Specialised cells detect colour differences

Researchers asked people with a form of colour blindness called deuteranomaly - the most common form of the condition - to assess pairs of colours.

The University of Cambridge team found these people were able to distinguish between pairs that looked identical to those with "normal" colour vision.

The study is published in the journal Current Biology.

Deuteranomaly is linked to a defect on the X sex chromosome.

The present findings recall reports from the Second World War, which suggested that 'colour blind' observers might be superior in penetrating camouflage

Dr John Mollon

Colours are detected by humans through the combined action of three different types of cone photoreceptor cells, each of which is optimally activated by different wavelengths of light.

These sensitivities are altered in deuteranomalous colour blind individuals because they possess a variant form of one of the cone photoreceptors.

The sensitivity of cones that should be "middle-wave" is shifted toward that of "long-wave" cones.

This results in decreased ability to differentiate between some colours that are easily distinguishable by those with normal colour vision.

Altered sensitivities

In theory, however, it is possible that owing to the altered sensitivities of their cone photoreceptors, people with deuteranomalous colour blindness may be sensitive to colour differences that are not apparent to those with normal colour vision.

The Cambridge team tested this idea by asking deuteranomalous and "colour-normal" individuals to report whether they were able to distinguish between pairs of colours that were theoretically predicted to look different to people with deuteranomalous colour blindness, but the same to those with normal colour vision.

The researchers duly found some colour pairs were only seen to be different by deuteranomalous individuals.

The finding suggests that although these individuals may be blind to some colours accessible to people who are not colour blind, they also have a sensitivity to a "colour dimension" that is inaccessible to those with normal colour vision.

In fact, the researchers found people with deuteranomalous colour blindness gave large difference ratings to pairs of colours which appeared indistinguishable to others.

The researchers, led by Dr John Mollon, said: "The present findings recall reports from the Second World War, which suggested that 'colour blind' observers might be superior in penetrating camouflage.

"In part, this might be because random colour variations are less salient than structural or textural variations for the colour blind.

"However, a second factor may be the one considered above: a camouflage paint may not be physically identical to the surrounding foliage but may be a good match to the normal eye - while being a poor match for an anomalous eye."

Dr Joseph Carroll, of the University of Rochester in New York, said there was enormous variation between people with deuteranomalous colour blindness.

He said the findings of the latest research would probably only apply to those people with the biggest gap in sensitivity between their long-wavelength sensitive and middle-wavelength sensitive cones.

However, he said it was possible that the tests used in the study, when combined with standard tests of colour vision, may be a more sensitive indicator of the severity of a person's colour vision defect.