Extracellular tissue impedance and conductivity

The extracellular medium is not an ideal conductor: brain tissue has a finite, frequency- and direction-dependent conductivity (roughly 0.3 S/m for grey matter) that determines how transmembrane currents translate into recordable potentials. Whether the tissue behaves as a purely resistive (ohmic) medium or as a genuinely dispersive one with frequency-dependent conductivity has been debated; most quantitative modeling treats the bulk medium as approximately ohmic in the BCI band while acknowledging measurable dispersion and anisotropy, particularly the strong directional conductivity of white-matter tracts.

Conductivity enters every forward model: the extracellular potential scales inversely with conductivity, and inhomogeneities (cell membranes, blood vessels, and at the macroscale the CSF and skull layers) reshape the field. Anisotropy means current spreads preferentially along fibers, which matters for both LFP interpretation and for predicting the spread of stimulation current. Uncertainty in tissue conductivity is a leading source of error in EEG/MEG source localization and in stimulation dosing.