Temporal variability in pupillary asymmetry reflects ADHD-related traits in preschool and early school-aged children

Attention-deficit/hyperactivity disorder (ADHD) often emerges in early childhood; however, objective validated biomarkers for its early detection remain limited. In this study, we aimed to identify pupil-based candidate physiological markers (i.e., physiological correlates) associated with ADHD-related traits in preschool- and early school-aged children. To this end, we recorded the pupil diameters of typically developing children without an ADHD diagnosis during a simple fixation task. From these data, we extracted multiple features, including the mean pupil size, temporal variability, and interocular asymmetry in both magnitude and variability. ADHD-related tendencies were assessed using the ADHD Rating Scale-5. Among the features analyzed, only the temporal variability of interocular asymmetry (VarLRdiff) showed a significant positive correlation with ADHD Rating Scale-5 total and subscale scores. This association likely reflects the combined effects of immature autonomic regulation and functional asymmetry in the neural circuits involving the locus coeruleus. In contrast, other pupil-based indices showed no significant correlations, potentially because of developmental ceiling effects and the short evaluation duration. These findings suggest that VarLRdiff may serve as a promising candidate physiological marker of ADHD-related traits in young children, requiring further validation. Future studies incorporating longer recordings, advanced analytical methods, and evaluation of test–retest stability, as well as longitudinal follow-up, are warranted to evaluate its potential utility for early screening, pending clinical and longitudinal validation.