H. Tokimura et al., SHORT-LATENCY FACILITATION BETWEEN PAIRS OF THRESHOLD MAGNETIC STIMULI APPLIED TO HUMAN MOTOR CORTEX, ELECTROMYOGRAPHY AND MOTOR CONTROL-ELECTROENCEPHALOGRAPHY AND CLINICAL NEUROPHYSIOLOGY, 101(4), 1996, pp. 263-272
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 su
bjects. 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 res
ponse to the pair of stimuli was much greater than the algebraic sum o
f the response to each stimulus alone. During contraction, fewer peaks
of facilitation were observed. Facilitation was evident if the stimul
i were 0.9-1.1 times threshold in the relaxed state, and 1.0-1.1 times
threshold during voluntary contraction. Experiments using either magn
etic followed by anodal electric stimulation, or pairs of anodal elect
ric stimuli, suggested that the facilitation most likely occurred with
in the cerebral motor cortex. Given the timings at which facilitation
is prominent, it seems likely that it reflects interactions between ci
rcuits normally responsible for production of I-waves.