There is no single technique that has proven to be ideal to study the interaction between two brain signals from scalp recordings. Coherence measures are susceptible to both volume conduction and electrode reference effects [101]–[103], although in the present study both effects were minimized through calculating coherence from closely spaced bipolar electrode pairs [102]. This strategy renders these confounding influences negligible for close bipolar pairs separated from one another by more than 4–5 cm [104]–[106], although volume conduction still may increase coherence for shorter distances depending upon the frequency band and the orientation of the dipole source [103]. It is highly unlikely, however, that any of the differences reported between the MDD and healthy control groups in the current study would arise from volume conduction or reference effects because the electrode montage and recording techniques were identical for both depressed and control groups. Nevertheless, future studies also should consider use of surface Laplacian [107] and Independent Component Analysis (ICA) [108]–[109] EEG methods, as well as phase synchrony [110] connectivity measures, that may help further minimize the effects of volume conduction.
