Determination of long-range resistance from high resolution impedance spectroscopy in human cadaveric heads

Long-range resistance (R_t) reflects the resistive component of current flow through intracochlear fluids, surrounding tissues, and bone before it returns to the extracochlear return electrode (RE). Unlike near-field resistances, which mainly capture the local tissue-electrode interface, R_t captures the global tissue pathway and therefore represents how efficiently stimulation current spreads throughout the cochlea and surrounding anatomical structures. In this study, we have determined the long-range resistance (R_t) between a stimulation electrode (SE) and the extracochlear RE from precision impedance spectroscopic measurements in three human cadaveric heads involving six sequentially cochlear implanted ears with the MED-EL FLEX28 and the Cochlear Ltd. CI622 electrode arrays. Equivalent electrical circuit (EEC) modelling, combined with finite element method (FEM)-based correction, was employed to separate the biological impedance components from non-biological artefacts arising from the measurement setup. Principal Component Analysis (PCA) was applied to detect and remove atypical impedance responses due to malfunctioning electrodes. Missing electrode values were interpolated using cubic splines to maintain continuity. Across both array types, R_t consistently increased from basal to apical electrode positions, consistent with the progressively smaller scala tympani cross-section in the apical turn. The overall trend of R_t observed in the present study also aligns with trends reported in in vivo telemetry studies, supporting the translational relevance of the measurements.