Optimization of the source derivations from the scalp surface.

The selectivity of source derivations (SD) is computer-calucla ted using different combinations of electrodes and different interelectrode differences, with a view to the optimization of the method. For this purpose, the method described by Nunez and Katznelson (1981) is used to calculate surfaces in the brain tissue from the points of which a single current source will determine the same potential in a given point on the scalp surface. The lines of these surfaces are given for different planes. The calculations show that the selectivity increases with the decrease of the interelectrode distances, as well as when more electrodes are used. With a small number of electrodes for SD and especially when one central and two lateral electrodes are used, there exist brain areas in which the generators cause a potential with opposite sign on the scalp surface.