The Unity of the Bodily Self

Project Information

SELF-UNITY

Grant agreement ID: 787386

DOI

10.3030/787386

Project closed

EC signature date 27 June 2018

Start date 1 January 2019

End date 30 June 2024

Funded under

• EXCELLENT SCIENCE - European Research Council (ERC)

Total cost

€ 2 583 560,00

EU contribution

€ 2 583 560,00

Coordinated by

KAROLINSKA INSTITUTET
Sweden

Periodic Reporting for period 4 - SELF-UNITY (The Unity of the Bodily Self)

Reporting period: 2023-07-01 to 2024-06-30

Summary of the context and overall objectives of the project

The SELF-UNITY project investigates how we experience ourselves as single physical entities. Under normal, healthy conditions, humans consistently perceive a single, undivided body as their physical self. But what cognitive processes and brain mechanisms create this unified sense of bodily self? How do we perceive a coherent body rather than fragmented parts? And how is information from different sensory modalities, including vestibular and interoceptive signals, integrated to form this unified sense of self?

To explore these questions, SELF-UNITY employs novel bodily illusion paradigms to “fragment” or manipulate the perception of parts or whole bodies as belonging to oneself in controlled behavioral and neuroimaging experiments. By examining the behavioral and neural principles underlying these illusory changes in perceived self-unity, we can better understand the neurocognitive mechanisms that support the experience of a singular bodily self.

We uncovered that the perception of a unified bodily self relies on distinct multisensory processes. We developed full-body illusion paradigms, revealing that the experience of a coherent bodily self is driven by the integration of signals from different sensory modalities, supported by neural mechanisms in frontal, parietal, and subcortical brain regions. Our studies further showed that the brain mechanisms responsible for full-body ownership differ from those governing individual body parts, and, through computational modeling, we gained insights into how ownership of an entire body is constructed from information from individual body parts through probabilistic inferential processes. We also demonstrated that vestibular and skin-based interoceptive signals (nociception and thermosensation) play a critical role in supporting the sensation of a unified body. Furthermore, our innovative “dual-body illusion” experiments provided insights into how the brain prevents the experience of multiple selves by maintaining the cohesion of a single bodily self. Collectively, these findings represent significant advancements in our understanding of the neurocognitive mechanisms that give rise to the sense of self-unity.

The knowledge generated by this project is relevant for future clinical research into psychiatric and neurological disorders that involve disturbances in self-unity. It provides new insights and hypotheses about the possible underlying neurocognitive pathophysiology.

Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far

SELF-UNITY successfully met all the major project aims and made significant progress in clarifying the cognitive processes and brain mechanisms that mediate the sense of a unified bodily self.

In Work Package (WP) 1, we developed illusion paradigms to investigate how people perceive their entire body as their own (full-body ownership) and how this experience relates to the ownership of individual body parts (body-part ownership). Our studies found that while the senses of ownership of body parts and the full body are correlated and follow similar rules, full-body ownership is more than the sum of its parts and reflects a global bodily percept. fMRI experiments revealed that body-part and full-body ownership involve different active areas in the premotor, subcortical, and posterior parietal regions, and that part and full-body ownership are associated with different fine-grained activity patterns within these regions, reflecting distinct processes.

In WP2, we conducted behavioral and neuroimaging studies to investigate how the vestibular sense contributes to the sense of bodily self. We found that full-body ownership can be triggered solely by congruent visual and vestibular stimulation, that tactile and vestibular signals work together to promote a unified bodily experience, and that vestibular influences on body ownership are mediated by activated areas in the parietal operculum.

In WP3, e developed novel illusion paradigms to trigger body ownership illusions by stimulating thermoreceptors and nociceptors in the skin—'skin-based interoception.’ We found that signals from these somatosensory receptors, which carry information about the physiological condition of the body, follow the same basic multisensory rules for body ownership as vision and touch. Our neuroimaging findings suggest that the insular and temporal cortices mediate the integration of skin-based interoceptive signals with other bodily-related signals for body ownership.

Finally, in WP4, we developed new “dual-body” and “supernumerary limb” illusions, wherein people simultaneously experience two separate bodies as their own or feel like they have an extra arm. These findings are important because they suggest that the sense of a unified bodily self located in a specific place results from an active and dynamic multisensory integration process. fMRI experiments have pinpointed a specific region in the inferior posterior parietal cortex that may play a critical role in the dual-body illusion and in maintaining a cohesive singular bodily self.
Additionally, we conducted several studies to validate our methods and developed signal detection theory approaches and computational models for bodily illusion experiments to quantify body ownership perception more rigorously, achieving stronger mechanistic and explanatory power. We disseminated our findings through review articles, book chapters, and presentations at numerous scientific conferences (e.g. Society for Neuroscience). We also organized a workshop on bodily illusions and body ownership and shared our findings with the general public through press releases, events, podcasts, and interviews.

Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far)

The published double-body illusion represents significant progress beyond the current state-of-the-art by demonstrating unprecedented flexibility in multisensory body representation, challenging basic assumptions about the singularity of human bodily self-awareness. Similarly, our behavioral and fMRI studies, which support a novel neurocognitive model of part and whole in body ownership, mark a major advancement in the field. We have also conducted the first computational modeling experiments on body ownership, both of parts and whole bodies, in which data from individual trials are fitted to Bayesian causal inference models. These models offer a new way to conceptualize the sense of bodily self based on probabilistic inference and provide more explanatory power and mechanistic understanding than traditional approaches. Building on this, we conducted the first model-based fMRI study on body ownership.

Moreover, our experiments involving electrical nerve stimulation have clarified the vestibular contribution to the sense of bodily self, offering a deeper understanding of the role of the vestibular sense in bodily awareness and perceived self-unity. Additionally, our novel approach to selectively activating C-fibers in the skin provides a new method for probing the role of nociception and thermosensation (skin-based interoception) in body ownership illusions and self-unity, marking a new direction in the field. Furthermore, we developed novel psychophysics tasks to probe the sense of body ownership more rigorously than current state-of-the-art methods, which provides important validation of the robustness of our findings and our illusion-based approach to investigating the sense of bodily self.

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