Pain in the eye of the beholder

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It goes without saying that we are capable of noticing changes to our bodies, but it’s perhaps less obvious that the way we perceive our bodies can affect them physically. The two-way nature of this link, between physicality and perception, has been dramatically demonstrated by a new study of people with chronic hand pain. Lorimer Moseley at the University of Oxford found that he could control the severity of pain and swelling in an aching hand by making it seem larger or smaller.

Moseley recruited 10 patients with chronic pain in one of their arms and asked them to perform a series of ten hand movements at a set intensity and to a set pace. The volunteers had to watch their arms as they went through the motions. On some trials, they did so unaided, but on others, they viewed their arms through a pair of binoculars that doubled their size, a pair of clear-glass binoculars that did not magnify at all, or a pair of inverted binoculars that shrunk the image. 

On each trial, Moseley asked the recruits to rate their pain on a visual sliding scale. He found that they were in greater pain after they had moved their arms – no surprise there. But the amount of pain they felt depended on how large their arm appeared to them. They experienced the greatest degree of extra pain when they saw magnified views of their arms, and took the longest amount of time to return to normal. Perhaps more surprisingly, the “minified” images actually evoked less pain than normal.

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You could argue that this was nothing more than a cute psychological effect, but changing the perceived size of the volunteers’ arms also evoked physical changes. Moseley measured the circumference of the volunteers’ fingers as they moved them about. Again, those who saw a magnified image had the most extreme swelling and those who saw the shrunken versions had less swelling. All of these changes were statistically significant.

Moseley isn’t clear why this effect happens, although he suggests that making things more visually apparent could also make them more sensitive to touch. After all, some studies have found that it’s possible to heighten someone’s sense of touch by magnifying their view of the area they’re touching. Alternatively, the sight of a bigger and more swollen limb could fool the brain into evoking the type of pain response that it would normally use to signal imminent danger.  And scientists have even suggested that a mere conflict between what we see and what we feel can cause localised pain and swelling.

For now, that’s all speculation, but Moseley is especially interested in the fact that he could soothe the degree of pain and swelling in a limb, simply by making it look smaller. That could certainly have clinical merit, so it’s worth understanding why it happens. Moseley suggests that people may feel less ownership over their own body parts, if they seem smaller than expected. Indeed, in a study he published earlier this year, he used an illusion to convince volunteers that an artificial limb actually belonged to them and, as a result, the temperature in their actual arm fell. 

Reference: Moseley GL, Parsons TJ and Spence C. 2008. Visual distortion of a limb modulates the pain and swelling evoked by movement. Current Biology 18: R1047

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