The recorded EEG potential at each electrode does not represent brain sources solely adjacent to the electrode location (Nunez, 1981), because of two limitations (Kayser and Tenke, 2015): (i) EEG is measured as a potential difference between the recorded electrode and the reference electrode, and (ii) the EEG signal is a mixture of propagated neuroelectric activity from multiple local and distal brain sources and is smeared by volume conduction. Both of these limitations can be mitigated by use of the surface Laplacian algorithm (Hjorth, 1975), which is a simple mathematical transformation applied to the EEG surface potentials (for a tutorial review on this topic, see Kayser and Tenke, 2015). The CSD provides information about local radial current density representing components of the primary neural activity in the scalp region (Nunez and Pilgreen, 1991). Thus, the CSD topographic map is a spatially enhanced representation of current generators with more sharply localized peaks than those of the scalp potential, while eliminating volume-conducted contributions from distant regions and sources (Tenke and Kayser, 2012). CSD topography thus represents reference-free estimates of radial current flow
