Key Takeaway:
- Study finds the brain can process speech and sounds during general anesthesia.
- The hippocampus continued recognizing language patterns despite unconsciousness.
- Patients showed no conscious memory of the audio after surgery.
A new study published Thursday in Nature finds the brain continues processing words, sounds, and speech during general anesthesia, though patients later report no memory of hearing them.
Researchers Detect Brain Activity During Surgery
Scientists studied seven patients undergoing surgery for severe epilepsy and found that the hippocampus, a brain region linked to memory and learning, remained active under anesthesia. Researchers said the findings challenge long-held assumptions that the brain fully shuts down during unconsciousness.
The study was led by researchers at the Baylor College of Medicine and published Thursday in the journal Nature.
“The hippocampus, over millions of years of evolution, became so specialized in taking this information in and parsing it into a useful structure that it’s doing this without awareness,” said Sameer Anil Sheth, co-senior author of the study.
The patients underwent anterior temporal lobectomy procedures, in which surgeons remove brain tissue to treat epilepsy. During surgery, researchers temporarily inserted thin probes known as Neuropixels into the hippocampus to monitor electrical activity from hundreds of neurons.
Patients Hear Tones and Podcasts While Unconscious
Researchers played audio recordings in the operating room while patients were anesthetized with intravenous drugs, primarily propofol. Some patients heard repeating tones interrupted by unexpected sounds, while others listened to episodes of “The Moth Radio Hour.”
The study found that neurons in the hippocampus gradually learned to distinguish unusual sounds from repeated ones over 10 minutes. Researchers said the unconscious brain appeared to reorganize its responses to recognize changes in audio patterns.
“The other important finding was that this recognition of the oddball sound emerged over time,” Sheth said. “It wasn’t decodable in the first few minutes.”
In the podcast experiment, scientists found neurons responded differently to nouns, verbs, and other parts of speech. Brain activity also reflected semantic relationships between words. Researchers said neurons treated words such as “cat” and “dog” as closely related while distinguishing them from unrelated words such as “pen.”
The study also suggested the brain predicted upcoming words in real time, a process similar to language comprehension in awake individuals.
General Anesthesia Findings Raise Ethical Questions
Despite the brain activity, none of the participants reported consciously remembering the sounds or stories after surgery.
“This aligns with reports that some patients recognize words presented during general anesthesia at above-chance levels despite lacking explicit memory for hearing them,” said Janna D. Helfrich, who was not involved in the research.
Researchers cautioned that the findings were limited to a small group of patients and one anesthetic approach. All seven participants received intravenous anesthesia, primarily using propofol.
“We want to be careful and say our findings were obtained under a particular anesthetic regimen,” Sheth said.
Scientists said additional research is needed to determine whether similar brain activity occurs with other anesthetics or during states such as sleep or coma.
Helfrich said the findings could influence how medical teams think about conversations and sounds in operating rooms.
“How much of the auditory environment do patients process during anesthesia, and should we be more intentional about what they hear?” she said.
