Transcranial electrical stimulation nomenclature

Abstract

Transcranial electrical stimulation (tES) aims to alter brain function non-invasively by applying current to electrodes on the scalp. Decades of research and technological advancement are associated with a growing diversity of tES methods and the associated nomenclature for describing these methods. Whether intended to produce a specific response so the brain can be studied or lead to a more enduring change in behavior (e.g. for treatment), the motivations for using tES have themselves influenced the evolution of nomenclature, leading to some scientific, clinical, and public confusion. This ambiguity arises from (i) the infinite parameter space available in designing tES methods of application and (ii) varied naming conventions based upon the intended effects and/or methods of application. Here, we compile a cohesive nomenclature for contemporary tES technologies that respects existing and historical norms, while incorporating insight and classifications based on state-of-the-art findings. We consolidate and clarify existing terminology conventions, but do not aim to create new nomenclature. The presented nomenclature aims to balance adopting broad definitions that encourage flexibility and innovation in research approaches, against classification specificity that minimizes ambiguity about protocols but can hinder progress. Constructive research around tES classification, such as transcranial direct current stimulation (tDCS), should allow some variations in protocol but also distinguish from approaches that bear so little resemblance that their safety and efficacy should not be compared directly. The proposed framework includes terms in contemporary use across peer-reviewed publications, including relatively new nomenclature introduced in the past decade, such as transcranial alternating current stimulation (tACS) and transcranial pulsed current stimulation (tPCS), as well as terms with long historical use such as electroconvulsive therapy (ECT). We also define commonly used terms-of-the-trade including electrode, lead, anode, and cathode, whose prior use, in varied contexts, can also be a source of confusion. This comprehensive clarification of nomenclature and associated preliminary proposals for standardized terminology can support the development of consensus on efficacy, safety, and regulatory standards.

Keywords: Brain stimulation; Classification; Nomenclature; Terminology; Transcranial electrical stimulation (tES).

Figure 1:

Tree chart of transcranial electrical stimulation (tES) classification. The terms are organized here as defined in this consensus paper based on principles we developed including referencing both method of stimulation application (dose) and/or intended use, with reference to how terms are primarily used in tES literature rather than strict definition of each word. The categories are therefore not mutually exclusive (e.g. a technique may be both IF and HD-tACS) and a full report of stimulation dose (see Section 2) is always required in each publication for reproducibility. As explained in our classification approach, terms can reflect aspects of electrode montage, waveform, and/or intended outcome (e.g. if a technique is classified as rtACS or as tACS can depend on indication) – terms are defined here as commonly used in the literature without adding new qualifications or terminology. However, this organization of terminology into a classification tree is original to the present paper (e.g. ECT is typically not referred to as a type of tPCS). The grey box groups methods for techniques focused on vision rehabilitation.

Figure 2:

Overview of terms used to describe waveform in tES. A–C address waveforms composed of rectangular pulses with expanding temporal scale, while D shows additional waveform types. A: The pulse train waveform is specified by parameters including the frequency, pulse shape, width, amplitude, and interphase delay as well as the pulse repetition frequency. B: The burst stimulation pattern includes the repetition time and number of pulses or cycles per burst; if no burst pattern is reported then the stimulation pattern is continuous. C: The on/off period (duty cycle) describes the time the stimulation pattern—continuous or burst—is active/inactive. D: Direct current has a fixed amplitude but may include ramp up/down and is, by definition, monophasic. Unless otherwise indicated, sinusoidal stimulation has a single frequency and is symmetric biphasic (no dc offset). Monophasic sinusoidal/pulse waveforms have single polarity. Amplitude-modulated sinewave is a high frequency sine modulated by a low-frequency envelope. There are various types of noise-based stimulation, conventionally with no dc offset.