Disentangling Arousal and Attentional Contributions to Pupil Size: Toward Simultaneous Estimation of Emotional and Cognitive States

Pupillary response serves as a noninvasive physiological marker of autonomic activity, modulated by both emotional arousal and the luminance of attended visual stimuli. However, these factors often co-occur, making it difficult to disentangle their individual contributions. This study investigated how auditory emotional stimuli and attention-guided changes in luminance independently influence pupil dynamics. Participants were instructed to direct their visual attention to either white or black moving dot patterns, and to shift their attention upon the onset of an emotional sound. Pupil responses were z-normalized on a per-trial basis, and optokinetic nystagmus (OKN) was recorded to verify attentional shifts. Results showed that emotional arousal robustly increased pupil size across conditions, while valence had no significant main effect. Pupil size was also modulated by the luminance of the attended stimulus, reflecting parasympathetic influences. No significant interaction between arousal and valence was observed, suggesting additive rather than interactive effects. These findings emphasize the methodological necessity of controlling attentional luminance in pupillometric studies and highlight the potential of pupil-based measures for emotion-aware system design. Crucially, because pupil dynamics reflect both luminance-guided attention and emotional arousal via distinct autonomic pathways, combining pupillometry with nonverbal indicators of attention--such as optokinetic nystagmus (OKN)--may enable the simultaneous estimation of attentional focus and emotional state in real time.