New study finds that anesthesia inhibits brain’s predictive processing

Posted by Mary-Lou Watkinson on Tuesday, October 8, 2024 in News Story, Research.

Our brains constantly work to make predictions about what's going on around us, ensuring that we can attend to and consider the unexpected.

However, in a new study led by Assistant Professor of Psychology Andre Bastos, researchers found that animal subjects under general anesthesia were unable to detect moderate and complex surprises. While a sensory region in the brain retained the capacity to detect simple changes, communication with a higher cognitive region was lost.

"This discovery deepens our understanding about the nature of consciousness and how it arises," Bastos said. "Waking consciousness arises from the interactions between sensory areas and higher-order prefrontal areas. These interactions depend on intact brain wave patterns that facilitate communication."

Bastos said the study, conducted in partnership with first author and graduate student Sophy Xiong and published in the Proceedings of the National Academy of Sciences, could enable better surgical outcomes. Study findings could help clinicians monitor brain waves in response to sensory stimuli to keep patients stably unconscious during surgery.

"Technology based on our scientific findings could also help us to better understand the level of consciousness present in patients that are unresponsive and unable to signal their consciousness with language," he said. "This could help understand if consciousness levels change over time as different treatments are applied and as healing takes place."

Members of the research team had previously described how brain rhythms enable the brain to remain prepared for surprises. Cognition-oriented brain regions use low frequency alpha and beta rhythms, which are waves that wax and wane 10 to 30 times per second, to suppress processing by sensory regions of stimuli that have become familiar and mundane in a person's environment, such as your co-worker's music. When sensory regions detect a surprise, like a fire alarm, they use faster frequency gamma rhythms, which are waves that wax and wane 40 to 90 times per second, to tell the higher regions about it, and the higher regions process that to decide what to do.

To conduct their study, the team measured the electrical signals, or spiking, of hundreds of individual neurons and the coordinated rhythms of their aggregated activity in two areas on the surface of the brains of two animals as the animals listened to sequences of tones. Sometimes the sequences would all be the same note, sometimes there'd be a simple surprise that the researchers called a "local oddball." But sometimes the surprise would be more complicated, or a "global oddball."

Prior work has suggested that a sensory region, in this case the temporoparietal area, can spot local oddballs on its own. Detecting the more complicated global oddball requires the participation of a higher order region, in this case the frontal eye fields.

"What our work shows is that even when we are deeply anesthetized, the sensory brain still registers information that is occurring in the external world," Bastos said. "So why do we not become conscious of sensory information while we are under anesthesia? Using a metaphor to understand our findings, conscious brain function is similar to many sports fans performing 'the wave' at a stadium. The wave allows functional interactions between fans, or the brain's neurons, situated at opposite ends of the stadium. Anesthesia disrupts this wave and makes it more chaotic and less capable of influencing the sports fans. So even if one fan tries to initiate 'the wave', fans on the other end of the stadium cannot engage. This disrupts the ability of the stadium as a whole to carry and understand information."

The animals heard the tone sequences both while awake and while under propofol anesthesia. In the waking state, neurons in both sensory and higher regions signaled the surprising stimuli and coordinated their activity using brain waves. However, under propofol, spiking activity declined overall, but when a local oddball came along, the temporoparietal area spiking still increased notably but spiking in the frontal eye fields did not.

"In the awake brain, brain waves give short windows of opportunity for neurons to fire optimally-the 'refresh rate' of the brain, so to say," said Xiong. "This refresh rate helps organize different brain areas to communicate effectively. In this study, we see that anesthesia both slows down the refresh rate, which narrows these time windows for brain areas to talk to each other. It also makes the refresh rate less effective, so that neurons become more disorganized about when they can fire. When the refresh rate no longer works as intended, our ability to make predictions is weakened."

Additional authors on the paper are Earl Miller, Jacob A. Donoghue, Mikael Lundqvist, Meredith Mahnke, Alex James Major, and Emery N. Brown. Funding for the research was provided by the National Institutes of Health, the JPB Foundation, and the Picower Institute for Learning and Memory.

Read the full paper on PNAS.

Looking Ahead:

Bastos received a National Science Foundation CAREER Award to further investigate the inhibitory brain mechanisms involved in predictive processing. Bastos's lab plans to utilize the funding in a study to understand the role of cortical inhibition, helping to inform how predictions are made in the brain and potentially transforming our understanding of the brain in both health and disease.

Tags: Andre Bastos, Department of Psychology, faculty awards, Psychology

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