Brain neural synchronization and functional coupling in Alzheimer's disease as revealed by resting state EEG rhythms

Alzheimer's disease (AD) is the most frequent neurodegenerative disorder and cause of dementia along with aging. It is characterized by a pathological extracellular accumulation of amyloid-beta peptides that affects excitatory and inhibitory synaptic transmission. It also triggers aberrant patterns of neuronal circuit activity at the network level. Growing evidence shows that AD targets cortical neuronal networks related to cognitive functions including episodic memory and visuospatial attention. This is partially reflected by the abnormal mechanisms of cortical neural synchronization and coupling that generate resting state electroencephalographic (EEG) rhythms. The cortical neural synchronization is typically indexed by EEG power density. The EEG coupling between electrode pairs probes functional (inter-relatedness of EEG signals) and effective (casual effect from one over another electrode) connectivity. The former is typically indexed by EEG spectral coherence (linear) or synchronization likelihood (linear-nonlinear), the latter by Granger causality or information theory indexes. Here we revised resting state EEG studies in mild cognitive impairment (MCI) and AD subjects as a window on abnormalities of the cortical neural synchronization and functional and effective connectivity. Results showed abnormalities of the EEG power density at specific frequency bands (<12Hz) in the MCI and AD populations, associated with an altered functional and effective EEG connectivity among long range cortical networks (i.e., fronto-parietal and fronto-temporal). These results suggest that resting state EEG rhythms reflect the abnormal cortical neural synchronization and coupling in the brain of prodromal and overt AD subjects, possibly reflecting dysfunctional neuroplasticity of the neural transmission in long range cortical networks.