Background: Mild cognitive impairment (MCI) is a stage between expected age-related cognitive decline and dementia. Dementias have been associated with changes in neural oscillations across the frequency spectrum, including the alpha range. Alpha is the most prominent rhythm in human EEG and is best detected during awake resting state (RS). Though several studies measured alpha power and synchronization in MCI, findings have not yet been integrated.
Objective: To consolidate findings on power and synchronization of alpha oscillations across stages of cognitive decline.
Methods: We included studies published until January 2020 that compared power or functional connectivity between 1) people with MCI and cognitively healthy older adults (OA) or people with a neurodegenerative dementia, and 2) people with progressive and stable MCI. Random-effects meta-analyses were performed when enough data was available.
Results: Sixty-eight studies were included in the review. Global RS alpha power was lower in AD than in MCI (ES = -0.30; 95% CI = -0.51, -0.10; k = 6), and in MCI than in OA (ES = -1.49; 95% CI = -2.69, -0.29; k = 5). However, the latter meta-analysis should be interpreted cautiously due to high heterogeneity. The review showed lower RS alpha power in progressive than in stable MCI, and lower task-related alpha reactivity in MCI than in OA. People with MCI had both lower and higher functional connectivity than OA. Publications lacked consistency in MCI diagnosis and EEG measures.
Conclusion: Research indicates that RS alpha power decreases with increasing impairment, and could-combined with measures from other frequency bands-become a biomarker of early cognitive decline.
Keywords: Alpha rhythm; brain waves; dementia; electroencephalography; mild cognitive impairment.