The condition in which a sensory experience normally associated with one modality occurs when another modality is stimulated. The Mind, Richard L. Gregory Overview Synaesthesia is a literal crossing of the senses.
Incidence Estimates on the incidence of synaesthesia vary considerably.
Types of Synaesthesia Since synaesthesia can be the crossing of two or more senses, there are a possible 31 different combinations of senses. Most synaesthetic combinations involve two senses only, however, leaving 10 different pairings. In addition, the connection is typically unidirectional, so that sound evokes colour, but colour does not evoke sound. Therefore, there are 20 potential forms of two sense synaesthesia possible. In most literature, a synaesthetic form is referred to by its secondary, or imagery sense first.