Anesthetized but active: Our brain can still process language even under anesthesia, an experiment reveals. Accordingly, our hippocampus reacts to sentences and stories we hear, even when we are unconscious. The anesthetized brain even recognizes semantic connections and sentence structures, as characteristic patterns of brain activity reveal. This demonstrates that the human brain is still capable of complex sensory processing even under general anesthesia, as researchers report in Nature.
When we faint or are anesthetized, we lose consciousness. We are no longer consciously aware of our environment and lose active control over our muscles. Nevertheless, our brain is not completely immobilized even under anesthesia, as studies show. Although the patterns of brain activity change in a characteristic way, the anesthetized brain still reacts to certain sensory stimuli – even if these no longer penetrate our conscious perception.
What happens in the unconscious brain?
But how much do we really notice under anesthesia? For example, can the unconscious brain process heard speech? “Prominent theories of consciousness posit that the necessary advanced pattern recognition, semantic interpretation, and predictive processing are only possible with access to consciousness,” explain Kalman Katlowitz of Baylor College of Medicine in Houston and his colleagues.
However, this is contradicted by studies that have found evidence of complex stimulus processing under anesthesia. “This suggests that neuronal circuits – even at higher levels far from receptors and motor effectors – can still process sensory impressions and their structures even when consciousness is disturbed,” say the researchers. They therefore took the opportunity to test language processing under anesthesia again during brain operations.
View into the hippocampus under general anesthesia
For their tests, Katlowitz and his team used special electrodes to record the activity of hundreds of individual neurons in the hippocampus of seven epilepsy patients. This brain region is not one of the primary language centers of the brain, but plays an important role in language comprehension and the semantic classification of linguistic information. The hippocampus therefore represents one of the higher stages of processing sensory perceptions, which were previously considered to be closely linked to consciousness.
To test the response of the hippocampus under anesthesia, the researchers first played sequences of the same tones to the anesthetized test subjects, which were occasionally interrupted by different tones. It turned out that the neurons of the hippocampus reacted to the tone sequences and different tones in a specific way. The reaction to the “outliers” also increased over the course of the test – a possible indication of neural learning, as the team explains.
Clear signals of semantic decoding
However, the results in the second part of the experiment were even more interesting. In this, Katlowitz and his team played excerpts from podcasts to the test subjects who were under anesthesia. Surprisingly, the neurons in the hippocampus responded to these word sequences in a similar way to when they were awake: their activity pattern reflected semantic connections and even the assignment of individual words to nouns, verbs or adjectives. “These results indicate that the hippocampus can still decode the semantic information of each word even in the unconscious brain,” write the researchers.
According to the neural signals, the anesthetized brain can even predict which word is likely to come next in a sentence – similar to how AI language models do. “We have previously only associated this predictive decoding with wakefulness and attention, but apparently this also happens in the unconscious state,” says Katlowitz’s colleague Benjamin Hayden.
New perspective on unconsciousness
According to the researchers, these results question the common idea of unconsciousness. “Recognizing semantic features of language requires advanced processing of acoustic information,” they explain. Under anesthesia, not only the simple, primary centers of stimulus processing react, but also the higher levels of the brain. Until now, this was only thought possible with full consciousness.
“Our results show that our brains are far more active and capable during unconsciousness than previously thought,” says senior author Sameer Sheth from Baylor College of Medicine. “Even in completely anesthetized patients, the brain continues to analyze the world around them.” This could also explain why some patients can remember events during the operation after waking up from general anesthesia – for example, snippets of conversation they heard.
“These insights require us to rethink what consciousness means,” says Sheth. “Our brains apparently do far more behind the scenes than we previously thought.” However, it is still unclear whether the observed language processing only takes place under anesthesia or also in other forms of unconsciousness such as coma. It also remains to be further investigated which brain regions other than the hippocampus are active in this state.
Source: Kalman Katlowitz (Baylor College of Medicine, Houston) et al., Nature, 2026; doi: 10.1038/s41586-026-10448-0