Pupil dilation indexes the metrical hierarchy of unattended rhythmic violations

When we listen to music, we perceive regularities that drive our expectations. This is reflected in beat perception, in which a listener infers a regular pulse from a rhythm. However, it is still an open question whether attention to the music is necessary to establish the perception of a hierarchy of stronger and weaker beats, or meter. In addition, to what extent beat perception is dependent on musical expertise is still unknown. We addressed these questions by measuring the pupillary response to omissions at different metrical positions in drum rhythms, while participants attended to another task. We found that the omission of the most salient first beat elicited a larger pupil dilation than the omission of the less salient second beat. These results show that participants perceived stronger and weaker beats without explicit attention to the music, suggesting that hierarchical beat perception is an automatic process that requires minimal attentional resources. In addition, we found that this perception of meter was independent of musical expertise. Finally, our results show that pupil dilation reflects surprise without explicit attention, demonstrating that the pupil is an accessible index to unattentive processing