Tetrachromat
Encyclopedia
Color
Color or colour is the visual perceptual property corresponding in humans to the categories called red, green, blue and others. Color derives from the spectrum of light interacting in the eye with the spectral sensitivities of the light receptors...
information, or possessing four different types of cone cell
Cone cell
Cone cells, or cones, are photoreceptor cells in the retina of the eye that are responsible for color vision; they function best in relatively bright light, as opposed to rod cells that work better in dim light. If the retina is exposed to an intense visual stimulus, a negative afterimage will be...
s in the eye
Eye
Eyes are organs that detect light and convert it into electro-chemical impulses in neurons. The simplest photoreceptors in conscious vision connect light to movement...
. Organisms with tetrachromacy are called tetrachromats.
In tetrachromatic organisms, the sensory color space
Color space
A color model is an abstract mathematical model describing the way colors can be represented as tuples of numbers, typically as three or four values or color components...
is four-dimensional, meaning that to match the sensory effect of arbitrarily chosen spectra of light within their visible spectrum
Visible spectrum
The visible spectrum is the portion of the electromagnetic spectrum that is visible to the human eye. Electromagnetic radiation in this range of wavelengths is called visible light or simply light. A typical human eye will respond to wavelengths from about 390 to 750 nm. In terms of...
requires mixtures of at least four different primary color
Primary color
Primary colors are sets of colors that can be combined to make a useful range of colors. For human applications, three primary colors are usually used, since human color vision is trichromatic....
s.
Description
The normal explanation of tetrachromacy is that the organism's retinaRetina
The vertebrate retina is a light-sensitive tissue lining the inner surface of the eye. The optics of the eye create an image of the visual world on the retina, which serves much the same function as the film in a camera. Light striking the retina initiates a cascade of chemical and electrical...
contains four types of higher-intensity light receptors (called cone cell
Cone cell
Cone cells, or cones, are photoreceptor cells in the retina of the eye that are responsible for color vision; they function best in relatively bright light, as opposed to rod cells that work better in dim light. If the retina is exposed to an intense visual stimulus, a negative afterimage will be...
s in vertebrates as opposed to rod cell
Rod cell
Rod cells, or rods, are photoreceptor cells in the retina of the eye that can function in less intense light than can the other type of visual photoreceptor, cone cells. Named for their cylindrical shape, rods are concentrated at the outer edges of the retina and are used in peripheral vision. On...
s which are lower intensity light receptors) with different absorption spectra. This means the animal
Animal
Animals are a major group of multicellular, eukaryotic organisms of the kingdom Animalia or Metazoa. Their body plan eventually becomes fixed as they develop, although some undergo a process of metamorphosis later on in their life. Most animals are motile, meaning they can move spontaneously and...
may see wavelength
Wavelength
In physics, the wavelength of a sinusoidal wave is the spatial period of the wave—the distance over which the wave's shape repeats.It is usually determined by considering the distance between consecutive corresponding points of the same phase, such as crests, troughs, or zero crossings, and is a...
s beyond those of a typical human being's eyesight, and may be able to distinguish color
Color
Color or colour is the visual perceptual property corresponding in humans to the categories called red, green, blue and others. Color derives from the spectrum of light interacting in the eye with the spectral sensitivities of the light receptors...
s that to a human appear to be identical.
Animals that are tetrachromats
- The zebrafish (Danio rerio) is an example of a tetrachromat, containing cone cells sensitive for red, green, blue, and ultraviolet light.
- The mantis shrimpMantis shrimpMantis shrimp or stomatopods are marine crustaceans, the members of the order Stomatopoda. They are neither shrimp nor mantids, but receive their name purely from the physical resemblance to both the terrestrial praying mantis and the shrimp. They may reach in length, although exceptional cases of...
(Gonodactylus smithii) sees at least four basic colors.
- Most birdsBird visionVision is the most important sense for birds, since good eyesight is essential for safe flight, and this group has 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"...
are tetrachromats.
- Tetrachromacy is also suspected among several species of fishFishFish are a paraphyletic group of organisms that consist of all gill-bearing aquatic vertebrate animals that lack limbs with digits. Included in this definition are the living hagfish, lampreys, and cartilaginous and bony fish, as well as various extinct related groups...
, amphibianAmphibianAmphibians , are a class of vertebrate animals including animals such as toads, frogs, caecilians, and salamanders. They are characterized as non-amniote ectothermic tetrapods...
s, reptileReptileReptiles are members of a class of air-breathing, ectothermic vertebrates which are characterized by laying shelled eggs , and having skin covered in scales and/or scutes. They are tetrapods, either having four limbs or being descended from four-limbed ancestors...
s, arachnidArachnidArachnids are a class of joint-legged invertebrate animals in the subphylum Chelicerata. All arachnids have eight legs, although in some species the front pair may convert to a sensory function. The term is derived from the Greek words , meaning "spider".Almost all extant arachnids are terrestrial...
s and insectInsectInsects are a class of living creatures within the arthropods that have a chitinous exoskeleton, a three-part body , three pairs of jointed legs, compound eyes, and two antennae...
s.
