by Fabrizio Lombardi (IST, Klosterneuburg, Austria).
Sleep periods exhibit numerous intermittent transitions among sleep stages and short awakenings, with fluctuations within sleep stages that may trigger micro-states and arousals. Despite the established association between dominant brain rhythms and emergent sleep stages, origin and functions of sleep-arousals and sleep-stage transitions remain poorly understood. Empirical observations of intrinsic fluctuations in rhythmic cortical activity, and the corresponding temporal structure of intermittent transitions in sleep micro-architecture, raise the hypothesis that non-equilibrium critical dynamics may underlie sleep regulation at short time scales, in co-existence with the well-established homeostatic behavior at larger time scales. In this talk, I will discuss recent results on the dynamics of dominant cortical rhythms across the sleep-wake cycle supporting such hypothesis (Lombardi et al. J. Neurosci. 2020; Wang et al. Plos. Comp. Biol. 2020 ). I will focus on cortical theta and delta rhythms in rats, which are associated with arousals/wakefulness and sleep respectively. I will show that intermittent bursts in theta and delta rhythms exhibit a complex temporal organization: Theta-burst durations follow a power-law distribution, whereas delta-burst durations follow an exponential-like behavior. Such features are typical of self-organized criticality, where the active phase is characterized by bursts with power-law statistics, while quiescent periods (inactive phase) are exponentially distributed. By interpreting theta-bursts as active phases of the cortical activity in the sleep-wake cycle, I will then draw a parallel with other non-equilibrium phenomena at criticality, and demonstrate that theta-bursts exhibit a peculiar organization in time described by a single scaling function (Gamma distribution) and closely reminiscent of earthquake dynamics. Overall, such results constitute a fingerprint of critical dynamics underlying the complex temporal structure of intermittent sleep-stage transitions at the behavioral level, and ideally complement previous observations of critical behavior at the neuronal level.
Spotlight talk presented on October 7th 2020, at the Brain Criticality Virtual Conference 2020 (Plenz D., Chialvo D., de Arcangelis L. & Battaglia D. organizers)