The surface Laplacian operator (second spatial derivative of the voltage distribution in tissue) calculates the volume current flow
out of the brain through the skull into the skin. That is why we use the term Current Source Density (CSD). The CSD montage
will show a large signal if the cortical surface (convexity) is active. This corresponds to a radial superficial dipole.
If the brain activity is in a fissure (tangential dipole) the CSD signal is much weaker. The current flows in a tangential direction,
so you will see two peaks of opposite polarity in the CSD maps, corresponding to where the volume current is leaving (+) and
entering (-) the skull. The two zones of maximum current are closer together than in voltage maps, because voltage is an integral
over current source density.
Units of current source density are voltage per unit distance per unit distance (e.g. �V/cm�) or current per unit area (e.g. �A/cm�).
Conversion from the second derivative of the voltage to current units is a complex issue because this requires knowledge about
conductivities of the skull and scalp surfaces. Therefore, �V/cm� is used, i.e. just the second derivative of the voltage distribution.