Design of an Extracochlear Prosthesis for the Profoundly Deaf.

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A new model of extracochlear prosthesis has been designed for deaf infants and children.(1) We have developed a Pt-Ir electrode coated with polyvinyl alcohol gel which contains a saline solution. The coated electrode is suitable for the extracochlear prosthesis because it can firmly adhere to the round window membrane without causing damage so that the residual hearing of patients may not be lost.(2) In our telemetry method, the output of the speech processor is transmitted with electromagnetic coupling from the first coil inside the earmold to the second coil implanted under the skin of an external ear. Using this method, the subject only has receive a very minor operation. Our cochlear prosthesis may possibly be applied to even a small child.(3) As a speech coding method, a new idea has been proposed to simultaneously transmit the pitch signal and the second formant frequency through an electrode. It took about 20 ms to detect the pitch signal and the second formant frequency by using a digital signal processor. This new method of speech coding has proven effective for discrimination of the five Japanese vowels based on simple tests.