Local arousals during sleep.

Increasing evidence suggest that human sleep is a global phenomenon in which locally modulated behaviors occur. Topographic differences in the distribution of Slow Wave Activity (SWA), the marker of sleep homeostasis, are evident. Moreover experimental studies showed that SWA can be selectively enhanced or dimished in confined brain regions suggesting that certain brain regions may sleep ‘‘deeper’’ than others. Clinical observations also indicate that sleep and wakefulness could occur simultaneously in different parts of the brain. Here we show that two distinct region of the brain, the pre-frontal cortex and the motor cortex, though mantaining similar global dynamics of SWA during NREM sleep, can exhibit dissociated behaviors. We analysed sleep EEG in five drug resistant epileptic patients studied with intracerebral implanted electrodes during presurgical investigation. Each subject had at least two electrode contacts which could be localized unequivocally within the pre-frontal region and the motor cortex. Scalp EEG was recorded from Fz-Cz. The pre-frontal and motor cortex showed the expected cycle by cycle decreasing trend of SWA (as observed on scalp EEG). Nevertheless, expecially at sleep onset and during the transition towards REM sleep, an uncoupling of EEG rhythms could appear lasting till some minutes, with wakefulness-like EEG activity (alpha and beta waves) in the motor cortex and sleep activity (delta waves) in the pre-frontal cortex. Moreover both visual scoring analysis and wavelet transform analysis of the EEG showed that many brief arousals occurring at the level of the motor cortex, lasting less than 15 seconds and characterized by the occurrence of alpha and beta activity, were accompanied by an evident opposite increase in delta activity over the prefrontal cortex suggesting an anti-arousal reaction that may protect the continuity of sleep. These findings may give new insights for the comprehension of arousal regulation during NREM sleep and for the explanation of anomalous sleep/wake phenomena (sleep walking, misperception insomnia).