Tracking the developing brain: Resting-state aperiodic activity reveals nonlinear cortical maturation in externalizing disorders.

Previous research has linked psychiatric disorders to alterations in the aperiodic exponent-a measure of excitatory-inhibitory (E/I) balance derived from resting-state EEG that reflects broadband, non-oscillatory brain activity. This measure also appears sensitive to age-related neurodevelopmental changes. In this study, we investigated whether the aperiodic exponent varies with age in children and adolescents diagnosed with adjustment disorder (AD) or oppositional defiant disorder (ODD), compared to age-matched healthy controls. While healthy participants showed no significant association between age and the aperiodic exponent, both clinical groups exhibited a pronounced nonlinear relationship. Specifically, the exponent was lower in early childhood and early adulthood, but peaked around 9-10 years of age. This U-shaped developmental trajectory suggests a deviation from normative brain maturation in AD and ODD and points to temporally specific alterations in cortical E/I balance. These findings underscore the potential of the aperiodic exponent as a developmentally sensitive neural marker of externalising psychopathology and may inform age-tailored diagnostic and intervention strategies in child and adolescent psychiatry.