Transcranial magnetic stimulation: specific and non-specific facilitation of magnetic motor evoked potentials.

Different physiological mechanisms of facilitation of latencies and amplitudes of magnetic motor evoked potentials (MEPs) were evaluated in a cohort of 140 healthy volunteers. The potentials were induced at the vertex and recorded at the abductor pollicis brevis. The aim of the present investigation was to compare physiological mechanisms which presumably facilitate motor pathways at the cortical level with those known to occur during contraction of small hand muscles. When compared with MEPs at rest, the maximum average decrease of latencies (1.5, SD 1.1 ms) as well as the highest increase of peak to peak amplitudes (2.6, SD 2.1 mV) was observed during exertion of a voluntary background force, at the muscle recorded from. Pre-innervation of a neighbouring muscle (abductor digiti minimi) led to a lesser average decrease of latencies by 1.0, SD 1.1 ms and an average increase of amplitudes by only 0.5, SD 1.5 mV. Non-specific manoeuvres, like sticking out the tongue or counting aloud, reduced mean latencies slightly by 0.4 ms, SD 0.8 ms and 0.3 SD 0.85 ms respectively, but increased amplitudes markedly by an average of 1.0, SD 1.6 mV and 0.8, SD 1.4 mV respectively. It is concluded that facilitation of MEPs by non-specific manoeuvres occurs and must be taken into account when evaluating MEPs.