Daily dynamics of resting-state EEG theta and gamma fluctuations are associated with cognitive performance in healthy aging.

Objectives: Healthy age-related cognitive changes are highly heterogeneous across individuals. This variability is increasingly explained through the lens of spontaneous fluctuations of brain activity, now considered as powerful index of age-related changes. However, brain activity is a biological process modulated by circadian rhythms, and how these fluctuations evolve throughout the day is under investigated.

Methods: We analyzed data from one hundred and one healthy late middle-aged participants from the Cognitive Fitness in Aging study (68 women and 33 men; aged 50-69 years). Participants completed five EEG recordings of spontaneous resting-state activity on the same day. We used weighted phase-lag index (wPLI) analyses as an index of the functional synchrony between brain regions couplings and we computed daily global PLI fluctuation rates of the five recordings to assess the association with cognitive performance and β-amyloid and tau/neuroinflammation pathological markers.

Results: We found that theta and gamma daily fluctuations in the salience-control executive inter-network (SN-CEN) are associated with distinct mechanisms underlying cognitive heterogeneity in aging. Higher levels of SN-CEN theta daily fluctuations appear to be deleterious for memory performance and were associated with higher tau/neuroinflammation rates. In contrast, higher levels of gamma daily fluctuations are positively associated with executive performance, and were associated with lower rate of β-amyloid deposition.

Discussion: Thus, accounting for daily EEG fluctuations of brain activity contributes to better understand subtle brain changes underlying individuals' cognitive performance in healthy aging. Results also provide arguments for considering time-of-day when assessing cognition for old adults in a clinical context.