Where does the mind go when we lose consciousness?
Not that long ago, doctors generally assumed that unconscious brains were simply turned “off,” but that idea seems increasingly difficult to sustain. As scientists probe the far reaches of human awareness, we’re learning that when the lights seem to be out upstairs, the brain may still be alive with activity.
One frontier of discovery is the phenomenon known as “covert consciousness.” Over the past two decades, neuroscientists have begun to understand that many people in so-called vegetative states can, in fact, respond to mental commands with their imaginations, answer yes and no questions, and possibly feel pain. Another borderland of research is dream engineering. During the first stages of sleep, we can still hear and process sounds, and scientists are finding that dreams can potentially be manipulated through audio signaling. Now scientists have discovered that the human brain may even be able to learn and process language when we’re under anesthesia.
A study published in Nature earlier this month reports that neurons in the human hippocampus can identify audio patterns and get better at recognizing them over a short period of time when under the influence of general anesthesia. The scientists showed that these hippocampal neurons may also process human language and grammar, and potentially build predictive models of the world as well.
“Our findings show that the brain is far more active and capable during unconsciousness than previously thought,” said Sameer Sheth, a neurosurgeon at Baylor University and co-author of the study, in a statement. “Even when patients are fully anesthetized, their brains continue to analyze the world around them.”
This defies the most prominent theories of consciousness, which hold that sophisticated cognition beyond basic sensory processing cannot happen without conscious awareness, particularly if abstraction or leaps in time are involved. If the results hold up, they have implications for how we understand memory and language and how we build brain-computer interfaces.
Read more: “What’s So Hard About Understanding Consciousness?”
The scientists, from the Baylor College of Medicine, conducted two experiments. In the first, they played repeating tones for three patients, occasionally interrupting these recordings with so-called “oddball” tones, while recording the activity in hundreds of hippocampal neurons. They used Neuropixels probes to record the neuron activity, a technology that had never been used on the hippocampus before. What they noticed: The neurons detected these oddball tones and got better at detecting them over a short period of 10 minutes. The neurons not only became more sensitive to the unusual sounds, they reorganized how they represented the tones, which is a more sophisticated form of learning than the team had expected.
In the second experiment, the scientists played a podcast for four patients and recorded how hippocampal neurons responded to word frequency, grammatical part of speech, and word category—whether the word described a place, emotion, or object. They found that the neurons responded to these variations about 86 percent of the time, similar to rates found in patients who were awake. The neurons seemed to even record information about upcoming words in a sentence, suggesting activation of a version of predictive coding, an influential and widely adopted framework of brain function that holds that instead of passively registering sensory inputs, the brain actively creates internal models of the world.
“The brain appears to anticipate what comes next in a story, even without conscious awareness,” said Sheth. “This kind of predictive coding is something we associate with being awake and attentive, yet it’s happening here in an unconscious state,” added co-author Benjamin Hayden, a professor of neurosurgery at Baylor.
“Could scientists use audio signals to 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,” continued Vigi Katlowitz, first author and a neurosurgery resident with Baylor.
There are limitations. The findings only apply to anesthesia under propofol, a method that alters brain connectivity, typically inducing unconsciousness by interrupting high-level communication between brain regions. (It’s often described as inducing a “sleep-like” state.) They also can’t be generalized to other forms of unconsciousness, such as coma, vegetative states, or sleep. And they only relate to the hippocampus—though other regions of the brain could show similar effects, the scientists propose. Another limitation to their study: The sample size was tiny, only seven patients. Furthermore, the patients were all epilepsy patients already undergoing temporal lobectomies, so the tissue being recorded from was slated for removal anyway, which is how they could do such invasive recording ethically.
Still, the study has important implications for our understanding of consciousness.
“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.” 
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Lead image: C. Fish Images / Adobe Stock
