In a recent study, researchers employed a clever experimental setup that used 3 different mirrors to make it appear that a person's hand was twice the size, half the size or the actual size of his/her real hand. To grasp the setup it helps to look at the figure. Looking at the right hand side of the "hand-view condition" panel, you can see that participants sat at a table with left hand behind the mirror and the right hand on the reflective side of the mirror. This setup induced the impression that what they were seeing reflected by the mirror was actually their left hand.
I know this may sound a bit difficult to imagine but I've tried it and believe me the illusion is uncanny -- yeah you know that what you are looking at is really just a reflection of your right hand but what you perceive nonetheless is that the mirror is not a mirror at all but a window and that you're looking at your left hand through the window (the sensation is particularly powerful if you move your right and left synchronously). This illusion has actually been used to successfully treat otherwise stubborn pain conditions such as phantom limb pain and complex regional pain syndrome.
Anyway, the whole reason for this complicated setup was so that the researchers could achieve the illusion of different hand sizes. After an adaptation period to get participants' brains "convinced" that their right hand was actually their left hand, they applied a heat probe that got gradually hotter until participants pushed a pedal to indicate the sensation was painful. They repeated this over a series of trails, swapping the mirror in a random sequence and rated pain threshold under conditions of different apparent hand sizes. One final wrinkle: instead of seeing their hands, some participants saw a block object.
Two interesting results emerged: First, merely observing the hand in the non-distoring mirror increased pain thresholds. That is, just focusing on the body part that was experiencing pain somehow made people less sensitive to pain, enabling them to accept higher heat levels before reporting pain. Second, for people in the hand (but not the object) view condition, visual enlargement of the hand enhanced the analgesic effect occurring when people look at the painful body part, whereas visual reduction decreased the analgesic effect.
Intriguing. If you have a pain in your hand, just stare at it and the pain will diminish. View the painful region under a magnifying glass and the pain will diminish even further!
Perhaps somewhat confusingly, these results are different from those obtained by Moseley et al (2008) who reported that pain and swelling in complex regional pain syndrome (CRPS) patients increased when patients viewed the affected limb enlarged. The authors of the current study suggest that the neurophysiological distinctions between acute and chronic pain may underly the opposite results. For example, they point out that CRPS in fact alters the representation of the affected limb in somatosensory regions of the brain. Such changes might lead to differences in the way the brain perceives the affected regions of the body.
It will remain to future studies to investigate the mechanisms mediating this fascinating finding.
Mancini, F., Longo, M. R., Kammers, M. P. M., & Haggard, P. (2011). Visual Distortion of Body Size Modulates Pain Perception. Psychological Science, 22(3), 325–330. doi:10.1177/0956797611398496
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