Patrolling the boundaries of synaesthesia: a critical appraisal of transient and artificially induced forms of synaesthetic experiences

Synaesthesia is a neurological condition in which people make unusual associations between various sensations. This chapter investigates conceptually whether alleged non-developmental (i.e. artificial) forms of synaesthesia could be counted as genuine synaesthetic experiences. It focuses in particular on post-hypnotic suggestions, drug habits, flavor perception, and use of sensory substitution devices. It discusses a number of criteria that have been taken as definitional of synaesthesia; namely, inducer-concurrent pairing, idiosyncrasy, consistency over time, and automaticity of the process, and subsequently investigates whether those alleged non-developmental cases could fulfill these criteria. Although the response provided here is negative, as each of the cases fail to fulfill one or several of the criteria, the comparisons between these cases and congenital synaesthesia prove useful to highlight key differences between different kinds of multisensory experiences.