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What makes us human? Neuroscientists are a step closer to finding out

Scientists have discovered a shared link between sleep and anesthesia in brain.

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The question of consciousness — the supposed brain activity which makes humans distinct in their ability to reason and awareness — plagues both neuroscientists and philosophers alike. Members of the first camp appear to have discovered a new clue in their search for a biology-based answer to this elusive question.

In a paper published Monday in the journal JNeurosci, a team of researchers analyzed the brain activity of 37 participants as they first slipped under anesthesia, and then into sleep. Looking at distinct brain wave patterns during these trials, the researchers found identical patterns of activity in a shared brain network.

Why it matters — Because this brain activity was observed independently of anesthetic presence or concentration, the researchers report this brain network must play a critical role in generating human consciousness.

Here's the background — Locating the nexus of human consciousness in the brain is one of the greatest challenges of neuroscience. The answer could fundamentally alter the course of science, and redefine how we as humans understand ourselves. But even the very definition of this state of being is open to debate.

When comparing human consciousness to the consciousness of artificial intelligence systems, for example, "consciousness" might be used to describe the seeming spark of self-awareness and purpose unique to living beings. But for neuroscientists, charting the exact division between consciousness and unconsciousness is a little more complicated.

In this study, the researchers define "connected" and "disconnected" states of consciousness. A "connected" condition represents wakefulness and the ability to respond to outside stimuli, like answering a question, while a "disconnected" condition describes a state in which there is no behavioral response to outside stimuli, for instance, not responding when spoken to, while still generating internal markers of consciousness, like dreams.

The researchers sought to understand the differences between these separate states of consciousness, hypothesizing that looking at brain activity during sleep and wakefulness, as opposed to anesthesia-induced states, may reveal something new about the brain networks governing consciousness.

"Neural correlates of consciousness are often claimed to be found by comparing brain activity data collected during two states: wakefulness and a presumed unconscious state. This paradigm is, however, controversial. In the current study, we aimed to separate changes in brain activity related specifically to consciousness from the overall effects of anesthesia and sleep," the authors write.

In order to compare the brain patterns of people in these disconnected and connected states of consciousness, the research team recruited 39 volunteers and stuck them in a PET brain scanner.

Researchers find that sleep and anesthesia light up the brain in the same way.Scheinin et al., JNeurosci 2020

What they did — The researchers first gave the 39 volunteers escalating doses of two different anesthetic drugs and placed them in PET scanners for observation. Throughout the trial, the participants were forced to wake up at different points, and underwent interviews with the research team about their state of consciousness. They were asked questions like: Were they aware of their surroundings before rousing?

Following this initial experiment, 37 of the participants then continued into a similar sleep study. After 30 hours of sleep deprivation, they were also placed in PET scanners and alternatively allowed to dose, or forced awake, again, evaluating their state of consciousness throughout.

What they discovered — Comparing the brain activity of participants in connected and disconnected states of consciousness, the researchers identified a network in the brain which showed certain patterns of activity in both states. Specifically, this network crossed through these brain regions:

  • The thalamus — which acts like a sensory signal hub in the brain
  • Anterior and posterior cingulate cortex — brain areas thought to play a role in cognition and memory
  • Angular gyrus — believed to play a role in language processing, memory, and other forms of cognition

When a participant entered a more disconnected state, blood flow through this network decreased, but when the participant entered a more connected conscious state, the blood flow increased once more. This pattern remained stable in both sleep and anesthesia-induced disconnected states, a result the researchers say suggests the mechanism was independent of both — and instead correlated specifically to connected consciousness.

What's next — The results of this study provide intriguing new insights into this important, yet seemingly intangible, mental state. Nevertheless, the researchers write improvements to their experimental design, and further validation of their results, are needed to move the field forward from this study.

One aspect of the study which reveals its limitations is the subjective nature of the connected and disconnected evaluations performed by the researchers. The evaluations are self-reported by participants, meaning they are subjective, rather than objective, measures, and prone to bias. Future studies might try to include more objective measurements of these states, as well to continue zeroing in on exactly what makes us human.

Abstract: What happens in the brain when conscious awareness of the surrounding world fades? We manipulated consciousness in two experiments in a group of healthy males and measured brain activity with positron emission tomography. Measurements were made during wakefulness, escalating and constant levels of two anesthetic agents (Experiment 1, n=39) and during sleep-deprived wakefulness and Non-Rapid Eye Movement sleep (Experiment 2, n=37). In Experiment 1, the subjects were randomized to receive either propofol or dexmedetomidine until unresponsiveness. In both experiments, forced awakenings were applied to achieve rapid recovery from an unresponsive to a responsive state, followed by immediate and detailed interviews of subjective experiences during the preceding unresponsive condition. Unresponsiveness rarely denoted unconsciousness, as the majority of the subjects had internally generated experiences. Unresponsive anesthetic states and verified sleep stages, where a subsequent report of mental content included no signs of awareness of the surrounding world, indicated a disconnected state. Functional brain imaging comparing responsive and connected vs. unresponsive and disconnected states of consciousness during constant anesthetic exposure revealed that activity of the thalamus, cingulate cortices and angular gyri are fundamental for human consciousness. These brain structures were affected independent from the pharmacologic agent, drug concentration and direction of change in the state of consciousness. Analogous findings were obtained when consciousness was regulated by physiological sleep. State-specific findings were distinct and separable from the overall effects of the interventions, which included widespread depression of brain activity across cortical areas. These findings identify a central core brain network critical for human consciousness.
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