Reduced Pupil-Linked Arousal Marks Implicit Memory for Complex Auditory Sequences

Observers rapidly form long-lasting implicit memories for recurring sensory patterns, supporting adaptive behaviour through learning of environmental contingencies. Here we tested how such memories interact with neuromodulatory systems that regulate arousal--whether implicit familiarity increases arousal through retrieval-related processes or instead decreases arousal via more efficient sensory processing. We examined these competing accounts using complex auditory tone-pip sequences in which a subset of patterns recurred approximately every three minutes. One group of participants (N=30) actively detected transitions from random to regular structure, whereas a second group (N=40) heard the same sequences while performing a decoy task. Consistent with prior work, recurring regular sequences elicited faster reaction times in the active group, confirming the formation of implicit memory. Subsequently, both groups listened to previously encountered regular patterns (REGr) and matched novel regular patterns (REGn) while pupil diameter was recorded. Across groups, REGr sequences evoked significantly smaller pupil dilations than REGn sequences, indicating attenuated arousal for familiar patterns. This reduction was sustained until sequence offset in the active group but was transient in the exposure group, suggesting that task demands during exposure shape the stability of pupil-linked memory expression. Importantly, the REGr-REGn pupil difference did not correlate with explicit post-session recognition, demonstrating that the effect was not driven by conscious awareness. Together, these findings identify pupil dynamics as a robust physiological marker of learned auditory structure and show that implicit auditory sequence memory attenuates arousal. The results support predictive processing accounts in which familiarity reduces uncertainty and, consequently, neuromodulatory drive. Significance StatementHumans rapidly acquire implicit memories for recurring sounds, yet how such memories influence arousal during subsequent exposure remains poorly understood. Using pupillometry, we show that implicitly remembered auditory sequences evoke reduced pupil-linked arousal relative to matched novel sequences, even when listeners are unaware that the patterns have been previously experienced. This effect dissociates from explicit recognition, which typically yields the opposite "old/new" pattern of larger pupil responses to familiar items. Together, these findings establish pupil dynamics as a sensitive, non-invasive marker of implicit auditory memory and support predictive processing accounts in which familiarity reduces uncertainty and, consequently, neuromodulatory drive.