  Loss of cortical integration and changes in the dynamics of
electrophysiological brain signals characterize the transition from wakefulness
towards unconsciousness. The common mechanism underlying these observations
remains unknown. In this study we arrive at a basic model, which explains these
empirical observations based on the theory of phase transitions in complex
systems. We studied the link between spatial and temporal correlations of
large-scale brain activity recorded with functional magnetic resonance imaging
during wakefulness, propofol-induced sedation and loss of consciousness, as
well as during the subsequent recovery. We observed that during unconsciousness
activity in frontal and thalamic regions exhibited a reduction of long-range
temporal correlations and a departure of functional connectivity from the
underlying anatomical constraints. These changes in dynamics and
anatomy-function coupling were correlated across participants, suggesting that
temporal complexity and an efficient exploration of anatomical connectivity are
inter-related phenomena. A model of a system exhibiting a phase transition
reproduced our findings, as well as the diminished sensitivity of the cortex to
external perturbations during unconsciousness. This theoretical framework
unifies different empirical observations about brain activity during
unconsciousness and predicts that the principles we identified are universal
and independent of the causes behind loss of awareness.
