I recall a tv documentary here in the UK on the subject of Synaesthesia . If you search BBC Horizon Synaesthesia you should find some video links watchable outside for the UK - not all BBC links are so.
Apparently at birth all of our senses are connected : they start to separate some months later. With some there is no complete and final separation.
On the subject of Savants the easiest ones to search for Daniel Tammet, who is a maths genius, and Stephen Wiltshire whose eyes act almost a camera would. I think the complete BBC series may be called Beautiful Minds