Findings suggest consciousness may not be necessary for complex brain tasks
General anesthesia shuts down consciousness, but what does our brain process while we are under anesthesia?
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 the types of words coming in a sentencewhile a patient is completely anesthetized, researchers report May 6 in Nature.
Scientists are gathering more and more evidence that even when unconscious, our brains can track certain aspects of speech. “The field was already evolving towards a more nuanced image [of what the unconscious brain can do]but this study pushes the boundaries considerably further,” says Athena Akrami, a neuroscientist at University College London who was not involved in the research.
To examine the unconscious brain, neurosurgeon Kalman Katlowitz of Baylor College of Medicine in Houston and his colleagues monitored the activity of the hippocampi of seven anesthetized patients. The team used technology developed over the past few years called 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 the patients’ hippocampus, into tissue intended to be surgically removed as part of the treatment of epilepsy.
While the patients were under general anesthesiathe researchers played various sounds through headphones. For some patients, this consisted of a series of pure, even tones interspersed with occasional, unexpected “weird” tones of a different frequency. For others, researchers delivered 10 to 20 minutes of educational videos and narrative podcasts, such as The Moth Radio Hourto assess how the brain processes natural speech.
In the sound experiment, more than 70 percent of the hundreds of neurons monitored responded to the audio and distinguished rare strange tones from standard tones. This neural response improved at distinguishing oddities from standard tones over the 10-minute session. And in the language experiment, individual neurons responded to the length, type, and meaning of the words spoken. Neuron firing patterns could even predict the meaning of the next word. Although these patients’ hippocampi showed similar patterns to those in the waking brain, the researchers are confident that the patients were not secretly awake during the study.
“Here we demonstrate that some of the most complex things a human brain can do, like adapting to the environment and understanding language, can operate completely independently of consciousness,” Katlowitz says.
This discovery fundamentally challenges major theories of consciousness. Historically, some frameworks have argued that speech processing and prediction of upcoming words requires awareness.
“The calculations look almost identical to those performed in waking brains, but they produce no consciousness, no memory, no ability to act,” Akrami explains. The book invites experts to consider a fundamental question: “If the unconscious hippocampus can encode meaning, learn and anticipate… then what exactly is consciousness for?
