Snyder looked over my shoulder. ''Well, how about that? Leonardo would be envious.'' Or turning in his grave, I thought.

As remarkable as the cat-drawing lesson was, it was just a hint of Snyder's work and its implications for the study of cognition. He has used TMS dozens of times on university students, measuring its effect on their ability to draw, to proofread and to perform difficult mathematical functions like identifying prime numbers by sight. Hooked up to the machine, 40 percent of test subjects exhibited extraordinary, and newfound, mental skills. That Snyder was able to induce these remarkable feats in a controlled, repeatable experiment is more than just a great party trick; it's a breakthrough that may lead to a revolution in the way we understand the limits of our own intelligence -- and the functioning of the human brain in general.

Snyder's work began with a curiosity about autism. Though there is little consensus about what causes this baffling -- and increasingly common -- disorder, it seems safe to say that autistic people share certain qualities: they tend to be rigid, mechanical and emotionally dissociated. They manifest what autism's great ''discoverer,'' Leo Kanner, called ''an anxiously obsessive desire for the preservation of sameness.'' And they tend to interpret information in a hyperliteral way, using ''a kind of language which does not seem intended to serve interpersonal communication.''

For example, Snyder says, when autistic test subjects came to see him at the university, they would often get lost in the main quad. They might have been there 10 times before, but each time the shadows were in slightly different positions, and the difference overwhelmed their sense of place. ''They can't grasp a general concept equivalent to the word 'quad,''' he explains. ''If it changes appearance even slightly, then they have to start all over again.''

Despite these limitations, a small subset of autistics, known as savants, can also perform superspecialized mental feats. Perhaps the most famous savant was Dustin Hoffman's character in ''Rain Man,'' who could count hundreds of matchsticks at a glance. But the truth has often been even stranger: one celebrated savant in turn-of-the-century Vienna could calculate the day of the week for every date since the birth of Christ. Other savants can speak dozens of languages without formally studying any of them or can reproduce music at the piano after only a single hearing. A savant studied by the English doctor J. Langdon Down in 1887 had memorized every page of Gibbon's ''Decline and Fall of the Roman Empire.'' At the beginning of the 19th century, the splendidly named Gottfried Mind became famous all over Europe for the amazing pictures he drew of cats.

The conventional wisdom has long been that autistics' hyperliteral thought processes were completely separate from the more contextual, nuanced, social way that most adults think, a different mental function altogether. And so, by extension, the extraordinary skills of autistic savants have been regarded as flukes, almost inhuman feats that average minds could never achieve.

Snyder argues that all those assumptions -- about everything from the way autistic savants behave down to the basic brain functions that cause them to do so -- are mistaken. Autistic thought isn't wholly incompatible with ordinary thought, he says; it's just a variation on it, a more extreme example.