Impact of cognitive load on pupil dilation: A comparative study of young and older adults

Age-related cognitive decline increases the risk of neurodegenerative disorders and contributes to loss of autonomy in older adulthood. The noradrenergic system, particularly the locus coeruleus-noradrenergic (LC-NA) network, plays a crucial role in sustaining cognitive function, and its dysfunction has been implicated in age-related cognitive deterioration. Pupillary responses during cognitively demanding tasks provide a non-invasive index of LC-NA system integrity, as they closely reflect cognitive effort and resource allocation. In the present study, younger and older adults performed an auditory working memory task involving the immediate oral recall of digits sequences of varying lengths (3, 6, or 9-digits) while pupil size was continuously recorded. The data showed that performance declined with increasing sequence length across both groups. However, under higher cognitive load (6 and 9-digit conditions), older adults exhibited significantly reduced pupillary reactivity compared to younger participants. Moreover, pupil dilation correlated with both age and Stroop interference scores, indicating an association between pupillary dynamics and executive (inhibition) control. These findings suggest that although working memory performance was largely preserved in older adults, their attenuated pupillary responses to increasing task demands may reflect early functional alterations in the LC-NA system. Pupillary reactivity thus emerges as a promising, non-invasive biomarker of cognitive control and age-related neural integrity.