Consciousness likely emerged early in vertebrate evolution, as the basic neural mechanisms supporting it are ancient and highly conserved across species. The stepwise emergence from general anesthesia can serve as a reproducible model to study consciousness across species. The neurobiological structure of the vertebrate central nervous system is evolutionarily ancient, and the neurophysiologic mechanisms supporting human consciousness are found at the earliest points of vertebrate brain evolution. Differences between species in the ability to experience the world are of degree, not kind, supporting Darwin's insight and the Cambridge Declaration on Consciousness in Non-Human Animals.
Consciousness often persists even when unresponsiveness occurs. In two experiments involving 76 healthy males, brain activity was measured during varying states of consciousness induced by propofol and dexmedetomidine (n = 39) and during sleep-deprived wakefulness and non-REM sleep (n = 37). Most participants reported internally generated experiences while unresponsive. Key brain regions, including the thalamus and cingulate cortices, were found critical for consciousness, showing consistent activity patterns regardless of anesthetic type or sleep state, highlighting a core network essential for conscious awareness.
Anesthesia-induced disconnectedness—unresponsiveness and unawareness of external events—is linked to reduced activity in the thalamus, but not to widespread cortical suppression, which instead reflects drug exposure itself. In a study of 160 healthy men, four anesthetics (propofol, dexmedetomidine, sevoflurane, S-ketamine) or saline placebo were administered at doses designed to make half of subjects unresponsive. Positron emission tomography showed that, for all anesthetics except S-ketamine, lower thalamic metabolic activity distinguished disconnected from connected states. Widespread cortical metabolic suppression occurred in both connected and disconnected subjects compared with placebo, indicating it is necessary but insufficient for loss of consciousness.