  The electrical properties of extracellular space around neurons are important
to understand the genesis of extracellular potentials, as well as for
localizing neuronal activity from extracellular recordings. However, the exact
nature of these extracellular properties is still uncertain. We introduce a
method to measure the impedance of the tissue, and which preserves the intact
cell-medium interface, using whole-cell patch-clamp recordings in vivo and in
vitro. We find that neural tissue has marked non-ohmic and frequency-filtering
properties, which are not consistent with a resistive (ohmic) medium, as often
assumed. In contrast, using traditional metal electrodes provides very
different results, more consistent with a resistive medium. The amplitude and
phase profiles of the measured impedance are consistent with the contribution
of ionic diffusion. We also show that the impact of such frequency-filtering
properties is possibly important on the genesis of local field potentials, as
well as on the cable properties of neurons. The present results show non-ohmic
properties of the extracellular medium around neurons, and suggest that source
estimation methods, as well as the cable properties of neurons, which all
assume ohmic extracellular medium, may need to be re-evaluated.
