Late facilitations of motor evoked potentials by contralateral mixed nerve stimulation.

The effect on motor evoked potentials (MEPs) of a peripheral afferent volley (a single square pulse delivered to the contralateral median nerve at the wrist) was studied in abductor digiti minimi muscle of 9 normal patients following magnetic stimulation of the motor cortex. Long intervals were used (200-700 msec) between the electrical conditioning stimulus and magnetic test stimulation. An MEP amplitude increase, with no change in latency, was observed when the C-T interval was about 500 msec, and the motor threshold was decreased by the conditioning stimulation. The observed long latency and duration of the facilitation effect would seem consistent with activation of suprasegmental areas.