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UNDERSTANDING THE NORMAL FUNCTION of the nervous system is central to understanding dysfunction caused by disease or injury and designing therapies. Such treatments include pharmacological agents, surgical interventions, and, increasingly, electronic medical devices. These medical devices fill an important gap between largely molecularly targeted and systemic medications and largely anatomically targeted and focal surgical lesions.

In this chapter, we focus on medical devices that measure or alter electrophysiological activity at the level of populations of neurons. These devices are referred to as brain–machine interfaces (BMIs), brain–computer interfaces, or neural prostheses. We use the term BMI to refer to all such devices because there is no standard distinction among them. BMIs can be organized into four broad categories: those that restore lost sensory capabilities, those that restore lost motor capabilities, those that regulate pathological neural activity, and those that restore lost brain processing capabilities.

BMIs can help people perform “activities of daily living,” such as feeding oneself, physically dressing and grooming oneself, maintaining continence, and walking. A type of BMI that we will discuss extensively in this chapter converts electrical activity from neurons in the brain into signals that control prosthetic devices to help people with paralysis. By understanding how neuroscience and neuroengineering work together to create current BMIs, we can more clearly envision how many neurological diseases and injuries can be treated with medical devices.

BMIs Measure and Modulate Neural Activity to Help Restore Lost Capabilities

Cochlear Implants and Retinal Prostheses Can Restore Lost Sensory Capabilities

One of the earliest and most widely used BMIs is the cochlear implant. People with profound deafness can benefit from restoration of even some audition. Since the 1970s, several hundred thousand people who have a peripheral cause of deafness that leaves the cochlear nerve and central auditory pathways intact have received cochlear implants. These systems have restored considerable hearing and spoken language, even to children with congenital deafness who have learned to perceive speech using cochlear implants.

Cochlear implants operate by capturing sounds with a microphone that resides outside the skin and sending these signals to a receiver surgically implanted under the skin near the ear. After conversion (encoding) to appropriate spatial-temporal signal patterns, these signals electrically stimulate spiral ganglion cells in the cochlear modiolus (Chapter 26). In turn, signals from the activated cochlear cells are transmitted through the auditory nerve to the brain stem and higher auditory areas where, ideally, the neural signals are interpreted as the sounds captured by the microphone.

Another example of a BMI is a retinal prosthesis. Blindness can be caused by diseases such as retinitis pigmentosa, an inherited retinal degenerative disease. At present, there is no cure and no approved medical therapy to slow or reverse the disease. Retinal prostheses currently enable patients to recognize large letters and locate the position ...

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