Contemporary Cyborgs: How Neuroprosthetics are Changing Lives

Gage Haden

Illustrations by Ayane Garrison

When most people hear the word cyborg, the image that jumps to mind is that of a crime-fighting half-man, half-robot. The exciting depictions of cyborgs in sci-fi movies and TV shows — such as Robocop or Inspector Gadget — leave us with fantastical assumptions surrounding science fiction and futuristic machinery. However, cyborgs are not far from existence in our current reality. While fabricated super strength or speed might still be a fantasy, mechanical assistance of physiological processes is already possible with today’s technologies. There are people walking around right now with machinery inside them, ranging from pacemakers and mechanical heart valves to cochlear implants and insulin pumps. These mechanical prosthetics can vastly improve a person’s quality of life. Now, neuroscience and biomedical engineering are collaborating to take these a step further: cybernetic prosthetics controlled by the brain.

Sending Mixed Signals: Building a Brain-Computer Connection 

When you open your laptop to write an essay, surf the web, or send an email, a flurry of largely invisible processes occur within your computer’s hardware. In many ways, the human brain functions similar to an organic computer. At the most basic level, your brain — like your laptop — is a complex network capable of sending, encoding, receiving, and interpreting a multitude of electrical signals called impulses or action potentials. However, though brains and computers work similarly, they do not speak the same language; this is where the Brain-Computer Interface (BCI) steps in. The BCI translates our brain’s electrical signals into technical coding [1, 2]. Essentially, it works as a link between the brain and a computer, interpreting electrical signals from the brain and communicating them to a computer in the same “language.” Since its first clinical application as a spelling device for paralyzed patients, the BCI has become varied in form and even more versatile in function [3]. 

The BCI is a two-way street, meaning it must be able to both send and receive signals. These neural signals are classified as afferent or efferent based on their directionality. Afferent signals originate in the body and travel to the brain, while efferent signals move from the brain, through the spinal cord, and out to the body. The dilemma when working with afferent signals is how information should be encoded [4]. For instance, imagine that you are writing a letter to someone in another country. If you know that the recipient doesn't speak English, but understands Spanish, the only logical thing to do is write the letter in Spanish. Similarly, when encoding afferent signals, we must send them in a way that the brain can understand. If we are able to encode signals like this, they can be used to mechanically restore or augment sensory systems. Efferent signals, on the other hand, pose the opposite issue: decoding. Using our earlier example, this would be like translating the response to our letter from Spanish back to English so that we can understand it. In the case of the BCI, the efferent neural signals must be translated for the computer. This process of decoding facilitates the flashier side of neuroprosthetics: translating neural signals into complex movement such as manipulating a prosthetic hand [5]. Whereas the afferent functionality of the BCI offers solutions for disrupted sensory systems, the efferent pathways can help patients recover lost mobility.

Neuroprosthetics in Action: The Technology

When something important in your home breaks, there are two options for what to do: fix it or replace it. Similarly, neuroprosthetics are intended to fix or replace disrupted physiological functions, such as the ability to hear or walk, and are widely varied in form and use. Manufactured sensory organs or enhancements, such as artificial retinas and cochlear implants, detect afferent signals to restore use of the senses [6, 7]. The first cochlear implants were designed in 1959 and have since become one of the most prevalent and accessible forms of neuroprosthetics [7]. Cochlear implants use an external microphone and speech processor to convert environmental sound into electrical impulses. These impulses then directly stimulate the auditory nerve and engage afferent signaling, which restores deaf patients’ hearing [8]. In 2014, cochlear implants had been fitted for 300,000 people worldwide, and this continues to be a beneficial elective procedure [7]. Research on sight restoration with technology such as artificial retinas is slightly behind that of auditory aids, but is nonetheless hot on its heels [6]. However, these technologies still represent only one sort of neuroprosthetic technology. 

The other function of neuroprosthetics — decoding efferent neural signals — is where technological advancements begin to sound even more like science fiction. Neuroprosthetics that utilize efferent signals enable the brain to directly control mechanical limbs [9]. Such complex engineering is only possible because of the brain’s intrinsic ability to learn and rewire its own neural pathways, a phenomenon known as neuroplasticity [10]. Neuroplasticity allows our brain to learn how to interact with a machine and signal prosthetics in a way that can be understood by the apparatus. In order to consider the complexity of this brain-computer interaction, take a moment to turn over your hand, and wiggle your fingers. Now consider all of the muscles and tendons you feel flexing in your hand and arm as you make these movements. Imagine if each of those muscles and their basic movements were controlled by a separate switch or knob. Already we have an image of an immense switchboard jam-packed with dozens of similar controls. It’s easy to imagine that the motions resulting from manual control of this board would be uncoordinated and jerky, similar to the classic 60s dance move “the Robot.” Now we can see just how broad and complex the information processing required for fine motor control of an arm must be. For years, decoding these signals seemed impossible — that is, until someone with an idea, a plan, and a good chunk of funding managed to succeed.

