Synaesthesia is a rare condition where people experience some percepts as a different sensory modality than the one they normally belong to – e.g., numbers as colours, or tones as shapes. It is, thus, a positive (and rather bizarre!) syndrome, where an abnormal trait is present, not absent, in the affected person.
Synaesthetes clearly posses brains that are differently wired up than non-synaesthetes. It has been speculated by some neuropsychologists, such as V.S. Ramachandran, that the sensory areas of the synaesthetes' brains are connected in an abonormal fashion, such that, for example, signals normally destined for their number areas end up in the colour area.
Experimental work casting light on such hypotheses is finally forthcoming, and a lot of what is presently known has now been collected in the new issue (February 2006) of the journal Cortex. Edited by Jamie Ward and Jason Mattingley, it contains contributions by just about every researcher currently working on synaesthesia. And remember: Cortex doesn't require a subscription to access!
Ward, J. & Mattingley, J., eds. (2006): "Cognitive neuroscience perspectives on synaesthesia. Cortex, vol. 42, issue 2.