Predictive coding in the cortex relies on predictions fed back from deep layers via alpha/beta oscillations (8–30 Hz) that inhibit gamma (40–100 Hz) and spiking carrying sensory input forward. Intracranial recordings in macaques during passive auditory oddball tasks showed that in the awake state alpha/beta oscillations inhibited processing of predictable sounds. Propofol-induced loss of consciousness eliminated this alpha/beta modulation in sensory cortex and reduced alpha/beta coherence between sensory and frontal areas. Consequently, oddball stimuli evoked enhanced gamma power, late spiking, and superficial layer sinks in auditory cortex, indicating a disinhibited state. However, differential spiking to oddballs in higher-order cortex was lost, likely due to disrupted spike-field coupling. These findings constrain theories of consciousness.
Ketamine, an anesthetic that blocks NMDA receptors, produces alternating bursts of gamma (25-50 Hz) and slow-delta (0.1-4 Hz) brain oscillations. A hidden Markov model fitted to local field potentials from two non-human primates and electroencephalograms from nine humans quantified these dynamics. Gamma activity lasted on average 2.2 seconds in one primate, 1.2 in the other, and 2.5 in humans; slow-delta lasted 1.6, 1.0, and 1.8 seconds respectively. Five sub-states with regular sequential transitions were identified. These findings provide quantitative constraints for models of rhythm generation underlying ketamine-induced altered arousal.
Two anesthetics with different molecular actions, ketamine and dexmedetomidine, both increase phase locking of neural oscillations in the prefrontal cortex of nonhuman primates during loss of responsiveness. Within a hemisphere, neighboring prefrontal subregions become less phase-aligned, possibly due to large traveling waves. However, homologous areas across hemispheres become more aligned in phase. These distinct patterns of cortical phase alignment, markedly different from waking states, may represent a common mechanism by which diverse anesthetics produce loss of responsiveness.