ABSTRACT
Transcranial magnetic stimulation (TMS), introduced just over 15 years ago (1), has become a key method of studying the normal conductivity and excitability of the corticospinal system, as well as the pathophysiology of cortical modulation circuitry in Parkinson’s disease (PD). TMS is applied using a quick (100 µs) electrical discharge from a series of capacitors through a wire coil. The coil is embedded in a nonconductive plastic or rubber material and fashioned into a figure eight, butterfly, round, or cap shape and held over the part of the brain under study. The high-intensity electric current passes through the wires and induces a magnetic field, which in turn induces electrical current in the underlying cerebral cortex. Unlike transcranial electrical stimulation, TMS is relatively painless, with only a brief acoustic click, occasionally some mild scalp/facial muscle activation, and a momentary sense of disorientation with maximal TMS stimulation and virtually no sensation at low stimulation intensities. Also, unlike transcranial electrical stimulation, which directly affects cortical long tracts, the induced electrical field with TMS preferentially excites neural elements oriented parallel to the surface of the brain, i.e., primarily interneurons (2-4).
