Generator Localization by Current Source Density (CSD): Implications of Volume Conduction and Field Closure at Intracranial and Scalp Resolutions

Craig E. Tenke & Jürgen Kayser

Division of Cognitive Neuroscience, New York State Psychiatric Institute, New York, NY, USA;
Department of Psychiatry, Columbia University College of Physicians and Surgeons, New York, NY, USA

Received 11 February 2012; revised 21 May 2012; accepted 4 June 2012; published online 14 July 2012. 

Abstract

The topographic ambiguity and reference-dependency that has plagued EEG/ERP research throughout its history are largely attributable to volume conduction, which may be concisely described by a vector form of Ohm's Law. This biophysical relationship is common to popular algorithms that infer neuronal generators via inverse solutions. It may be further simplified as Poisson's source equation, which identifies underlying current generators from estimates of the second spatial derivative of the field potential (Laplacian transformation). Intracranial CSD studies have dissected the "cortical dipole" into intracortical sources and sinks, corresponding to physiologically-meaningful patterns of neuronal activity at a sublaminar resolution, much of which is locally cancelled (i.e., closed field). By virtue of the macroscopic scale of the scalp-recorded EEG, a surface Laplacian reflects the radial projections of these underlying currents, representing a unique, unambiguous measure of neuronal activity at scalp. Although the surface Laplacian requires minimal assumptions compared to complex, model-sensitive inverses, the resulting waveform topographies faithfully summarize and simplify essential constraints that must be placed on putative generators of a scalp potential topography, even if they arise from deep or partially-closed fields. CSD methods thereby provide a global empirical and biophysical context for generator localization, spanning scales from intracortical to scalp recordings.

Key Words: surface Laplacian; current source density (CSD); volume conduction; inverse models; closed field; EEG; ERP