Mar 15 2018

Prosthetic Sensory Feedback

We continue to make incremental improvements in robotic artificial limbs, and now researchers report one more – kinesthetic feedback. This is a sense that your limb is moving under your control.

What the engineers did is include a mechanism to make the robotic arm vibrate when it moves. The vibration gives the user sensory feedback that mimics movement. This is similar to the haptic feedback that some video game controllers give, to enhance the illusion that you are wielding an actual item rather than just a plastic controller.

In the study this vibrational feedback was successful in creating the illusion of control, meaning that the users felt as if they controlled their robotic limb. However, they still did not feel as if they owned their robotic limb, like it was part of them, which is another hurdle engineers will have to solve.

I found it interesting that they referred to this effect as an illusion, which I think is accurate, but I want to emphasize that our normal sense of agency and ownership is an identical illusion. The same circuits in the brain that give you the subjective sense that you occupy, own, and control your various body parts are the same ones that would potentially create this sensation in a robotic limb.

This is precisely why a robotic limb can work and ultimately feel natural. All we need to do is close the loop on these brain circuits.

What those circuits are doing is comparing various sensory and volitional streams. For example, when we have the intention to raise our arm, and then we see and feel the arm raise at the same time, the synchronization of those circuits creates the illusion (the subjective sense) that we control our arm. If we see our arm being touched, and we feel the touch, that is often all it takes to create the sense of ownership – that the arm is ours, is part of us.

There is probably also a threshold effect from all of this – an amount of synchronization between various sensory streams and our movements that creates the illusion of ownership and control. This is why incremental improvements in robotic prosthetics are important – they inch us closer to that threshold.

The current advance was quite clever, using vibration to essentially mimic the effects of proprioception and kinetic feedback, the sensation of where our limbs are in three dimensional space, and the sensation of moving our muscles respectively. This crossed the threshold for feeling control, but not for feeling ownership. That would probably require more real time sensation.

A different team reported in 2016 that they developed a robotic limb with a sense of touch, specifically to create the sense of embodiment. Now they need to put it all together – make a limb with motor control, touch feedback, and movement feedback. That should create the full sensation of ownership and control.

The advantage of these features are partly obvious – they make the robotic limb seem more real and natural. Some amputees (about 10%) reject their robotic prosthesis because it does not feel natural, and the hope is these technologies will reduce that number.

But further, these types of sensory feedbacks improve motor control of the robotic limb. Without them, the user has to rely entirely on vision for their feedback – they need to look at the limb as it moves in order to control it. With sensory feedback, their control is improved and they don’t need to be staring at the limb all the time.

Neurologically speaking, there is no theoretical limitation here. The circuits in the brain that create the sense of control and ownership work just fine with artificial sensory feedback. Plasticity also allows the brain to adapt to new wiring, so subjects can learn to control their new limb even with repurposed nerves.

All the pieces are in place. We are now on a course of incremental technological improvement. Robotic limbs will just get better and better.

One technological limitation is power. Batteries are still a limitation. So prosthetic technology will benefit from improvements in battery or power cell technology. There are even engineers working on ways to harvest energy from biological systems to power implants and prosthetics, but these are a long way off probably for something as power hungry as a robotic limb. Pacemakers and other low energy devices are what they will be used for.

It is funny to think about the 1970s tv show, the Six Million Dollar Man. The “bionic” limbs envisioned in that series were literally 100 years ahead of their time. But we will eventually get there.

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