Motor cortex excitability changes during imagery of simple reaction time

Abstract

Imagining motor actions is enough to enhance cortical motor excitability. However, the fact that execution of the motor task has to be inhibited should have a correlate on the extent of cortical excitability enhancement. Therefore, we examined the possible differences between real and motor imagery of simple reaction time tasks (SRT) in the facilitation of the motor-evoked potential (MEP) to transcranial magnetic stimulation (TMS) and in the reduction of short-interval intracortical inhibition (SICI) taking place before the movement onset. Thirteen healthy volunteers were requested to perform visual real or imaginary SRT tasks (rSRT and iSRT) with their dominant hand. For rSRT, subjects had to perform a rapid isometric squeeze as soon as possible after the imperative signal. For iSRT they had to imagine the same movement without actually doing it. Electromyographic (EMG) signals were recorded from the first dorsal interosseus (FDI) muscle. The mean EMG onset was calculated for each subject in rSRT trials. Single-and paired-pulse TMS (at an interstimulus interval of 2.5 ms) were applied at rest and at time intervals of -25, -50, -75, -100 and -125 ms before the expected real (rSRT) mean EMG onset. In rSRT there was a significant increase of MEP to single-pulseTMS at the intervals of -50 and -25 ms, and in iSRT at -50 ms in comparison to the rest condition. % SICI changes were significantly reduced at the intervals of -75, -50 and -25 ms, before EMG the onset in rSRT and at -50 and -25 in iSRT (P < 0.05) in comparison to % SICI changes at rest. The differences between MEPs to spTMS and MEP to ppTMS were significantly larger at rest, -125 and -100 ms intervals in rSRT, and at all intervals in iSRT. Imagining to move causes an increase in corticospinal excitability and a decrease in intracortical inhibition that follow a temporal profile similar to those observed with real movements. However, complete removal of SICI happened only in rSRT at the shortest intervals before the EMG onset. Such action may delineate new tool in motor rehabilitation of patients who have limitation to move.