Motor Execution And Imagery With Greater Task Difficulty Increases Corticospinal Excitability

Abstract

PURPOSE: To elucidate the influence of task difficulty during actual and imaginary force control by the unilateral first dorsal interosseous muscle (FDI) on the corticospinal excitability of 1) the contracting and contralateral resting hands and 2) the resting hand, respectively. METHODS: Seventeen young adults were asked to perform isometric abduction with their left index fingers (contracting hand) at 5 and 15% of maximal voluntary contraction (MVC) with visual guidance in actual force control task and to imagine isometric abduction at 15% MVC in imaginary force control task with the same procedure to actual force control task. Task difficulty was adjusted by the size of range about target force displayed on the computer monitor, which was defined by upper and lower lines, i.e., ± 7% of target force between lines as ‘easy’ task and 0% of target force between lines as ‘difficult’ task. Subjects actually or imaginary attempted to match their abduction force within the target range as steady as possible. In each task, transcranial magnetic stimulation was applied twenty times to the optimal scalp position for eliciting the motor evoked potential (MEP) in FDI overlying right or left motor cortex. The averaged MEP was normalized by the maximal motor response (% Mmax) that was obtained during supramaximal electrical stimulation to the ulnar nerve. RESULTS: In actual force control task, the MEP of the contracting hand was significantly larger (P < 0.05) during difficult task (24.8% Mmax) compared with easy task (22.3% Mmax) when collapsed across force levels. The MEP of the resting hand was also significantly larger (P < 0.05) in difficult task (12.3% Mmax) than in easy task (9.5% Mmax) when collapsed across force levels. In imaginary force control task, the MEP for difficult task (11.0% Mmax) was significantly larger (P < 0.05) compared with that for easy task (8.1% Mmax) in the resting hand. CONCLUSION: These results indicate that 1) actual force control task with greater task difficulty increases corticospinal excitability of the contracting hand, and 2) corticospinal excitability of the resting hand is enhanced when subjects actually and imaginarily perform force control task with greater task difficulty by the unilateral hand.