Even when a patient is unconscious, the human brain is capable of complex thought processes around language, new research indicates.
The scientists recorded the patients’ brain activity while they were under anesthesia, and they found neurons organizing words, processing meaning, and even predicting what would happen next in the narrative. This has huge implications for how we understand the human brain and consciousness.
What the Study Actually Demonstrated
A widely held belief amongst scientists is that higher-level thought processes relating to language, such as comprehension of specific words and prediction of subsequent story events, were only possible when a person was conscious. But neurosurgeon Sameer Sheth and his colleagues at Baylor College of Medicine wanted to test this idea.
In order to do so, they worked with seven patients undergoing clinical procedures for severe neurological conditions.
Before beginning the procedure, each patient received a special electrode called Neuropixels which were inserted into the hippocampus, a region of the brain associated with learning and memory. Through this technology, scientists were able to monitor the activity of hundreds of neurons throughout the hippocampus at the same time.
The patients were then put under standard clinical anesthesia, a commonly used medication capable of putting patients into a deep sleep-like state. Throughout the course of the experiment, scientists played sounds and stories to the unconscious patients while the Neuropixels monitored the activity of their neurons.
The goal was to understand whether and how the hippocampus responded to external auditory stimuli while the patient was under anesthesia. “Even when patients are fully anesthetized, their brains continue to analyze the world around them,” said Sheth.
How Did the Brain Process Sounds While Unconscious?
The first step was to determine whether the unconscious brain was even capable of responding to external auditory stimuli. To do this, scientists played a simple pattern of sounds to each of the patients that involved repeating the same noise over and over again with the occasional different sound inserted into the mix.
This is commonly referred to as an oddball paradigm. Typically, as soon as an awake brain detects this anomaly, it will begin to react to it. Scientists wanted to know whether an unconscious brain would do the same. And it did. When analyzing the recordings, they found that many hippocampal neurons responded to the pattern, with a subset displaying a distinct reaction to the strange sound.
Scientists then took this a step further to determine if the unconscious brain was capable of actually learning. This was done by repeating the pattern of sounds over the course of ten minutes to see whether the responses of the hippocampus would change, which they would if the brain was indeed processing information and adapting.
As it turns out, after ten minutes of continuous sound, the hippocampus was able to distinguish the oddball sound from the repeated pattern more effectively. Additionally, the responses to this anomaly increased over the course of the ten minutes.
This was a sign that learning was taking place—a type of adaptive processing that had not been clearly demonstrated in this detail in anesthetized humans before.
Unconscious Language Processing
Having established that the unconscious brain was capable of processing patterns of sounds, it was only natural to wonder whether it could also process language. To investigate this question, the scientists began playing short stories to four of the patients while they were under.
These stories, taken from The Moth Radio Hour, were written in complete sentences and delivered in first-person perspective. Remarkably, the hippocampus of each patient responded to the language by firing unique patterns of neurons for different words.
By analyzing these patterns, it was possible to determine aspects of the words that were being processed by the brain. This information was later matched with the transcripts for each story with notable accuracy.
Moreover, certain words elicited stronger or more distinctive responses from the hippocampus than others. Specifically, the brain distinguished parts of speech such as nouns, verbs, and adjectives. Words that were similar to one another, such as “dog” and “cat,” elicited more related responses than dissimilar words, such as “dog” and “pen.”
The strength of these connections was also consistent with the responses of conscious patients listening to similar stories. This demonstrated that the unconscious brain was, at least partially, processing language in much the same way that a conscious one would.
Did the Brain Predict the Next Word?
Most impressively, the scientists found that the unconscious brain was even capable of predicting what would occur next in the narrative. They did this by analyzing the firing patterns of the hippocampus in relation to words that had not yet occurred in the story.
This way, they were able to see whether the firing pattern of the brain was indicative of the word that was about to appear. By doing this, the researchers were able to demonstrate that, for the first time in anesthetized humans with this level of single-neuron detail, the hippocampus actually did engage in the process of predictive coding.
In other words, the brain was generating hypotheses about what would happen next in the story. “This kind of predictive coding is something we associate with being awake and attentive, yet it’s happening here in an unconscious state,” said Benjamin Hayden, a professor of neurosurgery at Baylor and senior author on the study.
This does not mean that the unconscious brain is actually conscious in any way. Rather, it just means that certain higher-level thought processes, such as predictive coding, can take place even when a person is deeply anesthetized. It is still unclear whether the unconscious mind can actually think in the traditional sense.
A Broader Understanding of Consciousness
These insights can also help to redefine consciousness itself. According to the researchers, this study adds evidence to a long-held belief that consciousness is not simply the product of activity in single regions of the brain, but rather arises through connections and interactions between different areas.
This information could also have medical implications in the future. As Kalman A. Katlowitz (also referred to as Vigi Katlowitz in some coverage), a neurosurgery resident at Baylor College of Medicine and first author on the paper, and colleagues explained, the findings might help people who have lost the ability to speak, such as patients with severe speech and mobility impairments.
By demonstrating that the hippocampus is still capable of language processing under anesthesia, it becomes possible to consider developing advanced speech prosthetics to enable communication with locked-in patients. One such line of research examining this possibility in conscious patients is currently underway.
What the Study Says
Published in the journal Nature under the title “Plasticity and language in the anaesthetized human hippocampus,” the study provides significant insight into the functioning of the human brain by establishing that single-neuron firings in the hippocampal formation can decode semantic and grammatical features without requiring active consciousness.
What This Study Doesn’t Say
Despite the intriguing nature of these discoveries, they should not be blown up to be more than what they are. First of all, this study only used seven patients and only looked at processing of language under the effects of one particular type of standard clinical sedation.
Future research will be needed to see whether these results apply to other methods or to other profound states of non-responsiveness, such as natural sleep or deep non-conscious states.
Secondly, it is doubtful whether the findings are applicable to all aspects of language processing outside the features analyzed, such as full grammar and syntax in all contexts.
Sources
Unconscious brains can still process language and predict words
Even the unconscious brain can learn — and predict what you’ll say next
Plasticity and language in the anaesthetized human hippocampus