The quest for the EEG reference revisited: A glance from brain asymmetry research

Recent findings suggested that the choice of the EEG reference might be a c ritical issue for the study of anterior asymmetry in the alpha band. The pr esent paper investigates the validity of different reference schemes for th e measurement of alpha asymmetry. A 32-channel resting EEG was recorded wit h a common vertex reference (Cz), and transformed into computer-averaged ea rs (A1 + A2), average reference (AR), and current source density derivation s. A correlation analysis of an alpha asymmetry measure between all derivat ion schemes indicated a poor convergent validity for anterior sites but an excellent convergent validity for posterior sites. Further analyses suggest ed the presence of substantial alpha activity at the various reference site s (Cz, Al + A2, AR), which might be similar in magnitude to anterior but sm aller than posterior alpha. These findings suggest that the validity of a r eference scheme is a function of the signal-to-noise ratio of the electrica l activities at target and reference sites. The limitations of each referen ce scheme for the measurement of anterior alpha asymmetry are discussed.