A persistent issue when analyzing electrophysiologic data is the dependency of surface potentials on the recording reference, affecting measures in both frequency and time domain. The choice on a reference location (e.g., linked-mastoids, linked-ears, nose, average) crucially affects component interpretation (e.g., polarity, topography, generator) and statistical analysis. Although this issue is generally known and communicated through seminal papers, reviews and textbooks, its empirical implications for electrophysiologic research have barely or only superficially been addressed. As a consequence, the field at large lacks straightforward or practical solutions, and misconceptions prevail. Among these are notions that an average reference or principal components analysis are reference-independent strategies. However, its has long been recognized that a surface Laplacian transformation (i.e., second spatial derivative) provides a truly reference-free technique, which can easily be implemented as an intermediate step during EEG/ERP data analysis. This symposium reviews the biophysical properties of surface potentials and their current source density (CSD) estimates on scalp (surface Laplacian), which simplify volume-conducted redundancies resulting in sharper topographies that more closely correspond to the underlying current generators. We will compare the CSD approach to other EEG source localization methods, and address common reservations based largely on concerns that CSD methods represent only superficial generators and require a high-density EEG montage. This symposium joins two research teams that have been and are systematically applying CSD techniques. While focusing on CSD methodology as a bridge between montage-dependent scalp potentials and their underlying current generators, the merits of a CSD transform to quantify EEG spectra and ERP components are illustrated through empirical studies addressing a diversity of basic and clinical research objectives.
Keywords: surface Laplacian; current source density (CSD); reference-free; current generator