Facilitatory effect of thinking about movement on motor-evoked potentials to transcranial magnetic stimulation of the brain

Two experiments using transcranial magnetic stimulation (TMS) were undertaken to investigate facilitatory effects of thinking about a specific movement without voluntary discharges on motor-evoked potentials (MEP). First, surface electromyographic (EMG) responses from the abductor pollicis brevis were recorded with maximal stimulator output in the three conditions: the muscle being at rest, contracting with 10% of maximal muscle activity, and with the subject 'only thinking' about thumb abduction (nine subjects). Median value of MEP amplitudes during 'only thinking' was twice that at rest (P = 0.008) and one-half that during voluntary contraction (P = 0.008). Second, needle EMG responses from the first dorsal interosseus were compared at rest, during thinking about index finger abduction, and during TMS at threshold intensity. Four normal subjects were tested with stimulation of each cerebral hemisphere for a total of eight tests. The number of detectable MEP responses of 20 stimuli to one hemisphere was counted for each condition of rest or thinking. The mean MEP response rate during thinking (58%) was higher than that at rest (12%) (P < 0.005). These results demonstrate that thinking about a specific movement has facilitatory effects on MEP and that the degree of facilitation in thinking is smaller than in voluntary contraction.