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, the computed average reference is, like any other reference scheme, a single value (constant) that is subtracted from the values obtained at each recording site for each measurement point (e.g., a fixed waveform in case of ERPs).
