Standard Non-Personalized Electric Field Modeling of Twenty Typical tDCS Electrode Configurations via the Computational Finite Element Method: Contributions and Limitations of Two Different Approaches

Abstract

Transcranial direct current stimulation (tDCS) is a non-invasive brain stimulation procedure to modulate cortical excitability and related brain functions. tDCS can effectively alter multiple brain functions in healthy humans and is suggested as a therapeutic tool in several neurological and psychiatric diseases. However, variability of results is an important limitation of this method. This variability may be due to multiple factors, including age, head and brain anatomy (including skull, skin, CSF and meninges), cognitive reserve and baseline performance level, specific task demands, as well as comorbidities in clinical settings. Different electrode montages are a further source of variability between tDCS studies. A procedure to estimate the electric field generated by specific tDCS electrode configurations, which can be helpful to adapt stimulation protocols, is the computational finite element method. This approach is useful to provide a priori modeling of the current spread and electric field intensity that will be generated according to the implemented electrode montage. Here, we present standard, non-personalized model-based electric field simulations for motor, dorsolateral prefrontal, and posterior parietal cortex stimulation according to twenty typical tDCS electrode configurations using two different current flow modeling software packages. The resulting simulated maximum intensity of the electric field, focality, and current spread were similar, but not identical, between models. The advantages and limitations of both mathematical simulations of the electric field are presented and discussed systematically, including aspects that, at present, prevent more widespread application of respective simulation approaches in the field of non-invasive brain stimulation.

Keywords: current flow; current intensity; electric field; finite element method; tDCS.

Figure 1

Electric field modeling corresponding to the tDCS electrode configurations 1–4 (M1 and 20 cm² electrode size): 1–2, anodal left primary motor cortex (lM1)-cathodal right supraorbital region (rSOR), at 1 and 2 mA stimulation intensity, respectively, and 20 cm² electrode size; 3–4, anodal lM1-cathodal right M1, at 1 and 2 mA stimulation intensity, respectively, and 20 cm² electrode size. Dorsal, frontal, and lateral SimNIBS (A) and COMETS (B) outputs for configurations 1–2 at 1 (superior images) and 2 (inferior images) mA stimulation intensity are depicted. Dorsal, frontal, and lateral SimNIBS (C) and COMETS (D) outputs for configurations 3–4 at 1 (superior images) and 2 (inferior images) mA stimulation intensity are depicted. The normal electric field (normE) intensity (V/m) is represented by the color bar (online version). Brighter colors (corresponding to larger values in the color bar) indicate higher electric field intensity. Red and blue electrodes represent the anodal and cathodal electrode positions, respectively.

Figure 2

Electric field modeling corresponding to the tDCS electrode configurations 5–8 (M1 and 35 cm² electrode size): 5–6, anodal lM1-cathodal rSOR, at 1 and 2 mA stimulation intensity, respectively, and 35 cm² electrode size; 7–8, anodal lM1-cathodal right M1, at 1 and 2 mA stimulation intensity, respectively, and 35 cm² electrode size. Dorsal, frontal and lateral SimNIBS (A) and COMETS (B) outputs for configurations 5–6 at 1 (superior images) and 2 (inferior images) mA stimulation intensity are depicted. Dorsal, frontal, and lateral SimNIBS (C) and COMETS (D) outputs for configurations 7–8 at 1 (superior images) and 2 (inferior images) mA stimulation intensity are depicted. For the color bar, electric field intensity, and electrodes information, refer to Figure 1.

Figure 3

Electric field modeling corresponding to the tDCS electrode configurations 9–12 (DLPFC and 20 cm² electrode size): 9–10, anodal left dorsolateral prefrontal cortex (lDLPFC)-cathodal rSOR, at 1 and 2 mA stimulation intensity, respectively, and 20 cm² electrode size; 11–12, anodal lDLPFC-cathodal right DLPFC (rDLPFC), at 1 and 2 mA stimulation intensity, respectively, and 20 cm² electrode size. Dorsal, frontal, and lateral SimNIBS (A) and COMETS (B) outputs for configurations 9–10 at 1 (superior images) and 2 (inferior images) mA stimulation intensity are shown. Dorsal, frontal and lateral SimNIBS (C) and COMETS (D) outputs for configurations 11–12 at 1 (superior images) and 2 (inferior images) mA stimulation intensity are depicted. For the color bar, electric field intensity, and electrodes information, refer to Figure 1.

Figure 4

Electric field modeling corresponding to the tDCS electrode configurations 13–16 (DLPFC and 35 cm² electrode size): 13–14, anodal lDLPFC-cathodal rSOR, at 1 and 2 mA current intensity, respectively, and 35 cm² electrode size; 15–16 anodal lDLPFC-cathodal rDLPFC, at 1 and 2 mA stimulation intensity, respectively, and 35 cm² electrode size. Dorsal, frontal and lateral SimNIBS (A) and COMETS (B) outputs for configurations 13–14 at 1 (superior images) and 2 (inferior images) mA stimulation intensity are depicted. Dorsal, frontal, and lateral SimNIBS (C) and COMETS (D) outputs for configurations 15–16 at 1 (superior images) and 2 (inferior images) mA stimulation intensity are shown. For the color bar, electric field intensity, and electrodes information, see Figure 1.

Figure 5

Electric field modeling corresponding to the tDCS electrode configurations 17–20 (PPC and 20/35 cm² electrode size): 17–18, anodal left posterior parietal cortex (lPPC)-cathodal rSOR, at 1 and 2 mA stimulation intensity, respectively, and 20 cm² electrode size; 19–20, anodal lPPC-cathodal rSOR, at 1 and 2 mA stimulation intensity, respectively, and 35 cm² electrode size. Dorsal, frontal, and lateral SimNIBS (A) and COMETS (B) outputs for configurations 17–18 at 1 (superior images) and 2 (inferior images) mA stimulation intensity are shown. Dorsal, frontal, and lateral SimNIBS (C) and COMETS (D) outputs for configurations 19–20 at 1 (superior images) and 2 (inferior images) mA stimulation intensity are depicted. For the color bar, electric field intensity, and electrodes information, refer to Figure 1.