High-Resolution Directional Depth Electrodes: Open-Source FEM Lead-Field Modeling, Characterization, and Validation

Abstract: Depth electrodes used in stereoelectroencephalography (sEEG) and deep-brain stimulation (DBS) are essential tools for neural recording and stimulation. Traditional designs have limited spatial resolution, typically 8 to 16 cylindrical contacts (0.8 to 1.0 mm diameter) along a 5 to 10 cm shaft, restricting recordings from small or localized populations. Recent high-density, directional electrodes (HDsEEG) enable finer localization of local field potentials (LFPs) and spike timing. Yet, characterizing their directional sensitivity and validating modeling tools for lead field (LF) analysis remain critical. We compare finite element method (FEM) LF modeling of a novel HD-sEEG electrode using two tools: a commercial solver (ANSYS) and an open-source pipeline (Brainstorm-DUNEuro). Goals: (i) validate against analytical solutions, (ii) assess solver differences, and (iii) characterize HDsEEG directional sensitivity. LFs were modeled in simple and bio-relevant scenarios. Using Helmholtz reciprocity, we computed LFs by (a) electrode-based stimulation with ANSYS and (b) source-based recording with Brainstorm-DUNEuro. First, a multi-sphere head model with known solutions tested the solver's accuracy. Next, an HDsEEG electrode in a homogeneous conductor was simulated. Directional effects were assessed by comparing sensitivity with vs. without the insulating substrate. Source localization performance was also compared between HDsEEG and standard electrodes. Both solvers closely matched analytic solutions. In realistic settings, LF distributions were highly similar. Modeling showed clear directional sensitivity: contacts facing a source had higher sensitivity than those shadowed, reflecting a substrate shielding effect that vanished when the substrate was conductive. Crucially, HDsEEG improved source localization, as voltage differences across contacts provided robust directional LF information.