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.
• Electrical and/or chemical cross-activation in areas of the brain.
• Disinhibited feedback from 'higher order' cortical regions to early sensory areas.
• Semantic misinterpretation.
See Neural basis of synesthesia at Wikipedia.