http://en.wikipedia.org/wiki/Savant_syndrome

Savant syndrome, sometimes referred to as savantism, is a rare condition in which people with developmental disorders have one or more areas of expertise, ability, or brilliance that are in contrast with the individual's overall limitations.

Though it is even rarer than the savant condition itself, some savants have no apparent abnormalities other than their unique abilities. This does not mean that these abilities weren't triggered by a brain dysfunction of some sort but does temper the theory that all savants are disabled and that some sort of trade-off is required.