Tonal Surprisal and Contextual Shifts Evoke Distinct Pupil Dilation During Dynamic Sound Sequences

The human brain continuously forms predictions about the unfolding sensory environment, relying on contextual information to anticipate upcoming events while remaining sensitive to unexpected changes. This study examined how pupil-linked phasic arousal, a putative proxy for the locus coeruleus-norepinephrine system, reflects the interplay between tonal surprisal (unexpectedness) and precision (reliability of the inferred context) in dynamic auditory contexts. Twenty-eight participants passively listened to stochastic tone sequences transitioning between periods of low-entropy (informative context) and high-entropy (less informative context). We quantified tone-by-tone surprisal and precision using Bayesian modeling. Despite their slow time evolution, pupil dilation responses revealed sensitivity to both surprisal and precision, showing that arousal tracks momentary deviations and the stability of contextual predictions. Analyses of context boundaries showed that transitions between distinct low-entropy environments (LE-dLE) evoked significant pupil dilation, whereas shifts between low- and high-entropy environments (LE-HE and HE-LE) did not. These findings indicate that pupil-linked arousal primarily responds to salient contextual shifts involving stable environments rather than to changes in entropy per se. The results emphasize the role of the locus coeruleus-norepinephrine system in adaptive model updating during passive listening and demonstrate the brain's continuous and implicit monitoring of uncertainty to navigate dynamic auditory environments.