If we attempt to bridge between the intracranial CSD and the scalp, a three-dimensional volume-conduction model may be posited that also complies with Eq. 2. This model may then be expanded to include extraneural dura-bone-scalp transitions common to all inverse models, such as three- and four-shell models used in Brain Electrical Source Analysis (BESA; Scherg, 1990) and similar methods that fit isolated equivalent dipoles (i.e., oriented point dipoles that serve as replacement for physically-separated sources and sinks) to a scalp topography via forward solutions. Impedance transitions at gross anatomical landmarks (e.g., gyrus patterns, ventricles, etc.) may also be incorporated, and even those associated with the local microstructure (including cyto- and fiberarchitectonic patterns). Not coincidentally, the volume-conduction model is also critical to the definition and implementation of low resolution brain electromagnetic tomography (LORETA; Pascual-Marqui et al., 1994), local autoregressive averages (LAURA; Grave de Peralta Menendez, 2004), and similar inverse models. Although a discussion of the strengths and limitations of each of these methods is beyond the scope of this paper, it is sufficient to distinguish between a generator that is empirically
