Temporal processing variability as a scalable marker of Parkinson's disease progression.

Parkinson's disease (PD) is associated with disruptions in the temporal organization of motor and non-motor behavior, likely arising from dysfunction within basal ganglia-thalamocortical circuits. Despite substantial evidence for impaired timing in PD, the relationship between temporal processing variability, disease progression, and established clinical severity markers remains poorly defined. We conducted a systematic analysis of empirical studies to quantify temporal processing variability in PD using the coefficient of variation (CV), a standardized index that enables comparison across diverse timing tasks. CV values were directly extracted or derived from published studies spanning a range of paradigms. Analyses focused on associations between CV, participant age, and disease severity as indexed by Unified Parkinson's Disease Rating Scale (UPDRS) scores and Hoehn and Yahr (H&Y) stages. Results revealed a positive correlation between temporal processing CV and UPDRS score, independent of age, suggesting that increased temporal variability is associated with disease severity beyond normal aging effects. In contrast, CV correlated negatively with H&Y stage and age, a pattern that may reflect differences in measurement properties of severity indices, variability in study characteristics, and medication state during behavioural testing rather than clinical staging. Secondary analyses found no reliable differences in CV across task type, temporal range, or medication status, although interpretation of null effects is limited by heterogeneity across studies and sample sizes. Together, these results indicate that temporal processing variability is associated with clinical markers of Parkinson's disease severity and may represent a potential target for future longitudinal and mechanistic research.