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Experimental Brain Research

, Volume 143, Issue 2, pp 240–248 | Cite as

Two phases of intracortical inhibition revealed by transcranial magnetic threshold tracking

  • R. J. FisherEmail author
  • Y. Nakamura
  • S. Bestmann
  • J. C. Rothwell
  • H. Bostock
Research Article

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.

Keywords

Transcranial magnetic stimulation Threshold tracking Intracortical inhibition 

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Copyright information

© Springer-Verlag 2002

Authors and Affiliations

  • R. J. Fisher
    • 1
    Email author
  • Y. Nakamura
    • 1
  • S. Bestmann
    • 1
  • J. C. Rothwell
    • 1
  • H. Bostock
    • 1
  1. 1.Sobell DepartmentInstitute of NeurologyLondonUK

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