When the senses mix together

'see' colour in numbers or letters of the alphabet and 'feel' sensation in sound have synesthesia - meaning literally 'joined sensation' - an extraordinary condition that causes certain senses to leak' into one another. B People whose senses behave in this way are called 'synesthetes'. Some synesthetes take pleasure in it. To me it's like other people see the world in black and white, says one, who sees every letter, number, sound and pain in colour. Others leam to keep it a secret for fear of people laughing at them. But to neurologists investigating the brain it is of great interest. 'When scientists study normal perception,' says Daniel Smilek of the University of Waterloo in Ontario, Canada, there are lots of things we don't question because most of us perceive in the same way. Synesthesia, because it's abnormal, can give us new insights into normal perception. C Seventeenth-century English philosopher John Locke was the first westerner to describe synesthesia. He wrote about a man who experienced bright red as the sound of the trumpet. Later, the condition excited the imagination of nineteenth-century European painters, such as the non-synesthete Wassily Kandinsky, who believed synesthetes were like good,…