The perhaps somewhat tenuous link between the move from figuration to abstraction in graphic art and the move from tonality to atonality in music. Does this link give us any insight into the nature of atonality (or graphic abstraction, for that matter)?
Also see Light, sound and synaesthesia.
Look at the series of pictures of a bull by Picasso on the left. Notice the transformation from a highly figurative picture to something much more abstract. Think about this, and about how it might relate to music.