Major depressive disorder (MDD) is a heterogeneous condition characterized by deficits in emotional, cognitive and motor functioning. Commensurate with its symptomatic complexity, recent conceptualizations view MDD as a systems-level disorder that arises from dysregulation among large-scale functional brain networks [1–4]. Connectivity among these networks has been commonly probed by examining the correlation in blood oxygen level-dependent (BOLD) fluctuations between brain regions under task-free conditions using functional magnetic resonance imaging (fMRI). However, since this is limited to the speed of the hemodynamic response, fMRI-based connectivity is restricted to frequencies below 1 Hz, and it is unclear whether higher frequency neuronal synchronization contributes to connectivity disturbances in depression. This is important because it has been posited that each functional network may be characterized by a unique electrophysiological signature [5, 6], and the spectral specificity of this electrophysiological signature may represent a way in which the brain builds a hierarchical structure of interconnected networks [7]. Accordingly, differences in the spectral properties of resting-state networks in depression may point to differences in the hierarchical organization of these networks, which may underpin differences in the cross-talk between networks.
