An editor for the Telegraph, Roger Highfield, recently volunteered to allow a UK researcher to shut off the speech center of his brain with a high-powered magnetic pulse. Regular speech is controlled by a section of the brain called Broca's area. Once the precise location is determined in the subject, a magnetic pulse can temporarily disrupt speech without impairing other cognitive functions. The link contains a video in which you can watch Highfield stutter and twitch while attempting to recite a nursery rhyme. A later test shows that he's able to sing the rhyme without difficulty, since singing is controlled in a different part of the brain (as you may remember from Scott Adams' speech disorder). Researchers believe that the ability to stimulate or quell activity in specific areas of the brain may help in treating conditions like epilepsy and migraine headaches.