Jun 01 2018

Another Advancement in Prosthetic Limbs

I have been following the development of brain-machine interfaces and their application to artificial (prosthetic) limbs. A recent paper documents another incremental but significant advance – combining a new surgical technique for amputation with a prosthetic designed to take advantage of it.

The goal is to close the loop between voluntary muscle control and sensory feedback, which is critical to that control. The way our brains normally work is to constantly monitor various sensory streams in order to create the subconscious and conscious sensation that we occupy, own, and control the various parts of our body.

We may take these phenomena for granted, but they are an active construction of the brain that are critical to proper function. Without the sensation that we occupy our body, we would have “out-of-body” sensations, like we were floating in space, and this would make it difficult to interface with the physical world.

Without the sense of ownership, we don’t feel like a limb is part of us, and it is therefore not incorporated into our internal model of our own body. This is the current state-of-the art for normal prosthetics – they feel like they are attached to the user, but not part of them. It therefore takes more conscious effort to use them, and control is not as good as a normal limb.

We also need sensory feedback confirming that our planned movements were actually executed, which results in the sensation that we control our body. Without this feedback loop, the result is known as “alien hand syndrome.” People feel as if their arm, for example, will suddenly start acting on its own accord, doing weird things like emptying their pockets or purse onto the floor. It seems that the sensation of control is necessary for actual conscious control, otherwise the alien limb might start acting on subconscious noise.

Various researchers are therefore working on making artificial limbs that have sensation. I wrote just two months ago about one such effort, which uses vibrations to signal limb movement, resulting in some sense of ownership and control.

The new approach may be even better, or may ultimately be combined with the vibration approach. The subject had a badly injured and painful foot that needed to be amputated. Instead of a standard amputation, they connected the tendons of leg muscles that would normally move the foot in opposite directions, like lifting the foot up (called dorsiflexion) and pushing it down (plantarflexion). Therefore, the tendons would pull on each other, creating sensory feedback tied to the desired movement.

The robotic foot then used these movements to control movement of the foot. Therefore, the resulting movement was associated with a specific sensory feedback that was similar to normal sensory feedback of foot movements. The robotic limb also has sensors on it that will sense force and provide feedback to electrodes on the the skin of the real leg.

So how did this all work? The patient, Jim Ewing, reports:

Ewing could feel the sensation as soon as the robot foot was attached and tuned up. “I right away started using it as if it were my own foot,” he told STAT, without having to retrain his brain how to get the desired movements.

“Oh, wow, there’s something there,” he recalled thinking. “It’s responding. It felt kind of like my foot had returned.”

“I’m not somebody who’s really prone to dramatic emotions,” he added, “but later on when I was driving home, I really felt a strong urge to be connected to it again. Like, ‘I want to have that thing on again and feel like I have my foot back.’”

That sounds pretty successful. This fits with previous research which shows that it is fairly simple to reproduce the phenomena of ownership and control. All that is required is sensory feedback that matches other sensory feedback and internal planned movement. So, if you see and feel your arm being touched, that creates the sensation that the arm is part of you, that it is you. If you try to raise your arm and you feel it raise, then you have the sense of control.

The surgeons in this case also observed (and you can see the video at the linked page) that Ewing would fidget with his artificial foot while not paying attention. They report this is an entirely new phenomenon, not seen previously with prosthetic limbs. It shows that the subconscious loop has been closed, and Ewing’s brain is treating the artificial foot as if it were part of him.

Obviously this is a single case report. A larger study with multiple individuals needs to be complete to see what the success rate of this procedure is. I am also curious to see how applicable it will be, meaning how many patients requiring amputation will have anatomy suitable to this procedure (depending mostly on the level of the amputation). Also, can this procedure be retrofitted to patients who have already had their amputation?

The good news is that this is yet more evidence that brain-machine-brain interfaces are plausible and can be achieved with current technology. We can make robotic limbs that will feel entirely natural. Hacking the sensory feedback circuits of the brain turned out to be easier than originally suspected, and brain plasticity can adapt to new interfaces.

This puts us on a trajectory that mostly involves just incrementally advancing the technology. We are not quite at the “bionic man” level, but we can see a path to that eventuality.

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