Synesthesia is a neurological phenomenon in which stimulation of one sensory or cognitive pathway leads to automatic, involuntary experiences in a second sensory or cognitive pathway.
Widely documented examples include individuals who reliably and consistently 'see' colours for numerals, 'taste' musical notes, or 'smell' colours etc.
Although many research projects have demonstrated various ways in which synesthesia manifests itself, there is no agreement on how it arises.
Theories include:
See Neural basis of synesthesia at Wikipedia.
Note that the incidence of synesthesia can be substantially affected by Psychedelic drugsplugin-autotooltip__plain plugin-autotooltip_bigPsychedelic drugs
Psychedelic drugs currently have a circular definition. Classed as hallucinogenic psychoactive drugs which have the primary effect of triggering non-ordinary states of consciousness - termed 'psychedelic' experiences.
Many of the known psychedelic substances have been found to bind-to and activate the brain's serotonin 5-HT