between lifetime MDD+ and MDD− groups for these faces are partially influenced by facial activity, consistent with assertions that alpha power asymmetries may be vulnerable to contributions from facial muscle activity (e.g., Davidson, 1998; Friedman & Thayer, 1991. Although Coan et al. (2001) did not find that EMG asymmetry accounted for EEG asymmetry during the DFA task in healthy participants, they could not determine whether EMG accounted for between-subjects differences as they did not enroll depressed subjects in that study.4 Moreover, robustness of findings for the withdrawal condition when shared EMG-alpha variance was removed may also be due to greater frontal non-EMG residualized alpha power evident in both MDD+ and MDD− participants during the withdrawal condition than approach and rest conditions (see Figure 1). Although group asymmetry differences across conditions for CSD-transformed data were reduced to marginal significance when shared EMG-alpha variance was removed, this finding is difficult to interpret because CSD may not accurately measure EMG activity. The CSD algorithm estimates radial current flow into and out of the skull from underlying neural tissue (Tenke & Kayser, 2005) and, as such, is not designed to provide a meaningful estimate of potentials that originate from muscles overlaying the skull.
