The way sensory predictions change under anesthesia tells us how conscious perception works MIT News

Our brains are constantly working to make predictions about what is happening around us, ensuring that we can, for example, pay attention to and accommodate the unexpected. A new study examines how this works while conscious and also breaks down under general anesthesia. The results support the idea that conscious thought requires synchronized communication – mediated by brain rhythms in specific frequency bands – between basic sensory and higher-order cognitive regions of the brain.

Previously, members of the research team at the Picower Institute for Learning and Memory at MIT and Vanderbilt University described how brain rhythms enable the brain to stay prepared for surprises. Cognition-oriented brain regions (generally at the front of the brain) use relatively low-frequency alpha and beta rhythms to suppress sensory regions’ (generally at the back of the brain) processing of stimuli that have become familiar and mundane in the environment (e.g. your colleague’s music). When sensory regions sense a surprise (e.g. the fire alarm in the office), they use faster gamma rhythms to inform the higher regions about it, and the higher regions process this at gamma frequencies to decide what to do (e.g. leaving the building). ).

The new results were published on October 7th in the Proceedings of the National Academy of Sciencesshow that in animals under propofol-induced general anesthesia, a sensory region retained the ability to detect simple surprises but lost communication with a higher cognitive region at the front of the brain, rendering this region incapable to engage with their “supreme” perception. “Down” regulates the activity of the sensory region, keeping it equally insensitive to simple and more complex surprises.

What we have here is a lack of communication

“What we are doing here speaks to the nature of consciousness,” says co-lead author Earl K. Miller, Picower Professor in the Picower Institute for Learning and Memory and the Department of Brain and Cognitive Sciences at MIT. “Propofol general anesthesia deactivates the top-down processes that underlie cognition. It essentially cuts off communication between the front and back of the brain.”

Co-senior author Andre Bastos, an assistant professor in Vanderbilt’s psychology department and a former member of Miller’s MIT lab, adds that the study results highlight the key role of frontal areas in consciousness.

“These results are particularly important given the newfound scientific interest in the mechanisms of consciousness and the relationship between consciousness and the brain’s ability to make predictions,” says Bastos.

The brain’s ability to make predictions is dramatically altered during anesthesia. What was interesting was that areas in the front part of the brain associated with perception were more limited in their predictive abilities than sensory areas. This suggests that prefrontal areas help trigger a “firing event” that allows sensory information to become conscious. Activation of the sensory cortex alone does not result in conscious perception. These observations help us narrow down possible models for the mechanisms of consciousness.

Yihan Sophy Xiong, a graduate student in Bastos’ lab who led the study, says the anesthetic shortens the time in which interregional communication can occur within the cortex.

“In the waking brain, brain waves give neurons short windows of time in which they can fire optimally – the brain’s ‘refresh rate,’ so to speak,” says Xiong. “This refresh rate helps organize different areas of the brain to communicate effectively. Anesthesia slows the refresh rate, which narrows the time windows for communication between brain areas and makes the refresh rate less effective, leaving neurons more confused about when they can fire. When the refresh rate stops working as intended, our ability to make predictions is weakened.”

Learn from oddballs

To conduct the research, the neuroscientists measured the electrical signals, “or spikes,” from hundreds of individual neurons and the coordinated rhythms of their aggregate activity (at alpha/beta and gamma frequencies) in two areas on the surface, the cortex. of the brains of two animals while they listened to sequences of sounds. Sometimes the sequences were all the same note (e.g. AAAAA). Sometimes there was a simple surprise that researchers called a “local oddball” (e.g., AAAAB). But sometimes the surprise would be more complicated or a “global oddball.” For example, after seeing a series of AAAABs, suddenly there was AAAAA, which violates the global but not the local pattern.

Previous work suggests that a sensory region (in this case the temporoparietal area, or Tpt) can detect local oddballs on its own, Miller says. Detection of the more complicated global oddball requires the involvement of a higher order region (in this case the frontal eye fields or FEF).

The animals heard the tone sequences both while awake and under propofol anesthesia. There were no surprises while awake. The researchers reconfirmed that top-down alpha/beta rhythms from FEF conveyed predictions to the Tpt, and that Tpt would increase gamma rhythms when an oddball appeared, causing FEF (and the prefrontal cortex ) also responded with increases in gamma activity.

However, through various measurements and analyses, scientists were able to determine that this dynamic broke down after the animals lost consciousness.

For example, under propofol, peak activity overall decreased, but when a local oddball emerged, the Tpt peak still rose significantly, but now the peak of FEF no longer followed the same trend as when awake.

Meanwhile, when a global oddball was presented while awake, the researchers were able to use software to “decode” the representation of this phenomenon between neurons in the FEF and the prefrontal cortex (another cognition-oriented region). They could also decode local oddballs in the Tpt. But under anesthesia, the decoder could no longer reliably detect the representation of local or global oddballs in the FEF or the prefrontal cortex.

In addition, they found significant differences when comparing the rhythms in the regions in the awake and unconscious states. When the animals were awake, the oddballs increased gamma activity in both Tpt and FEF and the alpha/beta rhythm decreased. Regular, non-unusual stimulation increased the alpha/beta rhythm. But when the animals lost consciousness, the increase in gamma rhythms caused by a local oddball at Tpt was even greater than when the animal was awake.

“Under propofol-induced loss of consciousness, the inhibitory function of alpha/beta was reduced and/or eliminated, resulting in disinhibition of oddballs in the sensory cortex,” the authors wrote.

Other analyzes of interregional connectivity and synchrony revealed that regions lost the ability to communicate during anesthesia.

Overall, the study’s results suggest that conscious thought requires front-to-back coordination across the entire cortex, the researchers wrote.

“Our results therefore suggest that activation of the prefrontal cortex, in addition to activation of the sensory cortex, plays an important role in conscious perception,” the researchers write.

In addition to Xiong, Miller and Bastos, the paper’s other authors include Jacob Donoghue, Mikael Lundqvist, Meredith Mahnke, Alex Major and Emery N. Brown.

The study was funded by the National Institutes of Health, the JPB Foundation and the Picower Institute for Learning and Memory.