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鈥檚 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 鈥渏oined sensation鈥�. 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鈥攖hough 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.
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To a normal person鈥檚 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鈥� unusual response, which groups the
coloured numbers into shapes, suggests they are really sensing colour. 鈥淕rouping
is a diagnostic test of whether something is sensory,鈥� he says. 鈥淐oncepts don鈥檛
驳谤辞耻辫.鈥�
In another test, the researchers demonstrated that Arabic numerals but not
Roman numerals evoked the colour, showing that it was the visual image, or
鈥済rapheme鈥�, 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鈥攁bout the edge of your palm if held straight out鈥攖he colour
perception vanished, even if the researchers made the number bigger or brighter.
鈥淚t鈥檚 not some memory thing,鈥� says Ramachandran. 鈥淎ll three results are
unexpected, and as such are new information on synaesthesia. We have so little,
really. So all new information is of value,鈥� 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.
