Objective: We examined the hypothesis that cognitive dysfunction in Alzheimer's disease is associated with abnormal spontaneous fluctuations of EEG synchronization levels during an eyes-closed resting state. Methods: EEGs were recorded during an eyes-closed resting state in Alzheimer patients (N=24; 9 males; mean age 76.3 years; SD 7.8; range 59-86) and non-demented subjects with subjective memory complaints (N=19; 9 males; mean age 76.1 years; SD 6.7; range: 67-89). The mean level of synchronization was determined in different frequency bands with the synchronization likelihood and fluctuations of the synchronization level were analysed with detrended fluctuation analysis (DFA). Results: The mean level of EEG synchronization was lower in Alzheimer patients in the upper alpha (10-13 Hz) and beta (13-30 Hz) band. Spontaneous fluctuations of synchronization were diminished in Alzheimer patients in the lower alpha (8-10 Hz) and beta bands. In patients as well as controls the synchronization fluctuations showed a scale-free pattern. Conclusions: Alzheimer's disease is characterized both by a lower mean level of functional connectivity as well as by diminished fluctuations in the level of synchronization. The dynamics of these fluctuations in patients and controls was scale-free which might point to self-organized criticality of neural networks in the brain. Significance: Impaired functional connectivity can manifest itself not only in decreased levels of synchronization but also in disturbed fluctuations of synchronization levels. © 2004 International Federation of Clinical Neurophysiology. Published by Elsevier Ireland Ltd. All rights reserved.
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