EEG alpha reactivity in cognitive aging and dementia: clinical implications and cholinergic mechanisms.

Alpha reactivity-the attenuation of the alpha rhythms upon eye-opening-is a well-known phenomenon in electroencephalography (EEG). Altered alpha reactivity has increasingly been recognized as a potential biomarker for cognitive aging and various types of dementia. This mini-review synthesizes the existing literature on EEG alpha reactivity in older adults, highlighting its clinical implications and neurobiological underpinnings. Methodological issues in quantifying alpha reactivity are first addressed, including the choice of reactivity index, frequency bands, and spatial analysis methods. The review then summarizes evidence that alpha reactivity declines with healthy aging and is further reduced in dementia, especially Alzheimer's disease (AD) and Lewy body dementia (LBD). Importantly, distinct patterns of reduced alpha reactivity may aid in differential diagnosis. It has been reported that AD is often characterized by reduced alpha rhythm during eyes-closed rest, whereas LBD typically shows impaired alpha attenuation upon eye-opening. Neuroimaging and pharmacological studies suggest that alpha reactivity reflects cholinergic system integrity, particularly involving the nucleus basalis of Meynert and its projections to the visual cortex. In conclusion, EEG alpha reactivity is a promising, non-invasive biomarker of cognitive health and dementia subtypes. However, larger, harmonized, multimodal and longitudinal studies are needed to establish its diagnostic value and clarify its neurobiological mechanisms.