Dichromacy (from

Greek Greek may refer to: 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 of all kno ...

''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 retina of the vertebrate eye. Cones are active in daylight conditions and enable photopic vision, as opposed to rod cells, which are active in dim light and enable scotopic vision. Most v ...

s, in the

eye An eye is a sensory organ that allows an organism to perceive visual information. It detects light and converts it into electro-chemical impulses in neurons (neurones). It is part of an organism's visual system. In higher organisms, the ey ...

s. Organisms with dichromacy are called dichromats. Dichromats require only two

primary color Primary colors are colorants or colored lights that can be mixed in varying amounts to produce a gamut of colors. This is the essential method used to create the perception of a broad range of colors in, e.g., electronic displays, color prin ...

s to be able to represent their visible

gamut In color reproduction and colorimetry, a gamut, or color gamut , is a convex set containing the colors that can be accurately represented, i.e. reproduced by an output device (e.g. printer or display) or measured by an input device (e.g. cam ...

. By comparison,

trichromat 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. The normal expl ...

s need three primary colors, and

tetrachromat Tetrachromacy (from Ancient 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 ...

s need four. Likewise, every color in a dichromat's gamut can be evoked

monochromatic light {{More citations needed, date=May 2023 In physics, monochromatic radiation is electromagnetic radiation with a single constant frequency or wavelength. When that frequency is part of the visible spectrum (or near it) the term monochromatic light ...

. 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 Color blindness, color vision deficiency (CVD) or color deficiency is the decreased ability to color vision, see color or differences in color. The severity of color blindness ranges from mostly unnoticeable to full absence of color percept ...

in which one of the three cone cells is absent or not functioning and color is thereby reduced to two dimensions.

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''

s. When these primary colors are also the

unique hues Unique hue is a term used in perceptual psychology of color vision and generally applied to the purest hues of blue, green, yellow and red. The proponents of the opponent process theory believe that these hues cannot be described as a mixture of ...

, 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 color A spectral color is a color that is evoked by monochromatic light, i.e. either a spectral line with a single wavelength or frequency of light in the visible spectrum, or a relatively narrow spectral band (e.g. lasers). Every wave of visible ...

s 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 properties (called color appearance parameters) of a color, defined in the CIECAM02 model as "the degree to which a stimulus can be described as similar to or different from stimuli that are described as ...

, 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 an optical phenomenon caused by refraction, internal reflection and dispersion of light in water droplets resulting in a continuous spectrum of light appearing in the sky. The rainbow takes the form of a multicoloured circular ...

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 frequencies independently of light intensity. Color perception is a part of the larger visual system and is mediated by a co ...

trichromatic 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. The normal expl ...

, so dichromacy is achieved by losing functionality of one of the three

s. The classification of human dichromacy depends on which cone is missing: *

Protanopia Color blindness, color vision deficiency (CVD) or color deficiency is the decreased ability to see color or differences in color. The severity of color blindness ranges from mostly unnoticeable to full absence of color perception. Color bl ...

is a severe form of red-green color blindness, in which the L-cone is absent. It is

sex-linked Sex linkage describes the sex-specific patterns of inheritance and expression when a gene is present on a sex chromosome (allosome) rather than a non-sex chromosome (autosome). Genes situated on the X-chromosome are thus termed X-linked, and a ...

and affects about 1% of males. Colors of confusion include blue/purple and green/yellow. *

Deuteranopia Color blindness, color vision deficiency (CVD) or color deficiency is the decreased ability to see color or differences in color. The severity of color blindness ranges from mostly unnoticeable to full absence of color perception. Color bl ...

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, color vision deficiency (CVD) or color deficiency is the decreased ability to see color or differences in color. The severity of color blindness ranges from mostly unnoticeable to full absence of color perception. Color bl ...

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 in Milwaukee, Wisconsin, United States. The school was established in 1893 and is the largest research center in eastern Wisconsi ...

(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 u ...

