Cognitive adaptations for memory deficits in MCI and AD patients: A meta-analysis of EEG microstates.

AIM: EEG microstate analysis enables the exploration of the brain's neuronal network activity associated with distinct mental states and cognitive functions in neurodegenerative conditions, such as Alzheimer's disease (AD). Some studies indicated that AD and patients with mild cognitive impairments (MCI) show a reduced presence of microstate C, which is related to self-related memory functions and mind-wandering and involves brain areas of the default mode network (DMN). However, other studies reported an increased presence of microstate A, which is associated with auditory/language functions. Here, we aimed to systematically investigate alterations in the four canonical EEG microstates (A, B, C, and D) in MCI and AD patients compared to healthy older adults. METHODS: We performed a meta-analysis that compared microstate temporal parameters - mean duration, occurrence rate, and time coverage - among MCI and AD patients and healthy older adults. We included 12 experimental studies that examined resting-state, eyes-closed EEG microstate parameters in 1347 participants (448 MCI patients, 514 AD patients, and 385 healthy controls). RESULTS: We found thatAD patients showed increased duration and time coverage of microstate A and increased duration of microstate B, while the occurrence of microstates C and D was reduced. MCI patients also exhibited increased duration, occurrence, and time coverage of microstate A, while all three parameters of microstate D were reduced compared to healthy controls. CONCLUSION: These results suggest that MCI and AD patients suffer from disruption in their cognitive control, memory, and self-referential processes. They may compensate for these deficits by verbalizing and visualizing their inner thoughts to maintain cognitive engagement.