Study of Hippocampal Neuron Activity Under Anesthesia

Main Experimental Results

During the experiment, the activity of individual hippocampal neurons was recorded in patients under deep anesthesia. The data showed that even under anesthesia, the brain not only perceives sounds but can also distinguish meaning, identify parts of speech, and anticipate the next word. These findings expand our understanding of the capabilities of unconscious perception.

Traditional Views and New Data

It was previously believed that complex cognitive processes require full consciousness. Abstract concepts, understanding of context, and grammar were traditionally associated only with the awake brain. However, it is known that primary sensory areas of the cortex continue to respond to auditory and visual stimuli even during anesthesia. It remained unclear whether higher levels of the neural hierarchy, such as the hippocampus, could perform analytical work without conscious involvement.

Experiment Procedure

Seven patients participated in the study, all of whom were scheduled for brain surgery due to severe epilepsy. Before the surgical intervention, the patients were placed under deep anesthesia using propofol. At this point, Neuropixels microelectrodes were inserted into the hippocampal tissue, allowing the recording of activity from hundreds of neurons.

While the patients were asleep, they were played two types of audio materials: a series of identical monotone sounds with occasional unusual tones, and podcasts featuring ordinary stories. In the first case, hippocampal cells not only distinguished the unusual tones but also showed increased response accuracy over time, indicating neuroplasticity and the ability to learn even under anesthesia.

In the second case, neurons exhibited selectivity: their firing rates depended on the rarity or unexpectedness of specific words in the text. Using machine learning algorithms, researchers were able to determine from cell activity which semantic category the heard word belonged to (place, emotion, or object), as well as distinguish nouns from other parts of speech. Brain activity also allowed for statistical prediction of the category of the next word.

Significance of the Results

The study demonstrated that the brain continues to assemble sounds into meaningful constructs even when consciousness is completely shut down. The hippocampus likely receives already processed signals from other brain regions that also remain active under anesthesia.

The data obtained may help in understanding complex clinical conditions and the phenomenon of implicit memory, when patients register events occurring on the operating table without being aware of it. The mechanism of covert speech analysis partially explains why people in deep coma or during sleep sometimes respond to familiar voices and words.

Conclusions

The results of the study indicate that maintaining context and understanding meaning does not always require global, active attention. A significant portion of analytical work can be performed by neurons entirely in the background.