EEG Spectral Power Across Rest, Body Scan, and Task Conditions in Autism and Schizophrenia.

State-dependent neural dynamics are central to models of large-scale brain regulation in neurodevelopmental and psychotic disorders, but it remains unclear whether autism spectrum disorder (ASD) and schizophrenia (SCH) show altered oscillatory activity across internally and externally oriented cognitive states. This study examined EEG spectral power across resting, interoceptive, and cognitive conditions in adults with ASD, SCH, and matched neurotypical controls. Absolute and relative power were calculated for canonical frequency bands to distinguish overall power differences from spectral redistribution; group and condition effects were analysed while controlling for covariates. Robust condition effects emerged across frequency bands. Absolute delta power decreased during interoception compared to rest and task, while relative delta power was lowest during rest, suggesting spectral reweighting. The most prominent group effect was a condition-invariant elevation of theta power in SCH, suggesting a baseline shift with preserved state-dependent modulation. The ASD group showed modest detectable spectral differences. Condition-dependent modulation patterns were broadly similar across groups. Alpha, beta, and gamma activity were primarily condition-driven with less group differences. Group-by-condition interactions, altered state-dependent oscillatory modulation were not reliably detectable. Absolute and relative EEG power captured partly distinct spectral characteristics across conditions and groups.