Fig. 1.

Example of the extremal heuristic. Three dipoles in an arbitary source space are considered. If this space is small in relation to the distance from the sensors then the left-most and right-most sources will have essentially identical lead fields, as they share the same orientation. Whereas the central source is distinct. The extremal heuristic in this case will be two (as there are only 2 unique extremal pairs) even though there are three sources in the space (and, in the random field context, all neighbours will have uncorrelated residuals).