Do you know what proprioception before? It is the sense of the position, speed, and torque of one’s body parts. In other words, it is what allows us to know where our body parts are as they move, without looking at them. It is fundamentally distinct from the sense of touch, although equally important. And why is proprioception important? Because this sense allows humans to precisely control their body movements. It is what allows us to reach for a coffee cup without knocking it over, or catch ourselves when we trip unexpectedly. Without proprioception, it is impossible to precisely control the movement of a limb.

However, despite significant improvements to prosthetic devices in recent years, researchers have been unable to provide this essential sensation to people with artificial limbs, limiting their ability to accurately control their movements.

Now, researchers at the Center for Extreme Bionics at the MIT Media Lab have invented a new neural interface and communication system that is able to send movement commands from the central nervous system to a robotic prosthesis. In turn, the limb relays proprioceptive feedback describing movement of the joint back to the central nervous system.

The agonist-antagonist myoneural interface (AMI), as the system is called, involves a novel surgical approach to limb amputation in which dynamic muscle relationships are preserved within the amputated limb. The AMI was validated in extensive pre-clinical experimentation at MIT, prior to its surgical implementation in a human patient at Boston’s Brigham and Women’s Faulkner Hospital, and bionic implementation at MIT.

Two agonist-antagonist myoneural interface devices (AMIs) were surgically created in the patient’s residual limb: One was electrically linked to the robotic ankle joint, and the other to the robotic subtalar joint.
(c) MIT Media Lab/Biomechatronics group. Original artwork by Stephanie Ku.

Two opposing muscle-tendons

Surgeons inserted two agonist-antagonist myoneural interface devices in the patient’s residual limb, linking one to the robotic ankle joint and the other robotic subtalar joint. The coupled movement transmits the muscle length, speed and force information as natural joint sensation – just how it works naturally in human joints, Herr said. Connecting the AMI with electrodes, the researchers can detect electrical pulses from the muscle or apply electricity to the muscle for it to contract.

“Because the muscles have a natural nerve supply, when this agonist-antagonist muscle movement occurs information is sent through the nerve to the brain, enabling the person to feel those muscles moving, both their position, speed and load,” said senior author and project director Hugh Herr, a professor of media arts and sciences at the MIT Media Lab.

“When a person is thinking about moving their phantom ankle, the AMI that maps to that bionic ankle is moving back and forth, sending signals through the nerves to the brain, enabling the person with an amputation to actually feel their bionic ankle moving throughout the whole angular range,” he said. Herr said it is the first time information on joint position, speed and torque has been fed from a prosthetic limb into the nervous system to a human patient at Brigham and Women’s Faulkner Hospital. “Our goal is to close the loop between the peripheral nervous system’s muscles and nerves, and the bionic appendage,” Herr said.

“This is groundbreaking,” said Daniel Ferris, a professor of Engineering Innovation at the University of Florida, who was not involved in the research, in a press release. “The increased sense of embodiment by the amputee subject is a powerful result of having better control of and feedback from the bionic limb.” Ferris said he expects amputees to seek out this new method. “It could provide a much greater quality of life for amputees.”

Source: MIT Media Lab