Being a double-amputee himself, MIT’s High Herr knows a thing or two about the realities of replacement limbs, and as advanced as they’ve become in recent years, they are also still incredibly primitive in certain ways. Now, a new technique pioneered by Herr and a team of researchers could not only let amputees control a mechanical limb with ease and comfort, but let them keep a vestige of natural body awareness, as well. It’s an important step to letting the recipients of bionic limbs feel as though they have a new arm or leg, as opposed to an awkward arm- or leg-shaped tool.
“Using this framework, the patient will not have to think about how to control their artificial limb,” said Herr, one of the study’s authors and an MIT media arts and sciences professor. The goal is to provide amputees with an accurate feeling of where their limb is without having to look at it, a level of unconscious comfort that could do wonders for people struggling with a handicap.
The big problem with using thoughts to control a mechanical limb is that thoughts are hard to read, and several years ago researchers had the idea to use the brain’s own thought-filtering hardware to help them deal with this fact. Rather than reading motor signals from the brain, right next to every other thought that has no relation to limb movement, they decided to look at motor control further down the line, toward the muscle itself. By looking only at the nerve bound for the lost limb, they could easily see only the neural activity that actually mattered to limb movement.
The problem with this simple idea is that nerves die when they get severed, and so doctors came up with the idea of keeping them alive after amputation. When they sever a limb, the doctors preserve a portion of muscle they would otherwise have removed — it doesn’t nothing for movement, but it provides a scaffold for the severed nerves to fuse to, allowing them to continue to live and transmit signals. It’s these signals, bound for a limb that no longer exist, that can be read and interpreted to move the limb.
This means that patients can try to move their old arm, and watch the new one move in response. It’s worlds better than having patients re-learn how to move a whole new limb that just-so-happens to be where the old one was, but this MIT study takes the concept a major level further, by preserving two bits of muscle in direct opposition to one another. They conducted their experiment in rats, but having seen the technique successfully report sensory information to the rat brain, they hope to expand to human testing soon.
In both rats and people, muscles are very often found in pairs that directly offset one another’s motion. If a nerve-fused bit of muscle receives an order from the brain to contract, it does so — but that doesn’t matter because it’s just a little bit of vestigial muscle that doesn’t do anything. What’s important is that as the bionic system is translating these same motor control signals into the much more useful movement of the mechanical replacement limb, the second little useless bit of muscle the doctors have preserved is reacting in opposition to the first, which received the motor signals. Just like a tricep will relax as a bicep flexes, and vice versa, this second bit of preserved muscle will undergo an inverse version of whatever action the first one takes.
What this provides is a source of real, and thus natural-feeling, sensory information. It’s great to build a hand with texture-sensors in the fingertips, and but anything these purely technological solutions do pick up will ultimately be robot sensation; it will take a long time for science to make that sort of thing feel right to people. The sensations coming from bits of preserved muscle, on the other hand, are normal feelings reported in the normal way. The brain will still have to learn to interpret the sensations of this small muscle as the sensations of moving the whole arm, but that’s the sort of generalization that the brain is very good at.
As stated, the nerves and muscles can only be preserved like this by a surgeon — not by a car crash or an explosion in war. Still, the options for planned amputations are increasing quickly, and researchers like Herr are hard at work finding solutions for the rest.
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