How minds play tricks with words and colours

New Scientist, 21 August 2004

SYNAESTHESIA, a condition in which people make weird sensory associations, may rely more on the plasticity of the brain than on any genetic predisposition. This might mean that all of us are capable of having a synaesthetic experience.

People with synaesthesia often say that letters, words and numbers have innate colours. Even when tested years later, their associations remain consistent. But no one really knows why or how these odd associations form.

In 1996, Simon Baron-Cohen and his team at the University of Cambridge estimated that about 1 in every 2000 people had the condition and that it was likely to be a genetic trait encoded on the X chromosome (Perception, vol 25, p 1073). But now a study of blind and blindfolded people by Megan Steven and colleagues at the University of Oxford suggests that while genes almost certainly play a role, it may be a minor one.

Steven and her team recruited six "late-blind" subjects, all of whom were synaesthetes before losing their sight. For three of them, their synaesthesia changed after they became blind. One man, JF, for instance, had always thought of days and months as having colours. Instruments in an orchestra and even his pay scale at work were also colour-coded in his mind. After learning Braille, he began experiencing colours when he touched the raised Braille characters denoting letters, numbers or musical notes - or even when he simply thought about touching them.

Knowledge of Braille, instruments and remuneration are all learned, Steven points out, so people such as JF must be adapting their pre-existing synaesthesia to incorporate them. But that still leaves a big question: can all brains adapt to make these unusual associations, with the ability only being unmasked in a select few people? Or are the associations themselves rare?

Evidence from a blindfolding experiment hints that the associations might be universal. For instance, DB, who was not known to be synaesthetic, was blindfolded for five consecutive days, and saw very vividly a frightening face whenever he listened to a specific passage of Mozart's Requiem. It only happened while he was deprived of vision. Though this mental association was not genuine synaesthesia, it did have many of the hallmarks of the condition, the researchers claim. It could be reproduced, was consistent and was triggered by something specific.

This suggests that even non-synaesthetes may have the neural machinery for generating a synaesthetic experience and that changes to the brain might expose them, they say in a forthcoming issue of Perception. "It can't be entirely genetic," Steven says. She speculates that in non-synaesthetes, the input of visual signals may be inhibiting tactile and auditory inputs to the "visual" areas. "When there's no more visual stimulation, maybe other connections become more important," she says.

Baron-Cohen agrees that genes and environments are likely to interact in shaping synaesthesia. But he questions whether atypical cases like these can teach us much about more common varieties. His main work is in autism and he points out that up to 40 per cent of congenitally blind children show autistic behaviours in early childhood, such as persistent rocking. But blind kids tend to grow out of such rocking behaviour, while those with true autism don't. "We should be wary of assuming phenomena that resemble synaesthesia - such as coloured hallucinations in a subject blindfolded for five days - involve the same brain mechanisms that give rise to 'naturally occurring' synaesthesia."