Abstract
Intracortical inhibition was investigated in normal human volunteers by paired-pulse transcranial magnetic stimulation, using a new, computer-assisted threshold-tracking method. Motor threshold was defined as the stimulus amplitude required to evoke a motor evoked potential of 0.2 mV (peak-to-peak) in abductor pollicis brevis, and inhibition was measured as the percentage increase in threshold, when the test stimulus was preceded by a subthreshold conditioning stimulus. This method was used to investigate the dependence of intracortical inhibition on conditioning stimulus parameters and on voluntary activity. Interstimulus interval (ISI) was first stepped from 1 to 4.5 ms, as inhibition was measured using conditioning stimuli of fixed amplitude (50–90% resting motor threshold). Maximal inhibition was produced at ISIs of 1 and 2.5 ms. The effect of conditioning stimulus intensity was then assessed at these ISIs. Inhibition occurred at significantly lower conditioning stimulus intensities with ISI=1 ms than with ISI=2.5 ms. Voluntary activity reduced inhibition at both ISIs, but had a much greater effect on inhibition at ISI=2.5 ms. Inhibition during voluntary activity was also examined for single motor units in first dorsal interosseous by generating poststimulus time histograms. Inhibition, indicated by a reduction in the later peaks of increased firing, was observed with ISI=1 ms, but not with ISI=2.5 ms. We conclude that there are two distinct phases of inhibition, occurring at ISI=1 ms and ISI= 2.5 ms, differing both in thresholds and susceptibility to voluntary activity.
Similar content being viewed by others
Author information
Authors and Affiliations
Corresponding author
Rights and permissions
About this article
Cite this article
Fisher, R.J., Nakamura, Y., Bestmann, S. et al. Two phases of intracortical inhibition revealed by transcranial magnetic threshold tracking. Exp Brain Res 143, 240–248 (2002). https://doi.org/10.1007/s00221-001-0988-2
Received:
Accepted:
Published:
Issue Date:
DOI: https://doi.org/10.1007/s00221-001-0988-2