The brain may still be able to hear speech under anesthesia

A study of people who had surgery to treat epilepsy suggests that the hippocampus can process words and speech when people are under general anesthesia, even if the study participants don’t remember them.

By Jacek Krywko edited by Lewis asked.

Under general anesthesia, the conscious mind turns off – or so we thought for a long time. But a new study of people in this condition suggests that the anesthetized brain still picks up sounds, words and even conversations. However, the participants did not remember this afterwards.

Scientists have discovered that the hippocampus, a deep brain structure that plays a role in memory and spatial navigation, continues to listen, learn and predict the meaning of words. while a person is completely anesthetized. “The hippocampus, over millions of years of evolution, has become so specialized at assimilating this information and analyzing it into a useful structure that it does so without being aware of it,” says Sameer Anil Sheth, professor of neurosurgery at Baylor College of Medicine and co-senior author of the study, published today in Nature.

To test this hypothesis, researchers recruited seven people who were scheduled to undergo anterior temporal lobectomy, a type of surgery in which pieces of brain tissue are removed to treat severe epilepsy. While the patients were under general anesthesia, surgeons temporarily inserted thin probes into their hippocampus. These probes, called Neuropixels, allowed scientists to simultaneously listen to the electrical signals produced by hundreds of individual neurons.

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Once the Neuropixels were in place, the team streamed audio into the operating room. Some patients heard a sequence of repetitive tones interrupted by occasional unexpected frequencies, while others were treated to sound. Moth Radio Time podcast.

In the task involving different tones, the researchers found that individual neurons in the hippocampus distinguished between standard and unexpected tones by gradually making their response to the latter more distinct. “The other important discovery was that this recognition of the strangeness [sound] appeared over time. It was not decodable in the first few minutes,” Sheth says. The unconscious brain learned to distinguish the tones, continually reorganizing its neural responses to better detect the anomaly over the course of 10 minutes of playback.

The podcast experience has gone even further. The patients’ hippocampal neurons coded for specific semantic and grammatical features of the spoken words. “They were listening to the detailed architecture of the speech. Some drew [sent signals] for names; some pulled more on verbs than other parts of speech,” says Sheth.

The firing rates of the neurons also contained information about the semantic categories of the spoken words; they seemed to recognize that words like “cat” were semantically close to words like “dog” but distant from words like “pen.” “What fascinated us the most was that [neurons] we were doing [real-time] predictions of what the next word will be [was] is going to be,” Sheth says. Overall, language processing in the anesthetized hippocampus worked much like it does in an awake hippocampus.

“This is consistent with reports that some patients recognize words presented during anesthesia at above chance levels despite a lack of explicit memory to hear them,” says Janna D. Helfrich, an anesthesiologist at Yale University, who was not involved in the study. Participants in Sheth’s study, however, reported no conscious memory of the sounds and stories the team played for them.

“We want to be cautious and say that our results were obtained under a particular anesthetic regimen,” says Sheth. All seven participants were anesthetized intravenously, with the common drug propofol being the primary anesthetic. The team believes it is too early to say whether using other anesthetics would change the results or whether they would remain the same in other unconscious states such as sleep or coma. Nonetheless, the results raise intriguing questions. “How much of the auditory environment do patients process during anesthesia, and should we be more intentional about what they hear?” Helfrich said.