Scalp EEG reveals functional dissociable aperiodic timescales in divergence of mental health

Aperiodic neural activity is increasingly used as a marker of brain health, yet it remains unclear whether this signal reflects a single neural timescale or separable slow and fast processes with distinct relevance to ageing and disease. Here, using resting-state scalp EEG from approximately 1,700 participants across healthy, neurological and psychiatric disorders and chronic-pain cohorts, we show that spectral knees recovered from individual spectra converge into two reproducible population-level components. These slow and fast aperiodic timescales showed distinct functional profiles: the slow component remained comparatively stable across healthy ageing, brain-related disorders and chronic pain, whereas the fast component increased with healthy ageing, decreased in brain-related disorders and was unchanged in chronic pain. These findings establish that scalp EEG preserves functionally dissociable slow and fast aperiodic timescales, possibly reflecting body-cognitive interactions, and suggest that the fast component may serve as a scalable, non-invasive marker of brain ageing and dysfunction.