The Plasticity of the Bodily Self : Head Movements in Bodily Illusions and Their Relation to Gallagher’s Body Image and Body Schema

Abstract

Context: The integration of sensorimotor signals and prior beliefs contribute to our sense of body. An influential framework in the study of the bodily self is Gallagher’s distinction between body image and body schema, which are roughly comparable to the perceived body and the lived body, respectively. Through systematic manipulations of sensorimotor signals, it is possible to induce the illusion of agency or ownership over foreign limbs or full bodies. Yet, there is diverging empirical evidence regarding the coherence of sensorimotor signals necessary to elicit such illusions. Problem: The large amount of empirical evidence and its relation to the concepts of body image and schema is not well understood and requires more fine-graded distinctions of various aspects of sensorimotor coherences. Method: We systematically discuss literature on sensorimotor coherence during bodily illusions and argue for the importance of distinguishing between head- and limb-related coherences. To support this discussion, we present new experimental findings where participants experienced a first-person perspective (1PP) full-body illusion over another human through the manipulation of hand-related visuotactile or visuomotor coherence. Results: Participants showed no significant reduction in ownership after asynchronous visuotactile, but after asynchronous visuomotor stimulation. Based on these results and the literature, we propose that head-related temporal sensorimotor coherences are necessary to integrate limb-related incoherent signals during full-body illusions. Furthermore, we speculate that during full-body illusions, head-related coherences are a binding factor between the body image and the body schema; that is, only through the coherent manipulation of the visual field over a 1PP resulting from an immersive image (body image) is our body schema manipulated. Implications: While yet to be experimentally tested, distinguishing head- and limb-related sensorimotor integration and their influences on body image and body schema could refine the study of the bodily self. Constructivist content: The plasticity of the bodily self - as shown in bodily illusions - reflects the dynamism of the subject as observer and its binding to its environment.