The CSD transform computes an estimate of the surface Laplacian based on the EEG voltage
values across the scalp electrodes that have valid channel positions (radius larger than zero).
The unit of the output CSD data is �V/m�. An estimate of the CSD is calculated by applying the
spherical Laplace operator to the EEG voltage distribution on the surface of the head for each
data point in time. This entails the computation of the second spatial derivative of the EEG voltage
assuming spherical head geometry.