Tetrachromacy

Four cones in nature

Four-colour vision is the old pattern among animals with backbones. Most birds have a fourth cone that responds to violet or ultraviolet light, along with coloured oil droplets in their cones that sharpen each one's response. A pigeon or a peahen sees patterns in plumage that a human cannot. Early mammals lost two of the four cone types, probably while living by night in the age of the dinosaurs. Our own third cone evolved much later, in the ancestors of African and Asian monkeys and apes.

How a human could have four

The genes for the medium and long-wavelength cones sit on the X chromosome and come in slightly different versions. A woman has two X chromosomes. If they carry different versions, her retina can contain four types of cone. This is likely in the mothers and daughters of men with the mild, shifted forms of colour blindness, so a fair number of women have the raw material. The fourth cone usually sits very close to one of the others, and the brain may simply merge the two signals. Men, with one X, cannot be tetrachromats by this route.

What the evidence shows

The psychologist Gabriele Jordan at Newcastle University in England tested women who carried four cone types. In results published in 2010, only one of them passed a test that required a fourth channel, telling apart mixtures of light that looked the same to everyone else. So true human tetrachromacy seems to exist but to be rare. Press claims that such people see a hundred million colours are a rough calculation, not a measurement. Online tests cannot detect it, because a screen has only three colours of light to show.

Sources and further reading

• Tetrachromacy on Wikipedia
• BBC Future: The women with superhuman vision
• Scientific American: What Birds See