No robust online effects of transcranial direct current stimulation on corticospinal excitability

Transcranial direct current stimulation (tDCS) has been used for over twenty years to modulate cortical (particularly motor corticospinal) excitability both during (online) and outlasting (offline) the stimula-tion, with the former effects associated to the latter. However, tDCS effects are highly variable, partially because stimulation intensity is commonly not adjusted individually (in contrast to transcranial mag-netic stimulation, TMS). In Experiment 1, we therefore explored an empirical approach of personalizing tDCS intensity for the primary motor cortex (M1) based on dose-response curves (DRCs), individually relating tDCS Intensity (in steps from 0.3 to 2.0 mA) and Polarity (anodal, cathodal) to the online modulation of concurrent TMS motor evoked potentials (MEP), assessing DRC reliability across two separate days. No robust DRCs could be observed, neither at the individual nor at the group level, with the only robust effect being a (paradoxical) MEP facilitation during cathodal tDCS at 2.0 mA, but no modulation at traditional intensities of or near 1 mA. In Experiment 2, we therefore attempted to replicate the classical bidirectional online MEP modulation during 1 mA tDCS that had been reported by several of the early seminal tDCS papers. We either closely recreated stimulation parameters and tem-poral protocol of these original studies (Experiment 2A) or slightly modernized them according to current standards (Experiment 2B). In neither experiment did we observed any significant online MEP modulation. We conclude that an empirical titration of individually effective tDCS intensities may not be feasible as online tDCS effects do not appear to be sufficiently robust. (C) 2022 The Authors. Published by Elsevier Inc.

Automated summary

Transcranial direct current stimulation (tDCS) has been used for over twenty years to modulate cortical excitability. However, its effects are highly variable, partially due to the lack of individual adjustment of stimulation intensity. This study explored an empirical approach of personalizing tDCS intensity based on dose-response curves (DRCs), but found that online tDCS effects were not sufficiently robust. Additionally, attempts to replicate the reported bidirectional online MEP modulation during tDCS were unsuccessful.