The body swap illusion

Body ownership - the sense that one's body belongs to one's self - is central to self-awareness, and yet is something that most of us take completely for granted. We experience our bodies as being an integral part of ourselves, without ever questioning how we know that our hands belong to us, or how we can distinguish our body from its surroundings.

These issues have long intrigued philosophers and psychologists, but had not been investigated by neuroscientists until recently. Now researchers from the Karolinska Institute report that they have induced a "body-swap" illusion, whereby subjects perceived the body of another person as belonging to themselves. Their findings are published today in the open access journal PLoS One.

Our sense of ownership of our own body depends largely upon the brain's representation of the body, which is constructed largely from various types sensory inputs. For example, visual inputs provide us with information about body posture; tactile inputs provide information about objects which we come into contact with, and proprioceptive inputs inform us about the position of the body within its surroundings. Normally, the position of one's body corresponds directly to the way it feels and the way it looks and, as a result, one can identify their own body as belonging to themselves, and also distinguish very easily between the "self" and "non-self" (or objects in the external world).

Under certain circumstances, however, the sense of body ownership can be perturbed, and sometimes this has bizarre consequences. Take, for example, somatoparaphrenia, a condition in which ownership of the left hand or leg is denied, following damage to the right parietal lobe. Often, patients with this condition perceive their limb to belong to, and to be controlled by, another person. Or take hemispatial neglect, a related disorder, which also occurs as a result of brain damage. This causes a complete loss of awareness of half of the body, and causes the patient to act as if it didn't exist - she will apply make-up to one half of the face while ignoring the other, and eat food from only one half of a plate that is placed in front of her. If the plate is then rotated by 90°, the patient will continue to eat half of the remaining food!

As well as going awry in various neuropsychiatric conditions, the sense of body ownership can be distorted experimentally in healthy individuals, by manipulating the sensory inputs entering the brain. Henrick Ehrsson, who is now at the Stockholm Brain Institute, was one of the first to demonstrate this. In a 2004 study, Ehrsson and his colleagues induced in their subjects what is referred to as the rubber hand illusion. This was done by seating the subjects at a desk, with one hand hidden from view. In front of them on the desk was an artificial rubber hand. When the hidden and rubber hands are stroked simultaneously and in the same way, the visual information overrides the proprioceptive inputs, evoking the illusion that the sensation originates in the rubber hand, so that the subject perceives it as belonging to them.

This illusion is the exact opposite of somatoparaphrenia - the artificial limb is perceived as being part of one's own body. Ehrsson's subjects underwent functional neuroimaging whilst the illusion was induced; this showed that the illusion was correlated with activity in the premotor cortex, which contains neurons that are known to respond to both visual and tactile stimuli, and to integrate these stimuli. These neurons therefore register and process multisensory stimuli which are associated with the "self", and their activity is somehow  correlated with body ownership.

Last year, Ehrsson went one step further, and succesfully induced out-of-body experiences in the healthy participants of another study. The participants sat on a chair wearing a head-mounted display that was connected to video cameras placed behind their back. They could therefore see themselves from the perspective of another person sitting behind them, such that their "self", or centre of awareness, was located outside of their physical body. While standing next to the chair, Ehrsson used plastic rods to simultaneously touch the each participants' chest and the chest of an illusory body located just ahead of the cameras. When the movements of both rods corresponded to each other, the participants reported the experience of sitting behind their actual bodies

Because of the position of the eyes, we always perceive our bodies in exactly the same way,  from the first-person perspective, with our limbs appearing in a number of limited orientations beneath us. And this earlier work suggested that this perspective is critical for the sense of body ownership. On the basis of these earlier findings, Ehrsson and his colleague Valeria Petkova reasoned that they could induce the illusion that one "owns" the body of another, by manipulating the visual perspective so that the other body is viewed from the first-person perspective and providing the corresponding tactile stimuli.

In the first set of experiments, they used a life-sized mannequin fitted with two CCTV cameras, which were positioned in such a way that the images from each of them corresponded to the mannequin's eyes. The 32 participants wore a set of head-mounted visual displays, which were connected to the cameras in such a way that the images from the left and right video cameras were presented to the participants' left and right eyes, respectively. The participants therefore  recognized the mannequin clearly, but when they tilted their heads downwards, they viewed its body from the first-person perspective, as if it was their own.

The researchers then used short rods to stroke each participant's abdomen and that of the mannequin. In the "synchronous" condition, the number, length and location of the strokes applied to the participants and the mannequin were identical; in the "asynchronous" condition, the same strokes were applied to a dfferent part of the mannequin. After doing this for two minutes, each participant was asked to complete a questionnaire about the perceptual effects they had experienced. From the answers they provided, it was clear that the participants perceived the mannequin's body as their own in the synchronous, but not in the asynchronous condition, even though all the strokes were applied in full view.

