Facilitation of magnetic motor evoked potentials during the cortical stimulation silent period

We investigated the relationship between stimulus intensity and magnetic motor evoked potentials (MEPs) elicited 100 msec after a conditioning stimulus that was 25% of stimulator output above resting motor threshold (RMT) during tonic contraction of abductor pollicis brevis. In five subjects, MEPs elicited with stimuli less than 25% above RMT were inhibited during the EMG cortical stimulation silent period (CSSP) produced 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 CSSP relative to MEPs elicited at rest, such that MEPs elicited with stimuli 30 to 45% above RMT were facilitated during the CSSP. Increasing the 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 shortening of MEP latency, our observations point to supraspinal facilitory mechanisms. We suggest that facilitation of MEPs during the CSSP reflects temporal and spatial summation of conditioning and test stimuli.