Waveform Asymmetry as a Biomarker of Neural Aging: Spatial Degradation of Peak-Trough Dynamics Across the Adult Lifespan

Background: The shape of each neural oscillatory cycle - quantified by peak-trough asymmetry (PTA), the ratio of rise to fall time - may reflect excitatory-inhibitory balance in the generating circuit. Whether waveform shape degrades with aging is unknown. Methods: We measured broadband PTA across five frequency bands in resting-state EEG from 215 adults (age 20-77; LEMON dataset) and replicated all findings in 608 adults (age 20-70; Dortmund Vital Study), including 208 with 5-year longitudinal follow-up. Results: (1) Asymmetry is band-specific and spatially structured: slow rhythms show excitatory-type asymmetry while fast rhythms show inhibitory-type asymmetry, with a spatial double dissociation between posterior alpha and central beta. (2) Beta-band asymmetry decreases with age (LEMON: r = -0.326, d = 0.69; Dortmund: r = -0.314, d = 0.75), surviving control for aperiodic slope and strongest at central electrodes - opposite to electromyographic contamination. Longitudinal data confirmed the predicted direction (mean delta = -0.017) with moderate test-retest reliability (r = 0.45). (3) Spatial organization of asymmetry degrades with age, and theta spatial entropy predicts memory performance independent of age. Conclusion: Waveform shape provides a power-independent measure of neural aging that complements existing spectral biomarkers. Code: https://github.com/mool32/waveform-asymmetry-agingData: LEMON (https://fcon_1000.projects.nitrc.org/indi/retro/MPI_LEMON.html); Dortmund Vital Study (https://www.dcn.sowi.tu-dortmund.de/cms/de/dortmund_vital_study/index.html)