Researchers report that individual neurons in a brain region known for its role in memory consolidation can detect unexpected sounds, understand language semantics, and even predict the next words in a sentence, even when the patient is under full anesthesia, according to a report published by Science News citing the journal Nature.

Researchers have gathered growing evidence that the human brain, even in a state of unconsciousness, can track certain aspects of speech. Athena Akrami, a neuroscientist at University College London who was not involved in the research, says: 'The field was already moving toward a more nuanced picture of what the brain can do during unconsciousness, but this study pushes the boundaries much further.'

Neuropixels probe

To examine the brain during unconsciousness, neurosurgeon Kalman Katlowitz from Baylor College of Medicine in Houston and his colleagues monitored neural activity in seven patients under anesthesia. The research team used a technology developed in the past few years called the Neuropixels probe. These high-density microelectrodes can record the electrical activity of hundreds of individual neurons simultaneously, rather than monitoring the collective activity of neuron groups.

The researchers implanted these probes in the patients' hippocampus, within tissue scheduled for surgical removal for epilepsy treatment.

While the patients were under general anesthesia, the researchers played various sounds through headphones. For some patients, this included a series of regular pure tones interspersed with unexpected oddball tones of a different frequency. For others, the researchers played educational videos and narrative podcasts for 10 to 20 minutes to assess how the brain processes natural speech.

Brain and anesthesia (iStock)

Response exceeding 70%

In the tone experiment, more than 70% of the hundreds of monitored neurons responded to sound and distinguished rare oddball tones from standard tones. The neural response in differentiating oddball from standard tones also improved over the ten-minute session.

In the language experiment, individual neurons responded to the length, type, and meaning of spoken words, and the firing patterns of these cells could predict the characteristics and meaning of the next word. Although the hippocampus in these patients showed patterns similar to those in the brains of conscious individuals, the researchers confirmed that the patients were not conscious despite this neural activity.

Brain and anesthesia (iStock)

Fundamental challenge and puzzling question

This discovery poses a fundamental challenge to prevailing theories of consciousness. Some theoretical frameworks have long assumed that processing speech and predicting subsequent words require consciousness.

Akrami says: 'The computations seem almost identical to those occurring in the brains of conscious individuals, yet they produce no consciousness, no memory, and no ability to act.' She believes the study's results push experts to reconsider a fundamental question: 'If the unconscious hippocampus can encode meaning, learn, and predict, then what is the role of consciousness?'

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