Unveiling the Inner Self: Causal Evidence Reveals Anterior Precuneus’ Role in Processing Bodily Self

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In this blog post, I will summarize the main findings and implications of a recent study published in Neuron by Dian Lyu and colleagues, titled “Causal evidence for the processing of bodily self in the anterior precuneus”.

This study provides novel insights into the role of a specific brain region, the anterior precuneus (aPCu), in generating the sense of self and its relation to the physical body and the surrounding space.

The sense of self is a complex and multifaceted phenomenon that involves various aspects, such as self-awareness, self-identity, self-agency, self-location, and body ownership. Previous studies have suggested that the posteromedial cortex (PMC), a large area at the back of the brain that includes the precuneus, the posterior cingulate, and the retrosplenial regions, is involved in processing the sense of self.

However, the PMC is not a homogeneous structure, and its different subregions may have distinct functions and connections with other brain areas. Therefore, it is important to investigate the causal role of specific PMC subregions in generating different aspects of the sense of self.

To address this question, Lyu et al. studied a rare cohort of nine patients who had electrodes implanted bilaterally in the PMC for clinical reasons. These electrodes allowed them to perform a combination of neuroimaging, intracranial recordings, and direct cortical stimulations.

Neuroimaging was used to map the functional connectivity of the PMC subregions with the rest of the brain at rest and during tasks. Intracranial recordings were used to measure the electrical activity of the PMC subregions in response to different stimuli. Direct cortical stimulations were used to test the causal effects of activating specific PMC subregions on the patients’ subjective experiences.

The main finding of this study was that stimulating specific sites within the aPCu caused dissociative changes in the patients’ sense of self in both physical and spatial domains. For example, some patients reported feeling as if they were falling, floating, or flying; others reported feeling as if their body parts were distorted or detached; and others reported feeling as if they were outside their body or in a different location.

These effects were consistent across different stimulation parameters and tasks, and they were not observed when stimulating other PMC subregions or other brain areas.

Using single-pulse electrical stimulations and neuroimaging, Lyu et al. also mapped the effective and resting-state connectivity of these aPCu sites with the rest of the brain. They found that these sites were located outside the boundaries of the default mode network (DMN), a large-scale brain network that has been associated with self-referential processing and introspection.

However, these sites were reciprocally connected with several DMN regions, such as the medial prefrontal cortex and the posterior cingulate cortex. Moreover, these sites were also connected with other brain regions involved in spatial processing, such as the TPJ and the parahippocampal gyrus.

Based on these results, Lyu et al. proposed 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 within a spatial environment.

They suggested that this subregion acts as a hub that integrates multisensory information about one’s own body and its position in space with higher-order information about one’s identity and goals from other brain networks. By doing so, this subregion enables one to maintain a coherent and stable sense of self across different contexts and situations.

This study has several implications for our understanding of the neural basis of the sense of self and its disorders.

First, it provides causal evidence for the involvement of a specific PMC subregion in processing bodily aspects of selfhood, such as body schema and self-location.

Second, it reveals a novel functional subdivision within the PMC that is distinct from but connected with the DMN.

Third, it suggests that dissociative phenomena, such as out-of-body experiences or depersonalization, may result from abnormal activity or connectivity of this PMC subregion or its network partners.

Fourth, it opens new avenues for exploring how this PMC subregion interacts with other brain regions and networks to support other aspects of selfhood, such as self-agency or self-identity.

In conclusion, this study by Lyu et al. sheds new light on how a specific brain region contributes to generating our sense of self and its relation to our body and our environment. It also highlights the importance of studying rare patient populations with intracranial electrodes for advancing our knowledge of human cognition and consciousness.


In deep…

The bodily self is the sense of being a physical entity that occupies a specific location in space and time. It is composed of several aspects, such as the sense of ownership, agency, and embodiment. The sense of ownership refers to the feeling that one’s body parts belong to oneself and are under one’s control. The sense of agency refers to the feeling that one’s actions are caused by oneself and not by external factors. The sense of embodiment refers to the feeling that one’s body is integrated with one’s mind and identity.

The bodily self is influenced by various factors, such as sensory feedback, motor commands, cognitive processes, social interactions, and cultural norms. For example, sensory feedback from proprioception, touch, vision, and vestibular system provides information about the position, movement, and shape of one’s body. Motor commands from the brain and spinal cord initiate and coordinate voluntary movements of one’s body.

Cognitive processes such as attention, memory, and imagination can modulate or alter one’s perception of one’s body. Social interactions such as mimicry, empathy, and communication can affect one’s sense of self and others. Cultural norms such as clothing, cosmetics, and body modifications can shape one’s bodily self-image and expression.

The bodily self is not static but dynamic and flexible. It can change over time due to development, aging, injury, illness, or adaptation. It can also be manipulated or distorted by various experimental paradigms, such as the rubber hand illusion, the body swap illusion, or the full-body illusion.

