Short latency facilitation between pairs of threshold magnetic stimuli applied to human motor cortex

Pairs of threshold magnetic stimuli were applied over the motor cortex at interstimulus intervals of 1-6 ms, and EMG responses recorded from the relaxed or active first dorsal interosseous muscle of 7 normal subjects. In relaxed subjects, when the interval between the stimuli was around 1.0-1.5 ms, 2.5-3.0 ms or 4.5 ms or later, the size of the response to the pair of stimuli was much greater than the algebraic sum of the response to each stimulus alone. During contraction, fewer peaks of facilitation were observed. Facilitation was evident if the stimuli were 0.9-1.1 times threshold in the relaxed state, and 1.0-1.1 times threshold during voluntary contraction. Experiments using either magnetic followed by anodal electric stimulation, or pairs of anodal electric stimuli, suggested that the facilitation most likely occurred within the cerebral motor cortex. Given the timings at which facilitation is prominent, it seems likely that it reflects interactions between circuits normally responsible for production of I-waves.