Stimulus-induced Gamma rhythms are weaker in human elderly with Mild Cognitive Impairment and Alzheimer’s Disease

Abstract

Alzheimer's Disease (AD) in elderly adds substantially to socio-economic burden necessitating early diagnosis. While recent studies in rodent models of AD have suggested diagnostic and therapeutic value for gamma rhythms in brain, the same has not been rigorously tested in humans. We recruited a large population (N=247; 106 females) of elderly (>49 years) individuals from the community, who viewed large gratings that induced strong gamma oscillations in their electroencephalogram (EEG). These individuals were classified as healthy (N=227), mild-cognitively-impaired (MCI; 14) or AD (6) based on clinical history and Clinical Dementia Rating scores. Surprisingly, stimulus-induced gamma rhythms, but not alpha or steady-state-visually-evoked-responses, were significantly lower in both MCI and AD patients compared to their age and gender matched controls. This reduction was not due to differences in eye movements or baseline power. Our results suggest that gamma could be used as potential diagnostic tool for MCI/AD in humans.

Stimulus-induced narrowband gamma oscillations are test-retest reliable in healthy elderly in human EEG

“…All analyses were performed using bipolar reference scheme. Every electrode was re-referenced offline to its neighbour, yielding 112 bipolar pairs of electrodes from the 64 unipolar electrodes (Murty et al, 2020, 2021). The bipolar pairs among the visual electrodes considered for analysis (except for scalp maps) were: PO3-P1, PO3-P3, POz-PO3 (left anterolateral group); PO4-P2, PO4-P4, POz-PO4 (right anterolateral group) and Oz-POz, Oz-O1, Oz-O2 (posteromedial group).…”

“…Their cognitive performance was evaluated by a set of tests – ACE (Addenbrooke’s Cognitive Examination), CDR (Clinical Dementia Rating) and HMSE (Hindi Mental State Examination). More details have been provided in previous reports (Murty et al, 2020, 2021). Out of the 257 subjects recruited in the first year, we discarded data of 10 subjects due to noise.…”

“…Experimental setup and details of recordings have been explained in detail previously (Murty et al, 2020, 2021) and are briefly summarised here. EEG was recorded from 64-channel active electrodes (actiCap) using BrainAmp DC EEG acquisition system (Brain Products GMbH) and were placed according to the international 10-10 system.…”

“…In addition, gamma oscillations can be induced by presenting certain visual stimuli such as bars and gratings, and their amplitude and centre frequency dependent on the stimulus properties such as size, contrast, and spatial frequency (Henrie and Shapley, 2005; Gieselmann and Thiele, 2008; Ray and Maunsell, 2010; Jia et al, 2013; Murty et al, 2018). Recently, we have shown that these stimulus-induced narrowband gamma oscillations recorded using electroencephalogram (EEG) in elderly subjects (>49 years) weaken with age (Murty et al, 2020) and are weaker in subjects with Mild Cognitive Impairment (MCI) and early Alzheimer’s Disease (AD) compared to their age and gender matched controls (Murty et al, 2021). These results suggest that stimulus-induced gamma oscillations recorded using EEG could potentially be an inexpensive and easily accessible biomarker for diagnosis of mental disorders such as AD.…”

“…To address these limitations, we tested the reliability of visually induced narrowband gamma oscillations recorded using EEG in healthy elderly (aged > 49 years) with an interval of one year or more. This dataset is a part of double-blind case-control EEG study (N=257; 106 females), part of which has been used earlier to show that gamma oscillations weaken with healthy ageing (Murty et al, 2020) and with MCI/AD (Murty et al, 2021). Fifty-two subjects were retested with the same visual paradigm after a period of approximately one year.…”