Psychometric characteristics of resting frontal EEG asymmetry also support a trait-level individual difference, with excellent internal consistency (Towers & Allen, 2009) and modest test-retest reliability in both depressed and non-psychiatric samples (Allen et al., 2004b; Hagemann, Naumann, Thayer, & Bartussek, 2002; Tomarken, Davidson, Wheeler, & Kinney, 1992), and approximately 60% of the variance representing stable trait variance (e.g., Hagemann et al., 2002). Several EEG studies, however, have failed to confirm an association between left frontal hypoactivity and depression (e.g., Bruder et al., 1997; Pizzagalli et al., 2002; Reid, Duke & Allen, 1998), and although some evidence suggests that this pattern of asymmetrical frontal activity remains stable in the midst of fluctuating symptom severity (Allen et al., 2004b), there exists some evidence to the contrary (Debener et al., 2000). Before resting frontal EEG asymmetry can be considered a viable endophenotype for depression risk in a particular subset of individuals, these inconsistencies must be addressed.
