(a) The depth EBI consists of 3 essential components: (i) the implanted DBS electrode, which has an array of 4 metal contacts (pink, numbered here 0 to 3); (ii) the surrounding brain tissue (light blue); (iii) a peri-electrode space (dark blue), which is filled with extracellular fluid in the acute case, and reactive giant cells in the chronic case. (b) This description is modelled as a three-dimensional geometry using the software COMSOL Multiphysics 3.3. The dimensions of the electrode match those of the actual DBS electrode used (Medtronic model 3389), the peri-electrode space is arbitrarily defined to have a thickness of 0.25mm, and the surrounding brain tissue is modelled as a homogenous cylinder of 10mm radius.