. Cones function best in relatively bright light, called the photopic region, as opposed to rod cells, which work better in dim light, or the scotopic region. Cone cells are densely packed in the fovea centralis, a 0.3 mm diameter rod-free area with very thin, densely packed cones which quickly reduce in number towards the periphery of the retina. Conversely, they are absent from the optic disc, contributing to the blind spot. There are about six to seven million cones in a human eye, with the highest concentration being towards the macula.

In visual physiology, adaptation is the ability of the retina of the eye to adjust to various levels of light. Natural night vision, or scotopic vision, is the ability to see under low-light conditions. In humans, rod cells are exclusively responsible for night vision as cone cells are only able to function at higher illumination levels. Night vision is of lower quality than day vision because it is limited in resolution and colors cannot be discerned; only shades of gray are seen. In order for humans to transition from day to night vision they must undergo a dark adaptation period of up to two hours in which each eye adjusts from a high to a low luminescence "setting", increasing sensitivity hugely, by many orders of magnitude. This adaptation period is different between rod and cone cells and results from the regeneration of photopigments to increase retinal sensitivity. Light adaptation, in contrast, works very quickly, within seconds.

Dichromacy is the state of having two types of functioning photoreceptors, called cone cells, in the eyes. Organisms with dichromacy are called dichromats. Dichromats require only two primary colors to be able to represent their visible gamut. By comparison, trichromats need three primary colors, and tetrachromats need four. Likewise, every color in a dichromat's gamut can be evoked with monochromatic light. By comparison, every color in a trichromat's gamut can be evoked with a combination of monochromatic light and white light.

<span class="mw-page-title-main">Monochromacy</span> Type of color vision

Monochromacy is the ability of organisms to perceive only light intensity without respect to spectral composition. Organisms with monochromacy lack color vision and can only see in shades of grey ranging from black to white. Organisms with monochromacy are called monochromats. Many mammals, such as cetaceans, the owl monkey and the Australian sea lion are monochromats. In humans, monochromacy is one among several other symptoms of severe inherited or acquired diseases, including achromatopsia or blue cone monochromacy, together affecting about 1 in 30,000 people.

<span class="mw-page-title-main">Melanopsin</span> Mammalian protein found in Homo sapiens

Melanopsin is a type of photopigment belonging to a larger family of light-sensitive retinal proteins called opsins and encoded by the gene Opn4. In the mammalian retina, there are two additional categories of opsins, both involved in the formation of visual images: rhodopsin and photopsin in the rod and cone photoreceptor cells, respectively.

<span class="mw-page-title-main">Opsin</span> Class of light-sensitive proteins

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 prominently, they are found in photoreceptor cells of the retina. Five classical groups of opsins are involved in vision, mediating the conversion of a photon of light into an electrochemical signal, the first step in the visual transduction cascade. Another opsin found in the mammalian retina, melanopsin, is involved in circadian rhythms and pupillary reflex but not in vision. Humans have in total nine opsins. Beside vision and light perception, opsins may also sense temperature, sound, or chemicals.

In the study of visual perception, scotopic vision is the vision of the eye under low-light conditions. The term comes from Greek skotos, meaning "darkness", and -opia, meaning "a condition of sight". In the human eye, cone cells are nonfunctional in low visible light. Scotopic vision is produced exclusively through rod cells, which are most sensitive to wavelengths of around 498 nm and are insensitive to wavelengths longer than about 640 nm. Under scotopic conditions, light incident on the retina is not encoded vis-à-vis the spectral power distribution.

Intrinsically photosensitive retinal ganglion cells (ipRGCs), also called photosensitive retinal ganglion cells (pRGC), or melanopsin-containing retinal ganglion cells (mRGCs), are a type of neuron in the retina of the mammalian eye. The presence of something like ipRGCs was first suspected in 1927 when rodless, coneless mice still responded to a light stimulus through pupil constriction, This implied that rods and cones are not the only light-sensitive neurons in the retina. Yet research on these cells did not advance until the 1980s. Recent research has shown that these retinal ganglion cells, unlike other retinal ganglion cells, are intrinsically photosensitive due to the presence of melanopsin, a light-sensitive protein. Therefore, they constitute a third class of photoreceptors, in addition to rod and cone cells.

