Google on synaesthesia - it's a fascinating area. Wikipedia has a long article about it. Here's a tiny edit: "Over 60 types of synesthesia have been reported... While cross-sensory metaphors (e.g., "loud shirt," "bitter wind" or "prickly laugh") are sometimes described as "synesthetic," true neurological synesthesia is involuntary. It is estimated that synesthesia could possibly be as prevalent as 1 in 23 persons across its range of variants... Famous synesthetes include David Hockney, who perceives music as color, shape, and configuration, and who uses these perceptions when painting opera stage sets but not while creating his other artworks... Composers include Duke Ellington, Franz Liszt, Nikolai Rimsky-Korsakov, and Olivier Messiaen, whose three types of complex colors are rendered explicitly in musical chord structures that he invented... Other notable synesthetes include musicians Billy Joel, Itzhak Perlman..."