In a previous post on the self and the brain I briefly mentioned how our bodily self can be affected by disease, such as anorexia nervosa. And as mentioned here some individuals refuse ownership of the their own limbs and want to get rid of them.
It does not take disease or injury to interfere with our bodily self. In spite of the constant presence of our body it is surprisingly easy to trick healthy people into believing that fake body parts belong to them, and at the same time almost making them disavowing their own limb that is hidden from sight. This fascinating illusion was first described by Botvinick and Cohen in 1998. Here is how it was done.
The rubber hand illusion
Botvinick and Cohen placed a person´s arm behind a screen and put a rubber hand on the table in front of the person. The real hand and the rubber hand were then stroked simultaneously. When the visual input from the rubber hand and sensory input from the real hand blended the result was startling. As one of the subjects put it: “I found myself looking at the dummy hand thinking it was actually my own“. The rubber hand illusion persists even after the stroking stops. Furthermore, the illusory ownership of the rubber hand is so real that when the rubber hand is “threatened” recordings from the brain show that it reacts with an anxiety response just as it would if the real hand had been threatened. The subjects know that the rubber hand is not theirs, after all they look at it and speak of it as a rubber hand. Yet, the subjective feeling accompanying the illusion is very real. It has also been demonstrated that there is a slight drop in skin temperature in the real hand during the rubber hand illusion. It is certainly striking that a cognitive illusion results in drop in skin temperature but this finding shows that temperature regulation and our conscious sense of our physical sense are somehow linked.
Here is a fascinating video presented by Morgan Freeman. It describes how we can relatively easily trick the brain by overriding our bodily signals with visual input and thereby changing our sense of ourselves.
Clinical relevance of the rubber hand illusion
Schizophrenic patients have a stronger rubber hand illusion than others and the illusion sets in more rapidly than in healthy controls. People who have an unhealthy body image also have a stronger rubber hand illusion. So is it possible that the rubber hand illusion could have a clinical application? Recent studies suggest that this might be the case.
In a study published earlier this month, Lenggenhager and her colleagues point out that spinal cord injury can lead to a disturbance in body image and the sense of self. Bodily sensations are important in maintaining a normal sense of self and in spinal cord injury these sensations are impaired. Lenggenhager investigated two patients with spinal cord injury who had paralysis in the legs and trunk and partial paralysis of the arms. The patients were also unable to feel some of their fingers. Yet, during the rubber hand illusion both patients reported vivid tactile sensations in their numb fingers. They scientists suggested that this approach might be useful in the rehabilitation of spinal cord injury, for example by helping patients maintain their body image and by reducing neuropathic pain that is common in spinal cord injury.
Should you want to dazzle your friends with the rubber hand illusion you can buy a rubber hand here.
Botvinick, M. and Cohen, M. (1998). Rubber hands “feel” touch that eyes see. Nature, 39, 756.
Moseley, G. L., Olthof, N., Venema, A., Don, S., Wijers, M., Gallace, A. and Spence, C. (2008). Psychologically induced cooling of a specific body part caused by illusory ownership of an artificial counterpart. Proceedings of the National Academy of Science, 105(35), 13169–13173.
Lenggenhager, B., Scivoletto, G., Molinari, M. And Pazzaglia, M. (2013). Restoring Tactile Awareness Through the Rubber Hand Illusion in Cervical Spinal Cord Injury. Neurorehabilitation and Neural Repair. On-line version can be found here: http://nnr.sagepub.com/content/early/2013/06/10/1545968313491009
Ramakonar, H., Franz, E. A. and Lind, C. R. P. (2011). The rubber hand illusion and its application to clinical neuroscience. Journal of Clinical Neuroscience, 18, 1596-1601.
Photo credit: http://www.flickr.com/photos/7931817@N02/997459653 (Lighthouse photostream)