These illusions demonstrate that the bodily self is not based on objective reality but on subjective experience and interpretation. They also reveal that the bodily self is not unitary but multisensory and multiscale. It can be extended or reduced to include or exclude artificial limbs, tools, avatars, or other people.

The bodily self is an important topic for various disciplines, such as psychology, neuroscience, philosophy, medicine, robotics, and virtual reality. It has implications for understanding human cognition, emotion, behavior, health, and well-being. It also has applications for developing new technologies, therapies, and interventions that can enhance or restore one’s bodily self.

…and

The bodily self arises from the dynamic integration of bodily and environmental visual, tactile, proprioceptive, vestibular, auditory, olfactory, visceral and motor information. Unlike other worldly objects, the body is the source of its own perception, a subject and an object at the same time.

The human brain computes bodily information via different maps and networks, notably areas of tactile, proprioceptive, vestibular and interoceptive projection in the primary somatosensory cortex, as well as unimodal and heteromodal cortices, providing an unified and global representation of the lived body, which allows for experiencing it as a unique self and agent. To these sensory components, recent research has added spatial and social factors involved in self-other bodily interactions, and ego- and altercentric perspectives on bodily and action perceptions .

The sense of ownership refers to the feeling that a body or a body part is one’s own. This feeling can be disrupted or altered in various ways, such as in cases of hemiasomatognosia (the denial of one side of one’s body), feelings of disappearance or transformation of body parts, supernumerary phantom limbs (the perception of extra limbs), somatoparaphrenia (the attribution of one’s limb to another person), or autoscopic phenomena (the perception of one’s own body from an external perspective). These disorders often follow damage to the right cerebral hemisphere.

The sense of ownership can also be manipulated experimentally by inducing illusions of ownership over artificial or virtual limbs or bodies. For example, in the rubber hand illusion, participants feel that a rubber hand placed in front of them is their own hand when it is synchronously stroked with their hidden real hand. Similarly, in full-body illusions, participants feel that a virtual body seen from a first-person perspective is their own body when it is synchronously touched with their real body. These illusions show that multisensory integration plays a crucial role in generating the sense of ownership.

The sense of agency refers to the feeling that one is the source or cause of one’s actions. This feeling can be impaired or distorted in various ways, such as in cases of alien hand syndrome (the feeling that one’s hand acts independently from one’s will), anarchic hand syndrome (the inability to inhibit unwanted movements), delusions of control (the belief that one’s actions are controlled by external forces), or passivity phenomena (the experience of involuntary movements or thoughts). These disorders often follow damage to the frontal lobes or are associated with schizophrenia.

The sense of agency can also be manipulated experimentally by inducing illusions of agency over artificial or virtual limbs or bodies. For example, in the moving rubber hand illusion, participants feel that they control a rubber hand placed in front of them when it moves synchronously with their hidden real hand. Similarly, in full-body illusions, participants feel that they control a virtual body seen from a first-person perspective when it moves synchronously with their real body. These illusions show that sensorimotor integration plays a crucial role in generating the sense of agency.

The sense of peripersonal space refers to the space immediately surrounding one’s body. This space is important for interacting with objects and other agents in the environment. The sense of peripersonal space can be altered or expanded in various ways, such as in cases of neglect (the failure to attend to stimuli on one side of space), extinction (the failure to detect stimuli on one side of space when presented simultaneously with stimuli on the other side), or tool use (the extension of peripersonal space by using a tool).

These phenomena often follow damage to the parietal lobes or are associated with sensorimotor learning. The sense of peripersonal space can also be manipulated experimentally by inducing illusions of peripersonal space over artificial or virtual limbs or bodies. For example, in the crossmodal congruency task, participants show enhanced interference effects when tactile stimuli on their hand are incongruent with visual stimuli on a rubber hand placed near their hand. Similarly, in full-body illusions, participants show enhanced interference effects when tactile stimuli on their body are incongruent with visual stimuli on a virtual body seen from a first-person perspective. These illusions show that multisensory integration plays a crucial role in generating the sense of peripersonal space.

In conclusion, the bodily self is a fascinating and complex phenomenon that involves different dimensions of self-awareness and self-representation. The study of the bodily self has advanced significantly in recent years, thanks to the development of new experimental paradigms and techniques that allow for manipulating and measuring the bodily self in healthy and clinical populations.

However, many questions remain open and challenging for future research, such as how the different dimensions of the bodily self interact and influence each other, how the bodily self develops and changes over the lifespan, how the bodily self is influenced by social and cultural factors, and how the bodily self relates to other aspects of selfhood, such as personality, identity, and narrative.


References:
: Lyu, D., Stieger, J.R., Xin, C., Ma, E., Lusk, Z., Kalkach Aparicio, M., Werbaneth, K., Perry, C.M., Deisseroth, K., Buch, V., & Parvizi, J. (2023). Causal evidence for the processing of bodily self in the anterior precuneus. Neuron. https://doi.org/10.1016/j.neuron.2023.05.013

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