Science Now has a story about the memory methods of Daniel Tammet, who last year coughed up a whole lot of Pi in just over 5 hours. Tammet, an unusually articulate autistic savant, is able to explain what he does when he's thinking numbers. He sees numbers as landscapes:
"When I multiply numbers together, I see two shapes. The image starts to change and evolve, and a third shape emerges. That's the answer. It's mental imagery. It's like maths without having to think."
He could recollect the digits of Pi as if he were viewing a film. (I wonder what digit number 22515 was.)
These descriptions of what he's doing are very interesting to psychologists wanting to improve everyone's memories. The Science now blurb talks about an initial test of how important visualization is to Tammet's memorization prowess. Intriguingly, a seemingly mild perturbation-- presenting numbers printed in different size fonts-- greatly interfered with his recall. His performance was forced down to nearly normal levels.
Engaging more than one sense to assist recall is an effective mnemonic technique, so Tammet's speaking of a visual component to his recall (and his sensitivity to visual disruptions) is a very interesting hint. In particular Peg listing , assigns numbers to vivid sensory qualities- furry, red, slippery-- to boost recall of numbers.(link is to Wikipedia). This sounds a bit like Tammet's Pi-film. My other immediate though was of chess players whose memory, even when playing blindfolded, is highly visual.
A nice general site about memory, including the procedural memory of savants, is here . Also, check out Mixing Memory , a very well written cognitive science blog.