TASK-EVOKED BUT NOT STIMULUS-EVOKED PHASIC PUPIL-LINKED AROUSAL PREDICTS MEMORY SUCCESS

Brainstem arousal systems including the locus coeruleus noradrenaline system, respond transiently to behaviorally relevant events. Locus coeruleus activity also drives dilations of the pupil, which are often observed during cognitive tasks. Mounting evidence shows that 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, rather than merely correlative, effects of phasic, pupil-linked arousal on cognition. Here, we evaluated whether task-irrelevant white noise sounds affect memory formation and/or memory-based decisions.
In two long-term memory tasks using images or spoken words as memoranda, task-irrelevant white-noise sounds were played before, during or after the presentation of the memoranda. Memory was measured in recognition and free recall tasks the day after, and we tracked participants’ pupil diameter throughout.
Trial-to-trial variations in the amplitude of pupil dilations during word encoding without task-irrelevant sounds predicted memory success. Task-irrelevant white-noise sounds also robustly dilated the pupil but did not improve memory formation. Trial-to-trial variations of pupil responses during recognition were monotonically related to participants’ decision bias and non-monotonically (“inverted U-shape”) to sensitivity. But, again, the task-irrelevant sounds had no impact on the decision behavior.
We conclude that pupil-linked arousal processes triggered by task-irrelevant sounds differ from those recruited endogenously during memory formation or decisions from memory, for example in states of increased emotionality or attention.