[Synaesthesia - towards a cognitive neuroscience of coloured hearing]

This review is about synaesthesia, a well known but still poorly investigated phenomenon. The most common form of synaesthesia is coloured hearing. Epidemiological and genetical studies show that synaesthesia has a higher prevalence in the normal population than previously stated. Most subjects are female and there is a clear heritability. Psychophysical investigations of synaesthetic mechanisms in normal subjects show reliable colour-tone-attributions. Single case studies with paradigms from experimental psychology found a high consistency of synaesthetic associations and observed behaviourally relevant interferences between primary perception and secondary synaesthetic sensations. The first studies using functional neuroimaging methods found specific neuronal correlates of synaesthesia. Neuropsychiatric studies analyze synaesthesia-like phenomena in patients with optical nerve lesions, upper limb amputation or severe depression and as induced by hallucinogenic drugs. Data-based and more speculative theories and models of synaesthesia are presented and critically discussed.