Periodic Reporting for period 3 - SELF-UNITY (The Unity of the Bodily Self)
Reporting period: 2022-01-01 to 2023-06-30
The SELF-UNITY project investigates how we come to experience ourselves as single physical entities. Under normal healthy conditions, humans always experience a single body as our physical self, and this bodily self is undivided and perceived as a single whole. However, what cognitive processes and brain mechanisms mediate this unity of the bodily self? This question has been challenging to study in experimental studies because of a lack of behavioral paradigms that allow controlled manipulation of basic components of the self-unity. To address this issue, SELF-UNITY uses novel full-body illusion paradigms to “fragment,” or “duplicate" the sense of bodily self during controlled behavioral and neuroimaging experiments. By studying the behavioral and neural principles that determine specific illusory changes in a perceived self-unity, we can elucidate the neurocognitive mechanisms that support the sense of having a single unitary bodily self under normal conditions. Our behavioral paradigms utilize the virtual reality technologies, and these are combined with state-of-the-art neuroimaging using advanced analytical methods. The specific aims of the project are to clarify how we come to experience a single bodily self, as opposed to multiple ones; how we perceive a coherent bodily self instead of fragmented parts; and how information from different sensory modalities, including vestibular and interoceptive signals, is integrated to achieve this coherent sense of a singular bodily self. The new basic knowledge generated by this project may be important for future clinical neuroscience research into psychiatric and neurological disorders with disturbances in self-unity, such as schizophrenia, dissociative disorders, and stroke with body neglect, by providing novel ideas for hypotheses about the involved neurocognitive pathophysiology.
SELF-UNITY has made substantial progress toward clarifying the cognitive processes and brain mechanisms that mediate the sense of a unitary single bodily self. In work package (WP) 1, we have developed illusion paradigms to investigate the coherent perception of an entire body as one’s own (full-body ownership) and how this experience relates to the ownership of individual body parts (body-part ownership). Our studies thus far have found that although 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 ownership of body parts and reflects a global bodily percept. We are currently further investigating the neural mechanisms that support this unitary whole-body perceptual experience. In WP2 and WP3, we conduct behavioral and neuroimaging studies to investigate how the vestibular sense (the sense of balance) and interoception (the body’s inner sensations) contribute to the perceived coherence of the bodily self. In WP2, we have developed a novel full-body illusion paradigm, which shows that a sense of full-body ownership can be triggered solely by congruent visual and vestibular stimulation through electrical stimulation of the vestibular nerve. In ongoing studies, we are investigating how tactile and vestibular signals work together to promote a unitary bodily experience and pinpoint the active brain regions involved in this process. In WP3, we have developed several novel illusion paradigms to trigger a body ownership illusion by stimulating various interoceptive submodalities, which suggests that the input from c-fibers reaching the insular cortex plays a critical role in the bodily self-unity and bodily illusions. Finally, in WP4, a major achievement is a new “dual-body illusion,” wherein people experience two separate bodies as their own at the same time. This illusion shows that healthy people may experience ownership of two district bodies under certain conditions of multisensory perceptual ambiguity. This finding is important because it suggests that the unitary sense of a single bodily self located at a single place results from an active and dynamic multisensory integration process. We have also conducted a study in which we generalize the principles of the dual-body illusion to the case of a single body part and describe conclusive empirical evidence for the existence of a supernumerary limb illusion where people experience having two right hands, which provides further support for the general assumptions and hypothesis in this WP. In addition, we conducted several technical studies in which we improved our methods used to register changes in body ownership illusions under well-controlled conditions. Moreover, we are currently continuing to investigate the brain mechanisms of self-unity using neuroimaging techniques and the illusion paradigms described above. Collectively, the results from SELF-UNITY advance our understanding of the multisensory processes and cortical mechanisms that support the unity of bodily self and open up new horizons for empirical research into one of the most fundamental questions in cognitive and brain sciences today, that is, how we come to have a unitary experience of a single self.
The published double-body illusion constitutes significant progress beyond the current state-of-the-art study by demonstrating unprecedented flexibility of multisensory body representation that challenges basic assumptions about the singleness of human bodily self-awareness. Moreover, our findings from experiments with electrical nerve stimulation show that congruent vestibular and visual signals are sufficient to elicit a vivid full-body ownership experience and that body ownership boosts self-motion perception. These findings represent a significant advance in the field and provide a deeper understanding of the role of the vestibular sense in bodily awareness and a perceived self-unity. Finally, our approach to selectively activating c-fibers in the skin as a novel way to probe the role of interoception in body ownership illusions and self-unity constitutes a new direction in the field (which has previously mainly focused on the awareness of cardiac signals). Finally, beyond the original research plan, we have developed novel psychophysics tasks to probe the sense of body ownership more rigorously than allowed by current state-of-the-art methods and formulated novel computational models of body ownership as multisensory integration (based on probabilistic principles). This significantly advances the empirical and theoretical foundations of the SELF-UNITY project and our understanding of bodily self-awareness.
In the next 30 months of the project, we expect significant advances in all project areas. We will conduct additional behavioral studies to clarify key perceptual processes and multisensory binding mechanisms that support self-unity and full-body ownership will use advanced state-of-the-art neuroimaging to identify the associated neural mechanisms. First, experiments are planned to identify the cortical multisensory integration mechanisms whereby we come to perceive a whole body as our own, as opposed to fragmented parts. Second, we study the cortical mechanisms that support the integration of vestibular and interoceptive signals for bodily self-unity. Finally, we will use our double-body illusion paradigm to investigate the cortical mechanisms that support the experience of a single bodily self instead of multiple selves that are “split” into different locations.
In the next 30 months of the project, we expect significant advances in all project areas. We will conduct additional behavioral studies to clarify key perceptual processes and multisensory binding mechanisms that support self-unity and full-body ownership will use advanced state-of-the-art neuroimaging to identify the associated neural mechanisms. First, experiments are planned to identify the cortical multisensory integration mechanisms whereby we come to perceive a whole body as our own, as opposed to fragmented parts. Second, we study the cortical mechanisms that support the integration of vestibular and interoceptive signals for bodily self-unity. Finally, we will use our double-body illusion paradigm to investigate the cortical mechanisms that support the experience of a single bodily self instead of multiple selves that are “split” into different locations.