Although a recent meta-analysis suggests a moderately robust association exists between depression (and related constructs) and relatively less left frontal activity (Thibodeau, Jorgensen, & Kim, 2006), participants in EEG asymmetry investigations have been predominantly female (Coan & Allen, 2004; Thibodeau et. al, 2006). It thus remains to be determined how reliable and stable this pattern is for depressed men, especially given that studies including substantial male samples (not included in the meta-analysis) are inconsistent, one indicating that male depression is associated with relatively more left frontal activity (Miller et al., 2002), and one demonstrating the opposite pattern (Jacobs & Snyder, 1996). Small patient samples have also prevented within-study comparisons of EEG activity between currently depressed individuals and euthymic individuals with a history of depression (with the exception of Gotlib et al., 1998), a contrast needed to test the assertion that EEG asymmetry is a trait-like marker of depression risk. Conflicting results across studies may also be due to heterogeneity of depressed samples, potentially attributable to types of comorbid anxiety that are associated with different patterns of brain asymmetry than those displayed by non-anxious depressed individuals (e.g., Heller & Nitschke, 1998).
