The scalp Laplacian as an estimate of local cortical source .activity

An investigation into the relationship between scalp recorded EEG data and the underlying source generators using an analytic three concentric spheres model is presented. Correlation coefficients were computed for scalp potential and scalp Laplacian due to randomly distributed conical sources using electrode pairs of varying separation. Direct comparisons of scalp potential and Laplacian with underlying cortical potential were also made using highly correlated cortical sources. In the case of scalp potentials, electrodes 6 cm apart showed a squared correlation coefficient of 0.25 due to volume conduction done. Use of the Laplacian resulted in correlations approaching zero for 3 cm electrode spacing and random sources. Direct comparison of analytic scalp recorded data with local cortical potential showed that the scalp Laplacian is much more highly correlated with underlying source activity than scalp potential. Spline based estimates of the scalp Laplacian using 64 electrodes were shown to be in close agreement with the analytic Laplacian. The implications for clinical applications are discussed.