Mechanisms controlling motor output to a transfer hand after learning a sequential pinch force skill with the opposite hand

Objective: Training to perform a serial reaction-time task (procedural motor learning) with one hand results in performance improvements in the untrained as well as in the trained hand, a phenomenon referred to as intermanual transfer. The aim of this study was to investigate the neurophysiological changes associated with intermanual transfer associated with learning to perform an eminently different task involving fine force control within the primary motor cortex (M1). We hypothesized that intermanual transfer of learning such a task would reveal intracortical changes within M1. Methods: Speed (time to complete each sequence) and accuracy (% of accuracy errors) of motor performance were measured in both hands before and after right (dominant) hand practice. Transcranial magnetic stimulation (TMS) was used to characterize recruitment curves (RC), short intracortical inhibition (SICI), intracortical facilitation (ICF) and interhemispheric inhibition (IHI) from the left to the right M1. Results: Practice resulted in significant improvements in both speed and accuracy in the right trained hand and in the left untrained hand. RC increased in the left M1, SICI decreased in both M1s, and IHI from the left to the right M1 decreased. No changes were identified in ICF nor in RC in the right M1. Conclusions: Our results suggest that some neurophysiological mechanisms operating in the M1 controlling performance of an untrained hand may contribute to optimize the procedure for selecting and implementing correct pinch force levels. Significance: These results raise the hypothesis of a contribution of modulation of SICI and IHI, or an interaction between both to intermanual transfer after learning a sequential pinch force task.