Fundamentals of transcranial electric and magnetic stimulation dose: definition, selection, and reporting practices

Abstract

Background: The growing use of transcranial electric and magnetic (EM) brain stimulation in basic research and in clinical applications necessitates a clear understanding of what constitutes the dose of EM stimulation and how it should be reported.

Methods: This paper provides fundamental definitions and principles for reporting of dose that encompass any transcranial EM brain stimulation protocol.

Results: The biologic effects of EM stimulation are mediated through an electromagnetic field injected (via electric stimulation) or induced (via magnetic stimulation) in the body. Therefore, transcranial EM stimulation dose ought to be defined by all parameters of the stimulation device that affect the electromagnetic field generated in the body, including the stimulation electrode or coil configuration parameters: shape, size, position, and electrical properties, as well as the electrode or coil current (or voltage) waveform parameters: pulse shape, amplitude, width, polarity, and repetition frequency; duration of and interval between bursts or trains of pulses; total number of pulses; and interval between stimulation sessions and total number of sessions. Knowledge of the electromagnetic field generated in the body may not be sufficient but is necessary to understand the biologic effects of EM stimulation.

Conclusions: We believe that reporting of EM stimulation dose should be guided by the principle of reproducibility: sufficient information about the stimulation parameters should be provided so that the dose can be replicated.

Figure 1

Simulation of the electric (C) and current density (D) fields injected by transcranial electric stimulation in a realistic head model (A) for right unilateral electrode configuration (B) commonly used in ECT. The anode is centered 2.5 cm to the right of the vertex and the cathode is centered 2.5 cm above the midpoint of the line connecting the external canthus and tragus on the right. The electrode current is 800 mA. Further details of the model are given in (Lee et al., 2010). In (C) and (D) the color scale gives the magnitude of the field and the arrows indicate the magnitude and direction of the field.

Figure 2

Example transcranial EM stimulation waveforms. Electrode current (A) and voltage (B) waveforms in tDCS delivered by Phoresor II Auto (Model No. PM850, IOMED, Inc., Salt Lake City, UT, USA) with “dose” and “current” settings of 4 mA·min and 1 mA, respectively. Electrode current (C) and voltage (D) of a TES pulse delivered by Digitimer Model DS7AH (Digitimer Ltd., Hertfordshire, UK) with “pulse width” and “current” settings of 0.2 ms and 86 mA. Coil current (E) and search coil voltage (F) of a conventional TMS pulse generated by Magstim Rapid (Magstim Co., Whitland, UK) with “output” setting of 67% of the device maximum and a 70 mm figure-8 coil (P/N 9925-00). The search coil voltage is proportional to the TMS coil voltage and the induced electric field. The search coil was made of a single-turn rectangular winding with dimensions 1 cm × 30 cm, positioned perpendicular to the TMS coil plane, with one of the 1 cm sides standing 1 mm away from the TMS coil center, parallel to the electric field orientation (16,102). Coil current (G) and search coil voltage (H) of a cTMS pulse produced by a custom-built waveform generator (16) connected to a Magstim figure-8 coil (P/N 9925-00).

Figure 3

Summary diagram of transcranial EM stimulation dosing. The EM stimulation dose is described by the electrode/coil configuration parameters and the electrode/coil voltage or current waveform parameters. See Section C for further discussion.

Figure 4

Definitions of typical transcranial EM stimulation waveform parameters.