Savant syndrome is a rare, but extraordinary, condition in which persons with serious mental disabilities, including autistic disorder, have some ‘island of genius’ which stands in marked, incongruous contrast to overall handicap. As many as one in 10 persons with autistic disorder have such remarkable abilities in varying degrees, although savant syndrome occurs in other developmental disabilities or in other types of central nervous system injury or disease as well.

Source The savant syndrome: an extraordinary condition. A synopsis: past, present, future Philos Trans R Soc Lond B Biol Sci., 2009 May 27; 364(1522): 1351–1357.

The exceptional skills can include rapid calculation, artistic ability, map making, musical ability, calendar date calculations etc, and are often strongly associated with memory. It seems to be present in around one person per million.

There is as yet no explanation for the syndrome - either as to why it happens or as to how it happens.

Several investigators have shown that memory alone cannot fully account for savant abilities, particularly calendar calculating and musical skills. (Note: As an example, the current record for being able to accurately recite the randomplugin-autotooltip__plain plugin-autotooltip_bigRandom numbers

unknowable

"We can never decide for sure that a number is random, but what we can do is apply an increasing number of tests and treat our sequence of numbers as innocent until proved guilty."

Source : Prof. Colva Roney-Dougal, senior lecturer in Pure Mathematics at the University of St Andrews, speaking in digits of pi - purely from memory - stands at 100,000).

Also see: Memoryplugin-autotooltip__plain plugin-autotooltip_bigMemory

inexplicable

"Although it is commonly accepted that learning and memory occur via enduring changes in neuronal properties such as synaptic strength within a network of neurons, many details of these processes remain unknown, including the mechanisms responsible for the persistence and maintenance of memory over long periods of time.