How sensory predictions change under anesthesia could reveal how conscious cognition works

Our brains are constantly working to make predictions about what is happening around us, for example to ensure that we can cope and deal with the unexpected. A new study examines how it works during consciousness and also breaks down under general anesthesia. The findings add evidence to 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, research team members from the Picower Institute for Learning and Memory at MIT and Vanderbilt University described how brain rhythms keep the brain ready for surprises.

Brain regions oriented toward cognition (usually toward the front of the brain) use relatively low-frequency alpha and beta rhythms to suppress processing by sensory regions (usually toward the back of the brain) of stimuli that have become familiar and mundane in the environment (for example your colleague’s music). When sensory regions detect a surprise (e.g. the office fire alarm), they use faster frequency gamma rhythms to notify higher regions and the higher regions process this at gamma frequencies to decide what to do (e.g. quit the building).

The new results published on October 7 in the Proceedings of the National Academy of Sciencesshow that when animals were under propofol-induced general anesthesia, a sensory region retained the ability to detect simple surprises, but communication with a higher cognitive region at the front of the brain was lost, rendering that region unable to detect simple surprises. engage in your “higher brain”. “regulate” the activity of the sensory region and keep it insensitive to simple as well as more complex surprises.

“What we’re doing here speaks to the nature of consciousness,” said co-senior author Earl K. Miller, the Picower Professor in the Picower Institute for Learning and Memory and the Department of Brain and Brain Sciences. cognitive sciences from MIT. “General anesthesia with propofol disables the top-down processes that underlie cognition. It essentially disconnects communication between the front and back halves of the brain.”

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

“These findings are particularly important given the emerging scientific interest in the mechanisms of consciousness and how consciousness relates to the brain’s ability to make predictions,” Bastos said.

“The brain’s ability to predict is significantly altered during anesthesia. It was interesting to note that the front of the brain, areas associated with cognition, were more greatly diminished in their predictive abilities than sensory areas. This suggests that prefrontal areas help trigger an “inflammation” event that allows sensory information to become conscious. Activation of the sensory cortex by itself does not lead to conscious perception. mechanisms of consciousness.

Yihan Sophy Xiong, a graduate student in Bastos’ lab who led the study, said the anesthetic reduces interregional communication delays within the

“In the waking brain, brain waves give neurons short windows of opportunity to function optimally – the brain’s ‘refresh rate,’ so to speak,” Xiong said. “This refresh rate helps organize different areas of the brain to communicate effectively. Anesthesia both slows the refresh rate, which reduces those time windows during which areas of the brain communicate with each other, and makes the refresh rate less efficient, so neurons become more disorganized about when they can fire. When the refresh rate no longer works as expected, our ability to make predictions is weakened.

Learn quirks

To conduct the research, neuroscientists measured the electrical signals, “or spikes,” of hundreds of individual neurons and the coordinated rhythms of their aggregate activity (at alpha/beta and gamma frequencies), in two areas of the surface, or cortex. , from the brains of two animals as they listened to sequences of tones.

Sometimes the sequences would all be the same rating (e.g. AAAAA). Sometimes there was a simple surprise that researchers called a “local oddity” (e.g. AAAAB). But sometimes the surprise would be more complicated, or “weird on a global scale.” For example, after seeing a series of AAAAs, there would suddenly be an AAAAA, which violates the global model but not the local model.

Previous work has suggested that a sensory region (in this case, the temporoparietal area, or Tpt) can detect local oddities on its own, Miller said. More complex global strangeness detection requires the participation of a higher order region (in this case, the frontal eye fields, or FEF).

The animals heard the sound sequences both while awake and under propofol anesthesia. There was no surprise about the wakefulness. The researchers reaffirmed that descending alpha/beta rhythms from the FEF transmitted predictions to Tpt and that Tpt would increase gamma rhythms when an oddity occurred, also causing the FEF (and prefrontal cortex) to respond through an increase in gamma activity.

But thanks to several measurements and analyses, scientists were able to see that this dynamic collapses after the animals lose consciousness.

On propofol, for example, peak activity decreased overall, but when a local phenomenon occurred, Tpt peak again increased noticeably, but now the spike in FEF did not follow suit. the step as is the case during wakefulness.

Meanwhile, when a global oddity was presented while awake, researchers could use software to “decode” the representation of it among neurons in the FEF and prefrontal cortex (another region geared toward cognition). . They could also decode local oddities in the Tpt. But under anesthesia, the decoder could no longer reliably detect the representation of local or global oddities in the FEF or prefrontal cortex.

Additionally, when they compared the rhythms in regions between the awake and unconscious states, they found marked differences. When animals were awake, quirks increased gamma activity in Tpt and FEF rhythms and alpha/beta rhythms decreased. Regular, non-strange stimulation increases alpha/beta rhythms. But when the animals lost consciousness, the increase in gamma rhythms due to local oddity was even greater in Tpt than when the animal was awake.

“In propofol-mediated loss of consciousness, alpha/beta inhibitory function was diminished and/or eliminated, leading to disinhibition of sensory cortex oddities,” the authors wrote.

Further analyzes of inter-region connectivity and synchrony revealed that regions lost the ability to communicate during anesthesia.

Overall, the study results suggest that conscious thought requires coordination across the cortex, from front to back, the researchers wrote.

“Our results therefore suggest an important role for prefrontal cortex activation, in addition to sensory cortex activation, for conscious perception,” the researchers wrote.

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