The loss and return of consciousness is linked to the same core brain network for both sleep and anesthesia.

Differences in brain activity between connected and disconnected states of consciousness studied with positron emission tomography imaging. Activity of the thalamus, anterior (ACC) and posterior cingulate cortices (PCC), and bilateral angular gyri (AG) show the most consistent associations with the state of consciousness (A = general anesthesia, B = sleep). The same brain structures, which are deactivated when the state of consciousness changes to disconnected in general anesthesia or natural sleep (cool colors in the left columns), are reactivated when regaining a connected state upon emergence from anesthesia (warm colors in the right columns). Image credit: Scheinin et al., doi: 10.1523/JNEUROSCI.0775-20.2020.

Differences in brain activity between connected and disconnected states of consciousness studied with positron emission tomography imaging. Activity of the thalamus, anterior (ACC) and posterior cingulate cortices (PCC), and bilateral angular gyri (AG) show the most consistent associations with the state of consciousness (A = general anesthesia, B = sleep). The same brain structures, which are deactivated when the state of consciousness changes to disconnected in general anesthesia or natural sleep (cool colors in the left columns), are reactivated when regaining a connected state upon emergence from anesthesia (warm colors in the right columns). Image credit: Scheinin et al., doi: 10.1523/JNEUROSCI.0775-20.2020.

Trying to understand the biological basis of human consciousness is currently one of the greatest challenges of neuroscience.

While the loss and return of consciousness regulated by anesthetic drugs and physiological sleep are employed as model systems in experimental studies, previous results have been confounded by drug effects, by confusing behavioral ‘unresponsiveness’ and internally generated consciousness, and by comparing brain activity levels across states that differ in several other respects than only consciousness.

“One major challenge has been to design a set-up, where brain data in different states differ only in respect to consciousness,” said senior author Dr. Harry Scheinin, a researcher in the Turku PET Centre and the Institute of Biomedicine and Unit of Clinical Pharmacology at the University of Turku and the Department of Perioperative Services, Intensive Care and Pain Medicine at Turku University Hospital.

“Our study overcomes many previous confounders, and for the first time, reveals the neural mechanisms underlying connected consciousness.”

Dr. Scheinin and colleagues sought networks associated with human consciousness by measuring the brain activity of adult males with positron emission tomography as they fell asleep and went under anesthesia.

“This unique experimental design was the key idea of our study and enabled us to distinguish the changes that were specific to the state of consciousness from the overall effects of anesthesia,” said first author Annalotta Scheinin, a doctoral candidate in the Turku PET Centre at the University of Turku and the Department of Perioperative Services, Intensive Care and Pain Medicine at Turku University Hospital.

The researchers woke participants mid-experiment to interview them and confirm their state of connectedness.

Changes in connectedness corresponded to the activity of a network comprised of regions deep inside the brain: the thalamus, anterior and posterior cingulate cortex, and angular gyri.

These regions exhibited less blood flow when a participant lost connectedness and more blood flow when they regained it.

The pattern held true for both sleep and anesthesia, indicating the changes corresponded to connectedness rather than the effects of sleep or drugs, and that the network may be imperative for human consciousness.

“General anesthesia seems to resemble normal sleep more than has traditionally been thought,” Dr. Harry Scheinin said.

“This interpretation is, however, well in line with our recent electrophysiological findings in another anesthesia study.”

“Because of the minimal delay between the awakenings and the interviews, the current results add significantly to our understanding of the nature of the anesthetic state,” Annalotta Scheinin added.

“Against a common belief, full loss of consciousness is not needed for successful general anesthesia, as it is sufficient to just disconnect the patient’s experiences from what is going on in the operating room.”

The findings were published in the Journal of Neuroscience.

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Annalotta Scheinin et al. Foundations of human consciousness: Imaging the twilight zone. Journal of Neuroscience, published online December 28, 2020; doi: 10.1523/JNEUROSCI.0775-20.2020