Paired associative stimulation (PAS) with high-frequency peripheral nerve stimulation (PNS), called “high-PAS”, induces motor-evoked potential (MEP) potentiation in healthy subjects and improves muscle activity and independence in incomplete spinal cord injury patients. Data on optimal PNS intensity in PAS are scarce. In a high-PAS protocol, PNS intensity is defined as “minimal intensity required to produce F-responses”. We sought to further refine this definition and to investigate how PNS intensity affects PAS outcome. Two experiments were performed on 10 healthy subjects where MEP amplitude change was measured 0, 30, and 60 min after PAS. In the first experiment, the intensity required to achieve 7/10 persistence of F-responses was used to define PNS intensity level. In the second experiment, we used the intensity required to achieve 1/10 persistence (“baseline”). In addition, we applied this intensity at + 25%, − 25%, and − 50% levels. In the first experiment, PAS did not produce significant MEP potentiation. In the second experiment, PAS produced statistically significant MEP potentiation, with PNS intensity of “baseline” and “baseline − 25%” levels but not at + 25% or − 50% levels. In conclusion, for PAS utilizing high-frequency PNS, the intensity required to achieve 1/10 F-response persistence or the intensity 25% lower produces significant MEP potentiation in healthy subjects.
Highlights
PNS and motor imagery were applied to 5 patients with tetraplegia.
The setup was identical to a previously published PAS study, but TMS was omitted.
This intervention slightly improved MMT scores but not functional hand tests.
PNS is inferior to PAS, but might be useful for patients with TMS contraindications.
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