The Contribution of Real-Time Mirror Reflections of Motor Actions on Virtual Body Ownership in an Immersive Virtual Environment Mar González-Franco 1 , Daniel Pérez-Marcos 1,2 , Bernhard Spanlang 1,3 , Mel Slater 1,4,5 1 EVENT Lab, Universitat de Barcelona, Spain 2 Institut d’Investigacions Biomèdiques August Pi i Sunyer (IDIBAPS), Barcelona, Spain 3 Departament de LSI, Universitat Politècnica de Catalunya, Spain 4 Institució Catalana de Recerca i Estudis Avançats (ICREA), Spain 5 Department of Computer Science, University College London, UK ABSTRACT This paper reports an experiment that investigated people’s body ownership of an avatar that was observed in a virtual mirror. Twenty subjects were recruited in a within-groups study where 10 first experienced a virtual character that synchronously reflected their upper-body movements as seen in a virtual mirror, and then an asynchronous condition where the mirror avatar displayed pre- recorded actions, unrelated to those of the participant. The other 10 subjects experienced the conditions in the opposite order. In both conditions the participant could carry out actions that led to elevation above ground level, as seen from their first person perspective and correspondingly in the mirror. A rotating virtual fan eventually descended to 2m above the ground. The hypothesis was that synchronous mirror reflection would result in higher subjective sense of ownership. A questionnaire analysis showed that the body ownership illusion was significantly greater for the synchronous than asynchronous condition. Additionally participants in the synchronous condition avoided collision with the descending fan significantly more often than those in the asynchronous condition. The results of this experiment are put into context within similar experiments on multisensory correlation and body ownership within cognitive neuroscience. KEYWORDS: rubber hand illusion, body ownership, agency, virtual reality INDEX TERMS: J.4 [Social and Behavioral Sciences]: Psychology; I.3.6 [Computer Graphics]: Methodology and Techniques— Interaction techniques; I.3.7 [Computer Graphics]: Three- Dimensional Graphics and Realism—Virtual Reality. 1 INTRODUCTION How do we know what is and what is not part of our own body? This issue of body ownership has received increasing scientific attention in recent years. Current findings suggest that visuotactile and visuomotor correlations play an important role in how the brain decides ownership. By manipulating these correlations, it is possible to generate illusions that alien or even objects in virtual reality can be felt as part of one’s own body. The most well- known example of this is the rubber hand illusion [2]. In this case, the real arm of a participant is hidden from their view, and a rubber arm placed in a plausible position on a table in front of them. The experimenter synchronously taps and strokes the real hand of the person and the rubber hand. After a few seconds of stimulation most people have the illusion that the rubber hand is their own hand. This is demonstrated both subjectively, through the use of a questionnaire, and behaviorally using a proprioceptive drift measurement. This is the distance between the felt position of their hand as blindly pointed out before the stimulation and the felt position blindly pointed out afterwards. When the visual and tactile stimulation are synchronous the proprioceptive drift is typically significantly different from zero, and towards the position of the rubber hand. When the stimulation is asynchronous, both the questionnaire scores and the drift scores do not indicate the change in ownership illusion. Physiological response to threat towards the rubber hand has also been used as an objective measure of ownership [1, 5]. The rubber hand illusion has also been shown to work in virtual reality, where both the real and a computer-generated 3D arm were synchronously stimulated with real (tactile) touches and virtual (visual) touches, respectively [14]. This method has also been extended towards ownership of the whole body using similar techniques. This employs tactile stimulation to the person’s (unseen) real body and corresponding visual stimulation to a displaced video image of their body as seen through a head-mounted display (HMD). When the visuotactile stimulation is synchronous an out-of-the-body illusion occurs, as if the participants were in the displaced body, as evidenced by questionnaire based subjective responses and a behavioral response akin to proprioceptive drift [11] or physiological arousal in response to a threat to the virtual body [6]. A similar idea has been used to generate ownership of a manikin that appears to replace the body of the participant [12]. A recent review of this area of research can be found in [15]. The rubber hand illusion has been extended to visuomotor correlations between the movements of the hidden real hand and the rubber hand [14, 15]. This is related to agency, ‘The sense that e-mail: margonzalez@ub.edu e-mail: dperez1@clinic.ub.es e-mail: bspanlang@ub.edu e-mail: melslater@ub.edu