- Some animals, notably pigeons and butterflies, are believed to possess pentachromacy.
- Marsupials and Arctic reindeer are the only mammals found to date that can see ultraviolet light, although both are believed to be trichromatic and not tetrachromatic.
Use of tetrachromacy
Tetrachromacy is a tool utilized in specific species to gain and compete at a higher level in the overall environment. The main idea behind the development of a four colored visionary system plays on the idea of natural selectionNatural selection
Natural selection is the nonrandom process by which biologic traits become either more or less common in a population as a function of differential reproduction of their bearers. It is a key mechanism of evolution....
and competition
Competition
Competition is a contest between individuals, groups, animals, etc. for territory, a niche, or a location of resources. It arises whenever two and only two strive for a goal which cannot be shared. Competition occurs naturally between living organisms which co-exist in the same environment. For...
pressures. Species which share this color vision contain a small physiological gain.
Use of tetrachromacy in mantis shrimp
Mantis shrimpMantis shrimp
Mantis shrimp or stomatopods are marine crustaceans, the members of the order Stomatopoda. They are neither shrimp nor mantids, but receive their name purely from the physical resemblance to both the terrestrial praying mantis and the shrimp. They may reach in length, although exceptional cases of...
utilize tetrachromatic vision utilizing all four wavelengths. Unlike most aquatic
Aquatic
Aquatic means relating to water; living in or near water or taking place in waterAquatic may also refer to:* Aquatic animal, either vertebrate or invertebrate, which lives in water for most or all of its life...
organisms, these shrimp use the ultraviolet
Ultraviolet
Ultraviolet light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than X-rays, in the range 10 nm to 400 nm, and energies from 3 eV to 124 eV...
wavelength for predator detection and evasion.
Use of tetrachromacy in birds
Some species of birds such as the Zebra FinchZebra Finch
The Zebra Finch, Taeniopygia guttata, is the most common and familiar estrildid finch of Central Australia and ranges over most of the continent, avoiding only the cool moist south and the tropical far north. It also can be found natively in Indonesia and East Timor...
and the Columbidae utilize the ultraviolet wavelength (300–400 nm) specific to tetrachromatic color vision as a tool during mate selection and foraging
Foraging
- Definitions and significance of foraging behavior :Foraging is the act of searching for and exploiting food resources. It affects an animal's fitness because it plays an important role in an animal's ability to survive and reproduce...
. When selecting for mates, ultraviolet plumage
Plumage
Plumage refers both to the layer of feathers that cover a bird and the pattern, colour, and arrangement of those feathers. The pattern and colours of plumage vary between species and subspecies and can also vary between different age classes, sexes, and season. Within species there can also be a...
and skin coloration show a high level of selection.
Use of tetrachromacy in insects
Floral colors were categorized into two main wavelengths of light including 360–520 nm and 400–500 nm. Flowers now reflect four broad domains of wavelength including 300–400 nm, 400–500 nm, 500–600 nm, and 600–700 nm. These wavelengths represent the colors ultraviolet (UV), blue, green, and red respectively in the color spectrum. Flowers utilize these wavelengths to differentiate color patterns within species. It has been determined, these difference in color patterns are used for behavioral attractions in pollinatorPollinator
A pollinator is the biotic agent that moves pollen from the male anthers of a flower to the female stigma of a flower to accomplish fertilization or syngamy of the female gamete in the ovule of the flower by the male gamete from the pollen grain...
insects increasing survival. Many trichromatic pollinators such as honeybees utilize ultraviolet, blue, and green wavelengths. As the color space within insects has become more and more filled, the increase in the wavelengths flowers reflect has increased as a result. Flower which reflect certain wavelengths such as UV and red wavelengths attract a greater number of pollinators. Pollination is a mutualistic relationship between plants and pollinators leading to an extremely high level of competition
Competition
Competition is a contest between individuals, groups, animals, etc. for territory, a niche, or a location of resources. It arises whenever two and only two strive for a goal which cannot be shared. Competition occurs naturally between living organisms which co-exist in the same environment. For...
. This competition has led to a coevolution between plants and foraging insects increasing the color variation in both orders leading to directional selection
Directional selection
In population genetics, directional selection is a mode of natural selection in which a single phenotype is favored, causing the allele frequency to continuously shift in one direction...
.
