Magicians often use quick hands to fool the eye. Now scientists find that people react to injuries committed against fake rubber hands and even tables, as if they were parts of their own bodies. Researchers say these findings show that the brain's concept of the body is surprisingly impressionable.
The researchers started off with "hand illusion" experiments, in which volunteers can see a fake rubber hand while their own is hidden from view. Previously, scientists found that when both real and fake were simultaneously tapped or stroked, people said they felt as if the sensations came from the dummy hand. In the new study, cognitive neuroscientists Carrie Armel and Vilayanur Ramachandran of the University of California, San Diego, added a twist--perceived pain--and measured volunteers' skin conductance response with electrodes. This test recorded when a volunteer suddenly got sweaty palms, an indicator of stress.
Sixteen student volunteers sat down at a table that hid their right hand. Researchers touched the volunteers' real hand and a fake hand on the tabletop for a few minutes, then bent a finger on the real right hand backwards slightly while a finger on the fake was bent back to a "painful" position. In a paper published online 6 June in the Proceedings of the Royal Society, the researchers report that the subjects thought their real fingers went back much further than they actually did.
In addition, researchers put Band-Aids on both a table and volunteers' real hands. After 24 students had their hand and table stroked and tapped the same way as before, when the Band-Aid was ripped partially off only the table, many winced. In both experiments, volunteers displayed strong skin conductance responses. A few volunteers even reported pain, although such verbal accounts might have been metaphorical, Ramachandran stresses. Even so, he says, subjects react as if they have nerves in the table, which is "spooky." He suggests that even though the intellect sees through the illusion, the sensory parts of the brain override this with their own interpretation of events.
Armel says these findings show how malleable the brain's map of the body can be. She suggests it could be possible to use similar concepts to help anorexics learn a healthier self image, perhaps by viewing themselves in mirrors that alter their shape. Neuroscientist Michael Graziano of Princeton University in New Jersey suggests that the same idea could be used to help amputees better adjust to prosthetic limbs.