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Dr. Josef Parvizi recalls meeting a man with epilepsy whose seizures caused some very unusual symptoms.
« He came to my clinic and said, ‘My sense of self is changing,' » she says You parviseprofessor of neurology at Stanford University.
The man told Parvizi that he feels « like an observer of the conversations that are happening in my mind » and that « I feel like I’m floating in space ».
Parvizi and a team of researchers eventually traced the man’s symptoms to a « sausage-looking piece of brain » called precursor.
The finding could help researchers develop forms of anesthesia that use electrical stimulation instead of drugs. It could also help explain the antidepressant effects of mind-altering drugs like ketamine.
Finding the seat of the physical self
It took Parvizi’s team years of research to uncover the importance of this obscure piece of brain tissue.
In 2019, when the man first arrived at Stanford’s Comprehensive Epilepsy Program, Parvizi thought his symptoms were caused by seizures in the posteromedial cortexan area towards the back of the brain.
This area includes a brain network involved in the narrative self, a sort of internal autobiography that helps us define who we are. Parvizi’s team understood that the same network must also be responsible for the bodily self.
« Everyone thought, ‘Well, maybe all kinds of selves get decoded by the same system,' » she says.
A series of experiments on the initial patient and eight other volunteers pointed to a different explanation.
All patients had severe epilepsy and were in hospital trying to locate the source of their seizures. The process requires placing electrodes in the brain and then waiting for a seizure to occur.
These electrodes can also be used to deliver pulses of electricity. So Parvizi’s team was able to stimulate different areas of the brain to see if they affected a person’s sense of self.
When the team stimulated the anterior precuneus, « here, everyone has changes in the sense of what we call a bodily or physical self, » says Parvizi.
In other words, the stimulation produced an out-of-body experience. People felt detached from their thoughts and no longer anchored in their bodies.
The finding was surprising because the anterior precuneus is separated from the brain system to maintain a narrative self. Instead, he seems devoted to the sense that something is « happening to me, » not to another person, Parvizi says.
« We think this might be a way for the brain to label every experience in the environment as ‘mine,' » he says.
A change of perspective
That role for the anterior precuneus makes sense, he says Christopher Lopezresearcher at the National Center for Scientific Research in France.
Lopez has done research suggesting that our sense of a physical self comes in part from the inner ear, which detects movement and monitors the body’s position in space. And Parvizi’s team’s findings suggest that signals from the inner ear are processed by the anterior precuneus.
« When they stimulate the anterior precuneus, you can evoke that the body is floating or falling, » Lopez says.
This means that the brain has to make sense of conflicting information: For example, signals from the inner ear can tell the body is falling while signals from the eyes say it’s still.
As a result, Lopez says, the brain may try to cope by taking on a different perspective.
« Sometimes the brain’s best solution is to think it’s somewhere else, outside the body, » he says.
The brain can face a similar conundrum when people take drugs like ketamine, which affect the anterior precuneus.
« It looks like ketamine is producing this artificial rhythm [in the brain] this is disrupting the function of that area,” he says Patrick Purdoneassociate professor of anesthesia at Harvard Medical School.
That slow pace is similar to what Parvizi’s team saw when they paced the brains of epilepsy patients, Purdon says.
That could mean that it will one day be possible to use electrical impulses instead of anesthetic drugs like ketamine, he says.
« You could get the specific brain areas you want without having to cause a brain- and system-wide effect that could bring with it a lot of side effects, » says Purdon.