Transdiagnostic patterns of electroencephalography ( EEG )‐derived aperiodic activity across schizophrenia‐spectrum, bipolar, and depressive disorders: A systematic review

Aperiodic electroencephalographic activity is proposed as a scalable marker of cortical population dynamics relevant to excitation–inhibition balance. However, it remains unclear whether aperiodic parameters represent disorder‐specific abnormalities or context‐sensitive indices of clinically relevant brain states. Consistent with systematic‐review guidelines and a prospectively registered protocol, Ovid databases and PubMed were searched from inception through February 9, 2026, with updated manual searching conducted through July 14, 2026. Peer‐reviewed studies reporting electroencephalography‐derived aperiodic parameters in adults with schizophrenia‐spectrum disorders, major depressive disorder, treatment‐resistant depression, and/or bipolar disorder were eligible. Studies lacking aperiodic outcomes or separable findings for eligible diagnoses were excluded. Two reviewers independently conducted study screening, data extraction, and risk‐of‐bias assessments. Findings were synthesized narratively by diagnosis, recording context, and parameter. Thirty articles ( N  = 2298 aggregate participants) spanning cross‐sectional, longitudinal, and interventional designs were included. Findings in schizophrenia‐spectrum disorders were heterogeneous, with several studies reporting steeper slopes, higher offsets, or reduced temporal flexibility relative to healthy controls. Task‐related and dynamic measures often showed greater clinical relevance than static resting‐state parameters. Studies of major depressive disorder and treatment‐resistant depression more consistently reported flatter slopes and reduced offsets, particularly in central and posterior regions, with evidence from sleep and intracranial recordings. Aperiodic parameters were sensitive to treatment‐related modulation, although the direction of change differed across interventions. Bipolar disorder evidence was limited, and methodological heterogeneity precluded quantitative synthesis. Aperiodic electroencephalographic activity may be more informative as a context‐sensitive, within‐subject marker of cortical state and treatment‐related change than a fixed diagnostic signature. Methodological harmonization and direct physiological validation remain necessary before clinical implementation.