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.

Comparison of transcranial magnetic stimulation measures obtained at rest and under active conditions and their reliability

“…This information can be used to facilitate comparisons between studies. For example it confirms that when performing a SICI protocol, using an intensity of stimulation (for the conditioning stimulus) of 100% of the aMT (Di Lazzaro et al, 2006;Orth et al, 2003) or of 80 % of rMT (Fisher et al, 2002;Maeda et al, 2002) should yield similar results.…”

“…Lack of difference between the SICI measured at rest and during active conditions is somewhat surprising given that some authors have reported decreased inhibition under active conditions compared to resting conditions (Fisher et al, 2002;Ridding et al, 1995). However, the effect of muscle contractions on intracortical inhibition has been shown to vary according to the intensity of muscle contractions (SICI decreases as the muscle contractions increase, and a low contraction level (7.5±2.5% of MVC) was used in the present study), to the intensity of the conditioning stimulus as well as to the intensity of the test stimulus (Ortu et al, 2008;Roshan et al 2003).…”

“…Also, the clarification of the relationship between resting and active TMS measures is necessary to interpret and compare adequately the results of studies using different approaches to set their stimulation protocols. For example, several authors set the intensity of paired-pulse TMS or rTMS stimulations by using a percentage of the resting motor threshold (rMT) (Fisher et al, 2002;Maeda et al, 2002), while others base their calculations on a percentage of the active motor threshold (aMT) (Di Lazzaro et al, 2006;Filipovic et al, 2010;Orth et al, 2003;Ortu et al, 2008).…”

Neurophysiological Mechanisms Involved in Transfer of Procedural Knowledge

“…The test stimulus (TS) was adjusted to produce an MEP of ϳ1 mV at rest. TS was delivered 2.5 ms after CS, an optimal interstimulus interval for eliciting SICI and to avoid mixture of the two phases of inhibition (Fisher et al, 2002). Fifteen TS and CS were pre- sented randomly at each time interval, and responses were recorded for off-line analysis.…”

Increased motor cortical facilitation and decreased inhibition in Parkinson disease

“…ISI of 1 millisecond was also tested because the underlying mechanisms for SICI at 1 millisecond may be different from those at other ISIs. 15,16 Intracortical facilitation (ICF) represents a facilitatory process that a subthreshold S1 facilitates the response to a subsequent superthreshold S2 at ISIs of 6 to 30 milliseconds. 2 We tested ICF with ISIs of 10 and 15 milliseconds.…”