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NEUROSCIENCE

You Feel That?
It's not so hard to fool your sense of touch

Most of us are pretty confident in our body ownership. We see appendages emerging from our shoulders and think: These are my arms. That multiple-digit tool at the end of my arm is my hand. I need to trim my nails. And so on. The awareness we have of our body position in space is called proprioception. But science is good at probing the fragile superstructure of things we take for granted.
 
It turns out that proprioceptive cues can be fooled easily. Recently a neuroscientist named Henrik Ehrsson performed an experiment in which a subject was positioned in an MRI machine with his right hand resting on his leg beneath a solid surface; a realistic rubber right hand rested atop the surface. A researcher used a small brush to stroke the finger of the real hand, which the subject could not see, while simultaneously stroking the corresponding finger on the rubber hand that the subject could see.
 
Within 15 seconds the test subjects typically developed a profound sense that the rubber hand was the real hand. The test subjects would flinch when Ehrsson threatened to smash his fist on the rubber hand. They were surprised when they realized they were unable to lift a rubber finger. They knew what was going on, but no amount of rational thought could dispel the sensory illusion.
 
 "They don't just think it," says Ehrsson. "They feel it. They can't think it away." Ehrsson isn't the first to perform an experiment that shows proprioception can be foiled by illusion. But he is the first to use brain scans to study which portions of the brain are active during the experiment. He believes that the sense of body ownership is controlled by the premotor cortex, a region of the brain that integrates vision and body movement.
 
That doesn't sound controversial to most of us, but it's a leap of sorts, for it takes a hard-science, technological approach to a question philosophers have debated for centuries. To what extent can we actually believe that we and the world around us are real? In the 17th century René Descartes stated, "I think, therefore I am." But that didn't exactly prove that he wasn't just a brain soaking in a mad scientist's vat.
 
Eighteenth-century philosopher George Berkeley stirred up the debate by proposing his theory of idealism, which says that the real world exists only by virtue of our apprehension of it—that material objects exist only as conceptions, or ideas in our minds.

The world, we strongly suspect, is real, and not an illusion. But there is no getting around the fact that many of our perceptions are internally constructed. It's like a movie constantly being filmed, edited, and sometimes censored by an idiosyncratic director running around in our skulls. And there are plenty of special effects.
 
—Joel Achenbach
Washington Post staff writer

Did You Know?
Phantom Input

Awareness of body parts can remain in effect even after an arm or leg is gone. Amputees often experience sensations, sometimes painful, of phantom limbs, even to the point of believing they are moving them—clenching a fist, grasping a pencil, waving goodbye, shaking a leg. The sensations are not caused by activity in remaining muscles. Some scientists suspect that without tactile inputs to stimulate sensations, the brain establishes new neural connections that project the signals it no longer receives.

—Heidi Schultz


Web Links

More Articles by Joel Achenbach
www.washingtonpost.com/wp-dyn/style/columns/achenbach
Read some of writer Joel Achenbach's columns for the Washington Post.

Helping Phantom Limb Pain
hubel.sfasu.edu/courseinfo/SL98/phantom4.html
Learn more about phantom limb pain and phantom limb studies on this website.
 
The Brain and Pain
www.wellcome.ac.uk/en/pain/microsite/medicine2.html
This site offers information about the brain's role in pain.
 
Mayo Clinic
www.mayoclinic.com/invoke.cfm?id=DS00444
The Mayo Clinic offers more in-depth insight into phantom limb pain.


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Bibliography

Botvinick, Matthew. "Probing the Neural Basis of Body Ownership." Science (August 6, 2004), 782-3.
 
Botvinick, Matthew, and Jonathan Cohen. "Rubber Hands 'Feel' Touch That Eyes See." Nature (February 19, 1998), 756.
 
Ehrsson, H. Henrik, and others. "That's My Hand! Activity in Premotor Cortex Reflects Feeling of Ownership of a Limb." Science (August 6, 2004), 875-7.
 
Franz, Elizabeth, and V. S. Ramachandran. "Bimanual Coupling in Amputees with Phantom Limbs." Nature Neuroscience (October 1998), 443-4.
 
Naito, Eiichi, and others. "I Feel My Hand Moving: A New Role of the Primary Motor Cortex in Somatic Perception of Limb Movement." Neuron (December 5, 2002), 979-88.
 
Ramachandran, V. S., and D. Rogers-Ramachandran. "Synthaesthesia in Phantom Limbs Induced with Mirrors." Proceedings of the Royal Society of London (1996), 377-86.
 
Ramachandran, V.S., and others. "Touching the Phantom Limb." Nature (October 12, 1995), 489-90.
 
Shreeve, James. "Touching the Phantom—The 'Phantom Limb' Phenomenon." Discover (June 1993).




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