Researchers at Baylor College of Medicine found that the brain may still process language even when a person is fully unconscious under general anesthesia.
"This work pushes us to rethink what it means to be conscious," said co-senior author Dr. Sameer Sheth, professor and Cullen Foundation Endowed chair of neurosurgery and a McNair Scholar at Baylor. "The brain is doing much more behind the scenes than we fully understand."
For a long time, scientists believed the brain mostly 'shut off' during anesthesia. But this new study suggests the brain can still hear sounds, understand words, and even predict what words might come next, although people do not remember any of it afterward.
During the study, published in Nature journal, the researchers focused on the hippocampus, a part of the brain linked to memory and navigation. They studied seven epilepsy patients who were already scheduled for brain surgery. During the operation, doctors placed tiny probes called Neuropixels into the patients' hippocampus to record the activity of hundreds of brain cells.
While the patients were unconscious, the researchers played sounds and spoken audio. Some patients heard repeating tones with occasional unusual sounds, while others listened to episodes of The Moth Radio Hour.
The scientists discovered that brain cells reacted differently to expected and unexpected sounds. They also found that the brain could recognize relationships between words. For example, it understood that 'cat' and 'dog' are closely related in meaning, while 'pen' is unrelated. The brain cells even appeared to predict upcoming words in real time, similar to how the brain works when a person is awake.
"Our findings show that the brain is far more active and capable during unconsciousness than previously thought," Sheth concluded. "Even when patients are fully anesthetized, their brains continue to analyze the world around them."
The findings are eventually expected to help create better communication tools, such as speech devices for people who cannot speak.
However, the researchers said that more studies are needed as the current results only apply to one type of anesthesia, and they still do not know whether the same thing happens during sleep, coma, or in other parts of the brain.
"Can we use these signals to deploy and run a speech prosthetic for some of the parts of the brain that are damaged by stroke or injury? These are questions that we can now consider in relation to this part of the brain," noted first author Dr. Vigi Katlowitz, a neurosurgery resident with Baylor.
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