Effect of stimulus orientation and intensity on short-interval intracortical inhibition (SICI) and facilitation (SICF): A multi-channel transcranial magnetic stimulation study

PLoS One. 2021 Sep 22;16(9):e0257554. doi: 10.1371/journal.pone.0257554. eCollection 2021.

Abstract

Besides stimulus intensities and interstimulus intervals (ISI), the electric field (E-field) orientation is known to affect both short-interval intracortical inhibition (SICI) and facilitation (SICF) in paired-pulse transcranial magnetic stimulation (TMS). However, it has yet to be established how distinct orientations of the conditioning (CS) and test stimuli (TS) affect the SICI and SICF generation. With the use of a multi-channel TMS transducer that provides electronic control of the stimulus orientation and intensity, we aimed to investigate how changes in the CS and TS orientation affect the strength of SICI and SICF. We hypothesized that the CS orientation would play a major role for SICF than for SICI, whereas the CS intensity would be more critical for SICI than for SICF. In eight healthy subjects, we tested two ISIs (1.5 and 2.7 ms), two CS and TS orientations (anteromedial (AM) and posteromedial (PM)), and four CS intensities (50, 70, 90, and 110% of the resting motor threshold (RMT)). The TS intensity was fixed at 110% RMT. The intensities were adjusted to the corresponding RMT in the AM and PM orientations. SICI and SICF were observed in all tested CS and TS orientations. SICI depended on the CS intensity in a U-shaped manner in any combination of the CS and TS orientations. With 70% and 90% RMT CS intensities, stronger PM-oriented CS induced stronger inhibition than weaker AM-oriented CS. Similar SICF was observed for any CS orientation. Neither SICI nor SICF depended on the TS orientation. We demonstrated that SICI and SICF could be elicited by the CS perpendicular to the TS, which indicates that these stimuli affected either overlapping or strongly connected neuronal populations. We concluded that SICI is primarily sensitive to the CS intensity and that CS intensity adjustment resulted in similar SICF for different CS orientations.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Adult
  • Electromyography
  • Evoked Potentials, Motor / physiology*
  • Healthy Volunteers
  • Humans
  • Male
  • Motor Cortex / physiology
  • Neural Inhibition / physiology*
  • Transcranial Magnetic Stimulation / methods*
  • Young Adult

Grants and funding

This project has received funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (grant agreement No 810377), the Jane and Aatos Erkko Foundation, the Academy of Finland (Decisions No. 294625, 306845, and 327326) and the Finnish Cultural Foundation. Pavel A. Novikov was funded by Aalto AScI Visiting Researcher Programme and by RFBR, project number 20-315-70048. Maria A. Nazarova was funded by RFBR, project number 20-315-70048.