3,184 research outputs found
Fast Approximation of EEG Forward Problem and Application to Tissue Conductivity Estimation
Bioelectric source analysis in the human brain from scalp
electroencephalography (EEG) signals is sensitive to the conductivity of the
different head tissues. Conductivity values are subject dependent, so
non-invasive methods for conductivity estimation are necessary to fine tune the
EEG models. To do so, the EEG forward problem solution (so-called lead field
matrix) must be computed for a large number of conductivity configurations.
Computing one lead field requires a matrix inversion which is computationally
intensive for realistic head models. Thus, the required time for computing a
large number of lead fields can become impractical. In this work, we propose to
approximate the lead field matrix for a set of conductivity configurations,
using the exact solution only for a small set of basis points in the
conductivity space. Our approach accelerates the computing time, while
controlling the approximation error. Our method is tested for brain and skull
conductivity estimation , with simulated and measured EEG data, corresponding
to evoked somato-sensory potentials. This test demonstrates that the used
approximation does not introduce any bias and runs significantly faster than if
exact lead field were to be computed.Comment: Copyright (c) 2019 IEEE. Personal use of this material is permitted.
However, permission to use this material for any other purposes must be
obtained from the IEEE by sending a request to [email protected]
Numerical analysis of nanostructures for enhanced light extraction from OLEDs
Nanostructures, like periodic arrays of scatters or low-index gratings, are
used to improve the light outcoupling from organic light-emitting diodes
(OLED). In order to optimize geometrical and material properties of such
structures, simulations of the outcoupling process are very helpful. The finite
element method is best suited for an accurate discretization of the geometry
and the singular-like field profile within the structured layer and the
emitting layer. However, a finite element simulation of the overall OLED stack
is often beyond available computer resources. The main focus of this paper is
the simulation of a single dipole source embedded into a twofold infinitely
periodic OLED structure. To overcome the numerical burden we apply the Floquet
transform, so that the computational domain reduces to the unit cell. The
relevant outcoupling data are then gained by inverse Flouqet transforming. This
step requires a careful numerical treatment as reported in this paper
Modeling extracellular field potentials and the frequency-filtering properties of extracellular space
Extracellular local field potentials (LFP) are usually modeled as arising
from a set of current sources embedded in a homogeneous extracellular medium.
Although this formalism can successfully model several properties of LFPs, it
does not account for their frequency-dependent attenuation with distance, a
property essential to correctly model extracellular spikes. Here we derive
expressions for the extracellular potential that include this
frequency-dependent attenuation. We first show that, if the extracellular
conductivity is non-homogeneous, there is induction of non-homogeneous charge
densities which may result in a low-pass filter. We next derive a simplified
model consisting of a punctual (or spherical) current source with
spherically-symmetric conductivity/permittivity gradients around the source. We
analyze the effect of different radial profiles of conductivity and
permittivity on the frequency-filtering behavior of this model. We show that
this simple model generally displays low-pass filtering behavior, in which fast
electrical events (such as Na-mediated action potentials) attenuate very
steeply with distance, while slower (K-mediated) events propagate over
larger distances in extracellular space, in qualitative agreement with
experimental observations. This simple model can be used to obtain
frequency-dependent extracellular field potentials without taking into account
explicitly the complex folding of extracellular space.Comment: text (LaTeX), 6 figs. (ps
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