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Dichromacy (from
Greek Greek may refer to: Greece Anything of, from, or related to Greece, a country in Southern Europe: *Greeks, an ethnic group. *Greek language, a branch of the Indo-European language family. **Proto-Greek language, the assumed last common ancestor ...
''di'', meaning "two" and ''chromo'', meaning "color") is the state of having two types of functioning photoreceptors, called
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. Cone ...
s, in the eyes. Organisms with dichromacy are called dichromats. Dichromats require only two
primary color A set (mathematics), set of primary colors or primary colours (see American and British English spelling differences#-our, -or, spelling differences) consists of colorants or colored lights that can be mixed in varying amounts to produce a gamu ...
s to be able to represent their visible
gamut In color reproduction, including computer graphics and photography, the gamut, or color gamut , is a certain ''complete subset'' of colors. The most common usage refers to the subset of colors which can be accurately represented in a given circ ...
. By comparison,
trichromat 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 exp ...
s need three primary colors, and
tetrachromat Tetrachromacy (from Greek ''tetra'', meaning "four" and ''chromo'', 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 te ...
s need four. Likewise, every color in a dichromat's gamut can be evoked with
monochromatic light {{Short description, Electromagnetic radiation with a single constant frequency In physics, monochromatic radiation is electromagnetic radiation with a single constant frequency. When that frequency is part of the visible spectrum (or near it) the ...
. By comparison, every color in a trichromat's gamut can be evoked with a combination of monochromatic light and white light. Dichromacy in humans is a color vision deficiency in which one of the three cone cells is absent or not functioning and color is thereby reduced to two dimensions."Guidelines: Colour Blindness."
Tiresias.org. Retrieved 29 September 2006.


Perception

Dichromatic color vision is enabled by two types of cone cells with different spectral sensitivities and the neural framework to compare the excitation of the different cone cells. The resulting color vision is simpler than typical human trichromatic color vision, and much simpler than tetrachromatic color vision, typical of birds and fish. A dichromatic color space can be defined by only ''two''
primary color A set (mathematics), set of primary colors or primary colours (see American and British English spelling differences#-our, -or, spelling differences) consists of colorants or colored lights that can be mixed in varying amounts to produce a gamu ...
s. When these primary colors are also the
unique hues Unique hue is a term used in certain theories of color vision, which implies that human perception distinguishes between "unique" (psychologically primary) and composite (mixed) hues. A unique hue is defined as a color which an observer perceive ...
, then the color space contains the individuals ''entire'' gamut. In dichromacy, the unique hues can be evoked by exciting only a single cone at a time, e.g. monochromatic light near the extremes of the visible spectrum. A dichromatic color space can also be defined by non-unique hues, but the color space will not contain the individual's entire gamut. For comparison, a trichromatic color space requires three primary colors to be defined. However, even when choosing three pure spectral colors as the primaries, the resulting color space will never encompass the entire trichromatic individual's gamut. The color vision of dichromats can be represented in a 2-dimensional plane, where one coordinate represented brightness, and the other coordinate represents hue. However, the perception of hue is not directly analogous to trichromatic
hue In color theory, hue is one of the main properties (called color appearance parameters) of a color, defined technically in the CIECAM02 model as "the degree to which a stimulus can be described as similar to or different from stimuli that ...
, but rather a spectrum diverging from white (neutral) in the middle to two unique hues at the extreme, e.g. blue and yellow. Unlike trichromats, white (experienced when both cone cells are equally excited) can be evoked by monochromatic light. This means that dichromats see white in the
rainbow A rainbow is a meteorological phenomenon that is caused by reflection, refraction and dispersion of light in water droplets resulting in a spectrum of light appearing in the sky. It takes the form of a multicoloured circular arc. Rainbows c ...
.


Humans

Dichromacy in humans is a form of color blindness (color vision deficiency). Normal human
color vision Color vision, a feature of visual perception, is an ability to perceive differences between light composed of different wavelengths (i.e., different spectral power distributions) independently of light intensity. Color perception is a part of ...
is
trichromatic 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 ...
, so dichromacy is achieved by losing functionality of one of the three
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. Cone ...
s. The classification of human dichromacy depends on which cone is missing: *
Protanopia 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 ...
is a severe form of red-green color blindness, in which the L-cone is absent. It is
sex-linked Sex linked describes the sex-specific patterns of inheritance and presentation when a gene mutation (allele) is present on a sex chromosome (allosome) rather than a non-sex chromosome ( autosome). In humans, these are termed X-linked recessive, ...
and affects about 1% of males. Colors of confusion include blue/purple and green/yellow. *
Deuteranopia 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 ...
is a severe form of red-green color blindness, in which the M-cone is absent. It is sex-linked and affects about 1% of males. Color vision is very similar to protanopia. *
Tritanopia 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 ...
is a severe form of blue-yellow color blindness, in which the S-cone is absent. It is much rarer than the other types, occurring in about 1 in 100,000, but is not sex-linked, so affects females and males at similar rates. They tend to confuse greens and blues, and yellow can appear pink. File:Вечір на "інтегралі" - річка Південний Буг.jpg, Normal sight File:Deuteranopia sight.jpg, Deuteranopia sight File:Tritanopia sight.jpg, Tritanopia sight


