Magnetic stimulation of the brain: starting or stopping.

A very brief magnetic pulse is able to stimulate cortical neurons through the intact scalp. It can elicit movement of a limb by activation of pyramidal cells either directly or via interneurons. When making a voluntary movement, a corresponding program of the task is thought to be compiled beforehand. In patients with ideomotor apraxia who can not perform a task, a lesion of the left hemisphere is frequently found. A magnetic cortical stimulus given directly before a movement is executed is known to delay the onset without altering the pattern. When it will become feasible to deliver a localized stimulus to one hemisphere, this will induce a temporary pause in that area and thus simulating a lesion in intact brain. In this way information can be gathered on the localization of voluntary action in intact brains.