Spatial organization of EEG activity from alertness to sleep stage 2 in old and younger subjects

In order to elucidate brain mechanisms that contribute to the increased tendency for vigilance dysregulation in the elderly, we examined the spatial organization of brain electric activity [electroencephalogram (EEG)] during decreasing vigilance from alertness to onset of sleep stage 2, comparing 7 old and 10 younger, healthy subjects (60-79 and 18-41 years old, respectively). Two features were analyzed: (1) change of location of the brain electric source gravity centers of the EEG frequency bands, and (2) magnitude of fluctuation of these locations over time. Multichannel EEG was analyzed into source gravity center localizations for seven EEG frequency bands, using fast Fourier transform (FFT) Dipole Approximation (first principal component-single source modeling in the frequency domain). Multivariate analysis of covariance (MANCOVA) showed: source localizations were more anterior in old than younger subjects for beta-1 and more superior for all three beta bands; from alertness to sleep, delta and theta EEG sources (inhibitory activity) changed to more posterior and superior areas, and alpha-1 and -2 (routine activity) and beta-1 and -2 sources (excitatory activity) towards anterior and superior areas. Fluctuations of the source locations of delta and beta-2 were larger on the superior--inferior axis, and of beta-2 smaller on the left-right axis in the old than younger subjects. The results suggest functional specifications (inhibitory, routine, excitatory) of cortical positron emission tomography (PET) changes reported in sleep. In sum, aging exhibits changes in spatial organization of EEG-generating neuronal assemblies; during the transition wakefulness-to-sleep, aging affects the spatial-temporal dynamics of this organization. The latter is suggested to contribute to the increased risk for consciousness disturbances in the elderly.