Methods and Technologies for Low-Intensity Transcranial Electrical Stimulation: Waveforms, Terminology, and Historical Notes

Transcranial electrical stimulation (tES) encompasses all forms of research and clinical applications of electrical currents to the brain noninvasively using (at least one) electrodes on the head. The dose of tES is defined by the electrode montage and the stimulation waveform applied to the electrode [1]. There has been a resurgence of interest since 2000, but “modern” tES developed incrementally over a century. This review provides the first comprehensive organization of approaches and doses used in modern tES since 1900. This process involves defining the litany of terminology that has developed and evolved around tES. We make no attempt to re-define or qualify any approaches used, but explain the terminology as used contemporarily by researchers. Particular attention is paid to historically linked categories of tES, “streams,” of which we identify four that span decades plus “contemporary” approaches (Fig. 2.1). 1. Cranial electrical stimulation (CES) descended from electrosleep (ES) through cranial electro-stimulation therapy (CET), transcerebral electrotherapy (TCET), and neuroelectric therapy (NET). 2. Electroanesthesia (EA) went through several periods of waning interest and resurgence when new waveform variations were proposed, including transcutaneous cranial electrical stimulation (TCES), Limoge, and interferential stimulation (IS). 3. Polarizing or direct current stimulation includes recent transcranial direct current stimulation, transcranial micropolarization, high-definition transcranial direct current stimulation (HD-tDCS), and galvanic vestibular stimulation (GVS). 4. Electroconvulsive therapy (ECT), initially called electroshock therapy, evolved in technique and dose, such as focal electrically administered seizure therapy (FEAST). 5. Finally, we categorize “contemporary” approaches that have been explored intensely over the last decade, such as transcranial alternating current stimulation (tACS), transcranial sinusoidal direct current stimulation (tSDCS), and transcranial random noise stimulation (tRNS). Though analogues to these contemporary approaches can be identified in earlier literature, contemporary approaches contain dose features that motivate us to consider them novel. Contemporary approaches to some extent reflect a “re-boot” of the tES approach, typically employing basic, well-documented, and welldefined waveforms (e.g., one sinusoid [1] in contrast to the increasingly complex waveforms developed [though not always justified] over decades in some streams. As our technical focus is on dose clarification and classification, we minimize comments on the clinical efficacy or safety of any approaches except in special cases where findings resulted in historically notable and sudden changes in dose or terminology. We note specific conferences and regulatory agencies that helped identify and shape the field of tES including establishing terminology. Commercial (brand) names of devices are noted ad hoc for context and linked to dose terms where appropriate. We do not comment directly on mechanisms but emphasize that dose determines electric field in the brain [2] which, in turn, gives rise to neurophysiological responses [3]; thus B. Guleyupoglu (*) Department of Biomedical Engineering, The City College of New York, 160 Convent Avenue, 463 Steinman Hall, New York, NY 10031, USA e-mail: [email protected]