Despite the fact that EEG has excellent temporal resolution, its spatial resolution (i.e., the information about the specific location or source of the recorded neuroelectric activity) is poor, due to the “blurring effects” of volume conduction, as the head acts like a low-pass spatial filter, transmitting to the scalp broad, as opposed to focal, spatial patterns of activity (Srinivasan et al., 1998b). The EEG activity recorded from each scalp electrode does not represent the specific activity of local brain sources (i.e. directly underneath the electrode), but the “volume-conducted” activity from multiple spatially dispersed sources. While this problem of removing or reducing the volume conduction effects has been a huge challenge for EEG technology, several methods have been devised to spatially represent the local effects of the brain sources responsible for the recorded scalp potentials. One such method is to calculate the current source density (CSD) or the Laplacian (second spatial derivative) of the scalp voltage using one of a number of specific algorithms.
