These findings clearly demonstrate that the mere appearance of a limb—making it appear larger or smaller than normal—can directly modulate our experience of bodily pain. Exactly how distorting the body image in this way affects the perception of pain remains unclear.
One possibility is that magnifying the view of the hand enhances the sense of touch, whereas minifying it has the opposite effect. Some evidence for this comes from a 2001 study, which showed that magnifying subjects' arms improved their ability to discriminate between needle points placed close to each other.
Alternatively, the authors suggest that manipulating the image of the hand may have altered the participants' sense of "ownership" of their painful limb. Viewing magnified images of their hands may have made the participants more aware that the limb belonged to them, thus increasing their sensitivity to the painful sensations originating from it. Conversely, the minifying condition may have "alienated" the participants' arms, reducing their sense of ownership of their limb and thus desensitizing them to the pain felt in them.
Regardless of the mechanism, these findings could lead to new methods for improving the rehabilitation regimes administered to patients with a wide variety of conditions. Most obviously, they point to a simple way for the effective management of chronic pain. They may also prove to be useful in helping patients with alien hand syndrome and related conditions to regain ownership of their limbs.
It is now well known that physical changes to the body alter the brain's representation of it. In the case of amputation, for example, the changes that take place in the brain lead to phenomena such as phantom pain, in which painful sensations are felt from the missing limb. This relationship between body and brain is referred to as "bottom-up"; this study, however, suggests a "top-down" relationship—it seems that the visual inputs (the participants' views of their hands) were sufficient to override the tactile sensations (the felt pain), by fooling the brain into modulating them.
Are you a scientist? Have you recently read a peer-reviewed paper that you want to write about? Then contact Mind Matters editor Jonah Lehrer, the science writer behind the blog The Frontal Cortex and the book Proust Was a Neuroscientist. His next book, How We Decide, will be available in February 2009.