Awakening¹ from anesthesia（麻醉） is often associated with an initial phase of delirious²（神经错乱的） struggle before the full restoration of awareness³ and orientation⁴ to one's surroundings. Scientists now know why this may occur: primitive⁵ consciousness emerges first. Using brain imaging techniques in healthy volunteers, a team of scientists led by Adjunct Professor Harry⁶ Scheinin, M.D. from the University of Turku, Turku, Finland in collaboration⁷ with investigators⁸ from the University of California, Irvine, USA, have now imaged the process of returning consciousness after general anesthesia. The emergence⁹ of consciousness was found to be associated with activations of deep, primitive brain structures rather than the evolutionary¹¹ younger neocortex（新皮层） . These results may represent an important step forward in the scientific explanation of human consciousness. The study was part of the Research Programme on Neuroscience by the Academy of Finland.

"We expected to see the outer bits of brain, the cerebral¹² cortex (often thought to be the seat of higher human consciousness), would turn back on when consciousness was restored following anesthesia. Surprisingly, that is not what the images showed us. In fact, the central core structures of the more primitive brain structures including the thalamus（丘脑） and parts of the limbic system appeared to become functional¹³ first, suggesting that a foundational primitive conscious state must be restored before higher order conscious activity can occur" Scheinin said.

Twenty young healthy volunteers were put under anesthesia in a brain scanner using either dexmedetomidine or propofol anesthetic¹⁴ drugs. The subjects were then woken up while brain activity pictures were being taken. Dexmedetomidine is used as a sedative¹⁵ in the intensive care unit setting and propofol is widely used for induction¹⁶ and maintenance of general anesthesia（全身麻醉） . Dexmedetomidine-induced unconsciousness has a close resemblance to normal physiological¹⁷ sleep, as it can be reversed with mild physical stimulation¹⁸ or loud voices without requiring any change in the dosing of the drug. This unique property was critical to the study design, as it enabled the investigators to separate the brain activity changes associated with the changing level of consciousness from the drug-related effects on the brain. The state-related changes in brain activity were imaged with positron emission¹⁹ tomography (PET正电子成像术).

The emergence of consciousness, as assessed with a motor response to a spoken command, was associated with the activation¹⁰ of a core network involving subcortical and limbic regions that became functionally²⁰ coupled with parts of frontal and inferior parietal cortices upon awakening from dexmedetomidine-induced unconsciousness. This network thus enabled the subjective²¹ awareness of the external world and the capacity to behaviorally express the contents of consciousness through voluntary responses. Interestingly, the same deep brain structures, i.e. the brain stem, thalamus, hypothalamus and the anterior²² cingulate cortex, were activated²³ also upon emergence from propofol anesthesia, suggesting a common, drug-independent mechanism²⁴ of arousal. For both drugs, activations seen upon regaining²⁵ consciousness were thus mostly localized in deep, phylogenetically old brain structures rather than in the neocortex.

The researchers speculate that because current depth-of-anesthesia monitoring technology is based on cortical electroencephalography (EEG) measurement (i.e., measuring electrical signals on the surface of the scalp that arise from the brain's cortical surface), their results help to explain why these devices fail in differentiating²⁶ the conscious and unconscious states and why patient awareness during general anesthesia may not always be detected. The results presented here also add to the current understanding of anesthesia mechanisms²⁷ and form the foundation for developing more reliable depth-of-anesthesia technology.