Interesting thread. There was an artist by the name of Pamela Coleman Smith who had a rather extreme case of Synesthesia. Whenever she would hear symphonic music, she would see visions, usually landscapes. She would draw these landscapes although she would often feel "displaced" and "unattached" to the drawing after the synesthetic (word?) episode. It was almost as if someone else had executed the drawing. Pamela Coleman Smith is best known for her Tarot card designs. I believe that there are articles in Art History journals about Smith and her "visions".