Abstract
Motor cortex (M1) paired-pulse TMS (ppTMS) probes excitatory and inhibitory intracortical dynamics by measurement of motor-evoked potentials (MEPs). However, MEPs reflect cortical and spinal excitabilities and therefore cannot isolate cortical function. Concurrent TMS-EEG has the ability to measure cortical function, while limiting peripheral confounds; TMS stimulates M1, whilst EEG acts as the readout: the TMS-evoked potential (TEP). Whilst varying preconditioning stimulus intensity influences intracortical inhibition measured by MEPs, the effects on TEPs is undefined. TMS was delivered to the left M1 using single-pulse and three, ppTMS paradigms, each using a different preconditioning stimulus: 70%, 80% or 90% of resting motor threshold. Corticospinal inhibition was present in all three ppTMS conditions. ppTMS TEP peaks were reduced predominantly under the ppTMS 70 protocol but less so for ppTMS 80 and not at all for ppTMS 90. There was a significant negative correlation between MEPs and N45 TEP peak for ppTMS 70 reaching statistical trends for ppTMS 80 and 90. Whilst ppTMS MEPs show inhibition across a range of preconditioning stimulus intensities, ppTMS TEPs do not. TEPs after M1 ppTMS vary as a function of preconditioning stimulus intensity: smaller preconditioning stimulus intensities result in better discriminability between conditioned and unconditioned TEPs. We recommend that preconditioning stimulus intensity should be minimized when using ppTMS to probe intracortical inhibition.
Highlights
Transcranial magnetic stimulation (TMS) non-invasively measures motor cortex (M1) function
false-discovery rate (FDR) corrected Wilcoxon signed-rank tests showed that only the paired-pulse TMS (ppTMS) 70 TMS-evoked potential (TEP) waveform differed significantly from the single pulse TMS (spTMS) TEP waveform (Figure 1B) at approximately 15 ms, 60 ms and 180 ms
What about the juxtaposed position? We explored this in an additional experiment when the stimuli are matched (70% RMT × 70% RMT ppTMS) and found that there was no difference in the TEPs (n = 8) providing some support for this view
Summary
Transcranial magnetic stimulation (TMS) non-invasively measures motor cortex (M1) function. Premoli et al [19] found that this ppTMS paradigm resulted in a marked reduction of N100 and P180 (negative/positive deflections occurring 100/180 ms after the TMS pulse). These changes at N100 and P180 have been attributed to an active, cortical, inhibitory process via GABAA and GABAB receptors. A consistent and highly reproducible finding from studying ppTMS and MEPs, is that changing the intensity of the preconditioning stimulus [1,2] alters the balance between excitation and inhibition resulting in a larger or smaller MEPs, respectively. We expect that the greater synchrony typically seen after a single pulse to be diminished after ppTMS
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