Modulation of human primary motor cortex plasticity during proactive and retroactive interference between motor practice and paired associative stimulation.

Proactive and retroactive interference have been widely demonstrated behaviorally, but their neurophysiological effects have rarely been compared within the same experimental framework. This study compared the primary motor cortex (M1) plasticity responses when motor practice (MP) was performed before or after paired associative stimulation (PAS25) and explored whether these responses were accompanied by changes in intracortical inhibition. A within-subject design was employed. A right-hand precision grip task with real-time electromyographic feedback was used to examine practice-related refinement of feedback-guided motor control, and PAS25 was used to induce long-term potentiation-like plasticity in M1. Two combined intervention protocols were examined: MP followed by PAS25 (MP→PAS25) and PAS25 followed by MP (PAS25→MP), with PAS25 alone serving as the reference condition. Motor-evoked potentials (MEPs), short-interval intracortical inhibition, and long-interval intracortical inhibition were assessed before and immediately after each intervention protocol. MEP amplitude increased significantly following PAS25 alone, whereas significant pre-post reductions were observed across both the MP→PAS25 and PAS25→MP combined protocols. Post-intervention MEP amplitudes were significantly lower in both combined conditions than in the PAS25-alone condition, with no significant difference between the two combined conditions. No significant time × condition interactions were detected for short-interval intracortical inhibition or long-interval intracortical inhibition. Combining MP with PAS25 produced MEP responses that differed from PAS25 alone, while the two intervention orders showed similar post-intervention responses despite their opposite temporal order. These neurophysiological effects were not accompanied by detectable changes in the specific short-interval intracortical inhibition and long-interval intracortical inhibition measures assessed in the present study. The physiological processes underlying these effects remain to be determined.