Although brain activity consists of a large degree of parallel computing, consciousness needs an integration process and production of “streams”, i.e. serial information. Models of consciousness denote this streaming process with the term “global workspace”. The hypothesis that brain behaves like a system at criticality may explain this phenomenon and other features of consciousness. In physics, complex systems, either critical or dissipative, naturally result in stream-like information grow, through the dynamics of the so called “order parameters”. These are integrative, macroscopic variables, normally coupled to external environment (i.e. responding to stimuli), which display, at criticality, scale-free fluctuations in time and space. The dynamical counterpart is the presence of metastable states, with quasi-abrupt transitions, where the system components, both micro- or meso-scopic, undergo “avalanches”, namely scale-free domino-like cascades. Avalanches, in other words, are distributed as inverse power laws in terms of mass (number of components involved) and of duration times. We studied the aforementioned phenomena in brain activity by extracting events, i.e. Rapid Transition Processes (RTPs) between stationary states, from high-density EEG recordings. We extracted network events from 29 night EEG recordings that included pre-sleep wakefulness and all phases of sleep, where different levels of mentation and consciousness are present. We show that while critical avalanching kept unchanged, at least qualitatively, temporal complexity and event-related functional connectivity, present during conscious phases wakefulness and REM sleep), broke down during both shallow and deep non-REM sleep. We provide a heuristic theory that explains how temporal complexity is hindered by a fragmentation of brain activity, without any need for the brain to depart from criticality. This breakdown suggests that the main difference between conscious and unconscious states resides in the backwards causation, namely on the constraints that the emerging properties at large scale induce to the lower scales. In particular, while in conscious states this backwards causation induces a critical slowing down, preserving spatiotemporal correlations, in dreamless sleep we have a self-organized maintenance of moduli working in parallel. Critical avalanches are still present, and establish transient auto-organization, whose enhanced fluctuations are able to trigger mechanisms that protect sleep and sleep-related unconsciousness, by reinstating parallel activity in different brain moduli. This happens through a complex interplay between cortico-thalamic entrainment and the elicitation of neural bistability, whose EEG correlates are, respectively, sleep spindles and sleep slow (< 1 Hz) oscillations. The plausible role of critical avalanches in dreamless sleep is to provide a rapid recovery of consciousness, if stimuli are highly arousing.
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