[How does your brain create a physical sense of self? New insight
comes from zapping a region, known as the anterior precuneus, that
causes people to feel dissociated from their body.]
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WHERE AM ‘I’? YOUR BRAIN AND YOU  
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Diana Kwon
July 12, 2023
Scientific American
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_ How does your brain create a physical sense of self? New insight
comes from zapping a region, known as the anterior precuneus, that
causes people to feel dissociated from their body. _

Allen Institute for Brain Science, by Lavender Dreamer (CC BY-SA 2.0)


 

The 19th-century philosopher William James proposed that the self
[[link removed]] could
be split into two parts
[[link removed]].
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
[[link removed]], a term coined by
neurologist Marcus Raichle in 2001
[[link removed]],
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
[[link removed]].
“[This network] has kind of been baptized as the center for the
sense of self,” says Josef Parvizi
[[link removed]], 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
[[link removed]] 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
[[link removed]] 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
[[link removed]] 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
[[link removed](23)00386-0], 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
[[link removed]], 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
[[link removed](15)00412-1],
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 [[link removed]], 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
[[link removed]]. 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
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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.

_DIANA KWON is a freelance journalist who covers health and the life
sciences. She is based in Berlin. _

_Subscribe to Scientific American digital, $39 per year.
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