Modality-General Sensitivity of Pupil Responses to Regularity Violations

Pupil dilation responses are reliable physiological markers of arousal in reaction to unexpected events across sensory modalities. However, a direct comparison across modalities has not yet been conducted. In this study, we address this gap by examining pupil responses to regular and random sequences of visual dots and auditory tones, as well as transitions between these sequences. In Experiment 1, we showed dilation responses when a regular sequence of visual dots changed to a random sequence, but not to the reverse transition. A transition between two different regular sequences led to less dilation on average. In Experiment 2, we replicated these findings, confirming the reliability of the observed dilation patterns. In Experiment 3, we compared the pupil responses when sequences and transitions were presented in visual versus auditory modalities. We observed strong cross-modal similarity in pupil sizes, particularly for transitions between regular and random sequences. Notably, these similarities persisted even during baseline trials, when no explicit transition occurred. We also decomposed the time-series pupil size to approximate phasic pupil dilation events. The patterns of dilation events aligned with the overall dilation, with modality-specific dilation event size differences primarily in the context of transitions between different regular sequences. Overall, our findings suggest that pupil-linked arousal reflects imperfect inference of statistical structure and its violations, exhibiting substantial similarity across modalities.