Nature Medicine Review (206.8KB)
Other Reviewers Have Written
"Victor Chase has looked into the future of broken nervous systems and how we might fix them—with all of the corresponding hopes and perils. It is a fascinating book, both stimulating and exciting, and makes you think about what it means to be human."—Michael S. Gazzaniga, author of The Ethical Brain and member of the President's Council on Bioethics
"A marvelous synthesis of new ideas."—V. S. Ramachandran, M.D., author of Phantoms in the Brain
Shattered Nerves Introduction
The marvel of the human machine unavoidably inspires awe. The coordination within the massive complex of organs that make up our bodies is nothing short of miraculous. While each organ performs its individual function, it also operates in finely tuned concert with the other instruments of the body to create the music of life. The nervous system alone, consisting of billions of neurons, or nerve cells, that allow us to perceive and interact with the world around us, makes the finest of humankind's technological developments pale into insignificance.
Even scientists who devote their entire lives to understanding the workings of the sensory systems eventually arrive at a gap they've been unable to bridge short of taking a leap of faith. Modern technology allows them to watch an individual's brain waves fluctuate in response to a stimulus such as sound, light, or a pinprick. But they still can't look at a spike in waves on an oscilloscope or changes in images on a brain scan and really understand how that stimulus translates into perception. They don't understand how electrical activity in the brain corresponds to perception, pain, pleasure, or conscious awareness.
There's no doubt, however, that electricity is at the root of it all. Electricity, or the movement of electrons and ions, is such a fundamental aspect of nature that it was woven into the fabric of life. A long time before humankind ever walked the face of the earth, let alone thought about electronics, Mother Nature found that electrical signals provide the most efficient method of transmitting information within the body. No living creature could survive without electricity, because the body is, in essence, an electrical machine. Without electricity, neurons could not communicate the signals that allow us to see, hear, touch, smell, taste, and move about, and even think. We need electricity to interact with the world around us as much as an electric motor requires electric power to function. Without it the motor is dead. The same holds true for human beings. Without electricity there is no life.
Complete comprehension of how small spikes of electricity lead to perception and thought still lurks somewhere in the future. But scientists are making exponential leaps in understanding the mass of neurons that make up the brain and the rest of the nervous system that extends from it, though their task is akin to counting, categorizing, and understanding the activity of each star in the universe, as well as its relationship to the whole. Given this level of complexity, resulting from the vast number of elements that must operate perfectly to provide perception, movement, and thought, it is amazing that it is not the norm for things to go awry. Yet in the vast majority of people, the staggering number of components that make up the bodily systems that allow us to function in our environment work perfectly, or close to it.
Unfortunately, in some people, the circuitry that generates and conducts electrical signals goes bad, rendering them unable to fully partake of the miracle of the senses, as in the case of the blind, when the rod and cone photoreceptors inside the eye can no longer translate light into the electrical signals that send information to the brain. Or when the hair cells inside the cochlea of the inner ear, which process sound waves, die off, and a person loses the ability to hear. Failure of the body's electrical circuitry is also responsible for paralysis that occurs when spinal cord injuries damage the nerve cells that carry electrical signals from the brain's motor cortex to the muscles and from the skin's tactile receptors to the somatosensory portion of the brain. Until recently, these conditions were deemed irreversible. Now there is hope.
Through the ability to miniaturize integrated electronic circuitry, scientists can take concrete steps toward countering the ravages wrought on those whose internal circuitry has shorted out, without it being a total act of hubris. The same methods used to shrink electronic components down to pocket computer and digital watch size are now being used to create reliable, intricate devices small enough to be implanted inside the eye, the ear, the muscles, and the brain itself. These manmade, implantable marvels of modern technology are known as "neural prostheses," devices that directly interface with some component of the nervous system. They do so either by feeding electrical impulses into nerves or muscles or by recording signals from the nervous system and using those signals to operate some kind of machine, which itself may be implanted in the body.
