Intracortical origin of the short latency facilitation produced by pairs of threshold magnetic stimuli applied to human motor cortex

Under certain conditions, EMG responses evoked by pairs of transcranial mag netic stimuli over the motor cortex are larger than the sum of the response s to each stimulus given alone. This occurs with interstimulus intervals of around 1.3, 2.5 and 4.3 ms and could be due to interaction between the res ponses to each stimulus at either the cortex or spinal cord. We recorded th e descending volleys set up by such pairs of stimuli from the cervical epid ural space of five patients implanted with chronic stimulators for pain con trol. Interstimulus intervals of 1, 1.2, 1.4 and 2 ms were used to investig ate the first peak of facilitation. Enhanced EMG responses occurred after p airs of stimuli at 1, 1.2 and 1.4 ms, and these were accompanied by larger and more numerous descending volleys than expected from the sum of each sti mulus alone. We conclude that faciritatorv interaction between the stimuli can occur within the cerebral cortex. This may involve elements that produc e repetitive I-wave activity in response to a single stimulus.