But what about the average reference (e.g., Lehmann and Skrandies, 1980)? It has been argued that if the volume that generates the electrical signal (i.e., the brain case) is measured at many different surface locations and from all directions (i.e., by using a high-density EEG montage of 128 or more channels), the sum of the recorded EEG activity will approximate zero, and therefore, the average of this activity will approximate an inactive reference (e.g., Bertrand et al., 1985). A related argument pertains to the requirement of inverse solution algorithms that the net source activity within the brain sums to zero at each measurement (sample) point (e.g., Murray et al., 2008). Of course, given the lack of surface area, it is not possible to place EEG electrodes ventrally to the brain case, thereby rendering this theoretical requirement a practical impossibility. Another issue that arises from using an EEG reference computed from the measures obtained at all scalp locations is that it will vary with each EEG montage, depending on the montage density and specific scalp locations used. In any case, however,