Neural prostheses have the potential to aid the hundreds of thousands, or perhaps even millions, of individuals with neurological disorders that disrupt their ability to move or to communicate. These people have functioning brains, but because of injury or disease, cannot get the output of their brains to the parts of their bodies that should receive the signals or cannot receive impulses to their brains that would enable them to utilize the sensory-processing portions of their brains. Though the idea of mating neural prostheses to the body has been around for quite some time and a number of early researchers did experiments in the field, it is only relatively recently that scientists have had the knowledge of brain function and the technological arsenal to actually create viable neural prostheses.
The first widely used neural prosthesis to be added to the physician's arsenal against sensory deprivation was the cochlear implant, which was first embedded in the inner ears of people with profound deafness in the early 1970s. Since then, tens of thousands of people have had some measure of hearing restored through these devices. Typically, the wearer of a modern cochlear implant who was completely deaf prior to being implanted, can now carry on a relatively normal telephone conversation. The success of cochlear implants helped pave the way for work on retinal implants designed to give at least partial sight to people who are blind and beyond the help of purely medical ministrations. Utilizing electrodes placed directly on the delicate retina inside the eye, retinal implants are intended to replace damaged rod and cone light receptors that are no longer doing their jobs because of diseases such as retinitis pigmentosa and macular degeneration. In addition to feeding signals to the blinded eye and the deaf ear, researchers are designing systems that bypass the primary sensory organs and feed electrical impulses directly into the visual and auditory cortices of the brain to stimulate sight and hearing. Such systems can be used in patients whose eyes and ears cannot process any signals, even those fed in by means of cochlear and retinal implants because the nerves leading from the ears or eyes are too damaged to carry those signals to the brain. In such cases, bypassing these primary sensory organs by feeding signals directly to the brain through electrodes placed on or in the brain may be the answer.
Another family of electronic implants is currently returning hand movement to quadriplegics, and the ability to stand and step to paraplegics. In this facet of neural prostheses, called functional electrical stimulation, or FES, scientists are merging humans and machines by implanting electrodes directly into the muscles of people with paralysis. Computer-controlled jolts of electricity stimulate the muscles causing contraction and movement. This can be achieved because even though one is paralyzed, one's muscles are usually intact despite damage to the nerve pathways that feed signals to them. The first U.S. Food and Drug Administration
Scientists are also developing technology that may return the sense of touch to users of FES systems by using electrodes to record signals from a patient's own tactile receptors, which along with muscles, remain functional in spite of paralysis. Early efforts are aimed at improving the grasp capabilities of Freehand users, who have only visual feedback, which does not provide the subtlety of grasp available to the able-bodied. A touch-sensitive neural prosthetic system records signals from the tactile receptors in the user's hand and feeds them directly to the prosthesis's computer, which uses the information to adjust the pressure of the grip. Early systems do not enable the patient to feel what he or she is holding, even though the tactile feedback system uses the body's own sensing apparatus to determine the pressure required to grasp a cup, for example. The hope is to eventually return the sense of touch directly to the patient, initially by remote referral. Pressure on the hand would activate a stimulation device to apply pressure to a part of the body above the severed nerves where natural feeling still exists. The ultimate goal is to record signals from healthy tactile receptors and transmit them to microelectrodes implanted directly in the brain's somatosensory cortex, where skin, muscle, and joint information is processed.
Another twist on the same theme involves sending signals in the opposite direction. Instead of transmitting them to the brain, electrodes implanted in the motor cortex of the brain--where electrical impulses initiating movement, known as action potentials, are created--can capture intentions, which would then be used to activate FES devices. This would be accomplished by transmitting action potentials recorded by the electrodes in the brain via a computer to electrodes implanted in paralyzed muscles, thereby effectively bypassing the damaged nerves in the spinal cord and allowing wearers to operate devices, such as the Freehand, merely by thinking about it, much as able-bodied people move their limbs. This differs significantly from the current configuration, in which the Freehand is operated by a joystick mechanism mounted on a part of the body unaffected by paralysis, such as the shoulder. The same brain implants that send signals to the Freehand via thought could also be used to operate a robotic arm that would respond to the wishes of the patient, or to operate a wheelchair. This technology can also give "locked-in" patients, who can neither move nor speak because of stroke or disease, such as advanced amyotrophic lateral sclerosis (ALS, also known as Lou Gehrig's disease), the ability to communicate. Though many such patients remain intellectually astute, they find themselves in one of the most fearful dilemmas a human can confront--being totally sentient yet unable to communicate with anyone. With the neural prosthetic technology that records their thoughts and allows them to essentially think a computer into operation, bypassing the need to move a mouse or type on a keyboard, locked-in patients can again interact with their fellow human beings.
