A small brain structure, the anterior precuneus or aPCu, has been identified as a crucial component of our physical self or ‘I’.
The aPCu is part of a network of brain regions that integrate information concerning our location, motion, and bodily sensations to shape our self-awareness. When electrical activity in the aPCu is disrupted, altered perceptions of one’s position in the world are experienced by individuals.
Has anyone ever wondered where in our brains the intriguing character known as “I” resides?
Josef Parvizi, MD, Ph.D., a physician-scientist at Stanford Medicine, has some insights about its location.
Even if other people’s skulls were transparent, not much would be visible in their brains. Nevertheless, Parvizi has methods for examining the contents of people’s heads and uncovering what drives us. Through his experiments, specific brain regions essential for various abilities, such as perceiving faces and recognizing numerals, have been identified.
In a paper published on June 8 in Neuron, Parvizi’s most recent discovery unveils the surprising role played by a small structure located between the brain’s two hemispheres.
The structure, known as the anterior precuneus or aPCu, is the focal point of a system of distributed brain regions that work together harmoniously, with their activity rising and falling in coordination, indicating teamwork.
Parvizi, along with postdoctoral scholar Dian Lu, Ph.D., and their colleagues, found that this collaboration of brain regions led by the aPCu is of great importance in integrating information about one’s location, motion, muscle and joint positions, and sensations to create a mental map of our sense of bodily or physical self.
To illustrate the significance of this system, Parvizi referred to the peculiar pair, “I” and “me.”
“For every action taken, even during dreams,” he stated, “there is always an agent behind it: ‘I’ is the term we use for that agent, while ‘me’ encompasses everything stored in our memories about ‘I’.”
The Two Selves
Neuroanatomically, each self is governed by a distinct archipelago of brain structures. These two systems continuously interact with each other.
The physical or bodily sense of self, referred to as “I,” differs from another aspect of self, known as “me.” “Me” involves actively or passively reflecting on our past life or planning our future, encompassing aspects such as memory, habits, personality, emotions, feelings for others, and future considerations.
The narrative self, “me,” resides within a well-researched network called the default mode network. (To the non-expert, the three words “default,” “mode,” and “network” may seem like a random trio of discordant words. However, the term aptly describes a brain network that becomes active during activities like daydreaming and recalling past events.)
While there isn’t an official name yet for the bodily self-network, it is known to exist. Previous studies have identified a set of structures comprising this network but did not provide much information about its now-evident central player, the aPCu.
To further delineate the brain network overseen by the aPCu, brain-imaging data were obtained from five patient-participants and nearly 1,000 participants in the Human Connectome Project, initiated in 2010 to map the neural connections in the human brain.
“The precise location of key cells in the aPCu has been demonstrated, and Dian’s map clearly illustrates how they are connected to the rest of the brain,” explained Parvizi.
The aPCU is situated in close proximity to a brain structure considered a crucial node in the default mode network. However, it is not a part of that network, despite its intense communication with it.
“Stimulating the default mode network electrically has no impact on one’s sense of self or consciousness,” noted Parvizi.
“Your sense of physical or bodily self represents your organism in the immediate here and now, with a unique perspective that is exclusively yours — your first-person viewpoint on the surrounding world. This perspective is not shared by anyone,” emphasized Parvizi.
“You may not always be conscious of your viewpoint, but you will be if the network generating it is disrupted. Suddenly, your place in the world around you will appear unreal.”
Exploring the Brain
Observations are made by Parvizi in his patients who are undergoing evaluation for potential surgical treatment of recurrent, drug-resistant epileptic seizures. Fine needles, functioning as electrodes, are inserted into the brain under anesthesia by a neurosurgeon.
Patients remain connected to the monitoring apparatus for several days, during which the electrical activity within the brain is picked up by the electrodes and relayed to a computer.
The repeated seizures are ultimately captured by this procedure, enabling neurologists to determine the precise location in the brain of each patient where the seizures originate. A significant number of patients achieve freedom from recurring seizures through this invasive approach.
With the consent of these patients, tiny pulses of current are passed through a series of individual electrodes by Parvizi, stimulating or disrupting activity in small, discrete brain areas, while observing the outcomes. (It’s important to note that this procedure is safe, and the brain does not experience any pain.)
“Whenever I have a seizure, I experience a sense of depersonalization and dissociation. Everything seems unreal, as if it’s not happening to me.”
– a patient shared one day.
It was discovered that this patient’s seizures originated from the aPCu. To learn more about what was happening, Parvizi and his colleagues identified eight more patients with implanted electrodes passing through their aPCu, and these patients agreed to allow him to disrupt the activity of this structure with electrical pulses.
“All of them reported something unusual happening to their sense of physical self. In fact, three of them reported a distinct sense of depersonalization, akin to the effects of psychedelic substances.” – Parvizi
However, this feeling of detachment did not constitute an out-of-body experience.
“In an out-of-body experience, one sees oneself from above,” Parvizi explained.
“Our subjects did not report that at all. They still felt as though they were inside their bodies. However, they typically described a shift in their perception of their location and orientation.
“When the right side of the brain was stimulated, they felt as though they were floating; if the left side was stimulated, they felt as if they were falling. As they looked around, it didn’t make sense.
“They shouldn’t be floating or sinking, but it felt like they were. The world around them appeared unreal.”
Interestingly, the patients’ reports of flying or floating versus falling or plunging were accompanied by relatively positive versus negative emotions, respectively, depending on whether the right or left aPCu had been electrically stimulated. (Like many brain structures, the aPCu is present on both hemispheres of the brain.)
“Why this happens, I have no idea,” Parvizi remarked. “I am absolutely perplexed as to why stimulating the left versus the right side produces opposing effects, but we intend to unravel this mystery.”
Abstract
To probe the causal importance of the human posteromedial cortex (PMC) in processing the sense of self, we studied a rare cohort of nine patients with electrodes implanted bilaterally in the precuneus, posterior cingulate, and retrosplenial regions with a combination of neuroimaging, intracranial recordings, and direct cortical stimulations. In all participants, the stimulation of specific sites within the anterior precuneus (aPCu) caused dissociative changes in physical and spatial domains. Using single-pulse electrical stimulations and neuroimaging, we present effective and resting-state connectivity of aPCu hot zone with the rest of the brain and show that they are located outside the boundaries of the default mode network (DMN) but connected reciprocally with it. We propose that the function of this subregion of the PMC is integral to a range of cognitive processes that require the self’s physical point of reference, given its location within a spatial environment.
Highlights
- We studied anterior precuneus (aPCu) with fMRI, intracranial EEG, and electrical stimulations
- Stimulation of aPCu caused distortions in core body schema and self-dissociation
- Responsive aPCu sites were not part of, but were connected with, a default mode network
- We present a brain-wide map of structures that are causally connected with the aPCu