Even under general anesthesia, the brain may be listening to surrounding sounds and processing language in real time.
Most people who undergo general anesthesia before major surgery wake up to find the surgery is over and have no memory of what happened during that time. However, new research suggests that even under general anesthesia, the human brain may be listening to surrounding sounds and processing language in real time.
Plasticity and language in the anesthetized human hippocampus | Nature
Researchers discover advanced language processing in the unconscious human brain | BCM
https://www.bcm.edu/news/researchers-discover-advanced-language-processing-in-the-unconscious-human-brain
A Small Part of Your Brain May Still Be Listening Under Anesthesia : ScienceAlert
https://www.sciencealert.com/a-small-part-of-your-brain-may-still-be-listening-under-anesthesia
A research team led by Samir Sheth, a professor of neurosurgery at Baylor College of Medicine in the United States, analyzed brain activity in seven patients undergoing epilepsy surgery while they were unconscious under general anesthesia.
The research team used microelectrodes called Neuropixels probes, which measure the electrical activity of neurons in the brain, to analyze brain activity in the hippocampus of subjects under general anesthesia. The hippocampus is a region that plays an important role in learning and memory, and is unique in that it is located far from the cerebral cortex , where sensory information is first processed.
First, the research team repeatedly exposed subjects to specific sounds, occasionally mixing in unusual or strange sounds. They observed that neurons in the hippocampus distinguished between these sounds, and that the ability to distinguish between normal and unusual sounds improved over time.
The graph below shows neuronal activity when listening to normal sounds (black) and when listening to unusual sounds (green). It can be seen that the hippocampus is able to distinguish between the two sounds even under general anesthesia.
Next, the research team had subjects under general anesthesia listen to educational videos and narrative podcasts, recording the activity of their neurons. Surprisingly, the results showed that the subjects' hippocampi were processing language in real time.
Neuronal activity in the subjects revealed that the brain was classifying nouns, verbs, and adjectives and attempting to predict the next word in a sentence. 'It seems the brain is predicting what will happen next in the story, even without conscious awareness. This kind of predictive coding is associated with a state of wakefulness and attention, but here it's happening in an unconscious state,' says Sheth.
These findings suggest that certain information processing abilities are not linked to human consciousness and may be able to be performed even when the person is not awake. The research team points out that this is relevant to a broader field of research that explores how the brain performs tasks unconsciously.
It should be noted that in this study, propofol was administered as an anesthetic to all subjects, so it is unclear whether similar results would be obtained with other anesthetics. Furthermore, different results may occur in cases of sleep or unconsciousness due to factors other than anesthesia.
In the future, it may be possible to develop methods to reconstruct neural circuits when a part of the brain stops functioning, by utilizing the fact that the brain can still hear sounds and process language even when its function is declining.
Shess stated, 'Our research shows that the brain is far more active than previously thought and remains highly capable even while unconscious. Even when a patient is completely under anesthesia and unconscious, the brain continues to analyze the world around them.'
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