The 19th-century philosopher William James proposed that the self could be split into two parts. The first was an “I” that physically perceives and experiences the world, and the second was a “me” that encompasses a mental narrative about oneself, based on one’s past experiences. Neuroscientists equipped with high-tech tool kits have begun to achieve some success in the long-running search to find the brain areas responsible for creating these two aspects of the self.
The discovery of “me” came first. The default-mode network, a term coined by neurologist Marcus Raichle in 2001, has emerged as a key player in the “me” aspect of the self. This collection of brain areas is active when a person is not focused on a task, and researchers have found that it plays an important role in processing self-referential thoughts. “[This network] has kind of been baptized as the center for the sense of self,” says Josef Parvizi, a neurologist and a professor at Stanford University who researches the self.
The “I,” in contrast, has been harder to pin down—at least until very recently. The awareness we have that we inhabit a body (call it an essential “I-ness”) forms a bridge that constantly switches back and forth between a conscious and unconscious state of mind. Suppose you’re sitting at the kitchen table or standing waiting for a train. Unless you’re in pain, you have no moment-by-moment awareness of your hand, your shin, your big toe or even your body as a whole. But as soon as you think of any of these spots, you can feel their presence immediately. “I-ness” is that feeling that you indeed occupy your own body.
In searching for the “I” in the brain, researchers reasoned that the default mode network would be a logical starting point. Of particular interest was a segment of the network known as the posteromedial cortex (PMC), located near the back of the head in the region where the two hemispheres meet. Neuroimaging studies had shown that the PMC was active while people were recalling memories or engaging in the type of mind-wandering that tends to spur self-related thoughts. So scientists wanted to see whether disrupting brain activity in this region could somehow change a study participant’s physical sense of self. But when Parvizi and others disrupted brain activity in the PMC by injecting electric currents into the brains of individuals with epilepsy, they failed to alter the physical “I” feeling. (These studies are typically conducted in people who have epilepsy because doctors implant electrodes in the brains of these research volunteers to monitor their brain activity prior to surgery.)
Then, in 2018, Parvizi encountered a patient with epilepsy who came to him with an unusual set of symptoms. During seizures, the patient said, he would enter a strange state of dissociation that caused him to lose his sense of coordination and feel disconnected with his inner self. When Parvizi and his colleagues probed the patient’s brain to find the source of his seizures, the team found that they originated in a specific region of the PMC known as the anterior precuneus.
This serendipitous discovery led to Parvizi and his colleagues’ latest study, published in Neuron in June, in which they recruited eight people with epilepsy whose seizures stemmed from areas other than the PMC to ensure they were examining people who had healthy tissue in the region they were investigating. All eight participants had electrodes implanted into the PMC for electrical stimulation.
Zapping the anterior precuneus caused all eight individuals to report alterations in their subjective experiences similar to what the person with seizures stemming from that region reported. These changes included a feeling of floating, dizziness, a lack of focus and a sense of detachment from themselves. Some participants remarked that the detachment was reminiscent of what they’d felt while on psychedelics. “We discovered that by stimulating this particular region, we can cause distortions in our sense of physical being,” Parvizi says.
“The findings are original and highly interesting” and contribute to a better understanding of how the brain processes the sense of bodily self, says Henrik Ehrsson, cognitive neuroscientist at the Karolinska Institute in Sweden who was not involved in the recent study. Ehrsson adds that the authors’ results align with prior work from his own group, which found the anterior precuneus to be active when individuals’ bodily self-perception was altered using an out-of-body illusion that made participants feel as though their real body was no longer a part of themselves. (Participants were made to feel as though they occupied another body by viewing a video of a stranger’s body being touched while they received touches on the same parts of their own body at the same time.) Ehrsson adds that because Parvizi and his team relied on participant’s self-reports in their study, it would be beneficial to also examine how stimulating this brain region changes the bodily self through more objective means, such as behavioral experiments.
To determine how the anterior precuneus was related to the default-mode network, Parvizi and his team placed five of the participants in a functional magnetic resonance imaging (fMRI) scanner and recorded their brain activity while they were at rest. The researchers found that the parts of the anterior precuneus that led to changes in participants’ sense of bodily self were not part of the default-mode network, although they formed connections with regions within that network. This finding implies that there are two different systems for processing the self, says study co-author Dian Lyu, a postdoctoral scholar at Parvizi’s lab at Stanford. “One is a narrative self, based on memory, and the other is the bodily self,” she says. In other words, “me” and “I” are located in separate networks within the brain. One of the big question Lyu is looking to address in future studies is how, exactly, these two networks interact.
Parvizi hopes that this research will help illuminate what happens in conditions, such as depression, that are characterized by excessive rumination and negative thoughts about oneself. Such individuals can find themselves stuck in a pattern of seeing everything around them in terms of themselves while losing the ability to see thing from a third-person perspective, Parvizi says. Thus, he wonders whether an emerging understanding of how the “I” and “me” networks interact to color our memories based on our subjective experiences of the world—coupled with new insights into whether cross talk between the networks turns hyperactive in individuals with depression—could reveal a way to help people escape from this terrible cycle.
Sahib Khalsa, a psychiatrist and neuroscientist at the Laureate Institute for Brain Research in Oklahoma, who was not involved in this work, says that the study may help explain elements of out-of-body experiences that people report while on drugs such as psychedelics or through non-pharmacological means, such as lying in a sensory deprivation tank, where individuals float in a dark, water-filled container and are cut off from their senses. Khalsa notes that he and his colleagues have found changes in the precuneus—the brain area linked to the sense of bodily self in Parvizi’s study—and other related regions in people who have undergone floatation therapy. He adds that this research also provides a potential brain area to target therapeutically in people with conditions in which dissociation is a common symptom, such as functional neurological disorders, where problems in the functioning of the nervous system can lead to a wide range of symptoms, and trauma-related disorders. “There’s an exciting array of studies that can be conducted based on this work,” Khalsa says.