Dynamique de connectivité fonctionnelle et modulation expérimentale des oscillations cérébrales en sommeil dans le vieillissement

Sleep is a state of consciousness which shows a great potential in the clinical field. For instance, sleep has become a tool in the early diagnosis of certain neurodegenerative processes, as well as in the treatment of various physiological and psychological disorders. Its potential could even be increased via brain stimulation. Although the brain is a major communication center, sleep research has mainly focused on static measures of sleep. The study of the patterns of communication between the different regions of the brain nevertheless allows us to infer on their functional utility in humans. What if these patterns of communication allowed a more integrated understanding of sleep changes throughout life and their consequences during aging? This thesis investigates, from an innovative and dynamic angle, the interaction of neuronal activity and experimental modulation of sleep in normal aging. Using electroencephalography, functional connectivity is assessed at the scale of sleep stages and cycles, at the scale of the slow wave itself, and from an experimental perspective using the transcranial alternating current stimulation. The results in our first two studies demonstrate changes in EEG functional connectivity during sleep in aging while our third study showed the possibility of experimentally modulating functional connectivity as well as brain oscillations in the same population. At the sleep stage scale, the brain of older individuals is more connected during slow wave sleep and less connected during lighter sleep, compared to young adults. These age differences are predominant at the beginning of the night. Our results also demonstrate a decrease in functional connectivity associated with the slow wave depolarization phase in older individuals. In the young ones, brain connectivity associated to a slow wave is markedly increased. Functional connectivity during slow wave depolarization is also affected by the simultaneous presence of sleep spindles, suggesting either the admixture of their networks or the establishment of those underlying spindle occurrence. We also demonstrate, through the use of novel metrics, the presence of two types of slow waves, each endowed with specific connectivity dynamics. This suggests the presence of distinct functional implications. These slow waves types could also be inherently modulated by distinct physiological processes. During a nap, the use of transcranial alternating current stimulation has made it possible to experimentally modulate sleep spindles, their coupling with the slow waves, and functional connectivity in older individuals. These results, although modest, demonstrate the promising aspect of non-pharmacological sleep modulation.  This thesis provides an integrated view of functional connectivity changes in aging and also demonstrates the feasibility of experimental sleep modulation in older individuals. The lack of network flexibility that we described in the older population in term of connectivity could underlie changes in sleep-dependent memory consolidation processes. The implications of our results is relevant to the study of sleep-dependent plasticity processes.

Keyphrases: Sleep, Functional connectivity, Electroencephalography, Aging, Transcranial Alternating Current Stimulation (TACS), Consciousness, Slow waves, Sleep spindles, Cognition