The source of consciousness is widely thought to be within the brain, and anesthesiologists have their own operational definition based on observations during anesthesia. The full functional correlates of consciousness remain unclear, but several theories have gained varying support from experiments, including those using anesthesia to reversibly alter aspects of consciousness. Understanding these mechanisms could improve patient management by enabling monitoring devices that detect different states during anesthesia: unconsciousness, internal awareness with or without conscious perception of the environment (connected or disconnected consciousness). Unresponsiveness does not guarantee absence of connectedness or consciousness. This narrative review presents current knowledge from a system-level perspective, highlighting anesthesia's contribution to theories of consciousness and proposing future research directions.
In a small double-blind, placebo-controlled, cross-over trial with three adults who had prolonged disorders of consciousness after a coma, an intravenous sub-anesthetic dose of the atypical psychedelic ketamine increased brain complexity as measured by Lempel-Ziv complexity, but did not change the explainable consciousness indicator. Patients showed reduced spastic paresis and spent more time with their eyes open, yet their diagnosis of consciousness did not improve. No adverse effects occurred. The findings suggest a potential therapeutic role for ketamine in disorders of consciousness and support a link between brain complexity and conscious states.