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Most people's first reaction to hearing of synaesthesia is disbelief. Yet a growing body of neuroscientific evidence points to its being real. Synaesthesia is the name given to the condition in which an individual experiences a sensation in one modality automatically triggering a sensation in another modality. For example, on hearing a sound, the person instantly "sees" a colour. Or on seeing a colour, the person instantly "hears" a musical tone. Synaesthesia means literally "experiencing together". Can a mixing of the senses really happen? Or is this just an imaginative story, belonging more in the realm of poetry than of science?

Richard Cytowic's book is a detailed account of the condition, and of one individual with synaesthesia. This particular case is of a man who feels tastes as shapes. For example, he feels the taste of quinine as smooth polished wood, sugar makes things taste "rounder", and citrus makes the taste have "points". Importantly, this man claims his is not just a metaphorical way of talking - he actually feels these shapes in his hands as he experiences the taste of each food. This new case study builds on Cytowic's earlier book The Union of the Senses (1989), which was the first book in modern science on synaesthesia. He can therefore be seen as a leader in this field. In his new book, he updates the reader on what is known about the condition.

The most common form appears to be "coloured hearing". In such individuals, each sound triggers a different colour. For some such synaesthetes, colours are most vividly triggered by hearing spoken words. Cytowic's new case, of a man who experiences tastes as shapes, is altogether a rarer form of the condition. It is also unusual in that the "patient" is male, while the majority of synaesthetes are female.

Cytowic reviews the history of research on synaesthesia from both the bird's-eye perspective on the field and the personal viewpoint of his own initial contact with the topic. Regarding the field as a whole, synaesthesia research was active at the turn of the century, though for most of this century it was forgotten by science.

Evidence of its being a lively topic early this century is not hard to find. For example, the composer Alexander Scriabin wrote his symphony Prometheus, the poem of fire for orchestra and simultaneous light show, each musical note being accompanied by the colour it triggered in Scriabin's mind. The first performance of Prometheus in 1915 was reviewed in Scientific American. Wassily Kandinsky, better known for his paintings, also wrote an opera called The Yellow Sound (Der Gelbe Klang). And Arthur Rimbaud wrote a poem in 1871 called "The Vowels" (Voyelles), which begins "A black, E white, I red, U green, O blue". As Cytowic writes, "synaesthesia had always been accepted as a real phenomenon by the medical and psychological professions if only because it had been independently and repeatedly noted by many investigators over a 200-year period".

So why did synaesthesia become a neglected phenomenon within 20th-century psychology and neurology? Cytowic argues that this was due to Skinnerian behaviourism, which banished the mind from experimental psychology. Another problem also caused the demise of the study of synaesthesia: that different synaesthetes disagreed on the colour of any given sound, which introduced doubt about the genuineness of the phenomenon. And finally, synaesthetic science depended on self-report - a form of evidence, but not one easily open to validation.

Cytowic's own work, described in this book, has been an enterprising attempt to restore the topic to scientific credibility. His approach has involved two experimental methods: psychophysics, which works by recording that each musical tone produces a different colour or that each taste produces a different experience of shape, and notes the relation and magnitude changes among these stimulus-response pairings; and functional neuro-imaging, which scrutinises the brain of one individual known to have taste-shape synaesthesia. His results show that there are systematic relationships at the sensory level, and that brain activity is markedly abnormal in his patient.

Scientific interest has continued to grow. In 1994 New Scientist published a lengthy article on synaesthesia, and the BBC's Horizon made a television programme. Reports in the journal Perception have also appeared in recent years, showing that individuals with synaesthesia are virtually 100 per cent consistent in their self-reports, when tested over long periods. For example, if a synaesthete is presented with a long list of words and asked for the description of the colour triggered by each word, virtually identical colour descriptions will be given if that person is retested a year later. And a new neuro-imaging study using PET (positron emission tomography) confirms that when synaesthetes with coloured hearing are presented with spoken words, areas in the visual cortex are active - a pattern not seen in non-synaesthetes. Synaesthesia is thus better established than it was, though plenty of research remains to be done; (1994 also saw the launch of the International Synaesthesia Association).

Cytowic is therefore to be congratulated for bringing this topic back into serious scientific research. Some may find his book too informal and personal, and may feel that in his attempt to popularise the science, he has gone too far into autobiography and speculation, and away from the empirical approach. For others, this casual style will be an attraction. For his scientific colleagues, there will clearly be debate over the details of his theory. Is the limbic system the basis of synaesthesia, as he claims? And is synaesthesia always an emotional experience, as he asserts? But this book, like his earlier one, will undoubtedly earn a place in the history of this unusual area of neurology and neuropsychology.

Simon Baron-Cohen lectures in psychopathology, University of Cambridge.