A CSD is not a measurement per se, but rather an estimate based on the available spatial data.5 In particular, it is a composite of spatial and electrical measurements with unique properties that should not be confused with those of the field potentials from which they are derived. The goal of an intracranial CSD is not to accurately represent a continuous second spatial derivative of the LFP, but rather to match the resulting estimates to known anatomical and physiological properties of the tissue. A coarse, but interpretable measure of the underlying (ensemble) current generators has greater empirical value than a “more precise” estimate that is unintelligible. Although the underlying ERP generators may be traceable to the molecular scale of ion channels (cf. Nunez and Srinivasan, 2006a), a CSD model is not appropriate for such microscopic potential gradients. The CSD method necessarily depends on a presumed divergence volume, and the original one-dimensional derivation (Nicholson, 1973) assumed that the resulting volume estimate would reflect the laminar properties of the tissue.