This unprecedented advancement in the field of neuroprosthetics offers incredibly promising opportunities for tetraplegics and amputees in particular. When equipped with sufficient BCI training, a pair of implanted electrodes, and the brain's natural ability to alter neural pathways, coordinated neuronal control of a prosthetic arm has proven viable [9, 11]. However, prosthetics cannot fully replicate the efficient functionality of a natural limb. These types of efferent signaling prosthetics are unidirectional and thus still fail to produce a large portion of coordinated movement [9]. While simple movement can be controlled by efferent neural signaling alone, more complex human movement requires proprioceptive feedback. Proprioceptive feedback allows the body to receive environmental sensory information and translate it into signals that orient the body in space [12]. For example, close your eyes and touch your nose. Proprioceptive feedback should allow your brain to signal exactly where in space the two body parts are and connect them, even without visual cues from the eyes. Bidirectional neuroprosthetics are now in development to integrate motor control with sensory and proprioceptive functionalities [13, 14]. Simply put, this means that we are trying to develop a robotic limb that can not only move and feel, but also be perceived by the brain as an extension of the body.

We’ve Got a Signal! Alternative Neuroprosthetic Treatment

While it may seem like the only goal of neuroprosthetics is to replace a missing or nonfunctional piece of the body and link it directly to the brain, this method is not the only way to remedy such a problem. Another method of restoring function attempts to foster the regrowth or augmentation of the anatomically intact but functionally impaired structure [15, 16]. In cases like traumatic brain or spinal injury, stimulating nerve regrowth or therapies meant to promote  neuroplasticity can foster the regrowth of relevant damaged structures [15, 16]. Alternative neuroprosthetic techniques like these offer a useful means of maintaining existing structures with technological integration rather than merely replacing dysfunctional parts. For example, individuals paralyzed by spinal injury in the neck or upper back can regain mobility of their own hand and arm after implantation of a functional electrical stimulation (FES) system [16]. FES is the coordinated electrical stimulation of muscles and nerves; it essentially shocks your muscles into contracting when necessary. After a BCI is integrated into the brain, FES allows patients to control their previously paralyzed limb [16]. Essentially, we can bypass the faulty wiring and add artificial electrical connections, linking the brain and the body to complete the circuit. Much like restoring an old house, we keep the metaphorical light fixtures and power source, opting to only replace the wiring in the walls to return them to working order. The same system of BCI and FES also produces long-term functional recovery of mobility in limbs affected by a stroke [17]. Even without direct stimulation by the FES, BCI training has also shown strong associative learning benefits for lasting stroke recovery. Using a device to externally move a paralyzed limb in response to motor cues from the brain enhances learning and bolsters neuroplasticity [17]. When paired with physical therapy, this device improved recovery of mobility in stroke patients [18, 19]. Once again, the brain is more powerful than we give it credit for, and with a little assistance, it can recover and regain all sorts of functionality, even when following a severe injury. 

Therapies that regenerate lost neurons or directly repair severed neural connections also hold great promise in treating paralysis that stems from spinal cord injury [20]. One such therapy is cell-based transplantation, where healthy cells and scaffolding are added directly to damaged tissue to promote axonal regrowth in the spinal cord. Think of this therapy as a complex, partial organ transplant: the healthy cells are grafted into the injured or affected area and then supplied with the necessary materials to integrate into the pre-existing networks [20]. However, this therapy is greatly limited by the low survival rates of grafted cells, the tendency of implanted cells to migrate and spread out from localized injury, and the lack of effective strategies to direct cell growth [21]. Despite efforts to lessen these limitations, they still limit the clinical success of cell-based transplantation. Nevertheless, neuroprosthetics are much closer to becoming a viable and accessible option for amputees and other afflicted individuals.

Emerging From Science Fiction: The Future of Neuroprosthetics

The difference between science fiction and reality may just be a matter of time. Science fiction writers once wrote of fantastical networks that connected people across the world; now we have the internet. They wrote of flying cars, hoverboards that don’t touch the ground, and self-driving cars, all of which now have at least a viable prototype. The stories of cyborgs in science fiction have also been brought into reality, but in the form of integrated neuroprosthetics. While the colloquial purpose of such cybernetic additions, in sci-fi novels, have leaned toward superpowers or help fighting crime, scientists have exchanged these goals for improving quality of life. Neuroprosthetics offer treatment options to a variety of people, ranging from amputees to those born deaf and beyond. More incredible developments and discoveries about the brain and the body lie on the horizon. Even proposed inventions so far-fetched they sound like fiction often become reality in time, so the question always remains, what’s next?

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