Excitability of human motor cortex inputs prior to grasp

Transcranial magnetic stimulation (TMS) was used to investigate corticospinal excitability during the preparation period preceding visually guided self-paced grasping. Previously we have shown that while subjects prepare to grasp a visible object, paired-pulse TMS at a specific interval facilitates motor-evoked potentials (MEPs) in hand muscles in a manner that varies with the role of the muscle in shaping the hand for the upcoming grasp. This anticipatory modulation may reflect transmission of inputs to human primary motor cortex (M1) for visuomotor guidance of hand shape. Conversely, single-pulse TMS is known to suppress MEPs during movement preparation. Here we investigate the time course of single- and paired-pulse MEP modulation. TMS was delivered over M1, at different time intervals after visual presentation of either a handle or a disc to healthy subjects. Participants were instructed to view the object, and later to grasp it when given a cue. During grasp there was a specific pattern of hand muscle activity according to the object grasped. MEPs were evoked in these muscles by TMS delivered prior to grasp. Paired-pulse MEPs were facilitated, whilst single-pulse MEPs were suppressed. The pattern of facilitation matched the object-specific pattern of muscle activity for TMS pulses delivered 150 ms or more after object presentation. However, this effect was not present when TMS was delivered immediately after object presentation, or if the delivery of TMS was given separately from the cue to perform the grasp action. These results suggest that object-related information for preparation of appropriate hand shapes reaches M1 only immediately preceding execution of the grasp.