Brain and Nonlinear Dynamics: Slow-Wave Sleep Regulates to the Edge of Chaos

We present a theoretical modeling study that predicts that the EEG waveforms observed during the passage from wake through the stages of deepening natural sleep arise from the evolving interactions between symmetry-breaking transitions in the brain. These non-equilibrium transitions are brought on by modulations from naturally occurring neurotransmitters, primarily acetylcholine and GABA, whose concentrations vary dynamically during sleep. In particular, we find that the slow-wave oscillations of deepest nonREM sleep are fundamentally chaotic in nature, arising spontaneously from a competitive interaction between Turing (spatial) and Hopf (temporal) instabilities. We show that by introducing an activity-based regulation of inhibitory gap-junction diffusion, the sleeping cortex can move towards the edge of chaos defined by the boundary between the disordered slow-wave state and a pathologically ordered seizure-like state. We suggest that such self-organization could allow the brain to dwell in a state that is optimized for pruning and consolidation of memories.