According to the researchers, the concept of body self-identification arises out of the way humans perceive their bodies versus other objects in the world around them.
"Our mind represents our own bodies as different from other worldly objects," explained Matthew Botvinick, M.D., a psychiatrist at the University of Pittsburgh School of Medicine. "We're suggesting that this is because the body participates in intersensory matches in a way that other objects don't. When something touches our hand, we both see it and feel it. This gives us a dual perspective, sight and touch. The brain makes a match between the two perspectives. This match leads the brain to identify that part of the self. When we see another object like a chair or table being touched, we have only one perspective, sight, with no corresponding tactile perception. This is what makes the body and other objects different, and this difference counts for the brain."
Dr. Botvinick and Jonathan Cohen, Ph.D., professor of psychology at Carnegie Mellon University and professor of psychiatry at Pitt, developed an experiment through which they could observe how the brain integrates multiple sensory modalities in representing the body. Each of 10 subjects was seated with his or her left arm resting on a small table just left of the body midline. A screen was placed beside the arm to hide it from the subject's view. Then, the researchers placed a rubber model of a left hand and arm on a table directly in front of the subject. The participant was instructed to sit still and focus on the rubber hand while the experimenter used two small paintbrushes to simultaneously stroke the rubber hand and the subject's hidden left hand. The procedure was carried out so as to create as close a match as possible between what the subject saw and what the subject felt.
After ten minutes, each participant completed a questionnaire. The subjects' responses indicated they had experienced an illusion in which they seemed to feel not the touch of the hidden brush, but the touch of the brush they could see, as if sensed by the rubber hand. Eight out of 10 subjects who reported this illusion also reported that they had experienced the rubber hand as if it were their own. The subjects described the rubber hand with phrases like "It felt like the rubber hand was my own."
"This suggests that one of the ways our mind comes to determine the body as its own is the correlation of sensory experiences such as touch and sight," commented Dr. Cohen. "This is an important step in determining what defines us as ourselves."
"We humans have a sense of inhabiting our bodies," said Dr. Botvinick. "This sense of ownership depends on multiple senses and the continuing matches our brain makes between them. In this experiment the subjects felt the brush on their real hand and saw it on the artificial hand and their brain brought these two things together in space. As a result, our subjects developed a sense of inhabiting the rubber hand.
"The effect is surprisingly compelling," continued Dr. Botvinick. "If you ask someone to describe the sensation, they tell you the rubber hand felt like it was their own."
"This is not a subtle finding that will create a lot of debate," commented Dr. Cohen. "It is a really robust effect. It's possible that this is the beginning of how our mind says 'that's mine, that's yours'."
According to Dr. Botvinick, the illusion of ventriloquism is created in much the same way. "In both cases, the brain puts together two things that belong together. In ventriloquism, it's the voice and the dummy's moving mouth. In the rubber hand experiment, a touch is felt in one place, but seen in another. In order to make sense of this, the brain puts the two together. We think we hear the dummy talk, we think we feel through the rubber hand."
The researchers have developed a computational model of what is happening within the brain as the illusion occurs. The next step will be to relate this to such clinical conditions as phantom limb and "alienation phenomenon," where brain damaged patients fail to recognize parts of their bodies as belonging to themselves.
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