There are people who see shapes when they hear things or taste things, I recall someone saying that The curlicues in the chicken need to be unwound when it was too lemony, the guy who had to pull over instantly if that Alert tone beeped on his car radio as his entire visual field would just go orange, and so on. Fascinating stuff. I have a theory that it is a weakening of myelination somewhere around the thalamus, but have never seen any data to confirm or deny this possibility. We did have a researcher here at UVA who was studying it, the letter E being dark blue and narrow, for instance, but I can't recall who he was or I'd send you to his researcher...