A. What is the colour of five? What is the sound of blue? To most of us such questions are either meaningless or suitable only for poetry. But for some people these are questions to which very precise answers can be given. Five, for example, for some people is green, while others say the sound of a guitar is like someone blowing on their ankles. People who '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, much-played violins, which vibrate in all their parts and fibres. But it proved impossible to research and people lost interest. Recently, however, advances in brain imaging have sparked renewed interest.

D. Early claims that the multi-sensory experiences of synesthesia were linked with the hallucinations of mental illness have long been disproved. Until 1993, many researchers dismissed it as another name for a vivid imagination, but then an experiment by Simon Baron-Cohen of Cambridge University in the UK showed that synesthetes who, when tested, had linked particular colours or shapes to letters, gave the same answers in 92 per cent of instances when tested again a week later. Non-synesthetes, given similar examples to imagine, returned the same answers in only 37 per cent of instances.

E. Although there is some overlap between synesthetes as to what colour is linked to what - 56 per cent see the letter 'o' as white - most of the responses are individual. There are 30 possible sensory combinations, but links between sounds and colours are the most common. Women are between two and eight times more likely than men to have the condition.

F. As yet there is no explanation for any of this, but small pieces of the jigsaw are emerging. A brain scanning experiment by Baron-Cohen in 1995 found that when synesthetes were listening to words, areas of the brain lit up that are normally only active in response to vision and colour. From this comes the notion that we may all have synesthesia at birth, when many parts of the brain are linked, but, as we develop, connections are pruned, so our senses become separated and the synesthetic mechanism is no longer intact. Somehow, synesthetes have kept their synaptic connections intact. It's an idea that has been challenged, however, on the grounds that if you give people certain drugs they will have synesthetic experiences, which suggests the mechanism is intact in adults but repressed.

G. There are many more unanswered questions about synesthesia. For example, is the synesthete's colour response to a number five triggered by actually seeing the number five written on a page, or does the synesthete 'see' the colour just by thinking of the number five? The neurologist Vilyanur Ramachandran came up with a test for synesthesia which seemed to suggest it was the sight of the number that was important. He asked people to look at a page made up of specially drawn twos and fives that were a mirror image of each other. The fives were placed at random on the page but the twos were placed to form shapes such as circles or triangles. To most people the numbers just looked like a jumble without order, but to synesthetes the patterns made by the twos were very obvious as a different colour from the fives. 'This shows they were really sensing colour in the numbers they saw,' says Ramachandran 'Ideas don't form these kinds of patterns.'

H. However, Smilek subsequently came up with evidence that what really matters is the idea of a number rather than the sight of it on a page. He asked a synesthete to do some simple mental arithmetic while looking at different coloured papers. The subject did not write her answers, but only thought of them and said them aloud. Smilek found that when the colour of the paper clashed with the colour of the answer, the subject's response was slower than when the colours were the same. An actual colour (the colour of the paper) could interfere with the colour of a number that existed only in her head (the answer to the mental arithmetic question). 'My research suggests that colour experiences coincide with the processing of meaning,' says Smilek. 'It's the concept of a number that's coloured.'

I. So which is it? At present, we don't know, but in the future neurologists may be able to explain what's going on in the brains of people like the novelist Nabokov, who perceived the English 'a' as dark brown and the French 'e' as black.