How Monkeys Provide Insights into the Development of Prosthetic Limbs

Imagine a gorgeous day with the sun beaming down on your face. It’s a pleasant afternoon for a saunter, passing through crosswalks and roads that span the city. Unfortunately, your tranquility is suddenly shattered. A distracted driver runs a red light and slams into you, causing several injuries. But worst of all, one of your arms is damaged beyond repair. Doctors have no choice but to amputate.

For decades, the best option for amputees was a prosthetic limb, but thanks to new technology, we will likely have new and better choices.

One way to approach this problem is to conduct research on the motor cortex, a region of the brain responsible for movement control. It is easiest to think about the body as an extremely efficient company. The motor cortex is the muscle department manager. It relays signals to the spinal cord, which serves as the second-in-command. The spinal cord interprets instructions from the motor cortex and sends modified orders to the body’s muscles. The motor cortex has the ability to make an extraordinary number of computations within fractions of a second in order to perform simple tasks, such as grasping or reaching. If we decipher how the motor cortex sends and receives messages, we could possibly create prosthetic limbs that can be controlled using the same set of instructions. We could address the physical problems that most, if not all, amputees experience.

To conduct these types of studies, researchers have created models from monkey brains to understand human neurological and physical problems. Monkeys are studied because their neural organization and function are incredibly similar to human brains.

“Because of these similarities, [monkeys] make a great candidate for testing our ideas about how the brain controls movement and how we can tap into those brain signals to control prosthetic devices,” said Raeed Chowdhury, a postdoctoral scholar at the University of Pittsburgh.

“As we start to understand more about how the brain works, we can start developing more sophisticated controllers for these prosthetic devices.”

One set of devices under development fall into a category called brain-machine interfaces (BMI). These systems can translate neuronal information into robotic movements.

In 2017, amputee monkeys were used in a study to examine the use of BMI. The three monkeys involved in the research were divided into groups based on the locations where electrodes were placed in their brains. Two of the monkeys had electrodes implanted on the opposite side of the brain as their amputated limbs. The third monkey had electrodes placed on the same side as the amputated limb. Researchers then recorded the clusters of neurons that control reaching and grasping movements and measured the length of time it took for the monkeys to learn these simple actions with a robotic arm.

As BMI exposure grew, the time to complete each task decreased significantly for all of the monkeys in the study. Even though these animals underwent amputations several years prior, the study showed both sides of the brain can create new neural connections to master the use of BMI. This work has been confirmed and expanded upon in additional studies.

Although researchers have taken impressive steps forward thanks to studies in our simian cousins, we are still years, perhaps decades away from being able to create fully-functional robotic limbs. However, with this progress, losing a limb may not be such a life-changing experience. In the future, the combination of neuroscience and robotics will likely lead to arms and legs for amputees that rival the functionality of ordinary human limbs.