LEAD PARAGRAPH - In a concrete basement at the University of Sydney, I sat in a chair waiting to have my brain altered by an electromagnetic pulse. My forehead was connected, by a series of electrodes, to a machine that looked something like an old-fashioned beauty-salon hair dryer and was sunnily described to me as a ''Danish-made transcranial magnetic stimulator.'' This was not just any old Danish-made transcranial magnetic stimulator, however; this was the Medtronic Mag Pro, and it was being operated by Allan Snyder, one of the world's most remarkable scientists of human cognition.

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