The number purple

New Scientist, 08 April 2000

PEOPLE who perceive a colour when shown a particular number are often dismissed by researchers as simply seeing a visual metaphor or reliving childhood associations. But a team of scientists in California believe that people with this condition, known as coloured number synaesthesia, may have their wires crossed near the brain's colour centre. The team has developed a series of visual tests that add weight to the idea that coloured numbers are genuine sensations.

Synaesthesia means literally "joined sensation". People with this form of the condition insist that every number has its own indelible hue. The number 5, for instance, might be green and the number 3 pink -- though two synaesthetes will seldom agree.

Vilayanur Ramachandran and Ed Hubbard, both at the University of California at San Diego, subjected two volunteers with coloured number synaesthesia to a series of tests. In one, they created a pattern that consisted solely of computer-generated 2s and 5s, which were mirror images of each other. The 5s were scattered randomly on the page, but the 2s formed shapes, such as triangles or circles.

To a normal person's eye, the page just looked like a jumble of numbers. But to the synaesthetic subjects, the shapes made of 2s leapt out. This is similar to the way healthy people see coloured patterns in the Ishihara colour-blindness test.

The brain picks out patterns very early on in the processing of an image, says Ramachandran. He says his volunteers' unusual response, which groups the coloured numbers into shapes, suggests they are really sensing colour. "Grouping is a diagnostic test of whether something is sensory," he says. "Concepts don't group."

In another test, the researchers demonstrated that Arabic numerals but not Roman numerals evoked the colour, showing that it was the visual image, or "grapheme", of the number that is important, rather than the numerical concept.

In a third test, the researchers displayed the number 5 in front of the subjects, then gradually moved it away from the centre of their vision. Beyond 4 degrees -- about the edge of your palm if held straight out -- the colour perception vanished, even if the researchers made the number bigger or brighter. "It's not some memory thing," says Ramachandran. "All three results are unexpected, and as such are new information on synaesthesia. We have so little, really. So all new information is of value," says Simon Baron-Cohen of Cambridge University.

Ramachandran thinks that a part of the brain known as the fusiform gyrus, which deals with colour, may have an area close by that deals with number-graphemes. He suspects that people with coloured number synaesthesia may have links between these two areas. He announced his preliminary results at the First International Conference on Phantom Pain last month in Oxford, and will present a poster at the Society for Neuroscience meeting in November. The next step, says Ramachandran, is to use brain imaging to confirm the hunch.