A recent study by neuroscientists at The Picower Institute for Learning and Memory at MIT and Vanderbilt University has uncovered how general anesthesia disrupts the brain's predictive mechanisms - a key element of conscious thought. The research, unveiling the complexities of how different brain regions collaborate, reveals significant insights into consciousness and cognition. The findings were reported on October 7, 2024, in the Proceedings of the National Academy of Sciences, as reported by MIT News.
The brain's intrinsic ability to predict and respond to unexpected stimuli hinges on synchronized communication, facilitated by alpha, beta, and gamma brain rhythms, between its sensory and cognitive regions. According to the research team, under the influence of propofol-induced anesthesia, the brain's sensory regions can still detect surprises such as unusual sounds; however, the vital communication link with the cognitive regions, which orchestrate the brain's predictive and responsive functions, is severed.
Co-senior author Earl K. Miller, Picower Professor at The Picower Institute for Learning and Memory and MIT's Department of Brain and Cognitive Sciences, encapsulated the essence of the study's implications. "Propofol general anesthesia deactivates the top-down processes that that underlie cognition. It essentially disconnects communication between the front and back halves of the brain," Miller told MIT News. Andre Bastos, an assistant professor in the psychology department at Vanderbilt, underscored the significance for understanding consciousness, stating that frontal brain areas are integral to our conscious experience.
Researchers used sequences of tones with occasional deviations called "local oddballs" and "global oddballs" to measure the brain's predictive capabilities in two animals, both while awake and under anesthesia. While an alert brain responded to these anomalies with increased gamma rhythms and decreased alpha/beta rhythms, this dynamic changed under anesthesia. Yihan Sophy Xiong, a graduate student leading the study, observed, "Anesthesia both slows down the refresh rate, which narrows these time windows for brain areas to talk to each other and makes the refresh rate less effective, so that neurons become more disorganized about when they can fire," as noted by MIT News.
By examining the coordination of spiking activities and rhythmic patterns, the team concluded that conscious perception requires not just sensory cortex activation, but also an "ignition" by prefrontal areas. In wakefulness, oddballs spurred a coordinated response between the Tpt, a sensory region, and the FEF, a higher-order cognitive region. Anesthesia, however, suppressed this interplay - disrupting the predictive process and reflective consciousness.
The collective efforts of Xiong, Miller, Bastos, and their colleagues contribute to a burgeoning field aiming to decode the mechanisms of consciousness. This study, funded by the National Institutes of Health, the JPB Foundation, and the Picower Institute for Learning and Memory, pushes the boundaries on what we know about the brain's inner workings, particularly in altered states induced by general anesthesia.