PARIETAL ACTIVITY DURING ACTION PLANNING AFFECTS SENSORIMOTOR ASSOCIATIVE PLASTICITY IN THE HUMAN MOTOR SYSTEM

Goal-directed motor skills are thought to arise from circuit-level associations encoded through activity-dependent strengthening of functional connections between parietal action-related processes and motor cortical output via temporally contingent co-activation. However, it has been difficult to provide strong causal evidence for this claim in humans. Here, we examined how action-related activity in the posterior parietal cortex (PPC) influences sensorimotor associative plasticity in the parietal-motor circuit. Using a within-subject design, participants completed three stimulation conditions during right-hand object-directed grasping, a task that reliably engages PPC activity during movement planning. Continuous theta-burst stimulation (cTBS) was applied to the left PPC or a left visual cortex control site to transiently disrupt neural activity, followed by a state-dependent Hebbian plasticity induction protocol in the parietal-motor circuit in the left hemisphere using cortico-cortical paired associative stimulation (cPAS). To determine whether PPC suppression alone modulates motor cortical plasticity, a third condition combined cTBS to the left PPC with a non-Hebbian plasticity cPAS protocol with altered interregional timing. Plasticity was assessed by measuring changes in motor-evoked potentials (MEPs) elicited with single-pulse transcranial magnetic stimulation before and after each protocol. cTBS to the PPC significantly reduced the task-engaged, state-dependent cPAS-induced increase in MEP amplitudes, whereas cTBS to the visual cortex had no effect. cTBS to the PPC, followed by a non-Hebbian cPAS protocol, also did not change MEP amplitudes from baseline. These findings provide strong causal evidence that parietal activity during movement planning regulates the induction of sensorimotor parietal-motor associative plasticity necessary for dexterous hand function.