title: Two phases of intracortical inhibition revealed by transcranial magnetic threshold tracking.
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.
authors.affiliation: Sobell Department, Institute of Neurology, Queen Square, London WC1N 3BG, UK. r.fisher@ion.ucl.ac.uk
authors.forename: R J
authors.initials: RJ
authors.lastname: Fisher
authors.forename: Y
authors.initials: Y
authors.lastname: Nakamura
authors.forename: S
authors.initials: S
authors.lastname: Bestmann
authors.forename: J C
authors.initials: JC
authors.lastname: Rothwell
authors.forename: H
authors.initials: H
authors.lastname: Bostock
mesh_terms: D000328:Adult; D003213:Conditioning, Psychological / Q000502:physiology; D004576:Electromyography; D019054:Evoked Potentials, Motor; D005260:Female; D006801:Humans; D008280:Magnetics; D008297:Male; D008875:Middle Aged; D009044:Motor Cortex / Q000502:physiology*; D009119:Muscle Contraction / Q000502:physiology; D009433:Neural Inhibition / Q000502:physiology*; D012684:Sensory Thresholds / Q000502:physiology