Age-related differences in memory encoding as a function of pupillary dynamics

Pupillary dynamics are known to depend on tonic and phasic activity of the brainstem locus coeruleus (LC), a key neuromodulatory nucleus. The LC shows a decline in its structural integrity in older age that likely impacts long-term memory. While several studies have explored the link between pupil dilation and successful memory encoding, little is known about the effects of aging on this relationship. This study investigated pupil dynamics in young and older adults during a rapid categorization task that allowed incidental encoding of neutral visual scenes. Pupil responses were compared based on subsequent recognition performance assessed 24 h later. Our findings notably revealed that categorization success was associated with smaller pre-stimulus pupil size. Moreover, phasic pupil responses were attenuated in older individuals, possibly reflecting impaired LC-driven modulation with age. Pupil dilation was associated with memory performance in young adults only, with subsequently recognized stimuli eliciting greater dilation during encoding. Furthermore, recollected scenes evoked larger pupillary responses than those remembered with a sense of familiarity. Importantly, this pattern was not observed in older adults. We propose that an impairment in LC-driven neuromodulation during scene encoding may partly account for these age-related differences in pupil responses. Our findings highlight pupillometry as an interesting technique to assess physiological correlates of cognitive function in aging.