The number purple
New Scientist, 08 April 2000
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.
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.
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.
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.
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.