Pupil dilation responses are reliable physiological markers of arousal in response to unexpected events. We investigated how these responses generalise across different sensory modalities by using transitions between regular and random sequences of visual dots and auditory tones. In Experiment 1, we investigated sequences of visual dots and found that (a) transitions from a regular sequence to a random sequence induced pupil dilations, (b) transitions from one regular sequence to another regular sequence also induced pupil dilations, and (c) transitions from a random sequence to a regular sequence did not reliably induce them. In Experiment 2, we replicated these findings, confirming their reliability and thereby generalizing the literature from the auditory to the visual modality. In Experiment 3, we directly compared pupil dilations in visual and auditory modalities. We observed strong cross-modal similarity in pupil sizes, particularly for transitions between regular and random sequences. We also decomposed the pupil size time series to approximate phasic pupil dilation events. While the patterns of dilation events were quite similar, differences between modalities in dilation size (but not in rates) occurred during transitions from one regular to another regular sequence. Overall, our findings suggest that pupil-linked arousal reflects inference of statistical structure and its violations, exhibiting substantial (albeit not perfect) similarity across modalities.
