Dr. Josef Parvizi remembers meeting a man with epilepsy whose seizures were causing some very unusual symptoms.
"He came to my clinic and said, 'My sense of self is changing,'" says Parvizi, a professor of neurology at Stanford University.
The man told Parvizi that he felt "like an observer to conversations that are happening in my mind" and that "I just feel like I'm floating in space."
Parvizi and a team of researchers would eventually trace the man's symptoms to a "sausage-looking piece of brain" called the anterior precuneus.
This area, nestled between the brain's two hemispheres, appears critical to a person's sense of inhabiting their own body, or bodily self, the team recently reported in the journal Neuron.
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 discover the importance of this obscure bit of brain tissue.
In 2019, when the man first came to Stanford's Comprehensive Epilepsy Program, Parvizi thought his symptoms were caused by seizures in the posteromedial cortex, an area toward 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 figured that the same network must be responsible for the bodily self too.
"Everybody thought, 'Well, maybe all kinds of selves are being decoded by the same system,'" he says.
A series of experiments on the initial patient and eight other volunteers pointed toward a different explanation.
All the patients had severe epilepsy and were in the hospital as part of an effort 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 whether they affected a person's sense of self.
When the team stimulated the anterior precuneus, "lo and behold, everybody has changes in their sense of what we call the bodily or physical self," Parvizi says.
In other words, the stimulation produced an out-of-body experience. People felt detached from their own thoughts and no longer anchored in their own bodies.
The finding was surprising because the anterior precuneus is separate from the brain's system for maintaining a narrative self. Instead, it appears devoted to the sense that something is "happening to me," not another person, Parvizi says.
"We think this could be a way for the brain to tag every experience in the environment as 'mine,'" he says.
A shift in perspective
That role for the anterior precuneus makes sense, says Christophe Lopez, a researcher at the National Centre 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 motion and monitors the body's position in space. And the results from Parvizi's team 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 the body is falling," Lopez says.
That means the brain has to make sense of conflicting information: For example, signals from the inner ear may say the body is falling while signals from the eyes say it's stationary.
As a result, Lopez says, the brain may try to cope by taking a different perspective.
"Sometimes the best solution which is found by the brain is to think that you are somewhere else, out of the body," he says.
The brain may face a similar conundrum when people take drugs like ketamine, which affect the anterior precuneus.
"Ketamine seemingly is producing this artificial rhythm [in the brain] that is disrupting function of that area," says Patrick Purdon, an associate professor of anesthesia at Harvard Medical School.
That slow rhythm is similar to the one that Parvizi's team saw when it stimulated the brains of epilepsy patients, Purdon says.
That could mean it will someday be possible to use electrical pulses in place of anesthetic drugs like ketamine, he says.
"You could get the specific brain areas that you want without having to cause a brainwide and systemwide effect that might carry with it a lot of side effects," Purdon says.
