Tetrachromacy
Tetrachromacy is the condition of four-channel color vision.
Tetrachromacy comes from Greek words meaning "four" and "color." It describes having four separate channels for color information, or four types of cone cells in the eye. Creatures with this ability are called tetrachromats. Their color perception works in four dimensions, so to match any given light spectrum within their visible range, they need at least four primary colors mixed together. Many birds, fish, and reptiles show tetrachromacy. The earliest vertebrate ancestor was a tetrachromat, but an early mammal ancestor lost two of its four cone types and became a dichromat—likely because it evolved during a nocturnal period. Later, some primates, including the ancestors of humans, regained a third cone.
In terms of physiology, tetrachromacy usually means the retina has four kinds of cone cells, each sensitive to different wavelengths of light. (Cone cells handle bright light, while rod cells handle dim light.) This lets the organism see wavelengths beyond what typical humans can, and distinguish colors that look identical to someone without tetrachromacy. Species with this ability might have an unknown edge over competitors.
Among humans and other apes, as well as Old World monkeys, three cone types are normal, making them trichromats. Still, a small percentage of humans may have tetrachromacy. One woman, for instance, had an extra cone type—called L′—that fell between the usual M and L cones in sensitivity. She could tell apart colors in the 546–670 nanometer range (where the S cone is inactive) and could see a difference between a red-green mixture and plain orange. For true tetrachromacy, four independent photoreceptor classes must exist, along with a post-receptoral system to compare their signals. Humans normally have three opponent channels, based on opponent process theory, which gives trichromacy. Whether a fourth opponent channel is enough for tetrachromacy is unclear. Mice, which usually have only two cone pigments and two opponent channels, have been engineered to express a third cone pigment. They showed better color discrimination, possibly indicating trichromacy and suggesting they could create or reactivate a third opponent channel. This hints that humans might be able to use a fourth opponent channel for tetrachromatic vision, though the original study's claims about optic nerve plasticity have been disputed.
Tetrachromacy may also appea
- field
- Color vision physiology
- known_for
- Possessing four types of cone cell in the retina, enabling four-dimensional color space
- species_examples
- Birds, fish, reptiles, and some humans
Lore & Background
The normal explanation of tetrachromacy is that the organism's retina contains four types of higher-intensity light receptors (cone cells) with different spectral sensitivity. This means the organism may see wavelengths beyond those of a typical human's vision and may distinguish between colors that appear identical to non-tetrachromat humans. Species with tetrachromatic color vision may have an unknown physiological advantage over rival species. In humans, apes and Old World monkeys normally have only three types of cone cell and are trichromats, but human tetrachromacy is suspected to exist in a small percentage of the population. At least one woman has been implied to be a tetrachromat, having an additional cone type L′ intermediate between M and L in responsivity, and showing four-dimensional color discrimination for wavelengths 546–670 nm.
Reader's Guide
Tetrachromacy is significant because it expands the understanding of color vision beyond the typical human trichromatic model. The condition requires four independent photoreceptor cell classes with different spectral sensitivity, as well as appropriate post-receptoral mechanisms to compare signals from the four receptor classes. According to opponent process theory, humans have three opponent channels giving trichromacy, and it is unclear whether a fourth opponent channel is sufficient for tetrachromacy. Studies have theorized that females who carry recessive opsin alleles that can cause color vision deficiency could possess tetrachromacy. One study suggested that 15% of the world's women might have the type of fourth cone whose sensitivity peak is between the standard red and green cones, theoretically giving a significant increase in color differentiation. Another study suggests that as many as 50% of women and 8% of men may have four photopigments and corresponding potentially increased chromatic discrimination. In 2010, neuroscientist Gabriele Jordan identified a woman (subject 'cDa29') who could detect a greater variety of colors than trichromats could, corresponding with a functional or 'true' tetrachromat. Tetrachromacy may also enhance vision in dim lighting or in looking at a screen.
Did You Know?
- The common ancestor of all vertebrates was a tetrachromat, but a common ancestor of mammals lost two of its four kinds of cone cell.
- Some birds, such as the zebra finch and Columbidae, use ultraviolet wavelengths specific to tetrachromatic color vision during mate selection and foraging.
- Human rod cell sensitivity is greatest at 500 nm (bluish-green), which is significantly different from the peak spectral sensitivity of the cones.
- People lacking a lens (aphakia) see near ultraviolet light as whitish blue or whitish violet, though this does not denote tetrachromacy.
More in Sensory Systems 25-35
Elsewhere in the Sensory Systems universe
Spotted an error? Know more?
This is a living reference — every entry is fact-audited, and reader corrections feed straight into our audit queue. Suggest an edit · See this site's audit record