Consider as an example a bilateral cortical generator deep within the longitudinal fissure (Fig. 7A). The alignment of superficial sources and deep sinks results in extensive cancellation of the intracranial field, with an uncancelled residual remaining at the scalp that is reminiscent of a laminar closed field observed using one-dimensional intracranial methods (cf. Fig. 3 of Tenke et al., 1993). A reasonable simplification of this generator configuration is a pair of opposed dipole generators (i.e., a linear quadrupole), symmetrically positioned within midline gyral cortex and oriented normal to the cortical surface (corresponding to “cortical dipoles” with superficial sources; Figure 7C, D). This generator configuration results in a summation of midline sources, yielding a sharply localized source on the midline scalp. The remaining uncancelled activity, corresponding to the deep sinks, is projected onto the lateral cortical surfaces. The more superficially the generator is positioned within the longitudinal fissure, the larger is the midline positivity (compare waveforms at site FCz in Fig. 7E and F), and the smaller the displacement of the negative maximum from the midline (maximal at FC3/4 in Fig. 7F vs. FC5/6 in Fig. 7E). The corresponding surface Laplacian topographies show similar, but more sharply localized, differences (Fig. 7G).