Diagnosis

The three determining elements of a dichromatic opponent-color space are the missing color, the null-luminance plane, and the null-chrominance plane. The description of the phenomena itself does not indicate the color that is impaired to the dichromat, however, it does provide enough information to identify the fundamental color space, the colors that are seen by the dichromat. This is based on testing both the null-chrominance plane and null-luminance plane which intersect on the missing color. The cones excited to a corresponding color in the color space are visible to the dichromat and those that are not excited are the missing colors.


Color detecting abilities of dichromats

According to color vision researchers at the
Medical College of Wisconsin The Medical College of Wisconsin (MCW) is a private medical school, pharmacy school, and graduate school of sciences headquartered in Milwaukee, Wisconsin. The school was established in 1893 and is the largest research center in eastern Wisconsin ...
(including
Jay Neitz Jay Neitz (born 1953) is an American professor of ophthalmology and a color vision researcher at the University of Washington in Seattle, Washington. Education and career Neitz grew up in Montana. He attended San Jose State University for his un ...
), each of the three standard color-detecting cones in the retina of
trichromat 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 exp ...
s –
blue Blue is one of the three primary colours in the RYB colour model (traditional colour theory), as well as in the RGB (additive) colour model. It lies between violet and cyan on the spectrum of visible light. The eye perceives blue when obs ...
,
green Green is the color between cyan and yellow on the visible spectrum. It is evoked by light which has a dominant wavelength of roughly 495570 Nanometre, nm. In subtractive color systems, used in painting and color printing, it is created by ...
and
red Red is the color at the long wavelength end of the visible spectrum of light, next to orange and opposite violet. It has a dominant wavelength of approximately 625–740 nanometres. It is a primary color in the RGB color model and a secondar ...
– can pick up about 100 different gradations of color. If each detector is independent of the others, the total number of colors discernible by an average human is their product (100 × 100 × 100), ''i.e.'' about 1 million; Nevertheless, other researchers have put the number at upwards of 2.3 million. The same calculation suggests that a dichromat (such as a human with red-green
color blindness Color blindness or color vision deficiency (CVD) is the decreased ability to color vision, see color or differences in color. It can impair tasks such as selecting ripe fruit, choosing clothing, and reading traffic lights. Color blindness may ...
) would be able to distinguish about 100 × 100 = 10,000 different colors, but no such calculation has been verified by psychophysical testing. Furthermore, dichromats have a significantly higher threshold than trichromats for colored stimuli flickering at low (1 Hz) frequencies. At higher (10 or 16 Hz) frequencies, dichromats perform as well as or better than trichromats. This means such animals would still observe the flicker instead of a temporally fused visual perception as is the case in human movie watching at a high enough
frame rate Frame rate (expressed in or FPS) is the frequency (rate) at which consecutive images (frames) are captured or displayed. The term applies equally to film and video cameras, computer graphics, and motion capture systems. Frame rate may also be ca ...
.


