Human alpha oscillations in wakefulness, drowsiness period, and REM sleep: different electroencephalographic phenomena within the alpha band

Cortical oscillations in the range of alpha activity (8–13 Hz) are one of the fundamental electrophysiological phenomena of the human electroencephalogram (EEG). Evidence from quantitative EEG data has shown that their electrophysiological features, cortical generation mechanisms, and therefore, their functional correlates vary along the sleep-wake continuum. Specifically, spectral microstructure and EEG coherence levels between anterior and posterior cortical regions permit to differentiate among alpha activity spontaneously appearing in relaxed wakefulness with eyes closed, drowsiness period, and REM sleep, by reflecting distinct properties of neural networks involved in its cortical generation as well as a different interplay between cortical generators, respectively. Besides, the dissimilar spatio-temporal features of brain electrical microstates within the alpha range reveals a different geometry of active neural structures underlying each alpha variant or, simply, changes in the stability level of neural networks during each brain state. Studies reviewed in this paper support the hypothesis that two different alpha variants occur during human REM sleep: ‘background responsive alpha activity’, blocked over occipital regions when rapid eye movements are present, and ‘REM-alpha bursts’, non modulated by the alternation of tonic and phasic periods. Altogether, evidence suggests that electrophysiological features of human cortical oscillations in the alpha frequency range vary across different behavioural states, as well as within state, reflecting different cerebral phenomena with probably dissimilar functional meaning. Les oscillations corticales dans la bande alpha (8–13 Hz) sont un des phénomènes électrophysiologiques fondamentaux de lˈelectroencephalogramme humain (EEG). LˈEEG quantifié a montré que les caractéristiques electrophysiologiques, les mécanismes de genèse corticale et les corrélats fonctionnels de cette activité varient le long du continuum veille–sommeil. En particulier, la microstructure spectrale et les niveaux de cohérence entre régions corticales antérieures et postérieures permettent de différencier lˈactivité alpha qui apparaît au cours de la veille calme, yeux fermés, de celle observée en stade de somnolence ou en sommeil paradoxal (REM). Ceci reflète probablement les propriétés différentes des réseaux neuronaux impliqués dans leur genèse respective, ainsi que les rapports différents entre générateurs corticaux. En dehors de cela,