Feedback connections, which make up a large portion of neural links in the thalamocortical system, are thought to be essential for conscious perception, according to major theories of consciousness. These theories predict that feedback modulation should be reduced in nonconscious brain states such as non-REM sleep and anesthesia, and when sensory stimuli are not perceived. However, recent experiments on mismatch negativity, a phenomenon linked to top-down modulation, show that feedback modulation persists during nonconscious states, though it is generally dampened. These deviations challenge current theories and may require reevaluating how consciousness is assessed in supposedly nonconscious states.
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.