A strange sensation, but familiar to anyone who has ever been given local anaesthesia and watched while a doctor operated on their leg or arm: in that moment, your own body part seems foreign, as if it doesn’t belong to your body. One reason for this is that the brain still knows which position the limb occupied before the local anaesthetic took effect. As soon as it wears off, the spooky sensation disappears.

Persistent sense of estrangement

For people who have suffered a spinal cord injury or a stroke, however, this “estrangement” of their own limbs doesn’t go away, because an injury of this kind impairs or disrupts communication between the brain and the body. This impacts the anatomical reconstruction of the body in the brain, known as body representation.

Our brain and our limbs, such as arms, legs, feet and hands, constantly exchange data on position, orientation and condition – somatosensory information. The eye is involved in body awareness as well. The brain, for its part, processes this visual and somatosensory information to form an image of the body and “files” it in the cerebral cortex. Whether and to what extent damage to the spinal cord alters the body representation, however, is still a matter of dispute among experts and doctors. Studies carried out on this have reached conflicting conclusions.

Now researchers from a number of institutions have conducted a new study that sheds more light on the matter. One of the leaders of this study, which was just published in the journal Nature - Scientific Reports, was ETH Zurich Professor Roger Gassert from the Rehabilitation Engineering Laboratory.

Interpreting body images in our minds

In the study, the researchers used an established test to analyse whether and to what extent the body representation in paraplegics differs from that in healthy individuals.

To this end, during his research sabbatical at ETH Zurich, corresponding author Silvio Ionta (now lecturer at the University of Lausanne) developed a task in which participants were shown pictures of foreign body parts, such as a foot or a hand, or even of the entire body. While the images were being presented, the participants could not see their own hands and feet, but kept them either parallel to one another or in a crossed position.