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What Happens in the Brain During Unconsciousness

New research explores the breakdown in communication between brain regions.

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The first public demonstration of surgery under anesthesia occurred in 1846. By 1847, people were already theorizing about what might be going on in the brain while a person was unconscious. More than 170 years later, we’re still wondering. Mysteries remain about all forms of unconsciousness—sleep, sedation, deep anesthesia, and comas. But three recent papers from a team of researchers at Michigan Medicine have added a little bit to what we know about how our brain responds when we sink into unconsciousness.

“It’s not like a light switch going off, it’s more of a process of disconnection,” says anesthesiologist George Mashour, an author on all three papers. “The nature of how information is processed really is key.” In other words, the brain doesn’t shut down under anesthesia (although it can be made to do just that with very high doses of drugs). Instead, the way that neurons communicate and send signals to each other becomes increasingly localized, creating isolated islands of activity. In that state, there’s far less connectivity across the entire brain. The conversations are all local, not long-distance, according to Mashour and his team, which includes neurophysiologist Anthony Hudetz and physicist UnCheol Lee.

As director of the Center for Consciousness Science at the University of Michigan Medical School, Mashour is part of a rich tradition of anesthesiologists who investigate consciousness—not a surprising interest, since these doctors spend their days putting people under and pulling them out again. When he talks about consciousness, Mashour does not mean free will or self-consciousness: “We’re talking about experience primarily, and the general thought that integration or synthesis of information is critical for conscious experience.”

In 2004, as a resident at Harvard, he published a theory that sowed the seeds for the work he’s doing today: “I was asking myself the question, why do anesthetics work?” At the time, the role of the integration of information, also known as cognitive binding, was a big question in neuroscience. People wanted to know how different areas of the brain process information as it arrives from different sensory areas, like vision and hearing. “How does the brain get all this information together such that we have this very seamless, unified experience?” asks Mashour. “What I posited was that maybe anesthetics work not just by shutting everything down, but rather inhibiting these processes of integration. Maybe anesthesia is a kind of cognitive unbinding.”

Mashour was not the only one thinking this way. Most notably, neuroscientist Giulio Tononi of the University of Wisconsin, an expert in sleep and consciousness, put out an important theory the same year that related to the integration of information in the brain. Mashour, who started his academic life studying philosophy, has continued to work on the question ever since.

Each of his group’s recent papers, published in Trends in Neurosciences, The Journal of Neuroscience, and Frontiers in Human Neuroscience, tackles a slightly different part of the “communication breakdown” in the unconscious brain. Taken together, the work analyzed different neural areas during sedation, surgical anesthesia, and a vegetative state, and showed not only that communication becomes more localized, but also that information processing slows down as well. Using EEG to eavesdrop on the brain’s electrical activity, the team made the complex task of measuring information integration in the brain more manageable. They were able to identify the varying states of consciousness that they were studying based on their measurements.

Essentially, they were able to measure what they call “the state of modularity” in a brain network. “Modularity is a measure of how connected the brain is,” says Mashour. “The more islands you have, the higher the modularity and the lower the measure of integration.”

What about sleep? Consciousness researchers think of sleep and anesthesia as distinct states that share certain traits, says Mashour:

"[This work] does suggest that one of the key features of the anesthetized or unconscious state is that you’ve got a breakdown or a fragmentation of the network. This has also been shown for sleep. It’s been shown for certain disorders of consciousness. I think it’s emerging as a consistent theme."

Even though all these states of unconsciousness show similar communication breakdowns in the brain, an important and intriguing difference is how we come out of them. “Why is it you can reverse sleep in a couple of seconds by shaking someone, you can reverse anesthesia in a matter of minutes after you discontinue the drug, but it might take years, if at all, to recover from a pathologic state?” asks Mashour. He and others in the field hope that one day, if researchers can better understand what underlies these differences, such knowledge might be used to help people come out of comas or vegetative states. “We can think about engineering or re-engineering an emergence from this state the way the brain does when it’s coming out of anesthesia,” says Mashour.

Beyond such clinical implications, Mashour hopes that his work will continue to add to what we know about the human experience. “We’re trying to understand how we better monitor anesthesia,” says Mashour, “but the other side of the coin is that we’re using these anesthetics as tools to try to see what consciousness is all about in the brain.”

Copyright: Lydia Denworth 2018.

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Huang, Zirui, et al. "Timescales of intrinsic BOLD signal dynamics and functional connectivity in pharmacologic and neuropathologic states of unconsciousness." Journal of Neuroscience (2018): 2545-17.

Mashour, George A., and Anthony G. Hudetz. "Neural Correlates of Unconsciousness in Large-Scale Brain Networks." Trends in neurosciences (2018).

Kim, Hyoungkyu, et al. "Estimating the integrated information measure Phi from high-density electroencephalography during various states of consciousness in humans." Frontiers in Human Neuroscience 12 (2018): 42.

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