State-modulation of cortico-cortical connections underlying normal EEG alpha variants

Normal electroencephalographic (EEC) alpha variants appear during relaxed w akefulness with closed eyes, drowsiness period at sleep onset, and rapid ey e movement (REM) sleep in bursts without arousal signals. Previous results revealed that fronto-occipital and fronto-frontal alpha coherences became w eaker from wakefulness to drowsiness, and finally to REM sleep. The present work was aimed at determining whether a generalized or a unidirectional de activation of the long fronto-occipital fasciculi, previously proposed to b e involved in the alpha rhythm generation, could explain the above-mentione d results. Polynomial regression analyses, applied to the change of alpha c oherence with distance along the antero-posterior axis, suggested that the anterior and posterior local circuits show a similar level of activation in all brain states. Bivariate partial correlation analyses between local alp ha coherences revealed that such local circuits maintain a reciprocal depen dency during wakefulness, but unidirectional during drowsiness (anterior-to -posterior, A-P) and REM sleep (posterior-to-anterior, P-A). From these fin dings, both anterior and posterior cortical structures are suggested as bei ng involved in the generation of the three alpha variants. If the implicati on of a double cortical generation source (anterior and posterior) of alpha variants is assumed, these two generators seem to maintain a mutual inter- dependency during wakefulness, whereas during the transition to human sleep , the anterior areas work quite independently of the posterior regions. Fin ally, the occipital structures may be the driving force for the REM-alpha b ursts generation, since involvement of frontal regions demonstrated a high dependence on the posterior neural circuits in the genesis of this sleep ev ent. (C) 2000 Elsevier Science Inc. All rights reserved.