  Simulation of extracellular fields is one of the substantial methods used in
the area of computational neuroscience. Its most common usage is validation of
experimental methods as EEG and extracellular spike recordings or modeling of
physiological phenomena which can not be easily determined empirically.
Continuous experimental work has been re-raising the importance of polarization
effects between neuronal structures to neuronal communication. As this effects
relies on very small potential changes, better modeling methods are necessary
to quantify the weak electrical fields in the microscopic scale in a more
realistic way. An important factor of influence on local field effects in the
hippocampal formation is the heterogeneous resistivity of extracellular tissue.
The vast majority of modeling studies consider the extracellular space to be
homogeneous while experimentally, it has been shown that the stratum pyramidale
has two times higher resistivity then other hippocampal layers. Common
simulation methods for extracellular electrical fields based on the point
source approximation are bound to describe the resistance of the space with a
single, linear factor. We propose that models should be based on the space- and
time-dependent Maxwell equations in order to account for heterogeneous
properties of the extracellular space and specific arrangements of neurons in
dense hippocampal layers. To demonstrate the influence of heterogeneous
extracellular resistivity and neuronal spatial orientation on modeling results,
we combine solutions of classical compartment models with spatiotemporal PDEs
solved by the FEM. With the help of these methods, we show that the inclusion
of heterogeneous resistivity has a substantial impact on voltages in close
proximity to emitting neurons, increasing the extracellular potentials
substantially compared to the homogeneous variant.
