Synaesthesia is the crossing over of 2 or more senses and there can be 31 possibilities of different combinations of senses. But clinically, it is usually only 2 senses that cross and this knocks it down to a potential 20 different pairings. Usually the pairing is unidirectional e.g. sound evokes colour, but colour does not evoke sound.
Synaesthesia is divided into 2 categories.
- This is when stimulation of one modality triggers the perception in a second modality, in the absence of direct stimulation to this second modality.
- This is when 3 or more senses crossed.