I loved a talk given on the TED site by Oliver Sachs where he talked about perfectly normal, very vivid hallucinations that people who start losing their eyesight have... then he revealed that he, himself, suffered from them.

Huh, I've heard of that. A blind friend of mine who lost his eyesight gradually since childhood and has no vision now was telling me that he is constantly having "light shows" like flashes of colour and light even though his eyes aren't capable of seeing light anymore. Crazy!