), each of the three standard color-detecting cones in the retina of

s –

blue Blue is one of the three primary colours in the RYB color model, RYB colour model (traditional colour theory), as well as in the RGB color model, RGB (additive) colour model. It lies between Violet (color), violet and cyan on the optical spe ...

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 nm. In subtractive color systems, used in painting and color printing, it is created by a com ...

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 625–750 nanometres. It is a primary color in the RGB color model and a seconda ...

– 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, color vision deficiency (CVD) or color deficiency is the decreased ability to color vision, see color or differences in color. The severity of color blindness ranges from mostly unnoticeable to full absence of color percept ...

) 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, most commonly expressed in frame/s, or FPS, is typically the frequency (rate) at which consecutive images (Film frame, frames) are captured or displayed. This definition applies to film and video cameras, computer animation, and moti ...

Mammals

Until the 1960s, popular belief held that most

mammal A mammal () is a vertebrate animal of the Class (biology), class Mammalia (). Mammals are characterised by the presence of milk-producing mammary glands for feeding their young, a broad neocortex region of the brain, fur or hair, and three ...

s outside of

primate Primates is an order (biology), order of mammals, which is further divided into the Strepsirrhini, strepsirrhines, which include lemurs, galagos, and Lorisidae, lorisids; and the Haplorhini, haplorhines, which include Tarsiiformes, tarsiers a ...

s were monochromats. In the last half-century, however, a focus on behavioral and

genetic testing Genetic testing, also known as DNA testing, is used to identify changes in DNA sequence or chromosome structure. Genetic testing can also include measuring the results of genetic changes, such as RNA analysis as an output of gene expression, or ...

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 require ...

ic color vision in a number of mammalian

orders Order, ORDER or Orders may refer to: * A socio-political or established or existing order, e.g. World order, Ancien Regime, Pax Britannica * Categorization, the process in which ideas and objects are recognized, differentiated, and understood * H ...

. Mammals are now usually ''assumed'' to be dichromats (possessing S- and L-cones), with monochromats viewed as the exceptions. The common

vertebrate Vertebrates () are animals with a vertebral column (backbone or spine), and a cranium, or skull. The vertebral column surrounds and protects the spinal cord, while the cranium protects the brain. The vertebrates make up the subphylum Vertebra ...

ancestor, extant during the

Cambrian The Cambrian ( ) is the first geological period of the Paleozoic Era, and the Phanerozoic Eon. The Cambrian lasted 51.95 million years from the end of the preceding Ediacaran period 538.8 Ma (million years ago) to the beginning of the Ordov ...

, 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 The nocturnal bottleneck hypothesis is an evolutionary biology hypothesis to explain the origin of several mammalian traits. In 1942, Gordon Lynn Walls described this concept which states that placental mammals were mainly or even exclusively ...

, 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 distinguish ...

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 mammals that rely on marine ecosystems for their existence. They include animals such as cetaceans, pinnipeds, sirenians, sea otters and polar bears. They are an informal group, unified only by their reliance on marine enviro ...

s (both

pinniped Pinnipeds (pronounced ), commonly known as seals, are a widely range (biology), distributed and diverse clade of carnivorous, fin-footed, semiaquatic, mostly marine mammals. They comprise the extant taxon, extant families Odobenidae (whose onl ...

s and

cetacea Cetacea (; , ) is an infraorder of aquatic mammals belonging to the order Artiodactyla that includes whales, dolphins and porpoises. Key characteristics are their fully aquatic lifestyle, streamlined body shape, often large size and exclusively c ...

ns) which are cone

monochromat Monochromacy (from Greek ''mono'', meaning "one" and ''chromo'', meaning "color") is the ability of organisms to perceive only light intensity without respect to spectral composition. Organisms with monochromacy lack color vision and can only s ...

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 Ceboide ...

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 Am ...

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 Neotropical realm, Neotropics and are among the largest of the New World monkey, platyrrhines along with the muriquis (''Brachyte ...

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''.

References

Sources

External links

Visual comparisons of various types of color vision impairments
by Cal Henderson
Colblindor -- Color Blindness Viewed Through Colorblind Eyes
by Daniel Flück {{Eye pathology Color blindness