Mechanisms of Hebbian-like plasticity in the ventral premotor - primary motor network.

The functional connection between ventral premotor cortex (PMv) and primary motor cortex (M1) is critical for the organization of goal-directed actions. Repeated activation of this connection by means of cortico-cortical paired associative stimulation (cc-PAS), a transcranial magnetic stimulation (TMS) protocol, may induce Hebbian-like plasticity. However, the physiological modifications produced by Hebbian-like plasticity in the PMv-M1 network are poorly understood. To fill this gap, we investigated the effects of cc-PAS on PMv-M1 circuits. We hypothesized that specific interactions would occur with I -wave interneurons as measured by the short intracortical facilitation protocol (SICF). We used different paired-pulse TMS protocols to examine the effects of PMv-M1 cc-PAS on SICF, on GABAergic circuits as measured by short (SICI) and long (LICI) intracortical inhibition protocols, and varied the current direction in M1 to target different M1 neuronal populations. Finally, we examined the effects of cc-PAS on PMv-M1 connectivity using a dual coil approach. We found that PMv-M1 cc-PAS induces both a long-term potentiation (LTP)- or long-term depression (LTD)-like after-effect in M1 neuronal activity that is strongly associated with a bidirectional-specific change in I -wave activity (SICF = 2.5 ms ISI). Moreover, cc-PAS induces a specific modulation of the LICI circuit and separately modulates PMv-M1 connectivity. We suggest that plasticity within the PMv-M1 circuit is mediated by a selective mechanism exerted by PMv on M1 by targeting I -wave interneurons. These results provide new mechanistic insights into how PMv modulates M1 activity that are relevant for the design of brain stimulation protocols in health and disease. KEY POINTS: The I -wave is specifically modulated by the induction of ventral premotor cortex - primary motor cortex (PMv-M1) plasticity. After PMv-M1 cortico-cortical paired associative stimulation (cc-PAS), corticospinal excitability correlates negatively with I -wave amplitude. Different cc-PAS coil orientations can lead to a long-term potentiation- or long-term depression-like after-effect in M1.