Pupil behaviour across contexts: from resting oscillations to social reactivity

Pupil diameter is shaped by central arousal mechanisms, autonomic drive and the biomechanical constraints of the iris. Although tonic pupil size is often treated as an index of baseline arousal, pupillary dynamics reflect partially dissociable physiological axes. We aimed to characterise this network organisation in a group of young adults and to test whether pupillary profiles can be identified within the population. Sixty-seven healthy adults (18-31 years old; 35 females) completed three pupillometric blocks under controlled ambient illumination using a Tobii Pro Fusion eye-tracker (250 Hz): fixation (2-3 min) to quantify spontaneous oscillations (hippus), a pupillary light reflex (PLR) block to characterise pupil integrity and kinematics, and dynamic emotional face videos to elicit socio-affective dilation. We extracted hippus indices (baseline, frequency, amplitude), PLR metrics (baseline, amplitude, latency, constriction velocity, recovery), and face-evoked dilation measures (baseline, amplitude, latency). Cross-parameter associations were tested using Spearman's rank correlations with correction for multiple comparisons. Robustly normalised features were then clustered using k-means, and cluster separability was assessed with one-versus-rest Receiver Operating Characteristic Curve (ROC) analyses to evaluate clustering discrimination using Area Under Curve (AUC). Tonic measures of pupil activity were strongly coupled across paradigms, with hippus baseline correlating with baseline during face videos. Baseline pupil diameter was also associated with reflex amplitudes, consistent with a peripheral contribution. Clustering revealed three distinct profiles with good discrimination (AUC > 0.80): a high-tonus/fast profile (short PLR latency, high constriction velocity), a hypo-reactive profile (low tonic level with attenuated PLR and face responses), and a slow/hyper-dilated profile (delayed PLR kinetics with large face-evoked dilation). Overall, pupil dynamics grouped into three profiles, consistent with an interplay between the tonic central state, autonomic kinematics and iris-related mechanical constraints. This classification cautions against single-mechanism interpretations of pupillary measures, supports individual stratification, and motivates multimodal studies to disentangle central arousal from peripheral gain.