Healthy ageing and cognitive impairment alter EEG functional connectivity in distinct frequency bands

Abstract

Functional connectivity (FC) indicates the interdependencies between brain signals recorded from spatially distinct locations in different frequency bands, which is modulated by cognitive tasks and is known to change with ageing and cognitive disorders. Recently, the power of narrow‐band gamma oscillations induced by visual gratings have been shown to reduce with both healthy ageing and in subjects with mild cognitive impairment (MCI). However, the impact of ageing/MCI on stimulus‐induced gamma FC has not been well studied. We recorded electroencephalogram (EEG) from a large cohort (N = 229) of elderly subjects (>49 years) while they viewed large cartesian gratings to induce gamma oscillations and studied changes in alpha and gamma FC with healthy ageing (N = 218) and MCI (N = 11). Surprisingly, we found distinct differences across age and MCI groups in power and FC. With healthy ageing, alpha power did not change but FC decreased significantly. MCI reduced gamma but not alpha FC significantly compared with age and gender matched controls, even when power was matched between the two groups. Overall, our results suggest distinct effects of ageing and disease on EEG power and FC, suggesting different mechanisms underlying ageing and cognitive disorders.

Stimulus-induced gamma sources weaken but not shrink with healthy aging in human EEG

“…In other words, an overall weakening of gamma sources may have much less effect on FC compared to shrinkage. As shown previously, gamma FC reduced with age even when sensor power was accounted for (Kumar and Ray, 2023), which is more consistent with shrinkage rather than weakening of gamma sources. On the other hand, the relationship between power and FC may depend on factors beyond just source distribution.…”

“…In a previous study, we found that the reduction in FC with age persisted even after accounting for variation in sensor power (Kumar et al, 2023). We therefore tested whether weakening (or potential shrinkage) of gamma sources could be observed after accounting for the variation in sensor power with age.…”

“…Apart from power measured using EEG sensors, various ensemble features like functional connectivity (FC) (Kumar and Ray, 2023) and related compensatory mechanisms (Pathak et al, 2022) may also change with healthy aging, albeit in a different way compared to power (Murty et al, 2020). For example, FC decreases with aging in alpha and stimulus-induced gamma oscillations, even though alpha power does not and even when variation in gamma power is accounted for using regression (Kumar and Ray, 2023). Similarly, FC at Individual Alpha Peak Frequency (IAPF) may stay invariant with aging due to a compensatory mechanism where enhanced inter-areal coupling cancels the effect of increased axonal delays (Pathak et al, 2022).…”

“…To compare these alternatives, we considered the same EEG dataset reported in previous studies (Murty et al, 2020;Murty and Ray, 2022;Kumar and Ray, 2023) and performed brain source localization of visual gratings-induced gamma, along with alpha, using eLORETA (Exact Low-resolution Tomography Analysis) source reconstruction method.…”

Enhanced stimulus-induced and stimulus-free gamma in open-eye meditators

“…Both slow and fast gamma weaken with age ( 18 ) and are abnormal in patients suffering from brain disorders like Alzheimer’s Disease (AD) ( 19 ) and Schizophrenia ( 20 ). In addition, connectivity across brain regions reduces with onset of AD predominantly in slow gamma ( 21 ).…”