FACILITATION OF MAGNETIC MOTOR EVOKED-POTENTIALS DURING THE CORTICAL STIMULATION SILENT PERIOD

We investigated the relationship between stimulus intensity and magnet ic motor evoked potentials (MEPs) elicited 100 msec after a conditioni ng stimulus that was 25% of stimulator output above resting motor thre shold (RMT) during tonic contraction of abductor pollicis brevis. In f ive subjects, MEPs elicited with stimuli less than 25% above RMT were inhibited during the EMG cortical stimulation silent period (CSSP) pro duced by the conditioning stimulus, relative to MEPs elicited with the test stimulus given at rest. However, increasing the intensity of the test stimulus increased the amplitude of MEPs elicited during the CSS P relative to MEPs elicited at rest, such that MEPs elicited with stim uli 30 to 45% above RMT were facilitated during the CSSP. Increasing t he intensity of the test stimulus also increased the amplitude of MEPs elicited with paired stimulation at rest, and caused facilitation in one subject. Since facilitation of MEPs was never accompanied by short ening of MEP latency, our observations point to supraspinal facilitory mechanisms. We suggest that facilitation of MEPs during the CSSP refl ects temporal and spatial summation of conditioning and test stimuli.