Synesthesia is the most interesting thing I've ever heard about a human brain doing. When I was in grad school, we had a profesor studying it in my department (needless to say, back then in Neanderthal times, he didn't last), but it wasn't my area so I didn't get to do any real esearch in it. The closest I, personally, get is that the sound ee is, somehow, narrow. Unless I have help, which doesn't count.