The local regulation of human sleep: anatomo-functional bases and implications for behavior

The traditional view of sleep and wakefulness as two distinct and mutually exclusive states has been recently challenged by the discovery that they actually are locally regulated and that islands of sleep- and wake-like activity may often coexist in a same individual at a given time. Importantly, it has been suggested that the local regulation of sleep may be involved in many of the essential functions of sleep in physiological conditions. Particular attention has been given to the study of the so-called 'slow waves' of sleep, which represent in particular the main hallmark of nonrapid eye movement (NREM) sleep. In fact, local changes in slow wave activity have been shown to occur in brain regions that are more actively used during wakefulness, ultimately reflecting wake- and experience-dependent plastic processes. In addition, electrophysiological events similar to sleep slow waves have been found to occur also during wakefulness and have been suggested to reflect neuronal functional fatigue and the accumulation of sleep need. Of note, while experimental research has started to shed light on the mechanisms involved in the local regulation of sleep-like activity, and on how they affect cognition and behavior, many aspects still remain to be fully clarified. Given these premises, in the present Thesis, my aim was to advance the current knowledge on the local regulation of sleep in humans, with a specific focus on slow-wave-like activity. To this aim, I performed three different experiments. In the first study, I investigated the role of cortico-cortical white matter connections in the generation and propagation of sleep slow waves. To this aim I analyzed overnight high density (hd)EEG data collected in an extremely rare population of ‘split-brain’ patients and in two additional groups including neurologic patients and healthy control subjects. Obtained results demonstrated that the traveling of sleep slow waves is significantly affected by the resection of the corpus callosum, which leads to a reduced proportion of crosshemispheric slow waves. This result demonstrates that the way sleep slow waves propagate can inform us regarding the status of brain connections and may thus offer a valuable marker for functional or structural alterations caused by traumatic or neurodegenerative disorders. On the other hand, our analyses showed that the lack of inter-hemispheric connections is not associated with dissociations characterized by sleep rhythms in one hemisphere and wake-like activity in the other half of the brain. In addition, while we found that sleep slow waves tend to originate more often in the right than in the left hemisphere, such an asymmetry was found not to differ between split-brain patients and subjects with an intact corpus callosum. Overall, these results indicate that global state changes are coherently modulated across the cortical mantle by non-cortical (bottom-up) mechanisms. In two additional studies, I investigated the local regulation of sleep-like activity during wakefulness and its possible effects on cognition and behavior. In particular, in one experiment I applied a single-subject multi-session design to explore whether the regional distribution of morning-to-evening increases in local sleep-like activity is dependent on the degree of experiencedependent activation or rather it mainly reflects inter-regional differences in vulnerability to neuronal fatigue. In fact, it has been shown that low-frequency power increases during wakefulness and decreases after a night of sleep, and such changes are on average more pronounced over frontal areas. Our results showed that changes in low-frequency activity may peak in different brain regions. In particular, we observed at least two main morning-toevening variation patterns: one, more common and stronger, involving centro-frontal cortical areas, and one, less common, mainly involving sensory cortices. This observation does not support an inherent vulnerability of frontal areas and is instead potentially compatible with a use/experience-dependent regulation of electrophysiological indices reflecting functional fatigue and sleep need. Finally, I investigated whether the occurrence of local sleeplike episodes may influence behaviors with a social relevance, such as the ability to regulate one’s own emotional reactions. In particular, my aim was to test whether the occurrence local sleeplike activity within brain areas involved in emotional regulation could account for failures in the suppression of emotional expressions. Obtained results demonstrated, for the first time, that sleep-like activity in frontal and parietal areas precede emotion regulation failures. Moreover, I found that the incidence of behavioral failures is negatively correlated with a shorter sleep duration the night preceding the experiment, in line with previous evidence linking local sleep-like episodes and sleep loss. Taken together, these results indicate that transient, local 'neuronal sleep' may represent a direct functional cause of impairment in complex and socially relevant human behaviors

