The emergence of consciousness after general anesthesia has been imaged and found to be associated with activations of deep, primitive brain structures, rather than the evolutionary younger neocortex, scientists from UC Irvine and the University of Turku have found.

“We expected to see the outer bits of brain, the cerebral cortex (often thought to be the seat of higher human consciousness), would turn back on when consciousness was restored following anesthesia,” said Harry Scheinin, M.D. from the Turku PET Centre at the University of Turku in Finland.

“Surprisingly, that is not what the images showed us. In fact, the central core structures of the more primitive brain structures including the thalamus and parts of the limbic system appeared to become functional first, suggesting that a foundational primitive conscious state must be restored before higher order conscious activity can occur.”

The study may help in understanding the traumatic phenomenon of ”intraoperative awareness” (remembering surgery).

In the study, volunteers were put under anesthesia in a positron emission tomography (PET) brain scanner using either dexme-detomidine or propofol anesthetic drugs. The subjects were then woken up while brain-activity images were being recorded. Dexmedetomidine is used as a sedative in the intensive care unit setting and propofol is widely used for induction and maintenance of general anesthesia.

Dexmedetomidine-induced unconsciousness has a close resemblance to normal physiological sleep, since it can be reversed with mild physical stimulation or loud voices without requiring any change in the dosing of the drug. This unique property was critical to the study design because 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 emergence of consciousness, as assessed with a motor response to a spoken command, was associated with the activation of a core network involving subcortical and limbic regions that became functionally coupled with parts of frontal and inferior parietal cortices upon awakening from dexme-detomidine-induced unconsciousness. This network thus enabled the subjective awareness of the external world and the capacity to behaviorally express the contents of consciousness through voluntary responses.

Interestingly, the same deep brain structures —. the brain stem, thalamus, hypothalamus and the anterior cingulate cortex — were activated also upon emergence from propofol anesthesia, suggesting a common, drug-independent mechanism of arousal. For both drugs, activations seen upon regaining consciousness were thus mostly localized in deep, phylogenetically old brain structures rather than in the neocortex.

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

Ref.: Jaakko W. Långsjö, et al., Returning from Oblivion: Imaging the Neural Core of Consciousness, The Journal of Neuroscience, 2012; 32(14):4935-4943; [DOI:10.1523/​JNEUROSCI.4962-11.2012] (open access)

Also see: Toward a Science of Consciousness (conference in Tucson, April 9–14), which will include a session on intraoperative awareness.