Distorting the body image affects perception of pain

The term body image was coined by the great neurologist Henry Head and refers to a mental representation of one's physical appearance. Constructed by the brain from past experience and present sensations, the body image is a fundamental aspect of both self-awareness and self-identity, and can be disrupted in many conditions.

Disruption of the body image can have profound physical and psychological effects. For example, body image distortion is implicated in eating disorders such as anorexia nervosa, and also leads to phenomena such as phantom limb syndrome and body dysmorphic disorder; extreme cases of the latter can lead some people to request amputation of what they perceive to be a supernumery limb.  

A new study published in Current Biology now shows that visual distortions of the body image in patients suffering from chronic pain can significantly affect their perception of painful sensations. The findings could lead to improvements in the treatment of, and rehabilitation regimes for, a wide variety of neurological and psychiatric disorders.

Ten patients suffering from chronic pain in one arm participated in the new study. They were asked to watch their own arms while they performed a standardized repertoire of ten hand movements.These tests were carried out under four different and randomized conditions. In the "clear" condition, the participants looked at their arms through a pair of binoculars with no magnification; in the "magnified" condition, they looked through binoculars with 2x magnification; and in the "minified" condition they looked through inverted binoculars which made their arm appear smaller than it actually was. In the control condition,  they looked at their arms without any visual manipulation.

While they performed the tests, the participants were asked to rate how much pain they felt,  on a scale of 0-100, which was displayed on a computer screen and could be adjusted with a mouse controlled by the unaffacted hand. After each movement was executed, the researchers also assessed the extent of swelling, by measuring the circumference of the digits of the affected hand and comparing them to those on the unaffected hand.

In all the tests, the participants reported that the pain was worse immediately after the movements than it was before. Unexpectedly though, the extent to which the movements increased the participants' pain depended on how their vision had been manipulated. They reported the greatest increase in pain when the image of their arm was distorted by the magnifying binoculars, and the least increase when they viewed their arms through the minifying binoculars.

The visual manipulations also affected the time taken for the participants' pain to return to the pre-task level - the recovery was slowest during the "magnified" condition and fastest during the "minified" condition. Furthermore, the swelling that occurred following the movements increased more during the "magnified" condition than during the "minified", "clear" or control conditions. These findings therefore clearly demonstrate that making a limb look bigger than it actually is increases the pain and swelling evoked by movements and that, conversely, making a limb look smaller than it is reduces that pain.

It is well known that physical changes to the body alter the mental representation of  it. Following amputation, for example, there is significant alteration of the somatosensory cortex, the part of the brain onto which the body is mapped. This relationship between the body and the brain is referred to as "bottom-up". This study, however, points to a powerful "top-down" effect, and  therefore strongly suggests that this relationship is reciprocal.  

Exactly how a distortion of the body image modulates painful sensations is unclear. It may be the case that magnifying the body leads to an enhancement of the sense of touch. Some evidence for this comes from a 2001 study which showed that the ability to discriminate between two tactile stimuli was improved by magnification of the arm, and a neuroimaging study published earlier this year, which showed that magnification of the arm alters its somatosensory representation. By the same token, minifying a part of the body would have the opposite effect on tactile sensations. In terms of cognitive function, the authors suggest that minifying the limb reduces the felt pain because it decreases the participants' sense of "ownership" of it.

Regardless of the mechanism, these findings have implications in the clinical setting, and could lead to improvements in the rehabilitation regimes administered to patients with a wide variety of conditions. Obviously, the finding that manipulating the body image can reduce pain could prove beneficial to patients with chronic pain. Less obviously, manipulating it so that body parts appear larger than they actually are could be useful for various psychiatric conditions, such as, for example, alien hand syndrome, in which a patient denies ownership of his limb and claims that it is being controlled by external forces.      

Moseley, G. et al (2008). Visual distortion of a limb modulates the pain and swelling evoked by movement. Curr. Biol. 18 DOI: 10.1016/j.cub.2008.09.031.