Corticospinal Excitability During Actual and Imaginary Motor Tasks of Varied Difficulty

The present study examined corticospinal excitability of the contralateral and ipsilateral hemispheres during actual (ACT) and imaginary (IMG) unilateral hand force-matching tasks of different difficulty. Seventeen young male adults (21.2 ± 2.2 yrs) actually and imaginarily matched their left index finger abduction force to a displayed target force. Task difficulty was manipulated by varying the acceptable force range about each mean target force (5 and 15% MVC for ACT, 15% MVC for IMG). Specifically, easy (EASY) and difficult (DIFF) tasks were assigned an acceptable force range of ±7% and ±0% of target force, respectively. Single pulse transcranial magnetic stimulation was applied to the both hemispheres in ACT and over the left hemisphere in IMG. Motor evoked potentials (MEPs) were collected from the first dorsal interosseous muscle during tasks. In ACT, MEPs in both the contracting and resting hands were significantly larger (P < 0.05) during DIFF than EASY when collapsed across target force levels. In IMG, MEPs in the resting right hand were significantly larger (P < 0.05) during DIFF than during EASY. The relative change in MEP amplitude in the right hand from EASY to DIFF in ACT was positively correlated (r = 0.63) with that in IMG. These results indicate that greater task difficulty increases corticospinal excitability of the contralateral hemisphere in ACT, and increases corticospinal excitability of the ipsilateral hemisphere in both ACT and IMG. The relative changes in corticospinal excitability of the ipsilateral hemisphere with increasing task difficulty are correlated between ACT and IMG.