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For most people, seeing, hearing and tasting are distinct senses, but for a significant minority of individuals some senses may be inextricably linked. Such people, known as synaesthetes, may "see" colours when they hear words, or have a sensation of different shapes when they experience different tastes. It is estimated that one in 2,000 people may have such sensory mixing. In Synaesthesia: The Strangest Thing , John Harrison provides a populist account of what is known about synaesthesia, and what it might tell us about the operation of human sensory experience.

Synaesthesia has received modest attention over the past 100 years, and Harrison uses a historical review of possible synaesthetes from the past century to provide a context for his presentation. For the most part, this presentation illustrates the difficulties in studying synaesthesia. How can one be sure that someone who claims to have synaesthesia truly does? It is argued that many figures from history who are sometimes described as synaesthetes (such as the composer Aleksander Scriabin and the artist Wassily Kandinsky) were unlikely to have been so. For the most part there is little evidence either way, and Harrison adopts a conservative scepticism. Only one figure, the novelist Vladimir Nabokov, survives Harrison's review as a possible true synaesthete.

Synaesthesia may occur in a variety of forms, but by far the most common is "seeing" colours when hearing words, and it is on this form that Harrison focuses. Curiously, different colour-word synaesthetes may associate different colours with different words, but each individual maintains consistency over time in his or her colour-word associations. This is the key to demonstrating that synaesthesia is something beyond over-zealous association of one sense with another by individuals with particularly florid imaginations. To show this, Harrison introduces several techniques from contemporary experimental psychology and cognitive neuroscience. This is a clear strength of the book for a general audience, for it provides the reader with a glimpse into old and new methods of studying mind and brain.

The key issue concerns the objective determination of synaesthesia, given that the condition is concerned with the synaesthete's subjective or conscious experience. Experimental psychology provides some useful techniques, and these show that colour-word synaesthetes are exceptional in that they are able to match colours to hundreds of words, and match the same colours to the same words months or even years later. Brain-imaging techniques also show differences between the neural processing of words in colour-word synaesthetes and non-synaesthetes.

Given that synaesthesia concerns a subjective state, there is a real possibility that the phenomenon may tell us something about psychological or neurological aspects of conscious experience. While Harrison discusses several theories of synaesthesia, he shies away from going into detail on the relation to conscious experience. His concern instead is on the philosophical side, on what it might be like to be a synaesthete. This seems like a real missed opportunity, and from this perspective I felt the concluding chapter was disappointing. Nevertheless, general readers will find much that is both interesting and intriguing in this highly readable book.

Richard Cooper is reader in cognitive science, Birkbeck College, University of London.