Summary: Researchers investigate the influence of anesthesia on brain functions.
“Our brain can be seen as a full football stadium when we are awake,” explains Nikoloz Sirmpilatze, a scientist with the Functional Imaging Unit and lead author of the study.
“Our active neurons are like tens of thousands of spectators all talking at once. However, under anesthesia, neuronal activity is synchronized. You can measure this activity using EEG as uniform waves, as if all the spectators in the stadium were singing the same song.
“In deep anesthesia, this song is repeatedly interrupted by periods of silence. This is called burst suppression. The deeper the anesthesia, the shorter the phases of uniform activity, the bursts, and the longer the periodically recurring inactive phases, the so-called suppressions.”
The phenomenon is caused by many different anesthetics, some of which vary in their mechanism of action. And burst suppression is also detectable in coma patients. However, it is not known whether this condition is a protective response of the brain or a sign of impaired functioning.
It is also unclear where in the brain burst suppression occurs and which brain regions are involved, as localization by EEG alone is not possible.
To answer this question, Nikoloz Sirmpilatze and the research team used the imaging technique of fMRI. The method makes changes in the blood flow in the brain visible.
The increased activity of neurons in a certain area of the brain leads to an increase in metabolism, followed by an increased blood and oxygen supply at this location, which is ultimately visible in the fMRI image.
In the first part of the study, the researchers set up a system to evaluate fMRI data in humans, monkeys and rodents in a standardized way using the same method.
To do this, they used simultaneously measured EEG and fMRI data from anesthetized patients generated in a previously conducted study at the Technical University of Munich.
“We first looked at whether the burst suppression detected in the EEG was also visible in the fMRI data and whether a particular pattern was seen,” says Nikoloz Sirmpilatze.
“Based on that, we developed a new algorithm that allowed us to detect burst suppression events in the experimental animals using fMRI, without additional EEG measurement.”
The researchers then performed fMRI measurements on anesthetized long-tailed macaques, marmosets and rats. In all animals, they were able to detect and accurately localize burst suppression as a function of anesthesia concentration.
The spatial distribution of burst suppression showed that in both humans and monkey species, certain sensory areas, such as the visual cortex, were excluded from it.
In contrast, in the rats, the entire cerebral cortex was affected by burst suppression.
“At the moment we can only speculate about the reasons,” said Nikoloz Sirmpilatze, who received the 2021 PhD Thesis Award from the German Primate Center for his work.
“Primates mainly orient themselves through their eyesight. Therefore, the visual cortex is a highly specialized area that differs from other brain areas by special cell types and structures. This is not the case with rats. In future studies, we will explore what exactly happens in these regions during anesthesia to ultimately understand why burst suppression is undetectable there with fMRI.”
Susann Boretius, head of the Functional Imaging Unit and senior author of the study added: “The study not only raises the question of the extent to which rodents are suitable models for many areas of human brain research, especially when it comes to anesthesia, but the results also have many implications for neuroscience and the evolution of neural networks in general.”
About this neuroscience research news
Original research: Open access.
†Spatial features of anesthesia-induced burst suppression differ between primates and rodentsby Nikoloz Sirmpilatze et al. eLife
Spatial features of anesthesia-induced burst suppression differ between primates and rodents
During deep anesthesia, the brain’s electroencephalographic (EEG) signal alternates between bursts of activity and periods of relative silence (suppression). The origins of burst suppression and its distribution across the brain remain a matter of debate.
In this work, we used functional magnetic resonance imaging (fMRI) to map the brain regions involved in anesthesia-induced burst suppression in four mammalian species: humans, long-tailed macaques, marmosets, and rats.
First, we determined the fMRI signatures of burst suppression in human EEG fMRI data. Applying this method to animal fMRI datasets, we found distinct burst suppression signatures in all species.
The burst suppression maps revealed a clear difference between species: in rats, the entire neocortex was involved in burst suppression, while in primates most sensory areas were excluded – mainly the primary visual cortex.
We expect that the identified species-specific fMRI signatures and whole-brain maps will guide future focused investigations into the cellular and molecular mechanisms of burst suppression in unconscious states.
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