Neurophysiological Mechanisms Underlying Retention of Motor Learning

Acquisition of skilled motor performance can be translated into long-lasting behavioral changes also known as retention of motor learning. Importantly, retention can be heavily influenced in either a disruptive or enhancive way depending on how we practice our motor training. In animals, motor learning induces long-term potentiation (LTP) that can ultimately limit the ability to induce further LTP, a phenomenon known as occlusion. In humans, evidence of LTP-like plasticity and its occlusion can be probed indirectly using non-invasive brain stimulation. First, we investigated the neurophysiological mechanisms underlying retention of motor learning. We used depotentiation to abolish the induction of LTP-like plasticity in MI after training and found that it reduced occlusion of LIP-like plasticity and decreased retention the following day. Moreover, retention positively correlated with the magnitude of occlusion following motor training. Overall these results suggest a direct connection between occlusion of LIP-like plasticity and retention of learning. Subsequently we studied the neurophysiological mechanisms of how training of similar motor skills in close succession interferes with retention of learning. Learning interference occurs when learning something new causes forgetting of an older memory (retrograde interference) or when learning a new task disrupts learning and/or retention of a second subsequent task (anterograde interference). Indeed, if occlusion depicts utilization of LTP-like plasticity and is crucial for retention, then competition over limited LIP-like resources should result in reduced retention. Here, we found that larger occlusion of LIP-like plasticity after learning the first skill was associated with larger anterograde interference but more resilience to retrograde interference. These findings implicate competition of LIP-like plasticity as a factor that limits our ability to remember multiple tasks trained in close succession. Finally, we studied the neurophysiological mechanisms of how supplementing motor training with the observation of someone else engaging in the same motor task, known as action observation (AO), can enhance retention. Previous work has shown that physical practice results in cortical motor representational changes, referred to as use-dependent plasticity (UDP), which can be enhanced with congruent AO. We found that inducing a virtual lesion over PMv reduced the beneficial contribution of AO to UDP. These findings suggest PMv activation during action observation studies is essential for the beneficial effects that AO exerts over motor training.