Returning From Oblivion: Imaging the Neural Core of Consciousness

In this elegant study, Långsjö et al. use positron emission tomography (PET) imaging to identify the neuroanatomic sites that are activated during restoration of consciousness (ROC). ROC was tested using two different drugs (dexmedetomidine and propofol), and the study was carefully designed to distinguish between drug-induced epiphenomena and the actual neural correlates associated with ROC. For propofol anesthesia, the dose was decreased and the neural correlates of ROC were identified as the subjects regained consciousness. In contrast, the experiments with dexmedetomidine were performed without changing the drug dose, and gentle tactile and/or verbal stimulation were used to restore consciousness. Because the dose of dexmedetomidine was unchanged, this method allowed the investigators to tease apart the neural correlates associated with the behavioral state change vs. changes due to decreasing drug levels. In both cases, the areas of the brain that were ‘turned on’ during ROC were essentially the same: the anterior cingulate cortex, thalamus, hypothalamus, and brainstem. The implication is that emergence from general anesthesia occurs in a step-wise fashion, where only the activation of arousal-promoting deep brain structures is required for ROC, which apparently precedes the full recovery of neocortical processing.