Tuning of ventral premotor cortex neurons to distinct observed grasp types: a TMS-priming study.

The ventral premotor cortex is known to be involved in the observation of others' actions. We tested the tuning properties of neuronal populations in the premotor cortex to distinct observed hand-object interactions with a transcranial magnetic stimulation (TMS)-priming protocol. Participants were asked to classify as "fast" or "slow" target pictures preceded by prime pictures both representing a spectrum of grasp types from precision grip (PG) to whole-hand grasp (WHG). Single TMS pulses time-locked to targets were delivered over the left ventral premotor (L-PMv) and left dorsal premotor (L-PMd) cortices or as sham stimulation. Without TMS and with sham stimulation, a clear priming effect was observed as a shortening of reaction times and as a bias towards the priming grasp type in the classification responses. The perceptual advantage of priming was reversed by TMS over PMv but not over PMd. According to the TMS-priming paradigm, these results show that distinct neural populations in L-PMv but not in L-PMd are selectively tuned to different observed grasp types as PG or WHG.