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A four-channel bipolar cochlear implant developed at the University of California at San Francisco through a long series of animal experiments, engineering development studies, and speech processor design-optimization studies with experimental prosthetic devices in patients, is now being applied in a clinical investigation, as a joint project of the UCSF group and the manufacturer, Storz Medical Instruments.

Microelectrode mapping experiments have been conducted in the auditory cortex of 21 cats, using conventional microelectrode recording and sound stimulation techniques. Cats were anesthetized with urethane or ketamine. Best frequencies (frequencies with lowest response thresholds) were determined for neuron clusters isolated within a large number of penetrations from individual cats. Among the results were the following: (1) Best frequency was remarkably constant within penetrations directed normal to the cortical surface in the primary field (AI). The range of best frequency was less then 0.1 octave in 95% of 169 vertical penetrations into AI in which best frequency was determined at each of several depths within individual penetrations; it did not exceed 0.3 octave in any of these penetrations. (2) There is a highly ordered representation of the cochlea within the primary field. (3) Any given frequency band or any given sector of the basilar partition is represented by a band or strip of cortex that crosses the primary field. These frequency-band strips are oriented dorsoventrally, approximately parallel to the ectosylvian sulci. (4) There is evidence of a columnar organization of AI expressed in the fine detail of its frequency organization. (5) The relation of the primary field to the cortical surface landmarks was highly variable. The location of AI cannot be defined on the basis of surface landmarks alone. (6) Auditory responses were seen within the belt of cortex surrounding the primary field. There is an orderly representation of the cochlea within a field rostral to AI. (7) In individual animals, the cytoarchitectonic boundaries of AI appear to coincide with physiologically defined boundaries. [Supported by NIH Grant NS 10414.]

Neurophysiological and anatomical studies in cats have been directed toward determining the feasibility of development of a multichannel acoustic nerve stimulation prosthesis with potential application as an aid for profoundly deaf individuals with surviving acoustic nerve. From recording experiments taking advantage of the binaural frequency response characteristics of neurons within the central nucleus of the inferior colliculus, the pattern of excitation across the acoustic nerve array can be defined for any intracochlear electrode array. These single-unit studies reveal that it is possible to excite a series of restricted sectors of the acoustic nerve array from within the scala tympani, with appropriately positioned electrodes mounted in silastic carriers that fill the scala (displace perilymph). Precisely constructed electrode arrays in carriers molded to fill the scala can be fabricated using dies of the scala made from soft-metal cochlear casts. Implantation of these multichannel electrode arrays does not lead to significant direct destruction of the acoustic nerve in prior normal or prior neomycin-deafened ears, over a time course of many months. [Supported by NIH Grant NS-11804.]

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