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

## TL;DR

The study shows that pupil dilation reflects how the brain tracks unexpected sounds and stable contexts during listening.

## Contribution

The paper introduces a novel use of Bayesian modeling to link pupil dilation with tonal surprisal and contextual precision in dynamic auditory environments.

## Key findings

- Pupil dilation reflects both tonal surprisal and contextual precision in auditory contexts.
- Transitions between stable low-entropy contexts cause significant pupil dilation.
- High-entropy transitions do not evoke strong pupil responses.

## Abstract

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.

Using pupil dilation and Bayesian modeling, we show that listeners track both momentary unexpectedness and the reliability of auditory contexts.Model‐based analyses reveal that tone‐by‐tone surprisal and contextual precision jointly shape arousal across dynamic auditory environments.Notably, pupil responses increased following transitions between stable, low‐entropy contexts, but not during shifts involving high‐entropy states.

Using pupil dilation and Bayesian modeling, we show that listeners track both momentary unexpectedness and the reliability of auditory contexts.

Model‐based analyses reveal that tone‐by‐tone surprisal and contextual precision jointly shape arousal across dynamic auditory environments.

Notably, pupil responses increased following transitions between stable, low‐entropy contexts, but not during shifts involving high‐entropy states.