Methodological dimensions of transcranial brain stimulation with the electrical current in human

Abstract

Transcranial current stimulation (TCS) is a neuromodulation method in which the patient is exposed to a mild electric current (direct or alternating) at 1-2 mA, resulting in an increase or a decrease in the brain excitability. This modification in neural activities can be used as a method for functional human brain mapping with causal inferences. This method might also facilitate the treatments of many neuropsychiatric disorders based on its inexpensive, simple, safe, noninvasive, painless, semi-focal excitatory and inhibitory effects. Given this, a comparison amongst different brain stimulation modalities has been made to determine the potential advantages of the TCS method. In addition, considerable methodological details on using TCS in basic and clinical neuroscience studies in human subjects have been introduced. Technical characteristics of TCS devices and their related accessories with regard to safety concerns have also been well articulated. Finally, some TCS application opportunities have been emphasized, including its potential use in the near future.

Keywords: Non Invasive Brain Stimulation (NIBS); Transcranial Alternating Current Stimulation (tACS); Transcranial Direct Current Stimulation (tDCS); Transcranial Electrical Stimulation (tES).

Figure 1

The preliminary TCS requirements

Figure 2

A sample tDCS device; the “Time Remaining” part reverse counts the preset time; the “Current” part indicates the applied current intensity; Patient care can be dedicated to manually increase or decrease the intensity and abort the whole process if necessary; the “Impedance Scan” estimates the electrodes contact impedance and verifies its quality to optimize the place of electrodes, it will be optimal if the whole triangle gets colorful; “Duration and Intensity” knobs account for the preliminary stimulation adjustment. When set to the Active mode, Scan (scans and checks the contact's impedance), Tickle (applies an excess amount of current in cases of insufficient contacts), Pass (enables the main process of stimulation) and Buffer (isolates the device and electrical fields from environmental inputs –e.g. MRI) options should be adjusted, otherwise Sham mode should be selected; AC or DC types can be selected with the pertaining switch.

Figure 3

TCS Fundamental sample circuits a) Circuit model, b) Integrated circuit implementation using LM334, c) Feedback implementation.

Figure 4

Sponge Pads (left) containing rubber electrodes (right)

Figure 5

Different solid-conductor shapes and materials (Ag pellet, Ag/AgCl pellet, rubber pellet, Ag/ AgCl ring, Ag/AgCl disc respectively).

Figure 6

The 10-20 International EEG system is used to determine electrodes placement. For instance, to perform the anodal stimulation of dorsolateral prefrontal cortex the anode (active electrode) should be placed over F3 or F4 depending on the study.

Figure 7

The critical questions which need to be answered to generate a roadmap when designing a TCS-included study. Red boxes are the brief descriptions of each bold phrase, which is the important keywords of each critical question.

Figure 8

The roadmap to design a study which measures TCS effects in six steps: 1. Concept Design: Determining the total number of the target populations, which separates study into two directions: single target population vs. multiple-targets population in which each target should be dealt with separately as a single-target study (Critical Question (CQ) 1-3), then specifying neurocognitive function of interest, its assessment method, and region of interest. 2. Intervention Types: Choosing the intervention type to use in the procedure. 2.1. Specifying Active intervention (CQ 4) and reference electrode placement base on Electrode Montage. 2.2. Choosing a combination of control interventions (CQ 5). 2.3. Positioning the electrodes on the head based on a standard system and specifying the size of each electrode. 3. Session Design: Designing the procedure of each session based on our choice for the Target TCS Effect. 4. Stimulation Protocol: Setting the stimulator's properties such as current intensity (CQ 8). 5. Blindness: Clarifying the blindness status of the people involved in the experiment (CQ 9). 6. Study Design: Determination of single group vs. multiple groups design, its randomization and the statistical model to analyze the results.

Figure 9

Assumptions and requirements in a multiple stimulation study design.