Probing locus coeruleus functional network in healthy aging and its association with Alzheimer's disease biomarkers using pupillometry.

BACKGROUND

Alzheimer's disease (AD) is the leading cause of dementia, and the early detection of the disease-associated changes allows early interventions. The locus coeruleus (LC) has been reported to be the first brain region to develop tau pathology in AD. However, the functional brain network of the LC in both healthy aging and AD pathology is largely unknown due to technical difficulties associated with the small size of the LC. In this study, we used the measurement of spontaneous pupil constriction/dilation as a surrogate for LC activity to study LC brain network changes during healthy aging.

METHODS

Thirty-seven healthy younger and thirty-nine healthy older adults were included from the Emory Healthy Brain Study and underwent resting-state functional MRI while simultaneously tracking pupil diameter. The measurements of pupil diameter dynamics were used as reference signals in brain connectivity analysis. The connectivity of the identified networks was then compared between younger and older participants. Correlations of the identified regions with neuropsychological assessments and cerebrospinal fluid (CSF) biomarkers were also evaluated.

RESULTS

A brain network of 20 clusters associated with pupil diameter dynamics was identified, including the LC as well as brain regions functionally connected to the LC. The pupil diameter network was found to positively correlate with the salience network and negatively correlate with the central executive network. Functional connectivity decreased within the pupil diameter network with healthy aging. The pupil diameter connectivity was associated with memory, executive, and visuospatial functioning. CSF total tau closely correlated with pupil diameter network.

CONCLUSIONS

Pupil diameter dynamics provide valuable insights into LC-related processes. While they are not solely influenced by LC activity, spontaneous pupil constrictor/dilatory activity shows promise as a non-invasive approach to probe the LC network and warrants further studies to evaluate its value as an early biomarker of AD.