Project 20

Cortical motor function in conscious humans studied with transcranial magnetic stimulation

Transcranial magnetic stimulation (TMS) offers a means of studying cortical motor function in conscious behaving humans. Brief high-intensity magnetic pulses delivered through a coil positioned on a subject’s scalp over motor cortex excite underlying neurons and evoke electromyographic activity in the corresponding muscles. The size of the motor-evoked potential (MEP) reflects the sensitivity of the corticospinal system. Paired TMS pulses (presented in a conditioning – test sequence, with a brief interstimulus interval) have revealed the existence of intracortical circuits that exert inhibitory and excitatory influences on the output of the motor cortex. These intracortical circuits have been implicated in the control of motor output and in the capacity of the motor cortex to reorganize with experience. Studies are in progress on the cortical mechanisms of manual dexterity and handedness (the asymmetry of manual dexterity) and on use-dependent reorganization of motor cortex.