Also known as Synesthesia if you're from the US (note the lack of 'a'), is a condition that pretty much boils down to having your sensory wires crossed. This means that when one sense is stimulated, TWO react.
There's loads of different theories as to why it exists, and whether it's inherent or semantically driven. Heres a task for you -- how would you describe the colour red to a person blind from birth? -- I bet ALL your answers involve metaphor. That's semantics - the meaning we derive from, or create for, something.
Theres all sorts of synaethetes, from being able to see music or emotions, to tasting sights, there are some truly unusual cases.
One such involves touching another human being, and then feeling that touch on that specific are of their OWN body. fMRI's (functional magnetic resonance imaging) have proven this to actually exist, rather than them 'just saying it'.
Another case, GW, saw rings of colour around the names and faces of familiar people, and also around emotions. Brings a new perspective to the concept of "Auras", don't you think?
One person associated personality and gender to typography, ie, "the number 3 is male and a jerk" (Ward, 2009)
I admit, I did laugh at that.
I can only imagine what the world would look like when experiencing these. There's several composers, I forget their names, who could see music.... and no doubt something similar applies to those who paint it.
For once, I think this is a brain abnormality that I'd love to have....