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Experimental Brain Research

, Volume 237, Issue 12, pp 3071–3088 | Cite as

Current challenges: the ups and downs of tACS

  • Nicholas S. BlandEmail author
  • Martin V. Sale
Review
First Online:

Abstract

The non-invasive delivery of electric currents through the scalp (transcranial electrical stimulation) is a popular tool for neuromodulation, mostly due to its highly adaptable nature (waveform, montage) and tolerability at low intensities (< 2 mA). Applied rhythmically, transcranial alternating current stimulation (tACS) may entrain neural oscillations in a frequency- and phase-specific manner, providing a causal perspective on brain–behaviour relationships. While the past decade has seen many behavioural and electrophysiological effects of tACS that suggest entrainment-mediated effects in the brain, it has been difficult to reconcile such reports with the weak intracranial field strengths (< 1 V/m) achievable at conventional intensities. In this review, we first describe the ongoing challenges faced by users of tACS. We outline the biophysics of electrical brain stimulation and the factors that contribute to the weak field intensities achievable in the brain. Since the applied current predominantly shunts through the scalp—stimulating the nerves that innervate it—the plausibility of transcutaneous (rather than transcranial) effects of tACS is also discussed. In examining the effects of tACS on brain activity, the complex problem of salvaging electrophysiological recordings from artefacts of tACS is described. Nevertheless, these challenges by no means mark the rise and fall of tACS: the second part of this review outlines the recent advancements in the field. We describe some ways in which artefacts of tACS may be better managed using high-frequency protocols, and describe innovative methods for current interactions within the brain that offer either dynamic or more focal current distributions while also minimising transcutaneous effects.

Keywords

Transcranial Stimulation Electric field Oscillation Artefact Phase 
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Notes

Author contributions

Conceptualisation: NB and MS; formal analysis, investigation, and writing—original draft: NB; writing—review and editing: NB and MS.

Funding

NB and MS were funded by the Office of Naval Research (N62909-17-1-2139).

Compliance with ethical standards

Conflict of interest

The authors state that they have no conflict of interest.

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© Springer-Verlag GmbH Germany, part of Springer Nature 2019

Authors and Affiliations

  1. 1.Queensland Brain InstituteThe University of QueenslandSt LuciaAustralia
  2. 2.School of Health and Rehabilitation SciencesThe University of QueenslandSt LuciaAustralia

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