Figure 3: Human head model.

a) Geometry of the model. b) Location of the three selected brain structures: left hippocampus (blue), right pallidum (green) and FDI area of left motor cortex (red). c) Section of gray and white matter with arrows representing the preferred direction vectors ${\overrightarrow{n}}_{\mathrm{pref}}$. See Fig. S2b for an image of ${\overrightarrow{n}}_{\mathrm{pref}}$ in the whole brain. d) Electrodes on the skin surface of the model. Each simulated configuration in Studies 1 and 2 consisted of two electrodes on the left side of the head (blue) that supplied current I₁ and two on the right (red) that supplied I₂. The configuration shown here is the standardized configuration used in Study 1 with electrodes at the C1, C2, C5 and C6 locations of the 10-10 system.