Finite element (FE) model of the conventional 7 × 5 cm² rectangular-pad and 4 × 1 ring configurations. (A) Segmented compartments in the following order: Scalp, Skull, CSF and Brain. (B.1) FE model of the conventional rectangular-pad configuration and corresponding FE mesh (B.2). (C.1) FE model of the 4 × 1 ring electrode configuration and corresponding FE mesh (C.2). The two insets show the zoomed mesh images, highlighting finer detail. ‘Red’: Anode electrode; ‘Blue’: Cathode electrode(s); ‘Olive green’: sponge/gel.

Brain modulation during tDCS using conventional rectangular-pad (A.1) and the 4 × 1 ring electrode configuration (B.1). ‘Red’: Anode electrode; ‘Blue’: Cathode electrode(s); ‘Olive green’: sponge/gel. For each configuration, we calculated the induced cortical electric field (EF) magnitude. (A.2, B.2) Surface-magnitude plots of EF along the brain surface, same view as (A.1, B.1). The dashed region is expanded in inset. (A.3, B.3) Top view of the brain showing the induced surface-magnitude EF. The insets (A.4) and (B.4) show the zoomed surface-magnitude EF plots. These images are obtained with “lighting” on [COMSOL Multiphysics] to highlight gyri/sulci modulation. (A.5, B.5) Cross-section magnitude EF plotted on a series of successive cortical slices, same view as (A.1, B.1). The cross-section magnitude EF plots for two slices and their corresponding MRI scans are shown in (A.6), (B.6). One slice is chosen directly from underneath the rectangular-pad/active electrode of 4 × 1 ring configuration and another from the prefrontal area of the brain. The same slices are shown for each of the configurations. All plots are normalized between zero and the peak positive cortical EF magnitude: conventional rectangular-pad peak = 0.67 V/m (or 0.13 A/m² current density peak); 4 × 1 ring peak = 0.44 V/m. (Note that for the rectangular-pad, the EF magnitude peak is observed between the pads and not underneath).