Chunk #22 — 3. Impact of spatial scale on CSD implementations — 3.1. Empirical considerations for linear (one-dimensional) intracranial recordings — 3.1.1. The cortical dipole and field closure
Although scalp-recorded activity necessarily implies an open field (i.e., uncancelled activity), field closure must always be viewed quantitatively. Real ERPs are always the result of the summation (i.e., volume integration) of interrelated cortical processes, most of which are locally cancelled, and invisible at the scalp (i.e., the fields are largely closed). Tenke et al. (1993) simulated open- and closed-field laminae composed of distributed point sources and sinks, as described by Eq. 1. Layers of paired source and sink generators (“dipoles”) resulted in an open field, characterized by a linear potential gradient outside the lamina, and a simple inversion profile inside. In contrast, the addition of “inverted dipoles” led to partial or complete field closure, identifiable from a nonlinear inflection of the field within the “closed” edge of the lamina. Even a small bias in an otherwise closed filed lamina was found to effectively open the field, and consequently, even symmetric neurons (e.g., stellate cells) can contribute to the volume-conducted field. These simulations also demonstrated a somewhat counterintuitive finding: although the precise localization of focal activity may require a high-resolution CSD,
