Dichromacy (fromGreekdi'two' andchromo'color'[citation needed]) is the state of having two types of functioningphotoreceptors, calledcone cells, in theeyes. Organisms with dichromacy are called dichromats. Dichromats require only twoprimary colors to be able to represent their visiblegamut. By comparison,trichromats need three primary colors, andtetrachromats need four. Likewise, every color in a dichromat's gamut can be evoked withmonochromatic 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 acolor vision deficiency in which one of the three cone cells is absent or not functioning and color is thereby reduced to two dimensions.[1]
Dichromatic color vision is enabled by two types of cone cells with differentspectral 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 onlytwoprimary colors. When these primary colors are also theunique hues, then the color space contains the individualsentire 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 purespectral 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 represents brightness, and the other coordinate represents hue. However, the perception of hue is not directly analogous to trichromatichue, 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 therainbow.
Dichromacy in humans is a form of color blindness (color vision deficiency). Normal humancolor vision istrichromatic, so dichromacy is achieved by losing functionality of one of the threecone cells. It is rarer than anomaloustrichromacy. The classification of human dichromacy depends on which cone is missing:
The three determining elements of a dichromatic opponent-color space are the missing color, the null-luminance plane, and the null-chrominance plane.[3] 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.[4]
According to color vision researchers at theMedical College of Wisconsin (includingJay Neitz), each of the three standard color-detecting cones in the retina oftrichromats –blue,green andred – 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;[5] Nevertheless, other researchers have put the number at upwards of 2.3 million.[6] The same calculation suggests that a dichromat (such as a human with red-greencolor blindness) would be able to distinguish about 100 × 100 = 10,000 different colors,[7] but no such calculation has been verified bypsychophysical testing.
Furthermore, dichromats have a significantly higher threshold than trichromats for colored stimuli flickering at low (1 Hz) frequencies, meaning they perform worse than trichromats but better than monochromats. At higher (10 or 16 Hz) frequencies, dichromats perform as well as or better than trichromats but worse than monochromats.[8][9] 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 enoughframe rate.
Until the 1960s, popular belief held that mostmammals outside ofprimates were monochromats. In the last half-century, however, a focus on behavioral andgenetic testing of mammals has accumulated extensive evidence ofdichromatic color vision in a number of mammalianorders. Mammals are now usuallyassumed to be dichromats (possessing S- and L-cones), with monochromats viewed as the exceptions.
The commonvertebrate ancestor, extant during theCambrian, wastetrachromatic, possessing 4 distinct opsins classes.[6] Earlymammalian evolution would see the loss of two of these four opsins, due to thenocturnal bottleneck, as dichromacy may improve an animal's ability to distinguish colors in dim light.[10]Placental mammals are therefore–as a rule–dichromatic.[11]
The exceptions to this rule of dichromatic vision in placental mammals areold world monkeys and apes, whichre-evolved trichromacy, andmarine mammals (bothpinnipeds andcetaceans) which are conemonochromats.[12]New World Monkeys are a partial exception: in most species, males are dichromats, and about 60% of females are trichromats, but theowl monkeys are cone monochromats,[13] and both sexes ofhowler monkeys are trichromats.[14][15][16]
Trichromacy has beenretained or re-evolved in marsupials, where trichromatic vision is widespread.[17] Recent genetic and behavioral evidence suggests the South American marsupialDidelphis albiventris is dichromatic, with only two classes of coneopsins having been found within the genusDidelphis.[18]
