Deafness can have many causes. In general, there are two categories of deafness: conduction and sensorineural (or nerve) deafness. Conduction deafness involves a mechanical deficiency in transmission of sound waves from the external ear to the spiral organ and may often be corrected surgically. Hearing aids help people with such hearing deficiencies by boosting the sound volume reaching the ear. Sensorineural deafness involves the spiral organ or nerve pathways and is more difficult to correct.

Research is currently being conducted on ways to replace the hearing pathways with electrical circuits. One approach involves the direct stimulation of nerves by electrical impulses. There has been considerable success in the area of cochlear nerve stimulation. Certain types of sensorineural deafness in which the hair cells of the spiral organ are impaired can now be partially corrected. Prostheses are available that consist of a microphone for picking up the initial sound waves; a microelectronic processor for converting the sound into electrical signals; a transmission system for relating the signals to the inner ear; and a long , slender electrode that is threaded into the cochlea. This electrode delivers electrical signals directly to the endings of the cochlear nerve. High-frequency sounds are picked up by the microphone and transmitted through specific circuits to terminate near the oval window, whereas low-frequency sounds are transmitted farther up the cochlea to cochlear nerve endings near the helicotrema.

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