Researchers have looked at the brain activity of monkeys to better understand why some patients can regain consciousness after being anesthetized during surgery.
- To better understand the effects of general anesthesia on the brain, and in particular to determine whether there is a state of consciousness during this procedure, the researchers measured the brain activity of a couple of signs during several states of consciousness.
- They then found that this state of consciousness is not detectable in the prefrontal lobe, but rather in the deep cerebral areas, and that it is the complexity of the brain waves, and not their frequency, which indicates whether a subject is conscious or no.
This is probably the nightmare of many patients preparing to enter the operating room: waking up during the procedure, when they are supposed to be asleep thanks to general anesthesia.
For some, it really does happen. According to available data, one out of 19,000 operated patients regains consciousness during general anesthesia.
An extremely rare phenomenon and a medical mystery whose answer could be found… in brain activity. In a study published in the journal Cell Systems, researchers from the University of Wisconsin-Madison (United States) explain that they have succeeded in defining which parts of the brain are involved in this recovery of consciousness during medical procedures.
“What has been shown for a hundred years in an unconscious state like sleep are these slow waves of electrical activity in the brain”explains Yuri Saalmann, who led the work, who believes that “these may not be the right signals to exploit”. “In a number of conditions – with different anesthetic drugs, in people with a coma or brain damage or other clinical situations – there can be high frequency activity as well.”
Focus on deep areas of the brain
To find out, the researchers followed two monkeys, whose brain activity they recorded during several states of consciousness: under drug anesthesia, in light sleep, in a state of wakefulness at rest, and awakened from anesthesia in a state wakefulness by electrical stimulation of a deep point in the brain.
They then collected the data in different regions of the brain, and in different states of consciousness, which allowed them to combine all the signs traditionally associated with consciousness – including the speed or slowness of brain rhythms in different areas of the brain. – with more computational measurements that describe the complexity of signals and how signals in different areas interact.
In order to determine whether or not the monkeys were in a conscious state, the researchers used machine learning. They then fed their large dataset to a computer, told it which state of consciousness produced each pattern of brain activity, and asked which areas of the brain and which patterns of electrical activity best matched consciousness.
The results showed that the prefrontal cortex was not affected, whereas it was hitherto considered to be the part of the brain most likely to present the slow waves of activity considered to be typical of unconsciousness.
“In clinics, electrodes are placed on the patient’s forehead, recalls Mohsen Afrasiabi, the study’s other lead author. We propose that the back of the head is a more important location for these electrodes because we have learned that the back of the brain and deep areas of the brain are more predictive of the state of consciousness than the front.”
The complexity of brain waves, revealing the state of consciousness
The results also showed that, more than high and low frequency activity, it is the complexity of the brain waves that best indicates whether the subject is conscious or not. “It takes more complexity to convey more information, that’s why it is related to consciousness, explains Michelle Redinbaugh, who also participated in the study. If these important areas of the brain are less complex, they cannot transmit much information. You are in the presence of an unconscious brain.”
Researchers now hope to take more precise measurements on patients undergoing anesthesia. The ultimate goal will be to help people with impaired consciousness. “We could use what we’ve learned to optimize electrical patterns through precise brain stimulation and help people who are, for example, in a coma to maintain a continuous level of consciousness.”
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