The Dawn of Mind-Controlled Prosthetic Bodies

Many years ago, I asked a psychologist friend what she considered most miraculous about the mind. She smiled, closed her hand and lifted her arm, opening the fingers of her clenched fist. She said, “That I can do this with a thought is truly the most amazing thing I know.” I opened and closed my hand with a new awareness of how incredible it was that a simple thought had made this happen.

In the 30 years since that conversation, the field of mind-machine interface has burgeoned, with the terms “mind reading technology” and “brain-computer interface” quickly entering our vocabulary.

Amputees can today wear a headset called a “wearable brainwave visualizer” that detects activity in a person’s cerebral cortex enabling them to control the movement of their prosthetics merely by thinking. The list of applications is endless.

How the Brain Causes Muscles to Move

In an article in Nature Communications, a group of researchers reveal the complexity of the human hand:

From picking up a coffee cup to playing the piano, humans can engage in a wide range of manual behaviors, highlighting the staggering flexibility of the hand, whose 27 bones and 39 muscles give rise to 20 biomechanical degrees of freedom for volitional movement. This versatility is also supported by a sophisticated neural system. But how exactly does the brain control something as complex and versatile as the hand?

Our brains use motor neurons — single nerve cells in the spinal cord — to connect with and command our muscles. A thought in the brain causes these neurons to fire. When a neuron fires, an electrical impulse goes out to the particular muscle you are interested in moving.

The electrical impulse from the motor neuron travels on a long thin extension called an axon. When the electrical impulse arrives at the end of the axon, it releases a chemical that causes the muscle fibers to move. When the impulses from the motor neurons stop, the fibers return to their original positions and the muscle relaxes.

Fortunately for us, when we want to drink coffee from a cup, we don’t have to think about the specific muscle groups we want to activate or know precisely how they must move. That’s the job of the cerebral cortex which is connected to a control center housed in the spinal cord.

Think of the cerebral cortex as a project manager whose job it is to organize and execute the firing of the motor neurons to accomplish a specific set of motions in one or more muscle groups. The cortex plans the positions each muscle will assume in space. Each desired arm movement, for example, is a collection of specific commands to each motor neuron and muscle.

The amazing thing about movement is that the conscious mind has very little to do with what’s happening. Instead, the unconscious mind initiates a command, thought or desire in the cerebral cortex. And because we don’t experience a time lag from our thought to the movement, the action seems to happen instantaneously.

Imagine how impossible life would be if we had to direct each step in this process instead of having the cerebral cortex to handle it without our conscious volition. We’d never move at all. Most people don’t realize it, but muscle movements are also integral to our internal organs. If we had to consciously control the muscles of our hearts, it wouldn't be long before we died.

The unexpected complexity of everyday motions

Think about how difficult it must have been for scientists to map out which neurons control specific muscles in a prosthetic limb made of plastic and metal, and then to connect the motor neurons to cause the desired motions.

Yet they have achieved this goal. As a result, amputees and those with nerve damage have been given new hope that they can regain normal function with prosthetic limbs. A seamless connection between a robotic limb and the brain could signal the end of disability.

In 2016, a group of scientists at the Applied Physics Laboratory at Johns Hopkins University built a bionic arm and an attached hand that an amputee can learn to control with his/her thoughts. A researcher at the Media Lab at MIT whose leg had been amputated below the knee built a bionic replacement limb that he could operate with his thoughts.

Amputees generally retain the neural pathways that previously controlled movement. Scientists can now connect these pathways to a class of robotic limbs that attach to the body and are controlled by thinking. People whose legs were amputated now can regain the ability to walk on bionic limbs that exactly replicate the act of walking. The prosthetic limbs move as though they were made of human muscle and bone. Some users have even reported the appearance of sensation in their prosthetic limbs.

A Fully Functioning Prosthetic Arm

Dr. Albert Chi, a researcher at Walter Reed Hospital, gave a TED talk in 2018 entitled “The Most Advanced Prosthetic in the World.” In this talk, he described how his team’s creation of the Modular Prosthetic Limb changed the field of prosthetics.

In 1858 the first lifelike prosthetics were invented. Over 80% of veterans who survived the Civil War had lost a limb, spurring the government to invest in the further development of prosthetics.

A century and a half later, Dr. Chi worked with amputee soldiers in Iraq and Afghanistan. At that time, he noted that those who had lost a leg were much more likely to return to active duty with a prosthesis than soldiers who had lost an arm.

After further research, he and his team created the Modular Prosthetic Limb or MPL which has 26 individually articulated joints and more than 200 sensors.

Dr. Chi asserts that anything a human arm can do can be replicated with the MPL. It enables the user to feel temperature, position, velocity and acceleration. The wearer can also distinguish the textures and hardness of surfaces. Intuitive movements can be programmed through a cell phone to make the wearer’s experience feel totally organic. One of Dr. Chi’s early patients reported being able to perform normal household tasks, including trimming hedges, cooking and learning to play the piano.

At present, this technology is very expensive. But as the market for advanced prosthetics continues to grow, the MPL and its successors will become more economical and increasingly available.