How the Brain Becomes Self-Aware

In the 19th century, philosopher William James suggested that the “self” can be divided into two parts. The first is the “I,” which physically perceives and senses the world, and the second is the “me,” which includes a mental narrative about oneself from an outside perspective, based on past experiences. Today, neuroscientists armed with advanced technology have made significant progress in identifying the brain regions responsible for forming these two aspects of the self.

The discovery of the “me” came first. The default mode network (a term introduced by neurologist Marcus E. Raichle in 2001) plays a key role in this narrative aspect of the self. This network of brain regions is active when a person is not focused on a specific task. Researchers have found that it plays an important role in processing self-referential thoughts. “This network has been dubbed a central hub of self-awareness,” says Josef Parvizi, a neurologist and professor at Stanford University who studies consciousness.

The “I,” on the other hand, was harder to pinpoint—at least until recently. The awareness that we live in a body (sometimes called the essential “I-ness”) forms a bridge that constantly shifts between conscious and unconscious states of mind. Imagine sitting at a dinner table or standing while waiting for a train. If you’re not in pain, you’re probably not aware of what’s happening with your hand, shin, big toe, or even your body as a whole. But as soon as you think about any of these areas, you become aware of their presence. “I-ness” is the feeling that you truly have your own body.

In searching for the “I” in the brain, researchers reasoned that a logical starting point would be the default mode network. Special attention was given to a segment at the back of the brain, where the two hemispheres meet, known as the posteromedial cortex (PMC). Neuroimaging studies have shown that the PMC is active when people delve into memories or daydream, usually triggering self-related thoughts. Scientists wanted to know if disrupting brain activity in this area could alter a participant’s physical self-perception.

These studies are typically conducted on people with epilepsy. But when Parvizi and other researchers disrupted brain activity in this area by delivering electrical currents to the brains of people with epilepsy, they were unable to change the physical sense of “I.” Then, in 2018, Parvizi encountered a patient with epilepsy who presented an unusual set of symptoms. According to the patient, during seizures he would enter a strange dissociative state, losing his sense of coordination and feeling disconnected from his inner “I.” When Parvizi and his colleagues examined the patient’s brain to find the source of the seizures, they discovered that they originated in a specific area of the PMC known as the anterior precuneus.

This accidental discovery led to a new study by Parvizi and his colleagues, published in June 2023 in the journal Neuron. They recruited eight people with epilepsy whose seizures originated in areas outside the PMC, ensuring that the tissue in the area of interest was healthy. The participants had electrodes implanted in the PMC for electrical stimulation.

Stimulating the anterior precuneus caused all eight participants to experience changes in their subjective perception similar to those reported by the patient with seizures in this region. These changes included feelings of floating, dizziness, loss of concentration, and a sense of detachment from themselves. Some participants noted that the sense of detachment was similar to what they had experienced when using psychedelics. “We found that by stimulating this specific area, we could distort the sense of physical being,” says Parvizi.

“The results are original and extremely interesting, and they contribute to a better understanding of how the brain processes the sense of bodily self,” says H. Henrik Ehrsson, a cognitive neuroscientist at the Karolinska Institute in Sweden. Ehrsson adds that these findings are consistent with previous work by his own group, which found that the anterior precuneus was active when people’s bodily self-perception was altered using an out-of-body illusion, making participants feel as if their real body was no longer part of themselves. (Participants were made to feel as if they occupied another body by watching a video of someone else’s body being touched while simultaneously feeling touches on the same parts of their own body.)

To determine how the anterior precuneus is connected to the default mode network, Parvizi and his team placed five participants in a functional MRI 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 the participants’ sense of bodily self were not part of the default mode network, although they did form connections with regions within that network.

This finding suggests that there are two different systems for processing the self, says study co-author Dian Liu, a postdoctoral researcher in Parvizi’s lab at Stanford: one is the narrative self, based on memory, and the other is the bodily self. In other words, the “I” and the “me” are located in separate brain networks. One of the main questions Liu wants to address in future research is how exactly these two networks interact.

Parvizi hopes this research will help clarify what happens in conditions like depression, which are characterized by excessive rumination and negative self-thoughts. According to Parvizi, such individuals may get stuck in a pattern of seeing the world only from their own perspective, losing the ability to see things from a third-person point of view. He wonders whether understanding how both networks interact—coloring our memories based on subjective experience of the world, and whether cross-talk between the networks becomes hyperactive in people with depression—could open up ways to help people break out of this vicious cycle.

Sahib S. Khalsa, a psychiatrist and neuroscientist at the Laureate Institute for Brain Research in Oklahoma, says the study may help explain elements of out-of-body experiences reported by people when taking drugs, especially psychedelics, or through non-pharmacological means. For example, in a sensory deprivation tank, where people float in a dark, water-filled chamber and are cut off from their senses.

Khalsa notes that he and his colleagues have found changes in the precuneus related to the sense of bodily self in Parvizi’s study—and in other related regions—in people who have undergone flotation therapy in a sensory deprivation tank. He adds that this research also helps identify a potential brain region for therapeutic intervention in people with conditions where dissociation is a common symptom, such as functional neurological disorders (where problems in the nervous system’s functioning can lead to a wide range of symptoms) and trauma-related disorders. “A whole range of interesting studies could be conducted based on this work,” says Khalsa.