Resting magnetoencephalography alterations in severe mental illnesses: A systematic review

Schizophrenia, major depressive disorder (MDD), and bipolar disorder (Severe Mental Illnesses: SMIs), share genetic risk and brain structural changes, but their neural mechanisms remain unclear. Magnetoencephalography (MEG), with millisecond precision, enables direct examination of shared and disorder-specific disturbances in the timing of spontaneous brain activity. We reviewed resting-state MEG evidence on spectral power, connectivity, information-theoretic complexity, and brain dynamics across SMIs. A comprehensive search yielded 62 studies. Three reviewers screened, retrieved, and extracted data on MEG-related variables across regional activity (power), coordination of activity (connectivity, coherence, synchrony), regularity of activity (complexity, entropy), and overall dynamics. Three principal findings come forth. Beta oscillations emerge as a promising, albeit yet unconfirmed, candidate transdiagnostic signal, with increased power now reported across independent investigations of all three disorders and correlating with hallucinations, sleep disturbances, and cognitive deficits, consistent with disrupted predictive processing. However, direct within-study cross-diagnostic comparisons remain sparse (2 of 62 studies). Preliminary support exists for complexity-age dissociation to partition SMIs into neuroprogressive and state-dependent trajectories: neural complexity declines with age in schizophrenia and bipolar disorder (opposite to healthy aging), while depression preserves the normative trajectory. Slow-wave activity diverges sharply: schizophrenia shows a widespread increased slow-wave dominance, while mood disorders show a decrease in activity. We propose a temporal disorganisation spectrum of SMI from excessive fragmentation (schizophrenia) to pathological rigidity (depression), with bipolar disorder occupying an intermediate position. This framework offers biomarker-type utility for stratifying illness severity, tracking neuroprogression, and identifying transdiagnostic therapeutic targets, particularly beta-modulatory neuromodulation interventions applicable across SMIs. Protocol registration: Open Science Framework (http://osf.io/v2gtb/).