Synaesthesia.

Synaesthesia is the intriguing, involuntary experience of feeling one sensation in response to a different sensory stimulus. Recognised since described in 1890 by John Locke and clarified by Galton in the 1880s, it has been analysed in the last 50 years. Grapheme-colour synaesthesia is the commonest form, but many other sensory linkages are reported. Experiments show that it is a genuine immediate perception, not merely a memory or learned association. Many of the mechanisms posited are based on indirect methods, and we know little of the neurophysiological mechanisms. Copyright 2007 S. Karger AG, Basel.