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Savant skills included more fully developed talents, such as being able to name the elevation of both the sun and the moon at any time of day, on any specified date; being able to name the day of the week for any date in the distant past or future (a talent known as calendrical calculation) and perfect pitch. Importantly, the abilities and the skills had to be exceptional by the standards of the general population, but also well above the individual's overall level of ability. In total, roughly 30 per cent had some kind of special ability (Philosophical Transactions of the Royal Society B, vol 364, p 1359).

For musical savants like Paravicini, Happé suggests that a bias towards small details might have led their developing brains to focus more on the exact notes than the overall melody, leading to perfect pitch and an exceptional musical memory. In art, a focus on small regions of a picture could lead to accurate perspective drawing.

Most people find this harder when they are shown an unsegmented version of the pattern versus a segmented one, but people with autism don't have this preference, demonstrating their skill at seeing a whole in terms of its parts even if there are no obvious dividing lines (see diagram). "It shows they are able to do the segmentation in their minds," says Winner. The precocious realists did not have this preference either, indicating a talent for realistic drawing may arise from this isolated trait commonly found in autism (Philosophical Transactions of the Royal Society B, vol 364, p 1449).

Although these results help to pinpoint exactly what it is about autism that predisposes people to talent, it's still not clear why an eye for detail is more common in autistic people in the first place. Clues might lie in the work of Simon Baron-Cohen from the University of Cambridge, which suggests that people with autism are "hypersensitive" to sensory information

Daniel Tammet, a prodigious savant who has memorised pi to 22,514 digits, believes his own talents have arisen from a special ability to connect different pieces of information together. "Savant abilities are linked to a highly associative type of thinking, an extreme form of a kind that everyone does - examples would include daydreaming, puns and the use of metaphors," he says.

The few studies of savant brains certainly suggest they are physically different from the average brain. For example, when Happé and Wallace studied the brain of a savant gifted at art, calendrical calculation and memory, they found his cortex was thicker in the areas associated with visuospatial processing and calculation and thinner in other regions associated with social cognition, compared with people who were neither savants nor autistic. But whether these differences were innate or grew with lifelong practice was still unclear.

The answer to that question may come from an unlikely source - a study of London taxi drivers who have acquired an encyclopedic memory of the streets of London known as "the Knowledge". Given that taxi drivers must remember the layout of 25,000 streets and the location of thousands of places of interest, and retrieve the information instantaneously, some researchers like Happé believe the Knowledge qualifies as a savant-like skill.

Eleanor Maguire and colleagues at the Institute of Neurology at University College London and colleagues found that drivers with the Knowledge have a bigger rear hippocampus than bus drivers and adults who do not drive taxis. In addition, the hippocampus appears to be larger the longer a taxi driver has been working, and shrinks once they retire (Philosophical Transactions of the Royal Society B, DOI: 10.1098/rstb.2008.0288).

In fact, it seems the remaining mystery is not so much how savants achieve their talents, but what drives them in the first place. "Motivation is a big unknown," says Wallace. "It's an enormous driving force in giftedness and in savants, but we don't know a lot about it."

One person who has something of an inside view on what contributes to savant ability is Paravicini's mentor, Adam Ockelford, a professor of music at Roehampton University in London who has watched Paravicini's talent blossom since the age of 4. When they first met, Paravicini was entirely self-taught and bashed at his plastic keyboard with his fists and elbows to reproduce the sounds he was hearing. It was only after years of practice that his technical skills developed.

But as researchers like Wallace have suggested, Paravicini seemed motivated way beyond the average music student. In fact, he seemed to be playing as if his life depended on it, and Ockelford thinks it's this that truly sets savants apart from their peers. "The survival instinct gets turned with extraordinary force into something else - in Derek's case music," says Ockelford. "When people see Derek, they think it is amazing, almost religious. But to me, it's mainly just hard work."