The Unconscious Brain Is Still Listening, New Study Shows

The human brain appeared to process complex sensory information under general anesthesia, challenging long-held assumptions about consciousness, neural recordings from a small experiment suggested.

Hippocampal neurons and local brain oscillations responded to “oddball” sounds — interspersed high- and low-pitched tones — among epilepsy surgery patients with anesthesia-induced unconsciousness, suggesting a capacity for pattern recognition and plasticity, reported Sameer Sheth, MD, PhD, of Baylor College of Medicine in Houston, and colleagues in Nature.

When exposed to language stimuli from “The Moth” podcast, both single neurons and local oscillations encoded information about natural speech. In some cases, brain activity appeared to predict semantic aspects of upcoming words.

The findings suggest that the hippocampus, a brain region anatomically and functionally distant from primary sensory centers, may be capable of sophisticated sensory analysis during unconsciousness.

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

Sheth and colleagues used high-density Neuropixels probes to measure single-neuron firing and broader local brain activity in the hippocampus of seven adults undergoing anterior temporal lobectomy surgery for seizure control.

All patients were diagnosed with drug-resistant temporal lobe epilepsy. Their mean age was 39.6 years; three were female and four were male.

The researchers conducted neural recordings under general anesthesia for up to 30 minutes. All seven patients received total intravenous anesthesia, mainly with propofol. No patient reported explicit memory of the events during surgery when interviewed in the postoperative care unit or during recovery the following day.

After a brief baseline recording, the researchers presented patients with auditory stimuli. Three patients were exposed to pure tones, and four were exposed to podcast episodes.

Patients in the pure tones group were presented with identical 100 ms tones interspersed with oddball tones with a 20% higher or lower frequency. Their neural response to oddball tones grew more distinct over the course of the experiment, which lasted around 10 minutes, the researchers noted.

Patients in the podcast group were exposed to 10 to 20 minutes of stories from “The Moth” episodes. Neural activity in these patients indicated an ability to differentiate parts of speech like nouns and verbs. The researchers also found a consistent relationship between neural responses and the relative surprise of each word presented.

The results paralleled those obtained from a separate cohort of awake patients who performed a similar task in the epilepsy monitoring unit. The data indicated that neuronal firing patterns in response to one word could be predicted from responses to other semantically related words, implying that the unconscious hippocampus may have access to abstract relationships in language, Sheth and colleagues noted.

The findings suggest that some high-level process of sensory integration is preserved in anesthesia-induced unconsciousness but the ability to consolidate this information is compromised, the researchers observed. “These results provide a potential explanation for previous reports of post-anesthesia implicit recall, which would depend on sensory processing and plasticity processes,” they wrote.

The study had several limitations, Sheth and colleagues acknowledged. It’s unclear whether the findings apply to non-conscious states like sleep or coma, or to other anesthetics besides propofol. The processes described may not be unique to the hippocampus, and it’s unknown how widespread they may be across different regions of the brain.

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