Daniel Tammet is one of only 25 documented ‘savants’ in the world. He can recall the mathematical constant Pi (3.141..) to more than 22,500 decimal places. His ability to “see” numbers manifested after a he suffered epileptic seizures as a toddler.
The interesting thing about him is that he can describe what he sees and feels in his synaesthesia. Sequences of numbers form complex landscapes in his mind. For example number one is bright and shiny, like someone flashing a torch; number two is a drifting, right to left motion; five is “like the clap of thunder or the sound of a wave on a rock”; six is very small, it’s the one he finds hardest to experience in any meaningful way, so it often manifests as an absence, a hole or a chasm; while “Number nine is the biggest number. It’s very tall. It can be very intimidating.” He says he sees every number up to 10,000 in a distinct way.