Current Sources in Inhomogeneous and Isotropic Media
Potentials in inhomogeneous volume conductors depend on conductive properties of the different tissues in addition to the source-sink distributions. Whereas genuine tissue is anisotropic, this discussion is limited to isotropic media. Potentials generated by dipole sources in planar, layered media and a homogeneous sphere are first presented as simple examples of the effects of inhomogeneity. The “4-sphere model” is presented in this chapter as an idealized head model consisting of an inner sphere (brain) surrounded by 3 spherical shells representing CSF, skull, and scalp layers. Forward solutions with the 4-sphere model include surface potentials generated by dipoles and dipole layers representing large synchronous source regions. The relative magnitudes and distributions of (simulated) scalp potentials are obtained. Scalp potentials generated by dipole sources in smooth cortex (gyri) are compared with potentials generated in cortical folds (fissures and sulci). A small discussion of the effects of an inhomogeneous skull layer on scalp potentials is also included.
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