Dichromacy
Dichromacy is color vision with two functioning cone types.
Dichromacy (from the Greek for "two" and "color") describes having just two types of functioning cone cells in the eyes. These organisms, called dichromats, need only two primary colors to map out their full visible range. For comparison, trichromats require three primaries, and tetrachromats need four. In a dichromat's color space, every color can be produced by a single wavelength of light; in a trichromat's, each color requires a mix of monochromatic light and white light. In humans, dichromacy is a form of color blindness where one of the three cone types is missing or nonfunctional, collapsing color perception into two dimensions.
**Perception** Dichromatic vision relies on two cone types with different spectral sensitivities, plus the neural wiring to compare their signals. This yields a simpler color experience than typical human trichromacy, and far simpler than the tetrachromacy common in birds and fish. A dichromatic color space can be defined by just two primary colors. If those primaries are also the unique hues—colors that can be evoked by stimulating only one cone type, such as monochromatic light near the ends of the visible spectrum—then the space contains the entire gamut. If non-unique hues are used as primaries, the gamut is incomplete. For trichromats, even picking three pure spectral primaries never covers the full trichromatic gamut. A dichromat's color world can be plotted on a two-dimensional plane: one axis for brightness, the other for hue. But this hue axis isn't like a trichromat's; it runs from white (neutral) in the middle out to two unique hues at the extremes, such as blue and yellow. Notably, dichromats can see white in a rainbow, because monochromatic light can stimulate both cone types equally.
**Humans** Human dichromacy is a type of color vision deficiency. Normal human vision is trichromatic, so dichromacy arises when one cone type is lost. It is less common than anomalous trichromacy. The specific form depends on which cone is missing: - **Protanopia**: a severe red-green deficiency where the L-cone is absent. It is sex-linked and affects about 1% of males. Confusion occurs between blue/purple and green/yellow. - **Deuteranopia**: a severe red-green deficiency where the M-cone is absent. Also sex-linked and affecting about 1% of males, its color vision is very similar to protanopia. - **Tritanopia**: a severe blue-yel
- field
- Color vision and visual perception
- known_for
- Color vision deficiency requiring only two primary colors; forms include protanopia, deuteranopia, and tritanopia
- prevalence_in_humans
- Protanopia and deuteranopia each affect about 1% of males; tritanopia occurs in about 1 in 100,000 people
- common_in_mammals
- Most placental mammals are dichromatic, with exceptions such as old world monkeys and apes (trichromats) and marine mammals (cone monochromats)
Lore & Background
Dichromatic color vision is enabled by two types of cone cells with different spectral sensitivities and the neural framework to compare their excitation. The resulting color vision is simpler than typical human trichromatic vision and much simpler than tetrachromatic vision, which is typical of birds and fish. A dichromatic color space can be defined by only two primary colors; when these are also the unique hues, the color space contains the entire gamut. White, experienced when both cone cells are equally excited, can be evoked by monochromatic light, meaning dichromats see white in the rainbow.
Reader's Guide
Dichromacy is significant as a common form of color vision deficiency in humans and as the typical color vision state for most placental mammals. In humans, it is classified by which cone is missing: protanopia (L-cone absent), deuteranopia (M-cone absent), or tritanopia (S-cone absent). The condition is rarer than anomalous trichromacy. Research indicates that dichromats can distinguish about 10,000 colors, compared to roughly 1 million for trichromats, though this calculation has not been verified by psychophysical testing. Dichromats also have a higher threshold for low-frequency flickering stimuli but perform as well as or better than trichromats at higher frequencies. The study of dichromacy has informed understanding of color vision evolution, particularly the loss of two opsin classes during the nocturnal bottleneck in early mammalian evolution.
Did You Know?
- Dichromats require only two primary colors to represent their visible gamut, while trichromats need three and tetrachromats need four.
- In humans, protanopia and deuteranopia each affect about 1% of males, while tritanopia occurs in about 1 in 100,000 people and is not sex-linked.
- Dichromats see white in the rainbow because white can be evoked by monochromatic light when both cone cells are equally excited.
- Most placental mammals are dichromatic, with exceptions including old world monkeys and apes (trichromats) and marine mammals (cone monochromats).
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