Pupil responses to pitch deviants reflect predictability of melodic sequences.

Humans automatically detect events that, in deviating from their expectations, may signal prediction failure and a need to reorient behaviour. The pupil dilation response (PDR) to violations has been associated with subcortical signals of arousal and prediction resetting. However, it is unclear how the context in which a deviant occurs affects the size of the PDR. Using ecological musical stimuli that we characterised using a computational model, we showed that the PDR to pitch deviants is sensitive to contextual uncertainty (quantified as entropy), whereby the PDR was greater in low than high entropy contexts. The PDR was also positively correlated with the unexpectedness of notes. No effects of music expertise were found, suggesting a ceiling effect due to enculturation. These results show that the same sudden environmental change can lead to differing arousal levels depending on contextual factors, providing evidence for a sensitivity of the PDR to long-term context.