Matching geometry and stimulation parameters of electrodes for deep brain stimulation experiments--numerical considerations.

Deep brain stimulation, the electric stimulation of basal ganglia nuclei, is a treatment for movement disorders such as Parkinson's disease. The underlying mechanisms are studied in animals, e.g. rodents. Designs and materials of commercially available microelectrodes, as well as experimentally applied driving signals vary tremendously. We used finite integration modeling to compare the electric field and current density distributions induced by various electrodes. Current density or field strength "hot spots", which are located particularly at sites of high curvature and material interfaces coincided with corrosion and erosion at poles and insulation, respectively, as shown by scanning electron microscopy of stainless steel electrodes. Cell constants, i.e. geometry factors relating the electrode impedance to the specific medium conductivity, were calculated to determine the electrode voltage for a given stimulation current. Nevertheless, for electrodes of the same cell constant but of different geometry, current and field distributions may be very dissimilar. We found geometry-dependent limiting values of the stimulation current, above which electric tissue damage may occur. These values limit the reach of the stimulation signal for a given electrode geometry. Also, electrode geometries determine the shape of the stimulated tissue volume. This study provides tools for choosing the most appropriate geometry for targeting different-sized brain areas.