General anesthesia shuts off conscious awareness, but what do our brains process while we’re under?
Individual neurons in a brain region known for its role in memory consolidation can detect unexpected sounds, decode the nuances of language and even predict upcoming word types in a sentence, all while a patient is fully anesthetized, researchers report May 6 in Nature.
Scientists have been gathering mounting evidence that even when unconscious, our brains can track certain aspects of speech. “The field was already moving toward a more nuanced picture [of what the unconscious brain can do], but this study pushes the boundary considerably further,” says Athena Akrami, a neuroscientist at University College London who was not involved with the research.
To peer into the unconscious brain, neurosurgeon Kalman Katlowitz of Baylor College of Medicine in Houston and colleagues monitored activity in the hippocampi of seven anesthetized patients. The team used a technology developed within the last few years called a Neuropixels probe. These high-density microelectrodes can record the electrical activity of hundreds of individual neurons simultaneously, rather than listening to the collective activity of groups of neurons. The team inserted these probes into patients’ hippocampi, in tissue slated for surgical removal as part of epilepsy treatment.
While the patients were under general anesthesia, the researchers played various sounds through headphones. For some patients, this consisted of a series of uniform pure tones interspersed with occasional, unexpected “oddball” tones of a different frequency. For others, the researchers played 10 to 20 minutes of educational videos and storytelling podcasts, like The Moth Radio Hour, to evaluate how the brain processes natural speech.
In the tone experiment, more than 70 percent of the hundreds of monitored neurons responded to the audio and distinguished the rare oddballs from the standard tones. This neural response grew better at telling oddballs from standard tones over the 10-minute session. And in the language experiment, individual neurons responded to the length, type and meaning of the spoken words. The neurons’ firing patterns could even predict the next word’s meaning. Though these patients’ hippocampi showed patterns similar to those in awake brains, the researchers are confident the patients’ weren’t secretly awake during the study.
“We demonstrate here that some of the most complex things a human brain can do, such as adapt to the environment and understand language, can function entirely independently of consciousness,” Katlowitz says.
This discovery fundamentally challenges prominent theories of consciousness. Historically, some frameworks have argued that processing speech and predicting upcoming words require consciousness.
“The computations look nearly identical to those in awake brains, yet they produce no awareness, no memory, no ability to act,” Akrami says. The work invites experts to ponder a fundamental question: “If the unconscious hippocampus can encode meaning, learn and anticipate… then what exactly is consciousness for?”
