Facilitatory effect of thinking about movement on magnetic motor-evoked potentials.

To investigate the facilitatory effect of thinking about movement on motor evoked potential (MEP) amplitude, we recorded MEPs in two test muscles during rest, with the subject thinking about contracting the test muscle but without subsequent contraction, and during 10% maximum voluntary contraction. Stimuli were delivered at 10% above resting motor threshold and at 90-100% stimulator output. H-reflexes, recorded in flexor carpi radialis, were obtained during rest and think conditions. MEP threshold was lower during the think condition (P = 0.004). At both stimulus intensities, median MEP amplitudes and areas were significantly (P < 0.001) larger during the think paradigm compared with rest. This effect was greater at the lower stimulus intensity. There was no significant difference in latency (P = 0.15). In 4/8 subjects, H-reflex amplitudes were mildly facilitated (P < 0.05) during the think condition. We conclude that thinking about movement without detectable EMG activity has a facilitatory effect on magnetic MEPs. The absence of a MEP latency shift between rest and think conditions and absence of a consistent increase in H-reflex amplitude suggests this effect occurs largely at the cortical level. In some subjects, however, an increase in spinal motoneuron excitability may also contribute.