Task-irrelevant stimuli robustly drive phasic pupil-linked arousal but only weakly affect memory formation

Brainstem arousal systems including the locus coeruleus noradrenaline system, respond transiently to behaviorally relevant events. This also drives dilations of the pupil, which are often observed during cognitive tasks. The strength of pupil responses during encoding of stimulus material predicts the success of its later retrieval, which might reflect the impact of noradrenaline on synaptic plasticity and memory formation. The pupil also dilates in response to task-irrelevant sounds, which could therefore serve as a valuable tool for investigating causal effects of phasic, pupil-linked arousal on cognition. Here, we evaluated whether task-irrelevant white noise sounds affect memory formation and memory-based decisions. These sounds were played before, during or after the presentation of memoranda (images or spoken words). Memory success was measured in recognition and free recall tasks the day after. Trial-to-trial variations in the amplitude of pupil dilations during word encoding in the absence of task-irrelevant sounds predicted memory success. Task-irrelevant white-noise sounds also robustly dilated the pupil but did not yield strong evidence of improved memory success: later recall of words was unaffected, and recall of images was weakly boosted only when the sounds were presented concurrently with them. Our results suggest that pupil-linked arousal processes triggered by task-irrelevant sounds differ from those recruited endogenously during memory formation.