Resting EEG as a condition-dependent ageing phenotype

Resting-state electroencephalography is increasingly proposed as a scalable marker of healthy brain ageing, yet its interpretability depends on distinguishing stable individual characteristics from condition-dependent measurement effects. This secondary analysis of an open lifespan dataset examined periodic alpha activity, alpha-peak detectability, aperiodic spectral structure, short-term repeatability, and longitudinal change under eyes-open and eyes-closed conditions before and after an extended cognitive battery. Eye state strongly organised both whether a posterior alpha peak could be detected and the values of periodic and aperiodic measures. Age-related trajectories were prominent for the aperiodic exponent and full-sample alpha activity, whereas evidence that age modified post-battery change was weak. Across follow-up, detectable alpha frequency and the aperiodic exponent showed mean decreases, while alpha power was comparatively stable. Level measures demonstrated moderate-to-high repeatability and rank-order stability, but post-battery and difference-in-differences change scores were considerably less reliable. The findings position resting EEG as a condition-dependent ageing phenotype rather than a unitary baseline. They further show that peak detection, recording context, attrition, and the reliability of change must be modelled explicitly when resting EEG is used for lifespan inference or normative applications.