Inside the mind of a staggering genius

Daniel Tammet is doing mental arithmetic: 37 x 37 x 37 x 37. His fingers hover above the table, tracing shapes visible to no one but himself. A few seconds later, he smiles: the answer is 1,874,161.

Too easy, perhaps: the testers think he may have memorised the cube of every number up to 100. Can he divide 13 by 97? His hands take up the dance again, and he begins: '0.1340206...' He continues reciting decimal points long after the calculator has stopped displaying them.

Calculations such as these are unlikely to faze Tammet: last year, he set a new record when he flawlessly recalled pi to 22,514 decimal places. Aged 26, he speaks seven languages, and is developing an eighth of his own.

Technically, he's what is called a 'prodigious savant' " one of an exclusive club of only 50 people in the world with such extraordinary mental abilities. He also has Asperger syndrome.

Asperger syndrome is a form of autism. It affects the way a person communicates and relates to other people. According to the National Autistic Society, an estimated 212,000 people (out of an autistic population of 535,000) have Asperger's in Britain today. People with the condition find social relationships difficult, largely because they find it impossible to put themselves into other people's shoes. They struggle to read non-verbal signals such as facial expressions, tend to be over-literal in their understanding of language, and can be bewildered by jokes and metaphors.

Asperger's is frequently associated with above-average intelligence, and an impressive memory is not uncommon. As so well demonstrated this week by Andrew Cowan, the 11-year-old Asperger's sufferer who, with his specialist knowledge of the Star Wars trilogy, has beaten hundreds of other young candidates to win a place on Junior Mastermind, people with the condition can store away an extraordinary amount of information. Abilities such as Tammet's, however, are way off the normal scale. Any single aspect of his extraordinary mind would be enough to make him a savant, but he has three areas of outstanding expertise: his ability with numbers, his memory, and his gift for languages.

Specialists believe Tammet's case may offer groundbreaking insights into the workings of the brain. What marks him out is not just his prodigious intelligence but the fact that he is unusually " uniquely, in fact " articulate about the workings of his mind. He describes his experience of synaesthesia, which means he perceives words and emotions as colours, textures or pictures. The word 'complex', for example, is a braid of hair; 'fragile' makes him think of glass.

Tammet explains his fluency with numbers as the result of the way he perceives them: for him, every number up to 10,000 has a unique visual form. 'Different numbers have different colours, shapes and textures,' he says. '[The number] one is very bright and shining, like someone flashing a light into my face. Two is like a movement from right to left. Five is a clap of thunder or the sound of a wave against a rock. Six I find more difficult: it's more like a hole or a chasm. When I multiply numbers, I see two shapes in a landscape. The space between the images makes a third shape, like a jigsaw piece. And that third shape gradually crystallises: I see a fuzziness that becomes clearer and clearer.' The whole process takes place in a flash, 'like sparks flying off'.

Tammet has always loved numbers. He's always been different, too. When he was three, he had a series of seizures. Epilepsy is relatively common among people with autism, although no one at the time knew he was autistic. The epilepsy is thought to have rewired his brain; it may have triggered the synaesthesia.

When he was four, he was given a Ladybird book called Counting, and suddenly the world began to make sense. He remembers sitting for hours in exactly the same spot on the floor of his bedroom, lost in a world of numbers. 'Numbers were the most real thing for me,' he says. 'You could say that they form the way I look at the world. They are my first language.'

He soon discovered he was more comfortable with numbers than with people. School was never easy: he didn't fit in. He was fortunate to have a large and supportive family (he is one of nine) who accepted him as he was. Nonetheless, some aspects of family life were hard: he found certain noises, such as the sound of tooth-brushing, excruciating to listen to, and took pains to avoid the bathroom when one of his brothers or sisters was using a toothbrush.

'Luckily, I had my own room. I would sit there, listening to the silence. For me, silence is more than an absence of noise " it has a particular quality to it. It is soft and silvery, trickling around my head, rather like condensation. That's one reason why I find sudden noise very painful. It feels as if something entirely tactile is being shattered.'

Tammet had one or two friends, but he was hampered by what is known as mind-blindness. 'It is hard for someone with Asperger's to realise another person has thoughts entirely separate from your own. For me, that's not intuitive. I remember going on holiday with another family, and I got homesick and rang home from a phone box. Mum said she would ring me back. I waited for an hour. I didn't realise that I had to give her the number. I assumed because I had it, she did too.'