Temporal dynamics of ipsilateral corticospinal excitability during the motor learning process of a complex manual task.

The primary motor cortex (M1) of the brain is responsible for learning and executing motor skills. Previous studies have reported that performing a complex unilateral motor task activates both the contralateral and ipsilateral M1 (ipsi-M1) regions. Corticospinal excitability in ipsi-M1 has been shown to decrease as motor skills are acquired. However, the temporal changes in this excitability throughout the motor learning process remain unclear. Therefore, this study aimed to clarify how corticospinal excitability in ipsi-M1 changes over time during motor learning of a unilateral complex task. Fifteen right-handed healthy adults were enrolled in the study and asked to perform a ball rotation (BR) task. In this motor task, the participants rotated two balls with their right hand. Corticospinal excitability was assessed at rest, followed immediately by the first assessment during movement (T1). Subsequently, the first BR practice and performance evaluation (P1) was conducted. Thereafter, corticospinal excitability assessments (T2-T10) and performance evaluations (P2-P10) were alternated for up to 10 sets. The results showed that motor performance significantly improved from P1 to P6. The motor-evoked potential (MEP) amplitude significantly increased at T1 compared to rest and significantly decreased at T5-T6 and T8-T10 compared to T1. These findings demonstrate that ipsi-M1 excitability increases during the early stages of learning but decreases even before motor learning reaches a plateau. In summary, these findings demonstrate the temporal changes in ipsi-M1 excitability associated with motor learning and may facilitate future investigations on the role of the ipsi-M1 in neurorehabilitation contexts.