Our brains carry on whirling long after we drift off to sleep.
Each night, the hippocampus, a serious hub for learning, replays experiences from the day prior to this and etches them into memory. And even in deep sleep, neurons in sensory regions of the brain spark with activity once they receive latest stimuli, like sounds.
This raises a provocative query: How much is consciousness required to make sense of the world around us?
A brand new study suggests the unconscious brain can handle excess of easy 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 style of word spoken—nouns or verbs, for instance—and predicted the following word in sentences.
“Our findings show that the brain is much more energetic and capable during unconsciousness than previously thought,” study creator Sameer Sheth said in a press release. “Even when patients are fully anesthetized, their brains proceed to research the world around them.”
Scientists have long thought that language processing, a fancy computation, relied on awareness. Anesthesia disrupts large-scale communication across the brain, seemingly making complex language processing inconceivable. But the brand new findings suggest that at the same time as global brain dynamics break down, some local circuits retain the flexibility to process sophisticated information—and, at the 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 can also be up for debate.
But “this work pushes us to rethink what it means to be conscious,” said Sheth. “The brain is doing way more behind the scenes than we fully understand.”
Lights Out
We slip into unconsciousness every night. The brain shifts gears.
In comparison with after 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 vital for processing latest information from the surface world. These unique electrical signals are crucial for sorting latest experiences and integrating them into long-term memory.
The brain is clearly busy during unconsciousness, nevertheless it also seems largely sealed off from its surroundings. Over the past twenty years, nevertheless, scientists have increasingly realized the sleeping brain stays surprisingly alert.
In a single study, volunteers repeatedly exposed to unfamiliar sounds during sleep were capable of discover them after waking up. In one other, participants hearing their very own names or indignant voices triggered brain activity even in deep sleep, a phenomenon called “sentinel processing.”
Scientists have also recorded directly from the brains of individuals with epilepsy, who had electrodes implanted to pinpoint the source of seizures. The researchers confirmed that the auditory cortex—the primary region involved in processing sound—lit up with activity, nevertheless it appeared disconnected with regions accountable for interpreting meaning.
Similar patterns emerged under other states of unconsciousness. After receiving propofol, a standard drug used to induce general anesthesia, patients still showed activity of their auditory cortex, but information relay to higher regions involved in cognition looked as if it would break down.
Or did it?
“The brain has developed such amazing, sophisticated mechanisms for doing all these complex tasks all day long, that it could possibly do a few of this stuff even without us being aware,” Sheth told Nature. They decided to take one other look.
Someone’s Home
The team focused on the hippocampus, best generally known as the brain’s memory center. Linking it to language processing looks as if a stretch. But mounting evidence suggest the hub is accountable for excess of memory. It may help organize information more broadly, from the mapping of physical spaces to watching other unfolding events like language.
It’s still a distinct segment idea, said Sheth. However the hippocampus could play a much wider role in structuring the world around us—even without awareness. “How is the world organized? The hippocampus could also be a part of that as well,” he said.
To check the thought, 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 filled with over a thousand sensors that snoop on the electrical chatter of lots of of neurons directly.
The team first played repetitive beeps to 3 participants, occasionally interrupted by random boops at a unique pitch. At first, neurons were indifferent to the oddball sounds. But inside 10 minutes, their activity levels showed they were improving at separating the unexpected tones from the traditional 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 unusual words like “cosmos.” Others tracked grammatical structure, responding in a different way to nouns, verbs, or adjectives.
The neurons also cared about semantic meaning, or the relationships between words. For instance, they looked as if it would recognize that “cat” is conceptually closer to “dog” than an unrelated word like “pen.” The hippocampus also looked as if it would anticipate upcoming words based on the context of a sentence, with activity patterns just like those seen within the awake brain.
“We’re all the time making predictions about what we’re about to listen to next,” said Sheth. Even under anesthesia, these neurons appeared to maintain track of the narrative, indicating a “very sophisticated type 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 can have lingered unconsciously. In future studies, the team plans to check for this by exposing unconscious participants to different podcasts then later asking which of them feel familiar. Additionally they wish to explore whether the hippocampus processes stories told in unfamiliar languages.
The findings are preliminary, drawn from a small group of individuals under one style of anesthetic. The sleeping or comatose brain may fit in a different way. However the work could help scientists decipher brain activity in individuals with severe traumatic brain injuries in a vegetative state. It could also guide the event of implants to rewire damaged neural circuits to other parts of the brain and reboot communication.
“Perhaps crucial thing is what can we do about this,” said Sheth. For somebody who’s unconscious, “can we bring them back?”