Technology Insight: Future Neuroprosthetic Therapies for Disorders of the Nervous System

Richard A. Normann

Disclosures

Nat Clin Pract Neurol. 2007;3(8):444-452. 

In This Article

Summary and Introduction

Most disorders of the nervous system result from localized sensory or motor pathologies attributable to disease or trauma. The emerging field of neuroprosthetics is focused on the development of therapeutic interventions that will be able to restore some of this lost neural function by selective electrical stimulation of sensory or motor pathways, or by harnessing activity recorded from remnant neural pathways. A key element in this restoration of function has been the development of a new generation of penetrating microelectrode arrays that provide unprecedented selective access to the neurons of the CNS and PNS. The active tips of these microelectrode arrays penetrate the nervous tissues and abut against small populations of neurons or nerve fibers, thereby providing selective access to these cells. These electrode arrays are not only beginning to provide researchers with the ability to better study the spatiotemporal information processing performed by the nervous system, they can also form the basis for new therapies for disorders of the nervous system. In this Review, three examples of this new generation of microelectrode arrays are described, as are potential therapeutic applications in blindness and spinal cord injury, and for the control of prosthetic limbs.

Until recently, the concept of helping the deaf to hear, the blind to see, and the paralyzed to walk was more the province of science fiction or theology than of clinical medicine. Today, however, individuals with profound deafness who have been fitted with cochlear prostheses are able to hear, and to enjoy relatively normal conversations with family, friends and fellow workers. This approach to hearing restoration is rapidly becoming a widely accepted therapy.[1] In a similar vein, researchers in the US and Germany have implanted electrode arrays in the visual cortices[2] or on the retinas[3,4,5] of individuals who have lost all vision, and although these individuals have yet to experience useful patterned vision, they have once again been able to perceive points of light. Researchers in the US have also implanted electrodes into the motor regions of the brain in paralyzed patients, and have been able to use recorded neural activity to infer the desires of these patients, enabling them to control the cursor on a computer screen simply through volitional thought.[6] These attempts to restore lost sensory and motor function are the result of new neuroprosthesis-based therapy, a field that is still in its infancy. Although the restored functions fall far short of natural sensory and motor capabilities, these successes offer a tantalizing glimpse of what the future of neuroprosthetics might hold for individuals with profound sensory or motor dysfunction.

The neuroprosthetic approach to restoring these lost functions is based on arrays of microelectrodes implanted into neural tissues, which can 'talk' and 'listen' to large numbers of small groups of neurons in the CNS and PNS. These implanted electrode arrays enable direct communication with still-functioning parts of the sensory and motor neural pathways. By stimulating and recording from these neurons, it is possible to bypass, to a limited degree, regions of the nervous system that have been damaged by inherited or acquired disease, or by traumatic injury. This approach is, however, made difficult by the complexity of the CNS and PNS. Even the simplest musculoskeletal movements or the most basic sensory perceptions are the result of the coordinated activity of hundreds of neurons. Complex and graceful movements and complex sensory perceptions require the activation of hundreds of thousands of synapses between thousands of sensory and motor neurons. Given this architectural complexity, one might conclude that it would be impossible to interact selectively with sufficient numbers of sensory or motor neurons to evoke any useful sensory percepts or motor behaviors. The success of the cochlear neuroprosthesis, however, highlights two important features of our nervous system: first, the brain has a remarkable capacity to make use of even the most limited amount of sensory stimulation, and second, the plasticity of these neural circuits is such that the brain can interpret somewhat inappropriate but systematic stimulation of sensory pathways, and can use this information to make useful judgments about the world.

The emergence of the neuroprosthetic approach to treating nervous system dysfunction is directly tied in with the development of a new generation of microelectrode arrays. Much ongoing work aims to develop various electrode array designs,[7,8,9,10] but this Review will focus on two contemporary examples of neural interface devices that were developed in the author's laboratory and have been evaluated in scores of animal experiments. Research and commercial versions of these electrode arrays have also been implanted in a small number of human subjects.[6,11] This Review also describes several possible applications of this technology in sensory and motor disorders, and concludes with a brief description of some of the remaining hurdles that must be overcome before neural interface devices can become clinical tools.

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