Interactions between the
ventral premotor (PMv) and the
primary motor cortex (M1) are crucial for transforming an object's geometrical properties, such as its size and shape, into a motor command suitable for grasp of the object. Recently, we showed that PMv interacts with M1 in a specific fashion, depending on the
hand posture. However, the functional connectivity between PMv and M1 during the preparation of an actual grasp is still unknown. To address this issue, PMv-M1 interactions were tested while subjects were preparing to grasp different visible objects requiring either a
precision grip or a whole
hand grasp. A conditioning-test
transcranial magnetic stimulation (TMS) paradigm was used: a test stimulus was applied over M1 either in isolation or after a conditioning stimulus delivered, at different delays, over the
ipsilateral PMv.
Motor evoked potentials (MEPs) were recorded in the first
dorsal interosseus and abductor digiti minimi
muscles, which show highly differentiated activity according to grasp. While subjects prepared to grasp, delivering a conditioning PMv
pulse 6 or 8msec before a test
pulse over M1 strikingly facilitated MEPs in the specific
muscles that were used in the upcoming grasp. This degree of facilitation
correlated with the amount of
muscle activity used later in the trial to grasp the objects. The present results demonstrate that, during grasp preparation, the PMv-M1 interactions are muscle-specific. PMv appears to process the object geometrical properties relevant for the upcoming grasp, and transmits this information to M1, which in turn generates a motor command appropriate for the grasp. We also reveal that the grasp-specific facilitation resulting from PMv-M1 interactions is differently related to the upcoming grasp
muscle activity than is that from paired-pulse stimulation over M1, suggesting that these two TMS paradigms assess the excitability of cortico-cortical pathways devoted to the control of grasp at two different levels.
