Phantom Limb

Even when the exhibition hall was almost empty on Tuesday night, the Revolutionizing Prosthetics booth was surrounded three layers deep by admirers. DARPA's "bionic" arm was demonstrated by Jesse Sullivan, an electrician whose arms had to be amputated at the shoulder after he accidentally touched an active cable. The prosthetic arm is the result of a $50 million effort led by Johns Hopkins' Applied Physics Laboratory and the Rehabilitation Institute of Chicago. Though it is in the second phase of a program that is due for completion in 2009, the Proto-1* demonstration already looks like a finished product. Jesse is recognizably wearing a prosthesis, but after about five minutes talking to him, I couldnâ''t shake the conviction that he was wearing a robot-colored glove over a fully functional human armâ''like a kid on Halloween night in a skeleton suit.

I grew up watching The Six Million Dollar Man and The Bionic Woman, so I always thought the key to a convincing bionic arm is having some kind of convincing coverâ''skin that looks real enough to fool the eye. Now I think you could forego any â''naturalâ'' cover. Itâ''s not necessary. If the movement is as smooth and easy and intuitive-looking as Jesseâ''s arm, your brain fills in the gaps and convinces you that thereâ''s a real arm lurking under a robot suit.

Jesse is able to control the arm with residual nerves surgically connected to his pectoral muscles, which link to a processor in the arm. Not only does he control the arm, but it provides sensory feedback as well: Jesse said that he could feel pressure when I squeezed his index finger. Which is to say, the â''index fingerâ'' of the prosthesis. The Proto-1 arm has 22 degrees of freedom and 80 sensors. Johns Hopkins' Stuart Harshbarger says it took Jesse about an hour to start to use the limb intuitively. I can't imagine what it will be like when the team starts working with models that work via electrodes surgically implanted into the cortex.

The revolution here might be less about the person fitted with the new limb, and more about the people around him or her. In the 1946 movie The Best Years of Our Lives, Harold Russellâ''s character isolates himself from his family and from greater society because the people around him canâ''t face what has happened to him and donâ''t want to be confronted with the reality and profoundness of his injury. Most of the wounded soldierâ''s trauma comes from the pain of watching peopleâ''s faces change when they perceive the massive injury he has sustained.

A prosthesis that removes the separation of mechanical tool versus organic outgrowth of the human body functions as a psychological rather than a mechanical tool. The prosthesisâ'' four fingers donâ''t even move independently but I still intuitively went for Jesseâ''s hand as I left, to shake hands goodbye. The persistence of the illusion was incredibly stubborn. And is that really a misapprehension? Jesse says that sometimes when he moves the arm, it feels like heâ''s moving his phantom limb and the prosthesis is just along for the ride.

* The original blog post misidentified the arm as Proto-2. The arm demonstrated at DARPATech was the Proto-1.

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