Methods for identifying neuronal generators underlying a scalp-recorded ERP rely on the volume-conduction relation expressed by Equation (1). Traditionally, scalp topographies have been used to suggest generators, but the task is hampered by the blurring introduced by volume conduction itself. Scalp-CSD sharpens these topographies and reexpresses them in terms of effective radial current generators from subjacent anatomical regions and thereby provides constraints on possible generators. Inverse models (e.g., BESA, LORETA) can improve this anatomical specificity by implementing additional assumptions to allow for stable or plausible solutions, such as the reasonable presupposition of continuity. However, the partial closure of intracranial fields is characterized by abrupt field transitions (Tenke et al., 1993). Invasive methods provide stronger evidence for generators (e.g., intracranial CSD profiles), but even these may require additional evidence before their laminar and cellular mechanisms may be deduced (Schroeder et al., 1995). Convergent evidence from other sources (e.g., functional imaging) may also be useful, but only as it illuminates the relationship expressed in Equation (1).
