Keeping an eye on attentive listening: Task-evoked pupil size unveils fluctuations in auditory sustained attention

Sustained attention is essential for effective cognitive performance in daily life, yet it remains challenging to monitor continuously in real time. This study examined whether pupil-based physiological markers - specifically task-evoked pupil dilation (TEPD) and baseline pupil size (BPS) - reflect fluctuations in sustained attention and alertness during a prolonged auditory continuous performance task (A-CPT). In two experiments (total N = 54), participants listened to a stream of spoken syllables and responded to a predefined target while their pupil size was continuously recorded. Intra-individual variability in reaction time served as an index of sustained attention, with higher variability reflecting poorer sustained attention. Across both experiments, a consistent positive linear relationship was found between TEPD and intra-individual varability: smaller TEPD values were associated with greater response consistency, indicating higher sustained attention. Additionally, TEPD was positively correlated with self-reported ADHD symptoms across participants, and contributed to their prediction above and beyond the behavioral task performance. This suggests that TEPD reflects inattention, hyperactivity and impulsivity symptoms across neurotypical individuals. In contrast, BPS showed a less consistent relationship with sustained attention: while it was positively associated with intra-individual varability in one experiment, this effect did not replicate in the second. These findings suggest that pupil size can serve as a continuous, implicit marker of sustained attention and a predictor of ADHD symptoms. By capturing moment-to-moment fluctuations in attentional state, pupil size offers a promising, non-invasive index of attention in real-world listening environments.