Can you discern 15 different shades of khaki?© Punchstock

The most common form of colour blindness makes it difficult for those with the condition to distinguish between red and green. But scientists have found that it also helps these people to discern subtle shades of khaki that look identical to those with normal vision.

About six percent of men, and a much smaller fraction of women, have deuteranomaly, commonly known as red-green colour blindness. It is caused by a genetic mutation that affects one of the three pigments found in the cone-shaped cells in the retina that respond to different colours of light.

This mutation alters the pigment that responds to green light so that it behaves more like the red-sensitive pigment. Therefore, the two colours produce almost identical responses in the eye. This means that people with deuteranomaly often cannot see differences between shades of red and green on test cards used by scientists to investigate the disorder.

Now researchers based at the University of Cambridge, UK, and the University of Newcastle upon Tyne, UK, have turned the tables. They designed test cards that deliberately favoured people with deuteranomaly to show that these individuals can spot differences between shades of khaki that look identical to those with normal vision. Their work is published in the journal Current Biology¹.

Green man

Men are more likely to suffer from deuteranomaly because the gene responsible for making the 'green' pigment is found on the X chromosome, and is also recessive. Very few women suffer from the condition because they have two X chromosomes and so are likely to have a normal copy of the gene, which overrides the effect of the recessive mutant. But because men only have one X chromosome, a single mutation is enough to produce the condition.

The scientists investigated the condition by first studying the wavelengths of light that the mutant form of green pigment is sensitive to. "We worked out which stimuli only people with deuteranomaly should be able to differentiate," explains Gabriele Jordan, an expert on colour vision at the University of Newcastle upon Tyne.

They identified 15 shades of khaki that fitted the bill, and tested their prediction by showing two sets of subjects - one with deuteranomaly and the other with normal vision - a series of cards carrying pairs of different khaki shades. It proved to be almost impossible for people with normal vision to tell the colours apart. "It made me realize what it's like for people with colour blindness when they do the normal tests," says Jordan. "It's immensely frustrating to be asked to look for colours that are to you invisible."

Grass vision

Whereas it took people like Jordan around 90 minutes to assign scores rating how different each of the 105 pairs of colours looked to them, people with deuteranomaly typically got through the test in less than half the time.

"It is obvious if you think about what causes deuteranomaly," says Jordan, "but no-one had thought of it this way before."

"This is a really fascinating finding," says David Simmons, an expert in visual perception at the University of Glasgow. "Now it would be interesting to find out what these people experience in a more natural environment."

Simmons hypothesizes that because deuteranomaly is quite common in human populations, the gene responsible may have once provided an evolutionary benefit. For example, it may have helped them spot potential food items in complicated environments such as grass or foliage, he suggests. 

• References

  1. Bosten J., et al. Current Biology, 15. R950 - R952 (2005).