The confounding effect of a common reference is not entirely resolved with what are called reference-free recordings. The bipolar EEG, obtained by subtracting the potentials of two nearby electrodes, will remove all signals common to the two channels, including the common reference. In addition, a given bipolar montage will completely miss dipoles with certain locations and tangential orientations, and not all signals common to the two electrodes are from the reference. Caution against the use of bipolar EEG for coherence analysis was given [36], [37]. Although the average reference EEG and Laplacian EEG are reference free, caution against their use for synchronization analysis was given [32].
