In the 1880s, Francis Galton described a condition in which “persons…almost invariably think of numerals in visual imagery.” This “peculiar habit of mind” is today called synaesthesia”, and Galton’s description clearly defines this condition as one in which stimuli of one sensory modality elicit sensations in another of the senses.
Read this article on Scienceblog: neurophilosophy. This article got my attention since the expressionist artist Wassily Kandinsky was also a synaesthete and Dr Shock is an admirer of Wassily Kandinski. Kandinsky in whom musical tones elicited specific colours, was a tone-colour synaesthete. Kandinsky used his synaesthesia to inform the artisitic process – he tried to capture on canvass the visual equivalent of a symphony.