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In a sense, study has disarming result

Thursday, February 19, 1998

By Byron Spice, Science Editor, Post-Gazette

Using sleight of hand, Pittsburgh researchers have performed an eerie experiment in which people thought they could feel the stroke of a brush against a life-sized rubber arm -- almost as if the rubber arm were their own.

Dr. Matthew Botvinick with the third hand used in his experiments, and Dr. Johathan Cohen. (Steve Mellon - Post-Gazette)

In the experiment, one of their own arms was hidden from their sight. Then that arm was stroked with a brush at the same time the rubber arm was stroked with a brush. But they sensed that the tickle they felt was occurring in the rubber arm, not their own.

"There's a very vivid perception . . . that you are feeling what it should be feeling," said Dr. Matthew Botvinick, a psychiatrist at the University of Pittsburgh School of Medicine.

More than a Stupid Human Trick, the experiment explored how people come to recognize their body as part of the self. As described in today's issue of the journal Nature, the process is democratic. In the interplay of conflicting senses, majority rules.

It may sound obvious that a person would know that his arm is, in fact, his arm, but the mind can play the same tricks that neuroscientists play.

Amputees, for instance, sometimes report sensations coming from their missing limbs. Stranger still, some people who suffer strokes or other brain injuries are alarmed to find, say, an arm connected to their body that they don't recognize as being their own.

Dr. Jonathan Cohen, a psychiatrist at Pitt and Carnegie Mellon University and Botvinick's co-author, said this same mechanism for perceiving self might also apply to the nonphysical self -- the person's perception of herself as a funny person, or a smart person or a logical person.

Their work is still basic research, but this line of inquiry might yield some understanding of phantom limb syndrome or multiple personality disorder, Cohen said.

Michael Graziano, a neuroscientist at Princeton University, said the findings were consistent with his own studies of how brain cells in a monkey determine the position of the monkey's arm. As in the Pittsburgh experiments, touch has proven important in the brain learning how to recognize the arm's position.

Botvinick got the idea for the experiment after reading from Maurice Merleau-Ponty, a French philosopher concerned with how the body is perceived as different from other objects. Botvinick began to wonder: Could someone be tricked to perceive another object as self?

The first test came not in the laboratory, but as an impromptu stunt at a party where the hostess happened to have a rubber Halloween hand. A silly mood prevailed as candles were lit and the party took on the trappings of a seance.

To everyone's amazement, the illusion seemed to work. So Botvinick and Cohen transformed the party trick into a scientific experiment. Each of 10 subjects was seated with his or her left arm resting on a table, but hidden from the subject's view by a screen. A rubber model of a left hand was placed on the table in front of the subject.

While the participant looked at the rubber hand, an experimenter used two brushes to simultaneously stroke both the subject's hidden left hand and the rubber hand.

"It's sort of uncanny," Botvinick said. "It doesn't seem like it should work."

After 10 minutes, each subject filled out a questionnaire. Eight of the 10 said it felt as if the rubber hand had been their own hand.

That's not a result most scientists likely would accept, Botvinick admitted. "They might say X, but you can never prove that they weren't saying X just to be nice," he explained.

So he and Cohen added a second experiment, in which the viewing period was extended to half an hour. In this second phase, each subject was asked to move his right index finger under the table to the point below his left index finger, both before and after the experiment.

The investigators found that after the experiment, the subjects placed their right index fingers farther to the right -- closer to the rubber hand -- than they had before the experiment.

Recognizing self, Cohen and Botvinick suggested, appears to result from a combination of senses -- sight, touch and proprioception, the sense of position derived from the muscles.

In the experiment, proprioception would have told the subjects the true position of their left hand, but sight and touch seemed to be telling them something else. Sight and touch overruled proprioception.

Princeton's Graziano said he had seen similar phenomena in monkeys.

"Your sense of arm position is very dependent on sight of the arm," he said. Neurons in the brain's premotor cortex respond both to proprioception and to visual signals, he said, and touch seems to be the glue that the neurons use to combine those signals.

Graziano and his colleagues are now trying their own version of the Pittsburgh experiments, using a monkey arm from a taxidermist instead of a rubber arm.

Earlier this month, Cohen performed an unplanned, unintentional and very painful experiment about the conflict between the senses and self. As he worked on a carpentry project in his workshop, his hand slipped and a chisel severed a tendon in his arm.

His finger was left frozen straight, but as the severed tendon retracted into his arm, his proprioception told him something different.

"When I closed my eyes, I could swear the finger was bent," said Cohen, whose tendon was repaired surgically last week at UPMC Presbyterian.

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