Model of low-pass filtering of local field potentials in brain
Claude Bedard, Helmut Kroger and Alain Destexhe
Physical Review E 73: 051911, 2006.
Local field potentials (LFPs) are routinely measured experimentally
in brain tissue, and exhibit strong low-pass frequency filtering
properties, with high frequencies (such as action potentials) being
visible only at very short distances (approx. 10 microns) from the
recording electrode. Understanding this filtering is crucial to
relate LFP signals with neuronal activity, but not much is known
about the exact mechanisms underlying this low-pass filtering. In
this paper, we investigate a possible biophysical mechanism for the
low-pass filtering properties of LFPs. We investigate the
propagation of electric fields and its frequency dependence close to
the current source, i.e. at length scales in the order of average
interneuronal distance. We take into account the presence of a high
density of cellular membranes around current sources, such as glial
cells. By considering them as passive cells, we show that under the
influence of the electric source field, they respond by polarization.
Because of the finite velocity of ionic charge movement, this
polarization will not be instantaneous. Consequently, the induced
electric field will be frequency-dependent, and much reduced for high
frequencies. Our model establishes that this situation is analogous
to an equivalent RC-circuit, or better a system of coupled
RC-circuits. We present a number of numerical simulations of induced
electric field for biologically realistic values of parameters, and
show the frequency filtering effect as well as the attenuation of
extracellular potentials with distance. We suggest that induced
electric fields in passive cells surrounding neurons are the physical
origin of frequency filtering properties of LFPs.
Experimentally-testable predictions are provided allowing to verify
the validity of this model.
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