Although prosthetic hands give amputees a way to grasp objects, they do not offer a sense of touch. That means the person has to watch his or her robotic hand as it reaches to push or pick up an item.
Now researchers at the University of Chicago might have found a way to add touch to prosthetic limbs. The research, which appears in the Proceedings of the National Academy of Sciences, is funded in part by the Defense Advanced Research Projects Agency, and it's not hard to see why the military would be interested. Beyond dreams of cyborg warriors, there's the more prosaic matter of helping injured veterans.
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The study, led by Sliman Bensmaia, assistant professor in biology and anatomy, identified patterns of neural activity that occur when monkeys manipulate objects and then induced these patterns artificially.
First he and his team connected electrodes to areas of a monkey's brain that corresponded to each of its finger. The idea was to find out what kind of brain activity occurred when monkeys pick up or touch something.
Next, the researchers touched the animals' fingers, using a device that applied a specific amount of pressure. The monkeys were rewarded if they correctly identified which finger was touched - the monkey just had to look in the right direction. Next, the researchers repeated the same action, but in reverse, sending an artificial signal through the electrode to the monkey's brain, which caused the monkey to act the same way it would had it's fingers been touched by the device - identifying fingers as touched even when they weren't.
The next step was the sense of pressure. They trained the monkeys to identify whether the pressure on their fingers was smaller or larger. In this case, Bensmaia's group wrote a computer program to generate the same kind of electrical current that gave rise to pressure sensations. Once again, the animals reacted the same way as if they had actually touched something.
Finally, the scientists examined the brain signals that occurred when there was a "contact event." When the monkey's hand was initially touched or pressure released, their brains showed a spike in activity. This spike is in addition to the signals for pressure and the individual fingers - it's what tells the brain that there's something in the hand to begin with before the signal settles down. The scientists duplicated that brain activity spike with artificial signals as well.
That implies that by programming those signals into an artificial limb, it's possible to duplicate the sensation of touch. Just as natural limbs send signals to the brain, the artificial one would do so, too, except it wold be through electrodes linked to the relevant parts of the brain rather than nerve cells. An amputee would actually feel the object they are touching with such a prosthetic.
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The setup hasn't been tested in humans yet. But the monkey results are promising, and it could solve not only the problem of touch sense, but that of sensing limb position and possibly even help a person balance on his or her artificial legs.