Power and functional connectivity of alpha oscillations in mild cognitive impairment: A systematic review and meta‐analysis

Background Alzheimer’s disease (AD) is sometimes preceded by a less severe decline in cognitive abilities called mild cognitive impairment (MCI). Having affordable and non‐invasive biomarkers of MCI would ease diagnosis, improve prognosis, and open up possibilities to delay the transition to AD. One potential biomarker is alpha oscillations measured with electroencephalography (EEG) that have been found to differ between cognitively intact older adults (OA) and people with AD. To find whether these changes can already be detected in MCI, we reviewed studies that compared alpha power and functional connectivity in people with MCI and OA, in people with MCI and people with AD, and in people with MCI who progress to AD and those who do not. Method We searched for records published until 24 January 2020 using search terms “mild cognitive impairment” AND (eeg OR oscillations). 65 studies were included in the review. We conducted four random effects meta‐analyses. We compared resting state power in people with MCI and OA in the full alpha band (∼ 8‐12 Hz; k=5; n=198 MCI, 258 OA), in lower alpha frequencies (∼ 8‐10 Hz; k=5; n=196 MCI, 203 OA), and in upper alpha frequencies (∼ 10‐12 Hz; k=5; n=131 MCI, OA 139). We also compared resting state power in the full alpha band in people with AD and people with MCI (k=6; n=481 AD, 499 MCI). Result In the full alpha band, resting state power was lower in people with AD than in people with MCI (ES=‐0.30, 95% CI=‐0.51, ‐0.10, p=0.003), and in people with MCI than in OA (ES=‐1.49, 95% CI=‐2.69, ‐0.29, p=0.02). Reviewing the literature showed that resting state alpha power is lower in people with MCI who eventually progress to AD than in those who do not. During task performance, alpha activity changed less in people with MCI than in OA. Studies that compared functional connectivity between the three groups reported inconsistent results. Conclusion Lower resting state alpha power, that has been found in people with AD can, though to a lesser degree, also be detected in people with MCI, especially those who are on a trajectory towards AD.