While the old adage may say that "seeing is believing," new research shows that vision can lead the mind to do some unusual things -- like thinking a rubber hand is your own.
The rubber hand illusion was first developed in 1998 by Matthew Botvinick and Jonathan Cohen. Researchers put a subject's hand out of sight and placed a rubber hand in front of the subject. By stroking both the real and rubber hands at the same time, the researchers found they could get the subject to think that the rubber hand belonged to them.
Recently, researchers further probed just how much the brain can be tricked into thinking it is in control of a rubber hand rather than the limb it is actually connected to.
To do this, they placed a subject's hand out of sight and moved a rubber hand into the subject's line of sight as a replacement. By touching the subject's actual hand and the fake hand in a similar way, they were not only able to cause most subjects to believe the hand belonged to them, but they also found that the real hand would react to things done to the rubber one.
In this case, the scientists found that the body temperature of the actual hand would drop slightly in response to something done to the rubber one -- suggesting that the body had given up on the subject's actual hand.
"It's showing that the different senses are not hot lines from the skin to the brain, but there's a considerable intermingling of the senses," said V.S. Ramachandran, a neuroscientist at the University of California San Diego who was not involved in the study, which he described as "elegant."
"Using visual feedback, you can profoundly alter your perception of a body part, and that, in turn, alters other sensations that would ordinarily be found in that body part," he said.
In the latest study, scientists from Oxford University took the Botvinick-Cohen research a step further by measuring the temperature of subjects' hands after they felt that the rubber hand was theirs. They found that skin temperature dropped in the hidden, real hands, a change that did not take place in the rest of the body.
Lorimer Moseley, one of the paper's lead authors, said that the study shows that the effect of the rubber hand illusion is not limited to conscious thoughts alone.
"The way our body feels to us can actually influence the tissues of the body," he said.
Moseley said that a practical application for the research may be "a long way off," but he sees some potential in rehabilitation therapies.
"I wonder if it will one day lead to us being able to affect unhealthy tissues by manipulating the sense of how it feels," he said.
In the discussion of their findings, the researchers noted that a number of diseases result from the possession or disowning of a limb. Disownership can occur in schizophrenia and, they say, may cause some of the neglect of a certain area after strokes.
The authors say that while past research has focused on the idea that disownership was due to damage in the brain, the results of their study suggest that "higher order cognitive processing associated with the representation of our physical body in space may also contribute."
The next step in this research, said Moseley, is "seeing if we can manipulate [a sense of limb ownership] in patients, and if we can, what effect it has."
What the rubber hand illusion findings point to, explains Ramachandran, is how much we rely on our sense of sight.
"We primates are highly visual creatures," he said.
In a situation like the rubber hand illusion, the brain struggles to make sense of conflicting inputs from senses of sight and of feel.
Ramachandran noted that in other tests of the rubber hand illusion, subjects would often show a physiological response if something painful were done to the phantom hand.
Conversely, when something cold was applied to the real hand, the person would often have a more minimal reaction, because they had disowned it.
Ultimately, they hope this research will lead to treatments for a number of mental ailments.
Self-perception problems have been implicated in a host of disorders, which include phantom pain and schizophrenia, and also self-image problems like anorexia.
One way this principle has been used is to treat phantom pain. Ramachandran said that by using a mirror, a subject can be "tricked" into thinking that a phantom limb has been resurrected.
The mirror is placed in the center of the body, and by moving the existing limb, subjects will come to believe that they are in control of the one they have lost.
"It is again telling you that in neurology you are not talking about fixed connections," Ramachandran said. "You can press the reset button."
As for our self-image, Ramachandran notes that "it is constantly being modified due to changing senses of inputs. … When you do these experiments, you're surprised by how malleable it is."