The role of electrode direction during axonal bipolar electrical stimulation: a bidomain computational model study

Direct electrical stimulation of cortical and axonal areas is widely used for brain mapping of functional areas during intraparenchymatous resections. However, there are very few data (be they experimental or computational) regarding the exact volume of activated axons surrounding the bipolar electrodes. The aim of this study was to provide a computational model to estimate the regions in which electrical stimulation will generate an action potential in the axons. An axonal fasiculus was modeled as a homogenized bidomain medium. Passive membrane dynamics was implemented at the interface between the two domains. The resulting set of equations was numerically solved by the finite element method. Simulations show that the activated volumes are located in the vicinity of each electrode. The volume of the activated regions grows linearly with intensity. The direction of the bipolar tips (parallel or orthogonal to the fibers' axis) does not significantly influence the size of activated regions. This computational study suggests that directing the bipolar electrodes orthogonal to the axis of a fasciculus should facilitate its identification, as the chances are higher in this configuration that at least one of the electrode tips will be in contact with a fasciculus. Experimental studies are needed to confirm this model prediction.