Influence of the amount of use on hand motor cortex representation: effects of immobilization and motor training

Converging evidence from animal and human studies has revealed that increased or decreased use of an extremity can lead to changes in cortical representation of the involved muscles. However, opposite experimental manipulations such as immobilization and motor training have sometimes been associated with similar cortical changes. Therefore, the behavioral relevance of these changes remains unclear. The purpose of this study was to observe the effect of the amount of use on hand muscle motor cortex representation by contrasting the effect of unspecific motor training and immobilization. Nine healthy volunteers were tested prior and after a 4-day exposure to two experimental conditions using a randomized cross-over design: a motor training condition (to play Guitar Hero 2h/day with the tested (nondominant) hand on the fret board) and an immobilization condition (to wear an immobilization splint 24h/day). Before and after each condition, motor cortex representation of the nondominant first dorsal interosseous (FDI) muscle was mapped using image-guided transcranial magnetic stimulation (TMS). At the behavioral level, results show that the training condition led to a 20% improvement in the trained task, while the immobilization condition resulted in a 36% decrease in the FDI maximal voluntary contraction. At the neurophysiological level, corticospinal excitability (e.g. Motor-evoked potential amplitude) was found to be decreased in response to immobilization, while no change was observed in response to motor training. No change was found for other TMS variables (motor thresholds or map location/volume/area) in either condition. In conclusion, our results indicate that a 4-day decrease, but not increase, in the amount of use of nondominant hand muscles is sufficient to induce a change in corticospinal excitability. The lack of a training effect might be explained by the use of an unspecific task (that is nevertheless representative of "real-life" training situations) and/or by insufficient duration/intensity to induce long-lasting changes.