There's colour in the notes

The role of synaesthesia (a neurological condition where ordinary stimuli trigger extraordinary experiences - for example, sounds trigger the perception of colours) in art and science is the focus of Cretien van Campen's The Hidden Sense . Van Campen admits that when he first learnt about synaesthesia he saw synaesthetes "as extraterrestrials" and wondered: "Did they live in another reality, with coloured music played on harps by cherubs?" After meeting synaesthetes, he realises that they are not "the extraterrestrial dreamers... I had originally pictured", raising hopes for a more nuanced discussion of a phenomenon that has fallen victim to romanticisation and oversimplification.

With such promising beginnings it is disappointing to see van Campen fall rapidly into the same trap, ascribing almost mystical powers to this "gift" - a much overused word in this text. This is particularly unfortunate as beneath the froth lies a very interesting discourse on synaesthesia and the role it has played in the development and appreciation of the arts.

Van Campen's academic background includes philosophy and aesthetics, and he is at his best when discussing art and musical history and the ways in which both the biological phenomenon of synaesthesia and the idea of synaesthesia - particularly the synthesis of music and visual representation - have contributed to artistic development. For the most part he wisely avoids attempting to diagnose deceased artists, musicians and poets as synaesthetes, focusing instead on the influence of synaesthesia on artistic movements - his analysis of synaesthesia's role in 18th and 19th-century literary and visual aesthetics and the impact of ideas about synaesthesia on the development of abstract art is insightful.

The section concerning the quest of artists, scientists and philosophers to understand the relationships between music and colour is particularly crisp and well written, incorporating perspectives from Plato and Newton as well as Kandinsky and van Gogh. Of equal importance is his clear outline of what is and is not neurological synaesthesia (for instance, poetic metaphor, programme music and drug-induced hallucination).

A highlight of the book is the excerpts from van Campen's interviews with synaesthetes, which underscore the individuality of the synaesthetic experience. One synaesthete reports that the opening notes of a musical composition trigger the perception of "dark red three-dimensional 'bars' ", while another perceives "a white sound that looks somewhat like a half- parabola" in response to the same music. Van Campen himself provides some lucid descriptions of synaesthetic perception when he keeps a rein on the romantic imagery.

The weakest sections are those focusing on the science of synaesthesia. Van Campen's one-sided view of synaesthesia as a "gift" limits his discussion of learning, where synaesthesia can both help (word recally) and hinder (adding colours versus numbers in maths). His assumption that variations from "normal" brain function musts be "bad" (rather than simply different) prevents full engagement with the latest research regarding important differences between synaesthetes and non-synaesthetes.

As a historical exploration of the role of synaesthesia in the arts, The Hidden Sense has much to recommend it. As a contemporary discussion of the art and science of synaesthesia, it falls short. Van Campen notes that he "allowed intuition to prevail over reason" in his explorations of synaesthesia - a shame, as a bit more reason might have resulted in a more balanced discussion of a fascinating phenomenon.

Julian Asher is research director of the Synaesthesia Research Group in the department of psychiatry, Cambridge University.