Foraging insects have the ability to see all four color wavelengths. Plants display increasingly diverse amounts of color variation extending into the ultraviolet color scale. Plants that display higher levels of color will in return attract higher levels of pollinators. Pollinators which maintain a wider range of color can utilize tetrachromatic color vision to increase and maintain a higher foraging success rate over their trichromatic competitors. Within tetrachromatic insects, background displays play a large role in the view of flower color variation. Flower which display pure color hue
Hue
Hue is one of the main properties of a color, defined technically , as "the degree to which a stimulus can be describedas similar to or different from stimuli that are described as red, green, blue, and yellow,"...
s are easily distinguished by the pollinating insect. When a pollinator encounters a flower, the insect can distinguish the flower from the background based on reflectance within the petals. This use of reflectance then draws the insect in directing it to the center reproductive organs of the plant.
Possibility of human tetrachromats
Humans and closely related primatesCatarrhini
Catarrhini is one of the two subdivisions of the higher primates . It contains the Old World monkeys and the apes, which in turn are further divided into the lesser apes or gibbons and the great apes, consisting of the orangutans, gorillas, chimpanzees, bonobos, and humans...
normally have three types of cone cells and are therefore trichromat
Trichromat
Trichromacy or trichromaticism is the condition of possessing three independent channels for conveying color information, derived from the three different cone types...
s (animals with three different cones). However, at low light intensities the rod cells may contribute to color vision, giving a small region of tetrachromacy in the color space.
In humans, two cone cell pigment genes are located on the sex X chromosome
X chromosome
The X chromosome is one of the two sex-determining chromosomes in many animal species, including mammals and is common in both males and females. It is a part of the XY sex-determination system and X0 sex-determination system...
, the classical type 2 opsin genes OPN1MW
OPN1MW
Green-sensitive opsin is a protein that in humans is encoded by the OPN1MW gene.-External links:* -Further reading:...
and OPN1MW2
OPN1MW2
OPN1MW2 is an opsin....
. It has been suggested that as women have two different X chromosomes in their cells, some of them could be carrying some variant cone cell pigments, thereby possibly being born as full tetrachromats and having four different simultaneously functioning kinds of cone cells, each type with a specific pattern of responsiveness to different wave lengths of light in the range of the visible spectrum. One study suggested that 2–3% of the world's women might have the kind of fourth cone that lies between the standard red and green cones, giving, theoretically, a significant increase in color differentiation.
Further studies will need to be conducted to verify tetrachromacy in humans. Two possible tetrachromats have been identified: "Mrs. M", an English social worker, was located in a study conducted in 1993,
and an unidentified female physician near Newcastle, England, was discovered in a study reported in 2006.
Neither case has been fully verified.
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
Color blindness
Color blindness or color vision deficiency is the inability or decreased ability to see color, or perceive color differences, under lighting conditions when color vision is not normally impaired...
" (protanomaly or deuteranomaly). The biological basis for this phenomenon is X-inactivation
X-inactivation
X-inactivation is a process by which one of the two copies of the X chromosome present in female mammals is inactivated. The inactive X chromosome is silenced by packaging into transcriptionally inactive heterochromatin...
of heterozygotic alleles 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 within 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 lump 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.
Mice, which normally have only two cone pigments, can be engineered to express a third cone pigment, and appear to demonstrate increased chromatic discrimination, arguing against some of these obstacles; however, the original publication's claims about plasticity in the optic nerve have also been disputed.
People with four photopigments have been shown to have increased chromatic discrimination in comparison to trichromats.
Each of the three cone types in a trichromatic human retina can pick up about 100 different gradations of color, and the brain can combine those variations so that the average human can distinguish about a million different colors; a true human tetrachromat would have another type of cone, and its 100 shades theoretically would allow them to see 100 million different colors.
See also
- RGBY
- Monochromacy
- Dichromacy
- Trichromacy
- Pentachromacy
- Evolution of color vision
- Somatosensory amplificationSomatosensory amplificationSomatosensory amplification 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 depressive disorder, some anxiety disorders, Asperger syndrome,...
- SupertasterSupertasterA supertaster is a person who experiences the sense of taste with far greater intensity than average. Women are more likely to be supertasters, as are individuals of Asian and African descent. The cause of this heightened response is currently unknown, although it is thought to be, at least in...
External links
- 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.
- Holba, Á.; Lukács, B. "On tetrachromacy"
- Looking for Madam Tetrachromat By Glenn Zorpette. Red Herring magazine, 1 November 2000
- Ultraviolet vision
- The Human is a blocked tetrachromat A review of the spectral sensitivity of the human visual system. (Suggests that the human lens is responsible for blocking the ultraviolet frequencies, that we already have a UV sensor in the retina ready and waiting, and if the UV wasn't blocked, we'd all be tetrachromats.)