High-definition transcranial direct-current stimulation of the right M1 further facilitates left M1 excitability during crossed facilitation

Abstract

The crossed-facilitation (CF) effect refers to when motor-evoked potentials (MEPs) evoked in the relaxed muscles of one arm are facilitated by contraction of the opposite arm. The aim of this study was to determine whether high-definition transcranial direct-current stimulation (HD-tDCS) applied to the right primary motor cortex (M1) controlling the left contracting arm [50% maximum voluntary isometric contraction (MVIC)] would further facilitate CF toward the relaxed right arm. Seventeen healthy right-handed subjects participated in an anodal and cathodal or sham HD-tDCS session of the right M1 (2 mA for 20 min) separated by at least 48 h. Single-pulse transcranial magnetic stimulation (TMS) was used to elicit MEPs and cortical silent periods (CSPs) from the left M1 at baseline and 10 min into and after right M1 HD-tDCS. At baseline, compared with resting, CF (i.e., right arm resting, left arm 50% MVIC) increased left M1 MEP amplitudes (+97%) and decreased CSPs (-11%). The main novel finding was that right M1 HD-tDCS further increased left M1 excitability (+28.3%) and inhibition (+21%) from baseline levels during CF of the left M1, with no difference between anodal and cathodal HD-tDCS sessions. No modulation of CSP or MEP was observed during sham HD-tDCS sessions. Our findings suggest that CF of the left M1 combined with right M1 anodal or cathodal HD-tDCS further facilitated interhemispheric interactions during CF from the right M1 (contracting left arm) toward the left M1 (relaxed right arm), with effects on both excitatory and inhibitory processing. NEW & NOTEWORTHY This study shows modulation of the nonstimulated left M1 by right M1 HD-tDCS combined with crossed facilitation, which was probably achieved through modulation of interhemispheric interactions.

High-definition transcranial direct-current stimulation of the right M1 further facilitates left M1 excitability during crossed facilitation

“…Briefly, the sham condition found a CF effect present at baseline with MEP increases of 75%. However, after baseline, Cabibel et al (2018) reported no further modulation of the left M1 for all other conditions or time points.…”

“…A notable difference to previous work in which participants acted as their own controls (e.g., Carson et al 2008) is that, except for three participants, the sham group in Cabibel et al (2018) consisted of a different set of participants relative to the anodal/cathodal group. From a statistical perspective, the effects of sham stimulation over time (baseline, during, post, post-30) for the four TMS protocols (Resting, Resting ϩ CF, CSP, CSP ϩ CF) were not analyzed within the same statistical model as anodal and cathodal HD-tDCS.…”

“…In a present study by Cabibel et al (2018), their group evaluated the effects of high-definition tDCS (HD-tDCS) over the right M1 on CF to the left M1 (Cabibel et al 2018). Seventeen healthy adults (average age: 27.4 Ϯ 8.7) participated in the study.…”

“…It is worth noting the strong study design by Cabibel et al (2018), which not only provided valuable information regarding the polarity specificity of HD-tDCS but also provided insight into the time-dependent effects associated with each stimulation type. Specifically, Cabibel et al (2018) did not solely focus on either anodal or cathodal stimulation relative to sham but instead included all possible stimulation conditions (sham, anodal, cathodal) together with multiple testing points (baseline, during, post, and post-30), allowing for direct comparisons of the effects of excitatory and inhibitory stimulation on CF and their respective changes over time.…”

“…Instead, they were analyzed in a separate one-way repeated measures ANOVA (RMANOVA), while the effects of HD-tDCS session (anodal, cathodal) over time (baseline, during, post, post-30) were tested using a separate two-way RMANOVA. Because the one-way RMANOVA did not demonstrate an effect of time, while the two-way RMANOVA did albeit without a condition interaction, Cabibel et al (2018) used this as support for a difference in the effects of sham and polarity-independent HD-tDCS on CF over time. However, this should be interpreted bearing in mind that an absence of evidence may not always represent evidence for an absence of effect.…”