6 research outputs found
Variable Anisotropic Brain Electrical Conductivities in Epileptogenic Foci
Source localization models assume brain electrical conductivities are isotropic at about 0.33 S/m. These assumptions have not been confirmed ex vivo in humans. This study determined bidirectional electrical conductivities from pediatric epilepsy surgery patients. Electrical conductivities perpendicular and parallel to the pial surface of neocortex and subcortical white matter (n = 15) were measured using the 4-electrode technique and compared with clinical variables. Mean (±SD) electrical conductivities were 0.10 ± 0.01 S/m, and varied by 243% from patient to patient. Perpendicular and parallel conductivities differed by 45%, and the larger values were perpendicular to the pial surface in 47% and parallel in 40% of patients. A perpendicular principal axis was associated with normal, while isotropy and parallel principal axes were linked with epileptogenic lesions by MRI. Electrical conductivities were decreased in patients with cortical dysplasia compared with non-dysplasia etiologies. The electrical conductivity values of freshly excised human brain tissues were approximately 30% of assumed values, varied by over 200% from patient to patient, and had erratic anisotropic and isotropic shapes if the MRI showed a lesion. Understanding brain electrical conductivity and ways to non-invasively measure them are probably necessary to enhance the ability to localize EEG sources from epilepsy surgery patients
Nonlinearity and Topology
The interplay of nonlinearity and topology results in many novel and emergent
properties across a number of physical systems such as chiral magnets, nematic
liquid crystals, Bose-Einstein condensates, photonics, high energy physics,
etc. It also results in a wide variety of topological defects such as solitons,
vortices, skyrmions, merons, hopfions, monopoles to name just a few.
Interaction among and collision of these nontrivial defects itself is a topic
of great interest. Curvature and underlying geometry also affect the shape,
interaction and behavior of these defects. Such properties can be studied using
techniques such as, e.g. the Bogomolnyi decomposition. Some applications of
this interplay, e.g. in nonreciprocal photonics as well as topological
materials such as Dirac and Weyl semimetals, are also elucidated
A systematic study of head tissue inhomogeneity and anisotropy on EEG forward problem computing
In this study, we propose a stochastic method to
analyze the effects of inhomogeneous anisotropic tissue
conductivity on electroencephalogram (EEG) in forward
computation. We apply this method to an inhomogeneous
and anisotropic spherical human head model. We apply
stochastic finite element method based on Legendre polynomials,Karhunen–Loeve expansion and stochastic
Galerkin methods. We apply Volume and Wang’s constraints
to restrict the anisotropic conductivities for both the
white matter (WM) and the skull tissue compartments. The
EEGs resulting from deterministic and stochastic FEMs are
compared using statistical measurement techniques. Based
on these comparisons, we find that EEGs generated by
incorporating WM and skull inhomogeneous anisotropic
tissue properties individually result in an average of 56.5
and 57.5% relative errors, respectively. Incorporating these
tissue properties for both layers together generate 43.5%
average relative error. Inhomogeneous scalp tissue causes
27% average relative error and a full inhomogeneous
anisotropic model brings in an average of 45.5% relative
error. The study results demonstrate that the effects of
inhomogeneous anisotropic tissue conductivity are significant on EEG