Origin, synchronization, and propagation of sleep slow waves in children

Study Objectives: Sleep slow wave activity, as measured using EEG delta power (<4 Hz), undergoes significant changes throughout development, mirroring changes in brain function and anatomy. Yet, age-dependent variations in the characteristics of individual slow waves have not been thoroughly investigated. Here we aimed at characterizing individual slow wave properties such as origin, synchronization, and cortical propagation at the transition between childhood and adulthood. Methods: We analyzed overnight high-density (256 electrodes) EEG recordings of healthy typically developing children (N = 21, 10.3 ± 1.5 years old) and young healthy adults (N = 18, 31.1 ± 4.4 years old). All recordings were preprocessed to reduce artifacts, and NREM slow waves were detected and characterized using validated algorithms. The threshold for statistical significance was set at p = 0.05. Results: The slow waves of children were larger and steeper, but less widespread than those of adults. Moreover, they tended to mainly originate from and spread over more posterior brain areas. Relative to those of adults, the slow waves of children also displayed a tendency to more strongly involve and originate from the right than the left hemisphere. The separate analysis of slow waves characterized by high and low synchronization efficiency showed that these waves undergo partially distinct maturation patterns, consistent with their possible dependence on different generation and synchronization mechanisms. Conclusions: Changes in slow wave origin, synchronization, and propagation at the transition between childhood and adulthood are consistent with known modifications in cortico-cortical and subcortico-cortical brain connectivity. In this light, changes in slow-wave properties may provide a valuable yardstick to assess, track, and interpret physiological and pathological development

Role of corpus callosum in sleep spindle synchronization and coupling with slow waves

Sleep spindles of non-REM sleep are transient, waxing-and-waning 10-16 Hz EEG oscillations, whose cortical synchronization depends on the engagement of thalamo-cortical loops. However, previous studies in animal models lacking the corpus callosum due to agenesis or total callosotomy and in humans with agenesis of the corpus callosum suggested that cortico-cortical connections may also have a relevant role in cortical (inter-hemispheric) spindle synchronization. Yet, most of these works did not provide direct quantitative analyses to support their observations. By studying a rare sample of callosotomized, split-brain patients, we recently demonstrated that the total resection of the corpus callosum is associated with a significant reduction in the inter-hemispheric propagation of non-REM slow waves. Interestingly, sleep spindles are often temporally and spatially grouped around slow waves (0.5-4 Hz), and this coordination is thought to have an important role in sleep-dependent learning and memory consolidation. Given these premises, here we set out to investigate whether total callosotomy may affect the generation and spreading of sleep spindles, as well as their coupling with sleep slow waves. To this aim, we analysed overnight high-density EEG recordings (256 electrodes) collected in five patients who underwent total callosotomy due to drug-resistant epilepsy (age 40-53, two females), three non-callosotomized neurological patients (age 44-66, two females), and in a sample of 24 healthy adult control subjects (age 20-47, 13 females). Individual sleep spindles were automatically detected using a validated algorithm and their properties and topographic distributions were computed. All analyses were performed with and without a regression-based adjustment accounting for inter-subject age differences. The comparison between callosotomized patients and healthy subjects did not reveal systematic variations in spindle density, amplitude or frequency. However, callosotomized patients were characterized by a reduced spindle duration, which could represent the result of a faster desynchronization of spindle activity across cortical areas of the two hemispheres. In contrast with our previous findings regarding sleep slow waves, we failed to detect in callosotomized patients any clear, systematic change in the inter-hemispheric synchronization of sleep spindles. In line with this, callosotomized patients were characterized by a reduced extension of the spatial association between temporally coupled spindles and slow waves. Our findings are consistent with a dependence of spindles on thalamo-cortical rather than cortico-cortical connections in humans, but also revealed that, despite their temporal association, slow waves and spindles are independently regulated in terms of topographic expression