What happens inside the human brain when consciousness fades away during anesthesia and normal sleep? Scientists have long estimated that the answer to such a question might provide clues to the neurological origins of consciousness.

Until now, researchers had struggled to separate the brain patterns associated specifically with the loss of consciousness and the more generalized neural effects of falling asleep or being put to sleep.

For the first time, scientists have successfully isolated the specific brain changes related to losing or gaining consciousness.

For the study, published this week in the Journal of Neuroscience, researchers used positron emission tomography, or PET scans, to monitor the brain activity of healthy volunteers experiencing different states of consciousness.

Scientists performed brain scans as volunteers experienced wakefulness, and also recorded brain activity as volunteers were administered rising and falling levels of two anesthetic agents.

Additionally, researchers recorded the neural signatures of sleep-deprived wakefulness and Non-Rapid Eye Movement sleep.

In both experiments, test subjects put to sleep via anesthesia and allowed to fall asleep naturally were roused suddenly, triggering a rapid recovery to a responsive state. In addition to recording changes in the brain, researchers interviewed the volunteers about their experiences.

Researchers did not adjust the drug dosage of the anesthetized subjects before or after the arousal and recovery to a responsive state.

"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," study author Annalotta Scheinin, an anesthesiologist and doctoral candidate at the University of Turku in Finland, said in a news release.

When researchers compared the brain scans of responsive and unresponsive subjects, they found the activity of the thalamus, cingulate cortices and angular gyri -- all parts of the brain -- were linked with the loss and return of consciousness in subjects that fell asleep naturally and those put to sleep with drugs.

This, scientists say, is central core brain network essential to human consciousness.

The analysis also showed both drug-induced sleep and natural sleep triggered similarly widespread suppression of brain activity across cortical areas.

"General anesthesia seems to resemble normal sleep more than has traditionally been thought," said study co-author Harry Scheinin, an anesthesiologist and Turku researcher. "This interpretation is, however, well in line with our recent electrophysiological findings in another anesthesia study."

Despite being unresponsive, test subjects in post-experiment interviews revealed the continuation of internally generated experiences, like dreams.

Studies have previously documented dreaming in anesthetized subjects.

"However, 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," Scheinin said.

"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," Scheinin said.

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