Color Vision

Three kinds of cones, opponent channels, colour constancy and the dress: how the brain turns three signals into millions of colours — and why some people see colours differently.

Light itself isn't coloured — it has wavelengths. Colour is something brains make.

Three cones

In 1802 Thomas Young suggested that colour vision depends on just three kinds of receptor, an idea Hermann von Helmholtz developed — the trichromatic theory. It was confirmed when the three human cone types were measured: S cones (short wavelengths, peaking in the blue-violet), M cones (medium, green) and L cones (long, yellow-green to red). Every colour you see corresponds to a particular ratio of activity across the three. That's why screens can make any colour with red, green and blue pixels.

Opponent channels

Trichromacy doesn't explain everything. Why can't we see a "reddish green" or a "yellowish blue"? In the 1870s Ewald Hering proposed that colour is coded in opposing pairs: red versus green, blue versus yellow, and light versus dark — opponent-process theory. Both theories turned out right at different stages: cones are trichromatic, and the retina and thalamus then recode their outputs into opponent channels. Afterimages show it: stare at a red patch, then look at white, and you see green.

Colour constancy

A banana looks yellow in daylight and lamplight, though the light reaching your eye differs greatly. The brain estimates the lighting and discounts it — colour constancy, involving visual areas such as V4. When the brain's guess about the lighting is ambiguous, people disagree. In 2015 a photograph of "the dress" went viral: some saw it as white and gold, others as blue and black, largely depending on whether their brain assumed it was in shadow or in bright, bluish light.

Colour blindness

The genes for the M and L cone pigments lie on the X chromosome, so inherited red-green colour vision deficiency is much more common in males: roughly 8% of men of Northern European ancestry and about 0.5% of women. Most affected people see colours, just fewer distinctions. Some women may have a fourth cone type (tetrachromacy), though whether it gives richer colour vision is rare and debated. Many animals differ: most mammals have two cone types; many birds, reptiles and fish have four, including ultraviolet.