Mammals

Until the 1960s, popular belief held that most
mammal Mammals () are a group of vertebrate animals constituting the class Mammalia (), characterized by the presence of mammary glands which in females produce milk for feeding (nursing) their young, a neocortex (a region of the brain), fur or ...
s outside of
primate Primates are a diverse order of mammals. They are divided into the strepsirrhines, which include the lemurs, galagos, and lorisids, and the haplorhines, which include the tarsiers and the simians (monkeys and apes, the latter including huma ...
s were monochromats. In the last half-century, however, a focus on behavioral and genetic testing of mammals has accumulated extensive evidence of
dichromat Dichromacy (from Greek ''di'', meaning "two" and ''chromo'', meaning "color") is the state of having two types of functioning photoreceptors, called cone cells, in the eyes. Organisms with dichromacy are called dichromats. Dichromats requir ...
ic color vision in a number of mammalian
orders Order, ORDER or Orders may refer to: * Categorization, the process in which ideas and objects are recognized, differentiated, and understood * Heterarchy, a system of organization wherein the elements have the potential to be ranked a number of d ...
. Mammals are now usually ''assumed'' to be dichromats (possessing S- and L-cones), with monochromats viewed as the exceptions. The common
vertebrate Vertebrates () comprise all animal taxa within the subphylum Vertebrata () ( chordates with backbones), including all mammals, birds, reptiles, amphibians, and fish. Vertebrates represent the overwhelming majority of the phylum Chordata, ...
ancestor, extant during the
Cambrian The Cambrian Period ( ; sometimes symbolized C with bar, Ꞓ) was the first geological period of the Paleozoic Era, and of the Phanerozoic Eon. The Cambrian lasted 53.4 million years from the end of the preceding Ediacaran Period 538.8 million ...
, was tetrachromatic, possessing 4 distinct opsins classes. Early
mammalian evolution The evolution of mammals has passed through many stages since the first appearance of their synapsid ancestors in the Pennsylvanian (geology), Pennsylvanian sub-period of the late Carboniferous period. By the mid-Triassic, there were many synaps ...
would see the loss of two of these four opsins, due to the nocturnal bottleneck, as dichromacy may improve an animal's ability to distinguish colors in dim light.
Placental mammals Placental mammals (infraclass Placentalia ) are one of the three extant subdivisions of the class Mammalia, the other two being Monotremata and Marsupialia. Placentalia contains the vast majority of extant mammals, which are partly distinguishe ...
are therefore – as a rule – dichromatic. The exceptions to this rule of dichromatic vision in placental mammals are old world monkeys and apes, which re-evolved trichromacy, and
marine mammal Marine mammals are aquatic mammals that rely on the ocean and other marine ecosystems for their existence. They include animals such as seals, whales, manatees, sea otters and polar bears. They are an informal group, unified only by their reli ...
s (both
pinniped Pinnipeds (pronounced ), commonly known as seals, are a widely distributed and diverse clade of carnivorous, fin-footed, semiaquatic, mostly marine mammals. They comprise the extant families Odobenidae (whose only living member is the walru ...
s and
cetacea Cetacea (; , ) is an infraorder of aquatic mammals that includes whales, dolphins, and porpoises. Key characteristics are their fully aquatic lifestyle, streamlined body shape, often large size and exclusively carnivorous diet. They propel them ...
ns) which are cone monochromats.
New World Monkeys New World monkeys are the five families of primates that are found in the tropical regions of Mexico, Central and South America: Callitrichidae, Cebidae, Aotidae, Pitheciidae, and Atelidae. The five families are ranked together as the Ceboidea ...
are a partial exception: in most species, males are dichromats, and about 60% of females are trichromats, but the
owl monkey Night monkeys, also known as owl monkeys or douroucoulis (), are nocturnal New World monkeys of the genus ''Aotus'', the only member of the family Aotidae (). The genus comprises eleven species which are found across Panama and much of South Ame ...
s are cone monochromats, and both sexes of
howler monkey Howler monkeys (genus ''Alouatta'', monotypic in subfamily Alouattinae) are the most widespread primate genus in the Neotropics and are among the largest of the platyrrhines along with the muriquis (''Brachyteles''), the spider monkeys (''Atele ...
s are trichromats. Trichromacy has been retained or re-evolved in marsupials, where trichromatic vision is widespread. Recent genetic and behavioral evidence suggests the South American marsupial ''Didelphis albiventris'' is dichromatic, with only two classes of cone
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 pro ...
s having been found within the genus ''Didelphis''.


See also

*
Pentachromacy Tetrachromacy (from Greek ''tetra'', meaning "four" and ''chromo'', 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 tet ...
*
Mantis shrimp Mantis shrimp, or stomatopods, are carnivorous marine crustaceans of the order Stomatopoda (). Stomatopods branched off from other members of the class Malacostraca around 340 million years ago. Mantis shrimp typically grow to around in lengt ...
(dodecachromats) *
RG color space RG, Rg or rg may refer to: People * Pete RG (fl. 1998–2015), an American singer-songwriter * Razor Ramon RG or Makoto Izubuchi (born 1974), a Japanese professional wrestler * RG Sharma (born 1987), an Indian international cricketer * RG Snyma ...


References


Sources

*


External links


Visual comparisons of various types of color vision impairments
by
Cal Henderson Callum James Henderson-Begg (born 17 January 1981), known as Cal Henderson, is a British computer programmer and author based in San Francisco. Education Henderson attended Sharnbrook Upper School and Community College, and Birmingham City Un ...

Colblindor -- Color Blindness Viewed Through Colorblind Eyes
by Daniel Flück {{Eye pathology Visual disturbances and blindness Color vision