Biomedical Engineering Reference
In-Depth Information
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CAP radius (cm)
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CAP radius (cm)
Figure 1.6
(Top) Maximum scalp potential expressed as a percentage of transcortical potential
V
C
directly above cortical dipole layers of varying angular extent, forming superficial spherical caps on
the inner sphere (cortex) of a four-sphere head model are shown. Each curve is based on the labeled
model brain-to-skull conductivity ratio. (Bottom) The relative scalp Laplacian in the same head model
due to the same dipole layers is shown. Potentials are primarily sensitive to large dipole layers,
whereas Laplacians are sensitive to smaller dipole layers. (
From:
[2]. © 2006 Oxford University Press.
Reprinted with permission.)
summary, the surface Laplacian acts as a spatial filter that emphasizes smaller source
regions than those likely to make the dominant contributions to scalp potential.
Thus, the Laplacian measure supplements but cannot replace the potential measure.
The Laplacian is a spatial filter that removes low spatial frequency scalp signals due
to both volume conduction and genuine source dynamics, but it cannot distinguish
between the two. Further discussion of the strengths and limitations of the surface
Laplacian and other high-resolution methods may be found in Chapter 13 and [2].
1.9
Alpha Rhythm Sources
Several source properties of spontaneous EEGs are suggested by the following study
using the New Orleans spline Laplacian algorithm [2]. Figure 1.7 shows 9 of 111
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