How we come to know our bodies as our own
By taking advantage of a 'body swap' illusion, researchers at Karolinska Institutet have captured the brain regions involved in one of the most fundamental aspects of self-awareness: how we recognize our bodies as our own, distinct from others and from the outside world. That self-perception is traced to specialized multisensory neurons in various parts of the brain that integrate different sensory inputs across all body parts into a unified view of the body. The findings, reported in the journal Current Biology, may have important medical and industrial applications.
"When we look down at our body, we immediately experience that it belongs to us," says Valeria Petkova, one of the researchers behind the study. "We do not experience our body as a set of fragmented parts, but rather as a single entity. Our study is the first to tackle the important question of how we come to have the unitary experience of owning an entire body."
Earlier studies showed that the integration of visual, tactile and proprioceptive information (the sense of the relative position of body parts) in multisensory areas constitutes a mechanism for the self-attribution of single limbs, the researchers explained. But how ownership of individual body parts translates into the unitary experience of owning a whole body remained a mystery.
In the current study, the researchers used a 'body-swap' illusion, in which people experienced a mannequin to be their own, in combination with functional magnetic resonance imaging (fMRI). Participants observed touching of the mannequi's body from the point of view of the mannequin's head while feeling identical synchronous touches on his or her own body, which they could not see. Those studies revealed a tight coupling between the experience of full-body ownership and neural responses in brain regions known to represent multisensory processing nodes in the primate brain, specifically the bilateral ventral premotor and left intraparietal cortices and the left putamen.
Activation in those multisensory areas was stronger when the stimulated body part was attached to a body, as compared with when it was detached, they report, evidence that the integrity between body segments facilitates ownership of the parts. According to the researchers, the findings suggest that the integration of visual, tactile and proprioceptive information in body-part-centered reference frames represents a basic neural mechanism underlying the feeling of ownership of entire bodies.
"Understanding the mechanisms underlying the self-attribution of a body in the healthy brain can help developing better diagnostic and therapeutic strategies to address pathological disturbances of bodily self- perception," says Associate Professor Henrik Ehrsson, who led the study. "In addition, understanding the mechanisms of perceiving an entire body or a body part as belonging to oneself can have important implications for the design and production of mechanical prosthesis or robotic substitutes for paralyzed or amputated body parts."