Evidence for frequency-dependent extracellular
impedance from the transfer function between
extracellular and intracellular potentials.
Claude Bedard, Serafim Rodrigues, Noah Roy, Diego Contreras and
Alain Destexhe
Journal of Computational Neuroscience 29: 389-403, 2010.
(Special issue "Modeling Extracellular Potentials")
Abstract
We examine the properties of the transfer function F_T = V_m /
V_{LFP} between the intracellular membrane potential (V_m) and the
local field potential (V_{LFP}) in cerebral cortex. We first show
theoretically that, in the linear regime, the frequency dependence of
the extracellular medium and that of the membrane potential have a
clear incidence on F_T. The calculation of F_T from experiments and
the matching with theoretical expressions is possible for
desynchronized states where monopolar current sources can be
considered as independent. Using a mean-field approximation, we
obtain a method to estimate the impedance of the extracellular
medium without injecting currents. We examine the transfer function
for bipolar (differential) LFPs and compare to simultaneous
recordings of V_m and V_{LFP} during desynchronized states in rat
barrel cortex in vivo. The experimentally derived F_T matches the
one derived theoretically, only if one assumes that the impedance of
the extracellular medium is frequency-dependent, and varies as
1/sqrt(omega) (Warburg impedance) for frequencies between 3 and
500 Hz. This constitutes indirect evidence that the extracellular
medium is non-resistive, which has many possible consequences for
modeling LFPs.
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