Do I need a body to know who I am? Neural mechanisms of body ownership

Abstract: The feeling that our body belongs to us, and that it is an essential part of ourselves, is a fundamental aspect of human self-awareness and individual psychological identity. The intimate relation between the sense of body ownership and the sense of self emphasizes the importance of understanding the former in order to access and comprehend the latter. However, it also outlines the challenge of objectifying the body by turning it into a subject of controlled experimental research. The aim of this thesis was to create an experimental tool with which to overcome this immanent hindrance and study for the first time the mechanisms underlying the feeling of an entire body as belonging to oneself. Three studies, designed to address the perceptual and neural underpinnings of full-body ownership, are presented. In Study I we created a perceptual illusion in which healthy participants experience a whole artificial or real body as their own. This illusory sensation is elicited when participants perceive convergent multisensory input from their own body (which is out of view) and the new body (which they view from a first person perspective). Importantly, the feeling of ownership was not confined to the specific location of sensory stimulation, but instead encompassed the entire body. The illusory perception was abolished or significantly diminished when the input from the different sensory modalities was temporally desynchronized, or when the new body was replaced by a non-corporeal object of similar size. In sum, this first study showcases a novel experimental set-up that enables the scientific study of the perceptual mechanisms underlying full-body ownership, and provided evidence that in the core of this perceptual phenomenon lies the integration of convergent multisensory input from the body. In Study II we specifically investigated the role played by the visual perspective in the generation of a full-body ownership illusion. To that end, we compared the strength of the illusion when the new body was perceived either from a first person or from a third person perspective. The results indicated that attributing a new body to oneself is possible only when one perceives this body, and the multisensory signals deriving from it, from an egocentric (i.e. first person) perspective. In Study III we created an experimental set-up to induce the full-body illusion in a functional magnetic resonance imaging (fMRI) setting, which enabled us to shed light on the neural mechanisms underlying full-body ownership. In three independent fMRI experiments we specifically investigated (i) which neural activation is associated with the experience of full- body ownership, (ii) whether the multisensory processes giving rise to the full-body illusion operate in body-centered reference frames, and (iii) how ownership of individual body parts translates into the unitary experience of owning a whole body? We found that perceiving a body as one’s own is associated with increased activity in the ventral portion of the bilateral premotor areas, the anterior part of the left intraparietal sulcus and the left putamen. In addition, the activation in the ventral premotor cortex was found to reflect the construction of ownership of a whole body from its parts. In fact, it was stronger when the stimulated body part was attached to the body, present irrespective of the location of the stimulation evoking the illusion, and moreover displayed multivoxel patterns carrying information about full-body ownership. All together these findings suggest that the unitary experience of owning an entire body is produced by neuronal populations that integrate multisensory information across body segments in body-centered reference frames.