Combining EEG and MRI Techniques - Quantitative EEG Analysis Methods and Clinical Applications - page 326

(a)
(b)
Figure 12.6 Three-dimensional rendering of triangulated cortex: (a) Delaunay triangulation and (b)
voxel face triangulation.
12.1.3.2 Source Orientations
Source orientations may be constrained to be perpendicular to the surface of the cor-
tical sheet, thus modeling the principal orientations of the synapses of the gray mat-
ter pyramidal cells that are the sources of the surface EEG (see Chapter 1). On a
cortical triangle mesh, source orientations can easily be computed as the vector sum
of the normals of all triangles surrounding a given node. If a cortical triangle mesh is
not available, the three-dimensional intensity gradient computed from the MRI can
be used. With given source orientations, one unknown source component per
location needs to be computed.
12.1.3.3 Connectivity
The temporally correlated activity of large populations of nearby and similarly ori-
ented neurons is the basic building block of the EEG. The equivalent current dipole
representing activity from several cubic millimeters of cortical tissue is one way to
model this basic building block. For a cortical source space, connectivity needs to be
exploited. Cortical triangle meshes encode all information necessary for modeling