Our brains keep on whirling long after we drift off to sleep.

Each night, the hippocampus, a major hub for learning, replays experiences from the previous day and etches them into memory. And even in deep sleep, neurons in sensory regions of the brain spark with activity when they receive new stimuli, like sounds.

This raises a provocative question: How much is consciousness required to make sense of the world around us?

A new study suggests the unconscious brain can handle far more than simple sensory cues. Recording electrical activity from patients under general anesthesia, a team at Baylor College of Medicine and collaborators found the hippocampus continued processing sounds, words, and speech while patients listened to alternating tones and podcast clips.

Groups of neurons shifted their activity depending on the type of word spoken—nouns or verbs, for example—and predicted the next word in sentences.

“Our findings show that the brain is far more active and capable during unconsciousness than previously thought,” study author Sameer Sheth said in a press release. “Even when patients are fully anesthetized, their brains continue to analyze the world around them.”

Scientists have long thought that language processing, a complex computation, relied on awareness. Anesthesia disrupts large-scale communication across the brain, seemingly making complex language processing impossible. But the new findings suggest that even as global brain dynamics break down, some local circuits retain the ability to process sophisticated information—and, at least for storytelling, predict what comes next.

To be clear, it doesn’t mean that participants were secretly awake. Whether the brain retains local processing power during sleep, coma, or other states of unconsciousness is also up for debate.

But “this work pushes us to rethink what it means to be conscious,” said Sheth. “The brain is doing much more behind the scenes than we fully understand.”

Lights Out

We slip into unconsciousness every night. The brain shifts gears.

Compared to when we’re awake and alert, the mind’s activity patterns change dramatically. The hippocampus reactivates neurons involved in recent learning, rapidly replaying their activity patterns to strengthen neural connections. Elsewhere, the brain generates short bursts of electrical activity called sleep spindles, which shut off communication between regions necessary for processing new information from the outside world. These unique electrical signals are crucial for sorting new experiences and integrating them into long-term memory.

The brain is clearly busy during unconsciousness, but it also seems largely sealed off from its surroundings. Over the past two decades, however, scientists have increasingly realized the sleeping brain remains surprisingly alert.

In one study, volunteers repeatedly exposed to unfamiliar sounds during sleep were able to identify them after waking up. In another, participants hearing their own names or angry voices triggered brain activity even in deep sleep, a phenomenon called “sentinel processing.”

Scientists have also recorded directly from the brains of people with epilepsy, who had electrodes implanted to pinpoint the source of seizures. The researchers confirmed that the auditory cortex—the first region involved in processing sound—lit up with activity, but it appeared disconnected with regions responsible for interpreting meaning.

Similar patterns emerged under other states of unconsciousness. After receiving propofol, a common drug used to induce general anesthesia, patients still showed activity in their auditory cortex, but information relay to higher regions involved in cognition seemed to break down.

Or did it?

“The brain has developed such amazing, sophisticated mechanisms for doing all these complex tasks all day long, that it can do some of these things even without us being aware,” Sheth told Nature. They decided to take another look.

Someone’s Home

The team focused on the hippocampus, best known as the brain’s memory center. Linking it to language processing seems like a stretch. But mounting evidence suggest the hub is responsible for far more than memory. It may also help organize information more broadly, from the mapping of physical spaces to watching other unfolding events like language.

It’s still a niche idea, said Sheth. But the hippocampus could play a much broader role in structuring the world around us—even without awareness. “How is the world organized? The hippocampus may be part of that as well,” he said.

To test the idea, the team recruited seven people undergoing epilepsy surgery. While they were under propofol anesthesia, the team inserted tiny probes into the hippocampus. Called Neuropixels, the implants are thinner than a human hair but packed with over a thousand sensors that eavesdrop on the electrical chatter of hundreds of neurons at once.

The team first played repetitive beeps to three participants, occasionally interrupted by random boops at a different pitch. In the beginning, neurons were indifferent to the oddball sounds. But within 10 minutes, their activity levels showed they were getting better at separating the unexpected tones from the normal ones.

“They learned over time to pay more attention to oddball sounds,” even while the person was fully unconscious, said Sheth.

A second test took things further. The team played 10-minute snippets from The Moth Radio Hour, a storytelling podcast featuring speakers from all walks of life, each with distinct intonations, turns of phrases, and accents.

Across the recordings, specific groups of hippocampal neurons responded to different linguistic features. Some were attuned to uncommon words like “cosmos.” Others tracked grammatical structure, responding differently to nouns, verbs, or adjectives.

The neurons also cared about semantic meaning, or the relationships between words. For example, they seemed to recognize that “cat” is conceptually closer to “dog” than an unrelated word like “pen.” The hippocampus also seemed to anticipate upcoming words based on the context of a sentence, with activity patterns similar to those seen in the awake brain.

“We are always making predictions about what we’re about to hear next,” said Sheth. Even under anesthesia, these neurons appeared to keep track of the narrative, indicating a “very sophisticated form of processing of the natural speech that they’re listening to.”

Despite intense neural activity, patients didn’t remember any of the podcast stories upon waking. Still, traces of the experience may have lingered unconsciously. In future studies, the team plans to test for this by exposing unconscious participants to different podcasts then later asking which ones feel familiar. They also want to explore whether the hippocampus processes stories told in unfamiliar languages.

The findings are preliminary, drawn from a small group of people under one type of anesthetic. The sleeping or comatose brain may work differently. But the work could help scientists decipher brain activity in people with severe traumatic brain injuries in a vegetative state. It could also guide the development of implants to rewire damaged neural circuits to other parts of the brain and reboot communication.

“Maybe the most important thing is what can we do about this,” said Sheth. For someone who’s unconscious, “can we bring them back?”