Area F5, in the ventral premotor cortex of the macaque monkey, plays a critical role in determining the hand shape appropriate for grasp of a visible object. F5 neurones show increased firing for particular types of grasp, and inactivation of F5 produces deficits in visually guided grasp. But how is F5 activity transformed into the appropriate pattern of hand muscle activity for efficient grasp? Here we investigate the pathways that may be involved by testing the effect of single stimuli delivered through microwires chronically implanted in area F5 and in primary motor cortex (M1) of two macaque monkeys. The EMG responses from M1 test (T) stimulation were recorded from 4–11 contralateral hand, digit and arm muscles during reach-to-grasp of visually presented objects. Conditioning (C) stimulation of F5, at intensities subthreshold for motor effects, caused strong modulation (over twofold) of M1 test (T) responses. The pattern of facilitation was specific. First, facilitation of the T response was particularly evident at short C–T intervals of −1 to 1 ms. Second, this facilitation was only present in some muscles and during reach-to-grasp of a subset of objects; it did not appear to be simply related to the level of EMG activity in the muscles at the moment of cortical stimulation or indeed to the upcoming contribution of that muscle during grasp. At later C–T intervals (1–6 ms), F5 stimulation caused significant suppression of the test M1 response. The results are in keeping with the concept that during visually guided grasp, F5 modulates corticospinal outputs from M1 in a muscle- and grasp-specific manner
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