<span class="mw-page-title-main">OPN1LW</span> Protein-coding gene in humans

OPN1LW is a gene on the X chromosome that encodes for long wave sensitive (LWS) opsin, or red cone photopigment. It is responsible for perception of visible light in the yellow-green range on the visible spectrum. The gene contains 6 exons with variability that induces shifts in the spectral range. OPN1LW is subject to homologous recombination with OPN1MW, as the two have very similar sequences. These recombinations can lead to various vision problems, such as red-green colourblindness and blue monochromacy. The protein encoded is a G-protein coupled receptor with embedded 11-cis-retinal, whose light excitation causes a cis-trans conformational change that begins the process of chemical signalling to the brain.

<span class="mw-page-title-main">Evolution of color vision in primates</span> Loss and regain of colour vision during the evolution of primates

The evolution of color vision in primates is highly unusual compared to most eutherian mammals. A remote vertebrate ancestor of primates possessed tetrachromacy, but nocturnal, warm-blooded, mammalian ancestors lost two of four cones in the retina at the time of dinosaurs. Most teleost fish, reptiles and birds are therefore tetrachromatic while most mammals are strictly dichromats, the exceptions being some primates and marsupials, who are trichromats, and many marine mammals, who are monochromats.

Color vision, a proximate adaptation of the vision sensory modality, allows for the discrimination of light based on its wavelength components.

<span class="mw-page-title-main">Bird vision</span> Senses for birds

Vision is the most important sense for birds, since good eyesight is essential for safe flight. Birds have a number of adaptations which give visual acuity superior to that of other vertebrate groups; a pigeon has been described as "two eyes with wings". Birds are theropod dinosaurs, and the avian eye resembles that of other reptiles, with ciliary muscles that can change the shape of the lens rapidly and to a greater extent than in the mammals. Birds have the largest eyes relative to their size in the animal kingdom, and movement is consequently limited within the eye's bony socket. In addition to the two eyelids usually found in vertebrates, bird's eyes are protected by a third transparent movable membrane. The eye's internal anatomy is similar to that of other vertebrates, but has a structure, the pecten oculi, unique to birds.

Gene therapy for color blindness is an experimental gene therapy of the human retina aiming to grant typical trichromatic color vision to individuals with congenital color blindness by introducing typical alleles for opsin genes. Animal testing for gene therapy began in 2007 with a 2009 breakthrough in squirrel monkeys suggesting an imminent gene therapy in humans. While the research into gene therapy for red-green colorblindness has lagged since then, successful human trials are ongoing for achromatopsia. Congenital color vision deficiency affects upwards of 200 million people in the world, which represents a large demand for this gene therapy.

<span class="mw-page-title-main">Congenital red–green color blindness</span> Most common genetic condition leading to color blindness

Congenital red–green color blindness is an inherited condition that is the root cause of the majority of cases of color blindness. It has no significant symptoms aside from its minor to moderate effect on color vision. It is caused by variation in the functionality of the red and/or green opsin proteins, which are the photosensitive pigment in the cone cells of the retina, which mediate color vision. Males are more likely to inherit red–green color blindness than females, because the genes for the relevant opsins are on the X chromosome. Screening for congenital red–green color blindness is typically performed with the Ishihara or similar color vision test. There is no cure for color blindness.

<span class="mw-page-title-main">Vertebrate visual opsin</span>

Vertebrate visual opsins are a subclass of ciliary opsins and mediate vision in vertebrates. They include the opsins in human rod and cone cells. They are often abbreviated to opsin, as they were the first opsins discovered and are still the most widely studied opsins.

References

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