Banner by Charley Wan

Probing Prosthetics 

by Charley Wan

Neuroplasticity has been frequently associated with an internal process involving the brain’s adaptation to salient novel stimuli, as well as its ability to heal from some traumatic injuries. However, the field of neuroscience is certainly experiencing a paradigm shift with the rise of neuroprosthetics. Neuroprosthetics are artificial devices that interface with the nervous system to re-establish functions of the body that may have been lost due to injury. By stimulating neural pathways or interpreting brain waves to control these devices, the field unlocks capabilities that extend far beyond what neuroplasticity alone could achieve. This explosive advancement combining biomedical engineering, computer science, and neurology has encouraged researchers to reconceptualize the brain’s ability to adjust and handle synthetic forms of intervention, challenging fundamental assumptions about the boundaries of our nervous systems. 

Neuroprosthetics is an extension of the already prosperous field of prosthetics. Early prosthetics arose as crude extensions of existing limbs rather than substitutions for missing limbs, providing only limited movement and dexterity for their wearers. The majority of early prosthetics, such as the peg legs and hook hands used by sailors during the Middle Ages, were also external, typically requiring only a few straps or harnesses to keep the artificial limb secure. The foundational purposes of enhancing mobility and restoring functionality to amputees have not only endured but grown more sophisticated over the years. These devices have now become integrated into the human body and are intricately involved with the mechanisms of the brain. 

Essentially, there are two approaches to the rehabilitation process in neuroprosthetics: replacement or restoration. In addition to amputations, traumatic brain or spinal cord injury can lead to the complete severing of ascending sensory input from the extremities or descending motor output from the brain, cutting off the major “highways” by which the central nervous system (CNS), which consists of your brain and spinal cord, communicates with the peripheral nervous system (PNS), everything else. In this case, the best course of action would be to substitute the action of neurons with the action of electrode implants. A familiar example of this is the cochlear implant, a surgically inserted device within the ear that stimulates the auditory nerve to allow for a sense of hearing. However, lucky cases of injury leave some pathways intact, providing a chance for the prosthetics to trigger neuroplasticity or compensatory mechanisms in the body to hopefully recover at least a portion of the prior capabilities. 

 For individuals with quadriplegia (complete paralysis below the neck), brain-controlled interfaces (BCI) are used to interpret motor intention and replace their lost motor ability with the movement of an external device. A BCI has four components that make up a sort of loop that helps to prompt movement. Recordings are first taken from a region of the brain through various means. Second, the data is sent to a computer to extract the intended action. Then, the computer controls the external device (such as a robotic arm) to perform the action. Finally, biofeedback from the individual’s five senses such as vision completes the loop, further stimulating the brain in preparation for another action. In this case, brain activity is taken from the motor cortex in the form of electrocorticography (ECoG). ECoG involves putting grids of electrodes directly on the surface of the cortex to capture these electrical signals. There are indeed other forms of recording that are available such as electroencephalography (EEG), local field potentials (LFP), and Single Units (one neuron), but ECoG prevails as EEG has poor spatial resolution, and LFP and Single Unit recording pose more of a risk for tissue damage compared to ECoG’s partially invasive nature.  

 The most notable aspect of the extraction algorithms was the work done by Dr. Apostolos Georgopoulos in 1982 where he found that detailed movement information could be identified in the patterns of activity in cortical motor neurons. Specifically, it seemed that there was a uniform distribution of cells that fired intensely only when the movement was in the cell’s “preferred direction.” By using a form of vector addition with the cells as individual vectors, a “bulk” movement could be interpreted from the numerous amounts of incoming data, ultimately determining the movements of the robotic arm.  

 Patients who might be suffering from phantom pains (the perceived pain of a non-existent limb) due to amputation or major spinal cord injury can also rely on neuroprosthetics to restore their somatosensation. Compared to dealing with CNS to PNS communication during motor movement substitution, returning sensory feedback to amputees wearing prosthetics is PNS to CNS communication—the opposite direction. In these circumstances, nerve cuff electrodes (NCEs) are used to send information to the cortex about tactile stimuli from multiple regions of the body. In particular, spiral NCEs delicately wrap around the chosen peripheral nerve, capable of adjusting their size to be identical to the nerve size as the sheath wraps around the nerve twice. Also, the stimulation contacts on the electrode are evenly spaced to ensure selective stimulation. As a result, the spiral NCE can stimulate many different fascicles close to their implantation site. All these characteristics make the NCE a viable method of precisely replicating sensory stimuli transduction and may be implicated as a reasonable way to maintain long-term peripheral nerve stimulation. 

 A paper by Daniel Tan in 2014 corroborated these assertions as their experimentation with implanting cuff electrodes into two upper-limb amputees yielded stable touch sensations for more than a year. By periodically testing the subjects with different patterns of stimulation intensities, the patients were able to report varying perceptions of the sensations such as tapping, brushing motions, and vibration. Thus, this not only reflected the efficacy of the NCE but also its potential to maintain proper tactile reception over a long period of time. 

 With all these promising facets of neuroprosthetics, what is stopping it from becoming a main rehabilitation tool in the medical field? There are a multitude of ethical considerations to mention as it seems that in the future neuroprosthetics could potentially be a double-edged sword. Two main points emerge. According to Walter Glannon’s 2016 review of neuroprosthetics, if they were to become successful methods of rehabilitation, they run the risk of shifting from restoration to enhancement. Tension arises when one considers the consequences of having neuroprosthetics as an enhancement, such as shifting societal norms and blurring the lines of competition. Additionally, Glannon notes that integrating the prosthetics into the brain may invade one’s privacy and security. If the prosthetics become integral to daily life, the hacking and collection of sensitive neurological data may be an issue. However, Glannon does assure us that these worries are pretty far off in the future. As long as the subject continues to encounter continuous ethical dialogue, sensible regulations may be instituted to prepare for such a world. 

The nervous system has certainly presented an unprecedented level of complexity for the medical field, especially for the biomedical engineers and computer scientists pioneering the domain of neuroprosthetics. There are various methods for tackling the obstacle of returning motor movement and somatosensation to amputee patients and victims of traumatic brain and spinal cord injury. BCIs allow paralyzed individuals to regain their ability to move their limbs while the nerve cuff electrodes play a big role in reducing phantom pains and might restore the tactile sensations that patients lack. While there may be emerging worries about the power of neuroprosthetics in the future, we have tons of discussions to have before we come to a concrete conclusion on their risk. For now, we can continue probing!