Increased positive and disorganised schizotypy in synaesthetes who experience colour from letters and tones.

Synaesthesia is a condition in which one property of a stimulus induces a conscious experience of an additional attribute. For example, in grapheme-colour synaesthesia, a visually presented grapheme results in synaesthetic experiences of colour. These experiences occur in approximately 4% of the population (Simner et al., 2006) and the authenticity of the condition is well established (Cohen Kadosh and Henik, 2007). Despite this, our understanding of the neuropsychiatric profiles of synaesthetes remains limited and surprisingly few studies have addressed whether synaesthesia is linked to more widespread abnormalities in perception that extend beyond the synaesthetic experience itself. There is, however, growing evidence to suggest that synaesthesia may be linked to a broader phenotype. For example, synaesthetes who experience colour show early processing differences to stimuli which do not evoke synaesthesia (Barnett et al., 2008); and the presence of synaesthesia has been linked with other phenotypic manifestations including out-of-body experiences (Terhune, 2009), creativity (Ward et al., 2008), mental imagery (Barnett and Newell, 2008), and mitempfindung (Burrack et al., 2006). Here, we examined the relationship between synaesthesia involving colour and the abnormal perceptions observed in schizophrenia by assessing levels of schizotypy in synaesthetes and non-synaesthetes. We report that synaesthesia for colour is associated with greater levels of positive and disorganised schizotypy (Fig. 1A), suggesting widespread perceptual differences in synaesthesia that extend beyond the synaesthetic concurrent.