Under anaesthesia, the patient's brain is not completely switched off - they can follow what the doctors are saying

Important: this does not mean that the patient consciously hears the doctors' conversations or remembers them afterwards. It is about neural processing of information in the brain, not full conscious perception.

Details

The study was conducted by scientists at Baylor College of Medicine and other research centres. The work was published in the journal Nature. The authors recorded the activity of neurons in the hippocampus, a region of the brain associated with memory and learning.

The study involved seven patients undergoing surgery for epilepsy. During the surgery, they were under general anaesthesia and the researchers used high-density Neuropixels microelectrodes to record the activity of individual neurons and local field potentials in the hippocampus.

First, patients under anaesthesia were turned on to repetitive sounds, among which a different, distinct tone would occasionally appear. Hippocampal neurons responded to such "unexpected" sounds, and the distinction became more noticeable within about 10 minutes of the experiment. The authors interpret this as a sign of neural plasticity - the ability of the system to adapt even in unconsciousness.

The researchers then turned to speech. Four patients were given podcasts and short stories lasting about 10-20 minutes. Analysis showed that neuronal activity carried information about the semantic and grammatical features of natural speech. Moreover, the brain signals allowed predicting semantic information about the following words.

Simply put, the brain didn't just react to sound as noise. It continued to pick up the structure of speech: it discerned language features and, judging from neural signals, could build expectations about what would be said next.

Why it matters

The study challenges the simple idea that under anaesthesia the brain completely 'switches off'. General anaesthesia does suppress consciousness and behavioural response, but some of the complex information processing can be retained.

This is important for understanding consciousness. Work shows that some cognitive processes - such as recognising speech structure and predicting next words - can occur without conscious perception. Consciousness probably depends not on the activity of a single area of the brain, but on broader coordination between different areas.

The study has practical potential, too. If scientists better understand what speech signals can be read from the brain even when consciousness is impaired or speech areas are damaged, it could help in the development of neural interfaces and speech prostheses for people who cannot speak after a stroke or injury.

Background

The hippocampus is commonly associated with memory, learning and experience processing. It is not a primary auditory area, so it is particularly interesting that it is where the researchers found evidence of sound and speech processing during anaesthesia.

But the findings should be read with caution. The study was conducted on a small group of patients - seven people who were undergoing surgery for epilepsy. In addition, the authors explicitly state that the results may not transfer to other unconscious states, such as sleep or coma, and may not generalise to other types of anaesthesia other than propofol protocols.

Therefore, this is not a reason to be anxious before surgery, nor is it evidence that patients under anaesthesia consciously hear everything around them. Rather, it is evidence that the brain, even in a profoundly altered state, continues to do more "background work" than scientists expected.

Source

The study by Kalman A. Katlowitz, Sameer A. Sheth and co-authors Plasticity and language in the anaesthetised human hippocampus is published in Nature in 2026. The authors recorded hippocampal activity in patients under general anaesthesia and studied how the brain responds to sounds and natural speech.