What happens in the brain when our conscious awareness fades during general anesthesia and normal sleep? Scientists studied this question with novel experimental designs and functional brain imaging. They succeeded in separating the specific changes related to consciousness from the more widespread overall effects, and discovered that the effects of anesthesia and sleep on brain activity were surprisingly similar. These novel findings point to a common central core brain network fundamental for human consciousness.

Explaining the biological basis of human consciousness is one of the greatest challenges of science. While the loss and return of consciousness, as regulated by drugs or physiological sleep, have been employed as model systems in the study of human consciousness, previous research results have been confounded by many experimental simplifications.

«One major challenge has been to design a set-up, where brain data in different states differ only in respect to consciousness. Our study overcomes many previous confounders, and for the first time, reveals the neural mechanisms underlying connected consciousness,» says Harry Scheinin, Docent of Pharmacology, Anesthesiologist, and the Principal Investigator of the study from the University of Turku, Finland.

A new and innovative experimental set-up

Brain activity was measured with positron emission tomography (PET) imaging during different states of consciousness in two separate experiments in the same group of healthy subjects. Measurements were made during wakefulness, escalating and constant levels of two anesthetic agents, and during sleep-deprived wakefulness and Non-Rapid Eye Movement (NREM) sleep.

In the first experiment, the subjects were randomly allocated to receive either propofol or dexmedetomidine (two anesthetic agents with different molecular mechanisms of action) at stepwise increments until the subjects no longer responded. In the sleep study, they were allowed to fall asleep naturally. In both experiments, the subjects were roused to achieve rapid recovery to a responsive state, followed by immediate and detailed interviews of subjective experiences from the preceding unresponsive period. Unresponsive anesthetic states and verified NREM sleep stages, where a subsequent report of mental content included no signs of awareness of the surrounding world, indicated a disconnected state in the study participants. Importantly, the drug dosing in the first experiment was not changed before or during the shift in the behavioral state of the subjects.

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