If I Were You: Perceptual Illusion of Body Swapping
Valeria I. Petkova*, H. Henrik Ehrsson Department of Neuroscience, Karolinska Institutet, Stockholm, Sweden
Abstract
The concept of an individual swapping his or her body with that of another person has captured the imagination of writers and artists for decades. Although this topic has not been the subject of investigation in science, it exemplifies the fundamental question of why we have an ongoing experience of being located inside our bodies. Here we report a perceptual illusion of body-swapping that addresses directly this issue. Manipulation of the visual perspective, in combination with the receipt of correlated multisensory information from the body was sufficient to trigger the illusion that another person’s body or an artificial body was one’s own. This effect was so strong that people could experience being in another person’s body when facing their own body and shaking hands with it. Our results are of fundamental importance because they identify the perceptual processes that produce the feeling of ownership of one’s body.
Introduction
We all experience our body to be part of ourselves. The question of how this comes about has been discussed by philosophers and psychologists for centuries [1], [2], [3], [4]. Recent advances in experimental science have made it possible for cognitive neuroscientists to begin to investigate how we perceive our body as an object distinct from the external world [2], [5], [6], [7], [8]. Having the experience of being the owner of one’s body is clearly adaptive, and its function probably relates to the problem of localising and correctly identifying oneself in the sensory environment [9], [10], a problem faced by all central nervous systems [11]. Consider a fight between two or more individuals. Survival depends on rapid identification and accurate localisation of one’s own body. From neurology we know that these functions can break down as people with pathological conditions affecting frontal and parietal lobes can sometimes fail to recognise their limbs as belonging to themselves [12], [13], [14]. Similarly, damage to, or abnormal physiology of, frontal, parietal and temporal regions can be associated with feelings of being outside the body [15], [16]. Although these neurological observations suggest that certain brain regions might be responsible for generating the habitual experience of being located within one’s body and of owning it, they tell us little about the underlying processes.
If we want to understand why our centre of awareness, or sense of ‘self’, is located inside our body, illusions of bodily self-perception could be invaluable. The study of illusions is a classical approach adopted in psychology to learn more about the basic processes that underlie normal perception. Indeed, some important initial insights into the mechanisms underlying self-perception of one’s own body have been gained through illusions. One such is the so called ‘Rubber Hand Illusion’ where people have the experience that a prosthetic hand is actually their own hand [17]. In this illusion, synchronous touches applied to a rubber hand in full view of the participant, and the real hand, which is hidden behind a screen, produce the sensations that the touch originates from the rubber hand and a feeling of ownership of the artificial hand. This suggests that the temporal and spatial patterns of visual and somatosensory signals play an important role in how we come to experience that a limb is part of our own body [5], [7], [18], [19].
Another important factor in determining how we perceive our own body is the adoption of the first person perspective [6], [20]. When we look at ourselves directly, our limbs and body always present themselves in certain orientations because our eyes are fixed to our skull. By changing the visual perspective, it is possible to induce the feeling of being in a different place [21], [22], [23] or, even, illusory ‘out-of-body experiences’ where people seem to lose ownership of their own body when observing it from the point of view of another person (which we refer to as the third person perspective) [6].
On the basis of this previous knowledge, we hypothesized that it would be possible to induce illusions of owning an entire body other than one’s own by the experimental manipulation of the visual perspective in conjunction with correlated visual and sensory signals being supplied to the respondent’s body. Our experiments reveal that healthy volunteers can indeed experience other people’s bodies, as well as artificial bodies, as being their own. This effect is so robust that, while experiencing being in another person’s body, a participant can face his or her biological body and shake hands with it without breaking the illusion. The existence of this illusion (and the identification of the factors triggering it) represents a major advance because it informs us about the processes that make us feel that we own our body in its entirety.
Citation: Petkova VI, Ehrsson HH (2008) If I Were You: Perceptual Illusion of Body Swapping. PLoS ONE 3(12): e3832. doi:10.1371/journal.pone.0003832
Editor: Justin Harris, University of Sydney, Australia
Received: September 4, 2008; Accepted: November 6, 2008; Published: December 3, 2008 Copyright: (c) 2008 Petkova et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Funding: This work was supported by grants from the Swedish Medical Research Council, the Swedish Foundation for Strategic Research, the Human Frontier Science Programme, and the European Research Council. V.I.P was supported by a PhD fellowship from the KI-NIH graduate programme. The study was conducted as part of Stockholm Brain Institute, a consortium for Cognitive and Computational Neuroscience in Stockholm, and PRESENCCIA (Presence: Research Encompassing Sensory Enhancement,Neuroscience, Cerebral-Computer Interfaces, and Applications) project, a European Union-funded project under the Information Society Technologies program. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. Competing interests: The authors have declared that no competing interests exist.
* E-mail: Valeria.Petkova@ki.se
