Chunk #55 — 3. Impact of spatial scale on CSD implementations — 3.3. Empirical considerations for planar (two-dimensional) scalp-recorded EEG — 3.3.2. CSD as a conservative description of neural current generators
The defining role of volume conduction in the topography of N1 was demonstrated by Scherg and von Cramon (1985), who identified and localized pairs of equivalent dipoles to the vicinity of auditory cortex (one tangential and one radial). The CSD topography of N1 is likewise consistent with a tangential generator within the Sylvian fissure, while the temporal N1 is consistent with an overlapping radial generator on the convexity of the temporal lobe. Both approaches represent reference-free simplifications of the field potential topography, and the dipole solution is quite useful and appropriate for specific, localized generator configurations. In the case of auditory N1, they both represent informal properties of the cortical dipole as well: surface-to-depth polarity inversions, regional composite generators, and dipolar elements aligned with the axis of the cortical projection cells. However, whereas an equivalent dipole model requires these simplifications, a CSD topography does not. Conversely, an equivalent dipole may be used to simplify a field potential topography even when it does not accurately represent the neuroanatomical generators that produce it. As sharply concluded by Fishman et al. (2001a), intracranial
