We have all had a blue Monday or been green with envy. But for some people, such descriptions go beyond the metaphor — the word 'Monday' really is blue and 'envy' is green. The perception of colour when one encounters words, letters or numbers is the most common example of the phenomenon of synaesthesia. Although described more than a century ago, little is known about its underlying causes and most reports have focused on individual cases.

In the 29 March Nature, researchers led by Jason Mattingley of the School of Behavioural Science, University of Melbourne, Australia, report an investigation of 15 individuals who perceive colour when they see particular graphemes (letters and digits). Subjects were asked to report their synaesthetic colour for each item in a list containing 150 letters, digits and numbers, and were retested without warning three months later (Nature 2001, 410:580-583). The synaesthetes...

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