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A True Bionic Limb Remains Far Out of Reach

Even so, the system is brittle, easily flummoxed by anything outside the tai-chi startup sequence. The muscles and nerves in Lehman’s stump overlap, making the signal noisy, ambiguous. “If you raise your arm, you can get interference from other muscles,” says Todd Kuiken, who invented the reinnervation procedure at the Rehabilitation Institute’s Center for Bionic Medicine. “Your deltoid starts to fire, and the weight of the arm is enough to change the firing pattern. That could put it outside the envelope of what works.”

And if Lehman tells the arm to do something it doesn’t understand? To make a fluid move instead of one built out of its vocabulary of bends, swivels, and pinches? “The arm will do something,” says Levi Hargrove, one of its developers, “but it won’t do that. It’ll do its best guess.” This problem bedevils all of the current research in prosthetics. Today’s computer-controlled prosthetic arms can carry out only a few commands. “Sometimes patients come in and they’re expecting a super-arm that works as good as their old one,” Kuiken says. “And you have to say no, I’m sorry, they aren’t that good.”

They were supposed to be better. A decade ago, researchers seemed on the cusp of creating a working interface between body and machine. Even back then, arms controlled by myoelectric signals were old news; more-advanced limbs would read commands directly from the brain. In 2003 scientists at Duke announced that monkeys could control a robotic arm via electrodes implanted in their brains. A year later, a similar device allowed a quadriplegic human patient at Brown University to play Pong with his thoughts. In 2008 researchers at the University of Pittsburgh showed off a monkey that could use a neurally controlled robotic arm to eat marshmallows. Surely if a monkey could use a robot arm to feed itself, it wouldn’t be long before amputees used them to tie their shoes and pilots flew jets with their minds.

Such advances are needed more than ever. There are approximately 185,000 limb amputations in the US every year—the causes range from diabetes to workplace injuries to battlefield trauma. In 2005, the most recent year for which data is available, 1.6 million Americans were living without a limb. As of January 2012 there were 1,421 amputees from the Afghanistan war and the second Iraq war. Of those, 254 lost upper limbs, like Lehman, and 420 lost more than one limb. Many of the rest, presumably, lost single legs or feet, but the statistics don’t break out those details. In fact, the numbers seem low—but they’re the best ones the military will share.

What the military will share, though, is money to study limb replacements. In 2006 the Defense Advanced Research Projects Agency began a program to build, in four years, an arm “directly controlled by neural signals” that would have abilities “almost identical to a natural limb in terms of motor control and dexterity, sensory feedback (including proprioception), weight, and environmental resilience.” With its deadline now three years past, those ambitions now look wildly unrealistic. After $153 million in funding and years of engineering, the best any amputee can get is a heavy, clunky arm that moves slowly, can’t feel anything, and often misreads its user’s intentions.

“The human arm is amazing,” says Rahul Sarpeshkar, a bioengineer at MIT who pioneered the design of ultralow-power circuitry for bionic interfaces. “It does a lot of very intelligent local computation that the brain doesn’t even do. We don’t understand the coding schemes that biology employs. We don’t understand how its feedback loops work together.” In other words, the science hasn’t yet caught up with the fiction. A true bionic limb—one that responded to mental commands with precision and fluidity, one that transmitted sensory information, one that its user could feel as it moved through space—would require a depth of understanding and technological complexity that is simply beyond today’s prosthetic experts. “It’s not that we’re not going to be able to do it,” Sarpeshkar says. “But it’s higher-hanging fruit than people think.” In other words, this is more than just an engineering problem. It’s a problem of basic science.

Across the street from the Rehabilitation Institute of Chicago, a rhesus monkey named Thor is strapped into a chair. His tiny hand clutches a metal handle on the end of a makeshift arm mounted below a computer monitor. The arm is hinged; Thor can’t raise or lower it, but he can move it side to side and forward and back, like a salt shaker on a tabletop. And when Thor moves the handle, a yellow dot on the screen moves in the corresponding direction. The monkey gets a sip of juice when he steers the dot into a box on the screen. What Thor doesn’t know is that the handle isn’t wired to anything. It’s just a mechanical trick to make his brain issue the right commands. A tiny square array of 100 electrodes, wired to a computer, is resting on the part of his brain that controls his arm. The lab around Thor, run by a neuroscientist named Lee Miller, looks well used—scuffed floor tiles, jerry-rigged and discarded machine parts on shelves. A cat’s brain floats in a jar, unregarded, while a stack of computers and other electronic equipment whirs in the corner.

The researchers have spent months analyzing what Thor’s brain does when his hand moves. They start by reading his motor output—letting Thor work the handle when it’s wired to the cursor and trying to correlate his movements to patterns of neural activity. Then they set up the computer to do the reverse: to infer his arm motion by watching neural output.

While Thor works, dozens of pink, yellow, blue, and green lines writhe on a monitor, each symbolizing the intensity and frequency of individual neural firings. When a neuron fires, the computer emits a click. If you listen, you can tell how busy Thor’s brain is under the electrode array. It sounds like popcorn popping. But then, suddenly, every line on the monitor turns gray and freezes. The clicks stop. A grad student checks the cables leading to the titanium plug in Thor’s skull and finds that one has come loose. He pushes it back into its socket, and a moment later the monkey is back in business. The clicking resumes.

It’s a different approach from the one Kuiken has taken with Lehman. Intuitively, going straight to the brain seems smarter. The problem is, no one knows how the brain does what it does. Neuroscientists know how neurons work, sending waves of electrical charge along their lengths and then squirting out chemicals—neurotransmitters—to signal one another. But how an intention, a thought, a mind, arises from that network of electrochemistry-in-aspic is still largely a mystery. The brain changes from instant to instant. The same task might be handled by different neurons at different times. Moreover, any given set of neurons could be sending commands to his arm, processing sensory data, or responding to reflexive movements.