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by an external magnet, when they feel a seizure coming on. The results have been beneficial; studies show that a year after the device is implanted, nearly a quarter of patients have had their seizure rate reduced 90 percent or more.

A similar implanted device is used to alleviate symptoms of Parkinsons disease, a chronic and progressive disorder first called the Shaking Palsyin 1817 by the English physician James Parkinson. The disease kills certain neurons in the brain that normally produce the chemical dopamine, which transmits nerve signals among areas in the brain that control the muscles. The nerve damage affects body movements at mild to severe levels, with such symptoms as rigid muscles; tremors of the hands, arms, feet, or jaw; changes in speech and handwriting, and the inability to maintain balance. The symp- toms can be treated with drugs that replace the missing dopamine, although they do not halt the neural degeneration.A new approach to relieving the muscular symptoms uses a battery-powered implant, which, like the VNS device, generates electrical pulses, although in this case they are sent deep into a particular region of the brain.Origi- nally approved in 1997 by the Food and Drug Administration (FDA), for implantation on one side of the brain to control tremors on that side of the body, FDA approval was extended in 2002 to allow the implantation of dual systems that operate on both sides of the brain.

The electrical pulses used in VNS or the Parkinsons implant are not digitally encoded, but a more sophisticated type of neural implant does use digital methods to correct another human problem, hearing loss.The physical understanding of sound extends to ancient Greece, where it was realized that sound consists of vibrations in the air. Later, the physiological mechanisms of hearing were explored, illuminating how those vibrations are detected and transmitted in the body. In humans, hearing occurs when sound waves enter the ear canal and set the eardrum vibrating in step with the waves. Those vibrations are transmitted through bony structures to an inner structure called the cochlea.There, the mechanical motion is converted into impulses that travel along the auditory nerves to the brain, where they are analyzed and interpreted to give them meaning.

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