I am linking to this article simply because I think it is really interesting. The article discusses the phenomenon of Synaesthesia, which is defined as "the pairing of two bodily senses by which the perception of a determined stimulus activates a different subjective perception with no external stimulus." So, you see the color blue, but also have an associated smell or sound along with it, despite no environmental stimulus. There's more information here.
If you find this sort of thing interesting, you will definitely want to check out one of the many books by Oliver Sacks, as I also recommended here. He has written numerous books regarding interesting neuropsychological conditions, including many rare ones. I'll list a few in the panel to the right.