Transcranial magnetic stimulation: Neurophysiological and clinical applications

Transcranial magnetic stimulation (TMS) is a safe and noninvasive means of electrically stimulating the brain by electromagnetic induction. TMS is capable of probing intracortical circuits and modulating cortical activity in humans; as such it has been instrumental to studying the neurophysiology and functional neuroanatomy of the frontal lobes. For example, using TMS to induce "virtual lesions"-transient disruption of function in the targeted brain region-has yielded important insights into the functional organization of the prefrontal cortex (PFC) with respect to working memory, language, and other core cognitive functions. Whereas neuroimaging is typically limited to observing correlations between brain function and behavior, TMS, by interacting with neural circuits, can lead to causal inferences that bridge human, nonhuman primate, and other model system studies. Applied repetitively in trains of stimuli, TMS is also capable of normalizing aberrant patterns of cortical activity in the treatment of neurologic and psychiatric disorders. The earliest and most well-established clinical use of repetitive TMS is in the treatment of medication-resistant depression with high-frequency stimulation of the left dorsolateral PFC. Research efforts to identify other promising clinical applications-such as for stroke and Alzheimer's disease-are rapidly expanding; however, the majority of these indications have yet to have devices cleared by the FDA for on-label use.