Peripheral stimulation affects subthreshold Triple Stimulation Technique.

BACKGROUND: Compared to conventional transcranial magnetic stimulation (TMS), the triple stimulation technique (TST) strongly decrease the effects of desynchronization of descending discharges and accompanying phase cancellation that follow TMS and offers a more sensitive method to quantify motor evoked potentials (MEPs). NEW METHOD: Using the TST, we explored as to whether sub-threshold TMS evokes peripheral motor neuron discharges (MNs). We compared the number of MEPs elicited by TMS and by TST in fifteen healthy participants. We used the subthreshold intensity of 80 % resting motor threshold. To control the TST assessment of the corticospinal tract, we included a peripheral stimulation control condition, which consisted of peripheral stimulation alone, in a subgroup of five volunteers. RESULTS: Compared to TMS, TST at sub-threshold intensities did not detect significantly more responses unequivocally attributable to the cortical stimulation. In contrast, the peripheral supra-maximal stimuli produced confounding effects in the TST condition that were, in part, indistinguishable from cortical responses. COMPARISON WITH EXISTING METHODS: At subthreshold TMS intensities, the TST does not detect more discharges of spinal MNs than conventional TMS and, in addition, it is confounded by effects from peripheral stimulation. CONCLUSION: The TST can be useful in assessing the integrity of the MN pool and of the corticospinal tract. However, if used at near threshold intensity, the confounding effects of peripheral stimulation need to be considered; for instance, in paired-pulse stimulation paradigms assessing the cortical physiology.

Desynchronization does not contribute to intracortical inhibition and facilitation: a paired-pulse paradigm study combined with TST.

The paired-pulse (PP) transcranial magnetic stimulation (TMS) paradigms allow the exploration of the motor cortex physiology. The triple stimulation technique (TST) improves conventional TMS by reducing effects of desynchronization of motor neuron discharges allowing a precise evaluation of the corticospinal conduction. The objective of our study was to explore PP TMS paradigms combined with the TST to study whether the desynchronization contributes to these phenomena and whether the combined TMS-TST protocol could improve the consistency of responses. We investigated the PP paradigms of short intracortical inhibition (SICI) with 2 ms interstimulus interval (ISI) and of intracortical facilitation (ICF) with 10 ms ISI in 22 healthy subjects applying either conventional TMS alone or combined with the TST protocol. The results of the PP paradigms combined with the TST of SICI and ICF do not differ from those with conventional TMS. However, combining the PP paradigm with the TST reduces their variability. These results speak against a contribution of the desynchronization of motor neuron discharges to the PP paradigms of SICI and ICF. Combining the PP TMS paradigm with the TST may improve their consistency, but the interindividual variability remains such that it precludes their utility for clinical practice.NEW & NOTEWORTHY Combining the triple stimulation technique with the paired-pulse stimulation paradigm improves the consistency of short intracortical inhibition and facilitation and could be useful in research, but the interindividual variability precludes their utility for clinical practice. Our findings do not suggest that desynchronization of descending discharges following transcranial magnetic stimulation contributes to short intracortical inhibition or intracortical facilitation.