Signal-, set-, and movement-related activity in the human premotor cortex

Single unit recording studies in non-human premotor cortex have revealed ne urons with motor-related activity. Other neurons, however, seem to be invol ved in prior movement selection and preparation processes, and have activit y related to visual instruction signals or movement preparation ('set'). We have used single pulse transcranial magnetic stimulation (TMS) to identify similar processes in human subjects. In Experiment 1 subjects performed a cued movement task while being stimulated with TMS over three sites: sensor imotor cortex, posterior premotor cortex and anterior premotor cortex. TMS slowed movements when applied at 140 ms after the visual cue over the anter ior premotor site, at 180 ms after the visual cue over the posterior premot or site, and at 220 ms and later after the visual cue over the sensorimotor cortex. The results are consistent with a change from signal to movement-r elated processing when moving from premotor to motor cortex. In Experiment 2 there was a preparatory set period between the instruction signal that in formed subjects which movement to make and the 'go' signal that informed th em when to actually make the movement. TMS was applied over the anterior pr emotor site and the sensorimotor site during the set period. At both sites TMS had similar effects on slowing subsequent movements. The results sugges t set activity in both premotor and motor cortices in human subjects. (C) 1 999 Elsevier Science Ltd. All rights reserved.