Transcranial magnetic stimulation (TMS), a new noninvasive technique for studying brain activity, uses powerful magnetic fields that are applied to the brain from outside the head. It is a method of exciting or interrupting brain activity in a specific and controlled location, and thus is able to imitate aphasic symptoms while giving the researcher more control over exactly which parts of the brain will be examined. As such, it is a less invasive alternative to direct cortical stimulation, which can be used for similar types of research but requires that the subject's scalp be removed, and is thus only used on individuals who are already undergoing a major brain operation (such as individuals undergoing surgery for epilepsy). The logic behind TMS and direct cortical stimulation is similar to the logic behind aphasiology: if a particular language function is impaired when a specific region of the brain is knocked out, then that region must be somehow implicated in that language function. Few neurolinguistic studies to date have used TMS; direct cortical stimulation and cortical recording (recording brain activity using electrodes placed directly on the brain) have been used with macaque monkeys to make predictions about the behavior of human brains.