YOU FEEL THAT?


It’s not so had 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. (Better read that again- - You must get the mental image first off.)


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. AGAIN – 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



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