Slow-wave activity (SWA) in non-REM sleep and anesthesia is a hallmark of unconsciousness, but how it changes during the return to consciousness is unclear. Recording multi-area and laminar activity in mouse posterior parietal (PPC) and primary visual (V1) cortices during spontaneous awakening from isoflurane anesthesia, the authors show that spectral power is stronger in PPC (especially superficial layers) during deep unconsciousness but stronger in V1 upon awakening. Rostro-caudal (feedback-like) propagation of SWA is state-dependent, particularly in layer 5. Excitability of layer 2/3 neurons, suppressed at high isoflurane, recovers during awakening, when V1 and feedforward pathways reassert a strong role. SWA is a multiscale phenomenon with hierarchical and laminar properties.
The cerebral cortex generates diverse patterns of activity that shift across brain states such as sleep, wakefulness, anesthesia, and disorders of consciousness, yet a unified definition of brain states remains elusive. This review focuses on two extremes: synchronous states, which predominantly underlie unconsciousness, and asynchronous states, which predominantly underlie consciousness, though exceptions exist. The authors integrate data across levels from local circuits to whole-brain dynamics, examining properties like cortical complexity, functional connectivity, synchronization, wave propagation, and excitatory-inhibitory balance. They make experimental and clinical data, as well as computational models at micro-, meso-, and macrocortical levels, available to readers.