Superficial Ventral Premotor Pathways to Primary Motor Cortex Shape the Temporal Coordination of Precision Grasping

Goal-directed actions, such as picking up, manipulating, or using objects, are so ubiquitous that impairments in these skills can severely impact quality of life. Reaching-grasping behaviors are driven by a frontoparietal network, with the ventral premotor cortex (PMv) and the primary motor cortex (M1) serving as critical frontal nodes. PMv-M1 connectivity can be modulated using cortico-cortical paired associative stimulation (cc-PAS), which involves repeated paired transcranial magnetic stimulation (TMS) of both nodes. Stimulating M1 with an anterior-posterior (AP) current direction selectively enhances corticospinal excitability during isometric precision grip, but not during isometric power grip. However, it is unclear how the plasticity induction in the more superficial PMv-M1 connectivity may influence the preparation and execution of goal-directed, naturalistic reaching-grasping actions. In this study, participants performed reaching-grasping actions toward small or large objects, requiring precision or power grip, before and after applying the PMv-M1 cc-PAS_AP protocol. The plasticity-induction protocol selectively modulated the joint angles temporal synergies during precision grip actions, suggesting a reorganization of whole-arm reaching-grasping coordination. The analyses of the joint angles spatial synergies did not reveal comparable effects. Taken together, these findings suggest that the PMv-M1 plasticity-induction protocol primarily modulated the temporal, rather than the spatial, control of joint angles recruitment during precision grip actions. Given that such basic skills are often permanently lost in stroke patients, our findings may offer valuable insight for the development of innovative therapeutic approaches for this clinical population.