What Synesthesia isn’t

What synaesthesia isn't. Synaesthesia eludes not only an agreed explanation but even a clear definition. Here we examine critically some of the common assumptions about synaesthesia: that it is best defined by the consistency of the sensory associations evoked by the inducing stimulus; that it is rare and always inherited; that it is categorically different from normal cross-modal sensory integration or implicit sensory association; that synaesthetic associations are formed only very early in life; that they are simply an exaggerated form of sensory 'binding'; that synaesthetes fall into distinct classes of 'associators' and 'projectors'; that synaesthetic colours convey a distinct advantage in perceptual segregation; and that the condition is adequately explained by and correlates with hyperconnectivity of the cerebral hemispheres. We question or qualify all these assumptions. We argue that the 'synaesthetic brain', characterized by hyperconnectivity, is likely to give rise to alterations in function beyond the idiosyncratic associations of synaesthesia. We end by trying to clarify the definition of synaesthesia through a set of statements about what it is not.