Dichromacy

Dichromacy is a state of color vision where an organism possesses two functioning types of photoreceptors. These specialized cells in the eye are known as cone cells. Organisms that see this way are called dichromats. While most humans are trichromats, meaning we use three types of cones, dichromats use only two primary colors to represent their visible gamut. This type of vision is simpler than the trichromatic vision of humans. It is even simpler than the tetrachromatic vision found in birds and fish.

To understand how this works, we must look at the mechanism of the eye. Dichromatic vision is enabled by two types of cone cells that have different spectral sensitivities. This means each cell responds to different wavelengths of light. The brain uses a neural framework to compare how much each cone cell is excited. This comparison creates a two-dimensional color space. One coordinate in this space represents brightness, while the other represents the hue. In this system, white light can be evoked by monochromatic light alone. This happens because white is experienced when both cone cells are equally excited.

There are specific types of dichromacy in humans, which is a form of color vision deficiency. The classification depends on which specific cone is absent. Protanopia is a severe form of red-green color blindness caused by the absence of the L-cone. This condition is sex-linked and affects approximately 1% of males. People with protanopia may confuse blue and purple or green and yellow. Deuteranopia is another form of red-green color blindness where the M-cone is absent. Like protanopia, it is sex-linked and affects about 1% of males. Tritanopia is a much rarer condition involving the loss of the S-cone. It affects about 1 in 100,000 people and is not sex-linked. Tritanopes often confuse greens and blues, and yellow may appear pink.

Scientists use specific methods to diagnose these deficiencies. They look for three determining elements: the missing color, the null-luminance plane, and the null-chrominance plane. By testing these planes, they can identify the fundamental color space of the individual. The point where these planes intersect reveals the missing color. The cones that are excited by a color are visible to the dichromat. The colors that do not excite the remaining cones are the missing ones.

We can also use math to estimate how many colors a person can see. Researchers at the Medical College of Wisconsin, including Jay Neitz, have studied this. They found that each of the three standard cone types can detect about 100 gradations of color. For a trichromat, the total number of discernible colors might be 1 million, or even upwards of 2.3 million. For a dichromat, the calculation suggests they can distinguish about 10,000 different colors.

In the animal kingdom, the history of color vision is quite complex. The common vertebrate ancestor from the Cambrian period was actually tetrachromatic. This ancestor possessed four distinct classes of opsins, which are light-sensitive proteins. During early mammalian evolution, mammals lost two of these four opsins. This likely happened due to a nocturnal bottleneck. Losing these cones may have improved an animal's ability to see in dim light. Because of this, most placental mammals are now considered dichromats.

There are several notable exceptions to these rules in mammals. Old World monkeys and apes re-evolved trichromacy. Some marine mammals, like cetaceans and pinnipeds, are cone monochromats. New World monkeys show a mix of vision types. In most species, males are dichromats, but about 60% of females are trichromats. However, owl monkeys are cone monochromats, while howler monkeys are trichromats in both sexes. Marsupials also show interesting patterns. While trichromacy is widespread in them, the South American marsupial Didelphis albiventris appears to be dichromatic.