Trial-level pupil dynamics reflect attention-related variability and medication-associated modulation in ADHD

Children with Attention-Deficit/Hyperactivity Disorder (ADHD) often exhibit rapid fluctuations in attention that are difficult to capture using standard behavioural assessments. We investigated whether trial-level pupil dynamics provide a physiological signal associated with task performance during a working-memory task and whether these responses differ with stimulant medication condition. Eye-tracking data were analysed from 50 children (28 ADHD, 22 Ctrl; mean age 10.6 years) performing a visuospatial Sternberg task. Trial-level pupil signals were characterised using inter-trial variance, within-trial variability (SD), and task-locked mean pupil responses. A hierarchical location–scale model estimated approximately 23% greater residual inter-trial variance in Off-ADHD than in Ctrl after adjustment for memory load, age, and IQ. The corresponding participant-level difference was statistically significant before correction but did not remain significant after FDR correction \\((q=.051)\\) . Within-trial variability was lower, rather than higher, in Off-ADHD than in Ctrl. Inter-trial variance was negatively associated with task accuracy in the Off-ADHD condition, although the strength of this association did not differ significantly from Ctrl. The On-ADHD condition was associated with an increased relative probe-window pupil response and reduced inter-trial variance within participants. However, these differences cannot be interpreted exclusively as cognitive medication effects because direct pharmacological and session-related contributions could not be fully excluded. Trajectory-based analyses further showed that the On-ADHD group-mean pupil trajectory was closer to Ctrl than the Off-ADHD trajectory under the DTW analysis. These findings suggest that trial-level pupil dynamics provide complementary physiological information about variability and task performance in ADHD, while remaining an indirect and non-specific measure of attention-related processing.