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

Abstract

Transcranial electrical stimulation (tES) encompasses all methods of noninvasive current application to the brain used in research and clinical applications. We present the first comprehensive and technical review, explaining the evolution of tES in both terminology and dosage over the past 100 years of research to the present day. Current transcranial pulsed current stimulation (tPCS) approaches such as cranial electrotherapy stimulation (CES) descended from Electrosleep (ES) through cranial electrostimulation therapy (CET), transcerebral electrotherapy (TCET), and NeuroElectric Therapy (NET) while others like transcutaneous cranial electrical stimulation (TCES) descended from electroanesthesia (EA) through Limoge and interferential stimulation (IS). Prior to a contemporary resurgence in interest, forms of transcranial direct current stimulation were explored intermittently, notably as polarizing current. The development of these approaches alongside electroconvulsive therapy (ECT) and pharmacological developments is considered. Both the roots and unique features of contemporary approaches such as transcranial alternating current stimulation (tACS) and transcranial random noise stimulation (tRNS) are discussed. Trends and incremental developments in electrode montage and waveform spanning decades are presented leading to the present day. Commercial devices, seminal conferences, and regulatory decisions are noted, though emphasis is placed on relevance and insight into current practices.