al., 2007, depicting the shift and polarity inversion of a nose-referenced N1/N2 complex from inferior-lateral sites to mid-centroparietal sites when using a linked-mastoids reference), which may also influence which electrode sites are selected for statistical analysis. A particular bias is introduced with an asymmetric (unilateral) reference, which has the inherent capacity to modulate (i.e., amplify or decrease) existing hemisphere asymmetries. Whereas some researchers are aware of these problems (e.g., Baving et al., 2000, note the absence of negative ERP deflections in their data as a consequence of activity at the averaged-mastoids reference), others seem to erroneously assume that ERP condition or group effects observed at a particular recording sites reflect neuronal activity of the underlying brain regions. However, depending on the orientation of the underlying equivalent current generator responsible for a particular ERP deflection, which may differ between experimental groups or conditions, the (arbitrary) reference choice may mask existing effects if the reference region is itself differentially affected through volume-conducted activity by the experimental manipulations (e.g., Nunez and Westdorp, 1994). This indeterminancy is not resolved by using an average reference, which has the additional caveat of being different for different EEG montages and, due to practical limits of sampling from
