Effects of Transcranial Static Magnetic Stimulation on Motor Cortex Evaluated by Different TMS Waveforms and Current Directions

Abstract

Transcranial static magnetic stimulation (tSMS) modulates cortical excitability probably by interacting with the GABA-glutamate intracortical balance. Different transcranial magnetic stimulation (TMS) waveforms probe distinct GABA-mediated cortical inhibition networks. The goal of the present work is to further characterize tSMS-induced changes in motor cortex reactivity and inhibition-excitation (I/E) balance. We hypothesized that tSMS affects particular cortical networks and thus, the effects of tSMS would be different depending on the TMS waveform used to assess its results. 23 healthy young adults completed two sessions of real or sham tSMS. The order of the sessions was randomized across participants. Motor evoked potentials (MEPs), cortical silent period (CSP), short- and long-interval intracortical inhibition (SICI and LICI), and intracortical facilitation (ICF) were assessed with TMS monophasic posterior-anterior (mono_PA; n = 9), monophasic anterior-posterior (mono_AP; n = 7), or biphasic (bi_AP-PA; n = 7) pulses. Repeated measures analyses of variance and appropriate pairwise comparisons were performed for each TMS measure. After 15 min of real tSMS, the MEP amplitudes decreased compared to sham and baseline, SICI and LICI showed greater inhibition, and a tendency towards longer CSPs and less facilitation was found. These results were only observed with mono_PA TMS. MEP amplitude increased compared to sham with mono_AP TMS, with no clear changes in general intracortical I/E balance. Biphasic TMS was not able to capture any effects of tSMS. The results show that the effects of tSMS on cortical excitability and inhibition involve specific interneuron circuits that are selectively activated by mono_PA TMS.

Keywords: biphasic; current direction; monophasic; paired-pulse TMS; transcranial magnetic stimulation; transcranial static magnetic stimulation.

Figure 1:

Effects of real tSMS and sham interventions on motor evoked potentials (MEP) amplitude. A. Percentage of change (%Δ) (mean ±SE) from pre- to post-intervention evaluated in real and sham interventions for the three different waveforms. Negative values represent a decrease in MEP amplitude and an increase in inhibition in the motor cortex. B. Comparison of the percentage of change from baseline (%Δ) (mean ±SE) through time for the three different waveforms and current directions. Real intervention is shown in black and sham intervention is in light grey. The dashed lines represent the transition time from pre-intervention (Pre) to post-intervention (Post-T1–T4) where the intervention took place. Abbreviations: Mono_AP, monophasic anterior-posterior; Bi_AP-PA, biphasic anterior-posterior—posterior-anterior; Mono_PA, monophasic posterior-anterior.

Figure 2:

Effects of real tSMS and sham interventions on paired pulse TMS protocols. A-C-D. Percentage of change (%Δ)) (mean ±SE) from pre- to post-intervention evaluated in real and sham interventions for the three different waveforms. Negative values represent a decrease in motor evoked potentials (MEP) amplitude, in other words more inhibition after short- or long-interval intracortical inhibition (SICI or LICI, respectively) and less facilitation after intracortical facilitation (ICF). B-D-F. Comparison of the percentage of change from baseline (%Δ) (mean ±SE) through time for the three different waveforms and current directions. Real intervention is shown in black and sham intervention is in light grey. The dashed lines represent the transition time from pre-intervention (Pre) to post-intervention (Post-T1–T4) where the intervention took place. Abbreviations: Mono_AP, monophasic anterior-posterior; Bi_AP-PA, biphasic anterior-posterior—posterior-anterior; Mono_PA, monophasic posterior-anterior.