Working memory deficits under high cognitive load in older adults with insomnia disorder: associations with frontoparietal control network connectivity and glymphatic function

Insomnia disorder (ID) is associated with cognitive impairment, but the neuroimaging correlates of high-load working memory deficits in older adults remain unclear. We recruited 80 older adults, including 40 participants with ID and 40 healthy controls (HCs) matched for age, sex, and education. Working memory was assessed using digit span and N-back tasks. Resting-state functional magnetic resonance imaging was used to characterize functional connectivity (FC) within the frontoparietal control network (FPCN). Glymphatic function was assessed using the diffusion tensor imaging analysis along the perivascular space (DTI-ALPS) index. We compared groups, examined brain–behaviour associations, and tested whether the DTI-ALPS index moderated the association between functional connectivity within the FPCN and high-load working memory performance. Compared with HCs, participants with ID showed lower backward and total digit-span scores, reduced 2-back accuracy, and longer reaction times during the 1-back and 2-back conditions. They also showed significantly reduced FC between the left lateral prefrontal cortex and the left posterior parietal cortex. In addition, the ID group exhibited a lower DTI-ALPS index, indicating glymphatic dysfunction. Moderation analysis further showed that glymphatic function, as indexed by DTI-ALPS, moderated the association between left lateral prefrontal cortex–left posterior parietal cortex FC and 2-back accuracy. These findings link high-load working memory vulnerability in older adults with ID to altered functional connectivity within the FPCN and lower DTI-ALPS values. This multimodal neuroimaging profile provides a framework for investigating sleep-related cognitive vulnerability.