The same technology that can enhance the lives of people with severe disabilities also holds the potential to expand the capabilities of the able-bodied with as yet undreamed of consequences. The visible wavelength may be increased, or the ability to hear sounds that only animals with more sensitive ears can now perceive may fall within human capability. And learning capacity and memory may be increased. The U.S. Air Force has looked at the technology as a possible way of augmenting the ability of fighter pilots to operate the highly complex systems in their aircraft. And in what sounds like science fiction, but has realistic potential, a leading physician in the study of how the brain represents tactile information says he believes the brain is capable of incorporating a machine into its representation of the body. In other words, from a sensory standpoint, an autonomous machine could be made part of a person. The individual would experience the same sensations as the machine. This would, for example, enable an earthbound scientist to explore another planet by seeing and feeling what a robot actually located on that planet perceives. By the same token, a safely ensconced individual could have a machine do all sorts of nefarious deeds on his or her behalf, essentially without detection.
Though some of the hopes and goals of the scientists involved in developing neural prosthetic implants may seem far-fetched and perhaps impossible, experience has shown that if it can be conceived it can be done, given time, money, and the tools made available by modern technology. Consider, for example, the 300-year-old drawing by Isaac Newton of a man on a mountaintop throwing a ball into a parabolic arc around the earth. During Newton's time, the idea of putting a manmade satellite into orbit around our planet would have undoubtedly been considered the musings of a madman, yet, though it took hundreds of years, Newton's dream is today a reality.
It is, therefore, an extremely exciting time for those working in the field of neural prostheses as well as for those who may benefit from the fruits of their labors. But a word of caution is prudent. As is the case with any emerging field that holds great promise, overzealousness on the part of some of those involved can result. Thus, people with disabilities who may, in fact, someday be aided by developments in this new area of technology should not allow their hopes to get unrealistically high. Blind individuals, for example, should not expect full sight restoration, but instead perhaps the ability to see only points of light or shadows that may enhance mobility. And people with paralysis cannot expect to stand and walk with a normal gait and without the assistance of a walker anytime soon. Most researchers in the field themselves expect only relatively modest gains in the short term. Neural prosthetic technology does indeed hold the promise of returning almost normal functioning to those whose nervous systems are impaired, but that remains a hope for future generations. In the meantime, the step-by-step gains will likely be more modest. Yet as virtually every person who has volunteered as a test subject for the research and development currently being conducted has said, "Something is better than nothing."
But even if some of the loftiest goals of this work are never achieved, the act of striving toward them will not be for naught. For it is certain that neural prosthetic research--especially the facet of it pertaining to brain implants--will go a long way toward solving the mysteries of some of biological science's last frontiers, such as how the brain and sensory systems function. Through the electrodes implanted in the brains of human patients, scientists for the first time have an unobstructed view into the workings of the brain. "You can tell someone to imagine something, but you can't tell a monkey to do that," said Andrew Schwartz, a leading researcher in the field, at the University of Pittsburgh. "Through the use of language and comprehension you can do all sorts of experiments that you could never dream of before, and the data we could get would be very rich." Through such work, scientists might finally be able to understand how perception gets transformed into consciousness and how a pinprick actually does make you say ouch. In the meantime, the quest for better neural prostheses goes on, and though no one is claiming to be anywhere near bionic nirvana, the pursuit is indeed electrifying.