I just saw this stunning segment on ABC 20/20 on Stephen Wiltshire, an artist who as a child was diagnosed as autistic. He draws behemoth citiscapes from his memory.
Attached here is a clip from when UBS commissioned him to sketch the NYC skyline from memory. The sketch was completed in three days and then used as an ad for UBS at the JFK airport.
His website – http://www.stephenwiltshire.co.uk/