While a patient is fully anesthetized and unresponsive, neurons in the hippocampus continue to process language, distinguish different types of words, and generate neural activity consistent with predicting upcoming words.
Researchers at Baylor College of Medicine directly recorded this neural activity, offering new insight into how unconsciousness works at the cellular level. Their findings, published in Nature, show that hippocampal neurons continue to process aspects of language even when the brain is unconscious.
“Our findings show that the brain is far more active and capable during unconsciousness than previously thought,” said Dr. Sameer Sheth, professor and Cullen Foundation Endowed Chair of neurosurgery and a McNair Scholar at Baylor. “Even when patients are fully anesthetized, their brains continue to analyze the world around them.”
a Rare Look Inside the Anesthetized Hippocampus
Sheth’s team recorded from hundreds of individual neurons in the hippocampus, a region of the brain essential for memory formation. Researchers collected data during epilepsy surgeries, specifically anterior temporal lobectomies, when patients were under general anesthesia and the hippocampus was temporarily accessible. This rare opportunity allowed researchers to directly record hippocampal activity, something scalp EEG cannot.
The researchers used Neuropixels probes, high-density recording devices that can monitor hundreds of neurons simultaneously. This study represents the first use of this technology in the human hippocampus during anesthesia. Seven patients participated, all under propofol anesthesia with consistent sedation levels maintained throughout the recordings.
The Unconscious Brain Still Learns
In the initial test, researchers played patients a repeating tone with occasional unexpected pitches. Hippocampal neurons consistently detected these outlier tones, and their sensitivity increased over approximately ten minutes. Researchers usually observe this kind of adaptive neural response during wakefulness.
To dig deeper, the researchers built a computer model of a recurrent neural network, trained to perform the same tone discrimination task. The model showed the same pattern as the human brain recordings, suggesting that this increased sensitivity to outlier tones can arise from local brain circuits alone, without the need for higher-level conscious processing.
Predicting Words Before They’re Spoken
The researchers also played short stories to anesthetized patients while recording from the same neurons. Hippocampal activity tracked language in real time, distinguishing between nouns, verbs, and adjectives. Neural activity reflected both the current word and prediction of the upcoming word in the sequence. “The brain appears to anticipate what comes next in a story, even without conscious awareness,” Sheth said.
“This kind of predictive coding is something we associate with being awake and attentive, yet it’s happening here in an unconscious state,” added Baylor neurosurgery professor Benjamin Hayden.
What This Means for the Science of Consciousness
The results show that the hippocampus can support neural computations associated with complex cognition even during unconsciousness. This suggests that at least some components of language processing and prediction may not require conscious awareness. Sheth’s team proposes that consciousness does not arise from a single brain region but rather emerges from interactions among multiple brain regions. The hippocampus may process information that later becomes part of awareness.
The researchers noted a parallel with artificial intelligence. Large language models generate text by predicting the next word in a sequence; the anesthetized hippocampus showed a similar pattern of prediction. This similarity may provide neuroscientists and AI researchers with a shared framework for studying language prediction based on biological circuitry.
“Can we use these signals to deploy and run a speech prosthetic for some of the parts of the brain that are damaged by stroke or injury? These are questions that we can now consider in relation to this part of the brain,” said study author and Baylor neurosurgery resident Kalman Katlowitz.
While these results are promising, the team acknowledged the study’s limitations. The research focused only on propofol-based general anesthesia, so the results may not apply to other unconscious states such as natural sleep or coma. Additionally, the researchers examined only the hippocampus, so they cannot yet determine how broadly this type of unconscious processing occurs in other brain regions.
“This work pushes us to rethink what it means to be conscious,” Sheth concluded. “The brain is doing much more behind the scenes than we fully understand.”
Austin Burgess is a writer and researcher with a background in sales, marketing, and data analytics. He holds an MBA, a Bachelor of Science in Business Administration, and a data analytics certification. His work focuses on breaking scientific developments, with an emphasis on emerging biology, cognitive neuroscience, and archaeological discoveries.