Body Self Perception
Tilt your head forward and take a look at your body. How do you know that this body belongs to you? How do you actually come to perceive this body as part of yourself? This question has been discussed by philosophers and psychologists for centuries but remained outside the scope of experimental investigation. Henrik Ehrsson and colleagues have been addressing this question from a cognitive neuroscience perspective.
|About the author: Henrik Ehrsson is a group leader in the Department of Neuroscience at the Karolinska Institute, Stockholm, Sweden. He was an HFSP Long-Term Fellow from 2004-2007 in the laboratory of Richard Passingham at the Wellcome Trust Centre for Neuroimaging at University College London. In 2007 he moved to Stockholm with an HFSP Career Development Award. He has been successful in obtaining competitive grants such as the Starting Grant from the European Research Council.|
The first indication that the self-perception of the body is something that is actually produced by brain comes from the clinical literature. Patients who have suffered from stroke affecting frontal and parietal regions can develop conditions with disturbed perception of their own body. Some of these patients develop a deficit that can take the form of denying or disowning parts of the body. Although these cases indicate that the frontal and parietal association cortices are related to body self-perception they do not pinpoint the specific brain mechanisms involved because typically the lesions are large and affect multiple areas including the underlying white matter tissue.
We have used a classical approach in psychology to investigate body self perception: we studied illusions to learn more about the basic processes that underlie normal perception and combined this with state-of-the-art brain imaging techniques to identify the underlying brain mechanisms in healthy individuals. One particularly informative illusion is the ‘rubber hand illusion’ where people experience that a prosthetic hand is in fact their own hand. When synchronous touches are applied to a rubber hand, in full view, and the real hand, which is hidden behind a screen, most individuals will sense the touches on the rubber hand and experience that the artificial limb is their own. Even more dramatic is the ‘out-of-body’ illusion. In this setup the participants wear a set of head mounted displays in front of their eyes which are connected to two CCTV cameras placed one and a half meters behind them. The two cameras provide a stereoscopic image and the participants, who can thus see themselves from the point of view of the cameras, i.e. from the back. The experimenter then jabs a rod towards a location just below the cameras while simultaneously touching the participant’s chest, which is out-of-view. The visual impressions of a hand approaching a point below the cameras and the felt touches on the chest lead the participants to experience the illusion of being located one and a half meters behind their real body, with loss of self-identification with that body. Subsequent experiments demonstrated that people can perceive an entire artificial body, or another person’s body, as their own. In these experiments the two cameras were attached to a helmet worn by a life-size mannequin (or another individual) and positioned so that they were looking down on the mannequin’s body when it was touched synchronously with the real body (see Figure below).
Figure. Set-up used to elicit of the illusion that an artificial body is one’s own body (left); and the out-of-body illusion (top right for a side view and middle right for the participant’s perspective). Activity in a multisensory area (premotor cortex) reflects illusory body-self perception (lower right). (Photo: Staffan Larsson).
By clarifying the precise combination of factors that are necessary and sufficient to elicit these changes in body self-perception we can develop models of body self-perception. It turns out that the critical factors are that the information from the eyes, skin and muscles matches both in time and space, that there is an ego-centric visual perspective of the body, and that the object to be owned has a sufficient human-like appearance. We then use these principles to develop testable hypotheses about the neuronal mechanisms of body self-perception.
To this end we used functional magnetic resonance imaging to show that neuronal substrates of body self perception involve areas in the frontal and parietal lobes that receive convergent visual, tactile, and proprioceptive afferent input, so called multisensory areas. Of particular interest were neuronal populations in the ventral premotor cortex and areas in the intraparietal that integrate visual, tactile and muscle sense information in body-part-centered reference frames in the space near the body. These neurons most probably mediate the perception of a limb as one’s own because our fMRI experiments have found significantly increased activation in these areas when people experience the rubber hand illusion and full-body illusion (see Figure above), and that the activity in this area closely matches the perceptual principles determining these illusions so that the stronger the activity the stronger the illusory self perception.
Taken together these studies represent a major advance in our understanding of the brain mechanisms mediating body self-perception. By applying these principles we can develop new clinical and industrial technologies where the self-perception of the body is deliberately manipulated. For example, one can use the rubber hand illusion to enhance the feeling of ownership of artificial limbs used by amputees, and the projection of ownership onto simulated bodies represents a new direction in virtual reality research which could enhance user control, realism, and the feeling of ‘presence’ in industrial, educational and entertainment applications.
Ehrsson HH, Spence C and Passingham RE. 'That's my hand!' Activity in the premotor cortex reflects feeling of ownership of a limb. Science, (2004) 305:875-877.
Ehrsson HH. The experimental induction of out-of-body experiences. Science (2007), 317:1048
Ehrsson HH, Weich K, Weiskopf N, Dolan RJ and Passingham RE. Threatening a rubber hand that you feel is yours elicits a cortical anxiety response. Proc. Natl. Acad. Sci. USA (2007), 104:9828-9833.
Ehrsson HH, Rosén B, Stockselius A, Ragnö C, Köhler P, Lundborg G. Upper limb amputees can be induced to experience a rubber hand as their own. Brain (2008) 131, 3443-3452.
Petkova VI & Ehrsson HH. If I were you: perceptual illusion of body swapping. PLoS One (2008), 3(12):e3832,
Slater M, Perez-Marcos D, Ehrsson HH and Sanchez-Vives MV. Inducing illusory ownership of a virutal body. Frontiers in Neuroscience (2009), 3:214-220