In a second set of experiments, the researchers sought to obtain objective evidence of these initial findings. As before, they used rods to stroke the participants' abdomens, both in and out of synchrony with the strokes applied to the mannequin, for one minute. This time though, they simulated a threatening situation by "cutting" the mannequin's body with a knife, while at the same time recording the participants' skin conductance response, as a measure of anxiety. This physiological response was found to be significantly greater when the mannequin was threatened with the knife in the synchronous condition than in the asynchronous condition, or when the mannequin was approached with a spoon in either condition.

This demonstrated that, in the synchronous condition with the knife, the participants responded as if it was their own body being threatened. But these responses were not limited to just one part of the body - the same results were obtained when synchronous tactile stimulation was applied to the hands. However, when the mannequin was replaced with a rectangular box of the same size, the threat-evoked response in the synchronous condition was significantly weaker. Thus, synchronous stimulation of any part of the body (or at least the abdomen and hands) is sufficient for the mannequin's artificial body to be experienced as one's own. However, this illusion only works with objects that resemble one's own body. 

Experimental set-up for inducing the body swap illusion, in which the participant views the mannequin's body in the first-person perspective. (From Petkova & Ehrsson, 2008)

Finally, Ehrsson and Petkova sought to determine whether the illusion could be extended so that the participants would perceive themselves to be localized within someone else's body, or to literally swap bodies with another person. In this experiment, one of the researchers wore the cameras that had previously been mounted on the mannequin's head, so that their visual perspective was conveyed to the head-mounted display worn by the participants. In this way,  they viewed the upper part of their own bodies from the third-person perspective. They were then asked to stand opposite the researcher and to take hold of their hand and squeeze it. In the synchronous condition, the participant and researcher squeezed each other's hands simultaneously, while in the asynchronous condition, they squeezed each other's hands in an alternating rhythm.

Afterwards, the participants were interviewed, so that their perceptual experiences could be established. They reported that in the synchronous condition they had perceived the researcher's arm as their own, and that they sensed their entire body behind it.  Some even spontaneously remarked with comments such as "Your arm felt like it was my arm, and I was behind it", "I felt  that my own body was someone else" or "I was shaking hands with myself!". Remarkably, they also reported that the sensations evoked when the researcher squeezed their hands seemed to originate from the researcher's hand and not from their own. This illusion was vivid  and robust - it persisted even though their own body was in full view, and  regardless of either the sex of the researcher or the shape of their body. Furthermore, the participants exhibited anxiety when a knife was placed just above the researcher's wrist, but not when it was placed near their own.

Thus healthy volunteers can easily be made to perceive themselves as "owning" the body of both a humanoid mannequin and another person. This body-swap illusion can only be induced with another person or something resembling a person; this must be viewed from a first-person perspective, and there must be a continuous match between the visual and tactile information about the state of the body. These findings support the view that the sense of owning one's own body depends on multisensory cues - visual and tactile - from all parts of the body and that these cues are integrated and processed together. They also demonstrate that visual information from the first-person perspective is critical for the experience of owning a body.

Why can the sense of body ownership be manipulated so easily? In a real life situation, identifying one's own body in space quickly and accurately can be of vital importance. It may therefore rely on memory during any decision-making process, because we have lifelong experience of seeing our own bodies from the first-person perspective, and because our bodies generate typical patterns of sensory signals in from the different modalities (vision, proprioception, and so on). Thus, information from memory is used to generate an estimate of the body's location in space, and this estimate is continuously refined by the incoming sensory information.

This refined information is then processed by populations of neurons in the premotor cortex which integrate multisensory cues and map them onto co-ordinates within which the body is at the centre. In the body-swap illusion, these same neuronal populations encode and integrate visual information from a new visual perspective, but because of their egocentric co-ordinates, the individual's centre of awareness is shifted, so that she perceives her "self" to be located at the new position in space. Consequently, the sense of ownership of one's own body can be modified simply by altering the perspective from which the body is viewed.    

As well as providing insights into how the brain constructs its representation of the body and of the "self", these fascinating findings could have useful applications in the clinical and industrial settings. For example, body swapping could prove to be a valuable research tool for researchers investigating self-identity or body image disorders. The findings could  provide improved control within virtual reality situations, and could also be applied to the field of tele-robotics, which is concerned with the control of robots from a distance. Remote-controlled robots are routinely used in the exploration of space and the deep sea; those operating such devices could possibly gain better control of them if they swapped bodies with them.

Related:

• Distorting the body image affects perception of pain
  

Petkova, V.I. & Ehrsson, H.H. (2008). If I Were You: Perceptual Illusion of Body Swapping. PLoS One 3 (12): e3832. doi: 10.1371/journal.pone.0003832.

Ehrsson, H.H. (2007). The experimental induction of out-of-body experiences. Science 317: 1048.

Ehrsson, H.H. et al (2004). That's my hand! Activity in premotor cortex reflects feeling of ownership of a limb. Science 305: 875-877.

Botvinick, M. (2004). Probing the neural basis of body ownership. Science 305: 782-783.