Tetrachromia and colour blindness
Stories from Physics for 11-14 14-16
- Whilst data on prevalence is still somewhat uncertain, between 15-47% of women and around 8% of men, are potential tetrachromats. That is, they have four cones and a greater ability to perceive differences in colours — some tetrachromats may be able to distinguish two million shades of colours.
- In contrast to the prevalence of tetrachromacy, men are 16 times as likely as women to be colour blind. Complete colour blindness, achromatopsia, is rare, affecting only 3 in 100,000 people. However, on the island of Pingelap in Micronesia, the prevalence is somewhere between 4-10% as the islanders are descended from survivors of a typhoon in the 1700s, one of whom carried the gene for the condition.
- The human retina has a smaller number of blue than red or green cones, and, in the fovea, there are no blue cones at all. Therefore, a sufficiently small blue dot, directly in the centre of the visual field cannot be recognised as blue. Viewed at a distance, white and yellow, blue and black, and red and magenta, will look identical. This effect has been known for many years, and naval signalling flags and the colours in heraldry were designed such that the colours could not be confused when viewed at a distance.
- Unlike the three cones humans possess, some species, for example mantis shrimps and butterflies, possess up to 12 spectral sensitivities. It is surprising, then, that mantis shrimp have a poorer ability to distinguish colours than humans. The shrimp can only distinguish colours differing in wavelength by 25 nm, whilst humans can detect differences of between 1-4 nm. Instead, it is hypothesised that the 12-receptor system gives an advantage by increasing the speed of colour processing.