| Cognition is regulated across the 24-h sleep-wake cycle by circadian rhythmicity and sleep homeostasis. To better understand these mechanisms, we used functional neuroimaging (PET, fMRI, EEG/fMRI) both during wakefulness and sleep in normal human volunteers . In a first set of studies, we characterized the neural correlates of cognition during a normal sleep-wake cycle and during sleep-loss. The data show that dynamic changes in brain responses evolve across the sleep-wake and circadian cycles in a regionally-specific manner in such a way that the allocation of cortical resources through subcortical activation is constrained by sleep pressure and circadian phase. In a second set of studies, we characterized the neural correlates of the main non rapid eye movement (NREM) sleep oscillations. For instance, slow waves were associated with increased responses in several cortical and subcortical structures, supporting the view that NREM sleep is an active state during which brain activity is synchronized to the slow oscillation in specific cerebral regions. In a further study, we show that spontaneous brain activity (i.e., spindles) has a major influence on cortical responses to external stimuli. A last set of studies aimed at showing that the regional brain activity is modified by previous experience, especially during NREM sleep following spatial learning. These studies provide some insights on the interaction between sleep and waking activity, and their influence on regional brain activity in humans. |
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