Preservation of auditory brainstem response thresholds after cochleostomy and titanium microactuator implantation in the lateral wall of cat scala tympani.

HYPOTHESIS: The safety of implanting a titanium microactuator into the lateral wall of cat scala tympani was assessed by comparing preoperative and postoperative auditory brainstem response (ABR) thresholds for 1 to 3 months. BACKGROUND: The safety of directly stimulating cochlear perilymph with an implantable hearing system requires maintaining preoperative hearing levels. This cat study is an essential step in the development of the next generation of fully implantable hearing devices for humans. METHODS: Following GLP surgical standards, a 1-mm cochleostomy was drilled into the lateral wall of the scala tympani, and a nonfunctioning titanium anchor/microactuator assembly was inserted in 8 cats. The scala media was damaged in the 1 cat. ABR thresholds with click and 4- and 8-kHz stimuli were measured preoperatively and compared with postoperative thresholds at 1, 2, and 3 months. Nonimplanted ear thresholds were also measured to establish statistical significance for threshold shifts (>28.4 dB). Two audiologists independently interpreted thresholds. RESULTS: Postoperatively, 7 cats implanted in the scala tympani demonstrated no significant ABR threshold shift for click stimulus; one shifted ABR thresholds to 4- and 8-kHz stimuli. The eighth cat, with surgical damage to the scala media, maintained stable click threshold but had a significant shift to 4- and 8-kHz stimuli. CONCLUSION: This cat study provides no evidence of worsening hearing thresholds after fenestration of the scala tympani and insertion of a titanium anchor/microactuator, provided there is no surgical trauma to the scala media and the implanted device is securely anchored in the cochleostomy. These 2 issues have been resolved in the development of a fully implantable hearing system for humans. The long-term hearing stability (combined with histologic studies) reaffirm that the microactuator is well tolerated by the cat cochlea.

Development of a test environment to evaluate performance of modern hearing aid features.

This article describes a new test environment and materials with the potential to measure performance differences among different hearing aid signal processing methods and features. Normative data suggest a linearly predictable increase in difficulty on a speech-in-noise task as the masker changes from random noise to multiple-talker speech, and the number of talkers increases. Data collected with normal listeners revealed no differences across four test sites for the single loudspeaker (Noise-Front) results and some differences across test sites for the multiple loudspeaker results. Room dimension differences among audiometric test enclosures and diffusion (or lack thereof) of the maskers in the vertical and horizontal dimensions of the sound fields appear to account for performance differences for the multiloudspeaker arrays, confirming the need to limit maskers to aperiodic signals in rooms with controlled ceiling height or to establish norms for each test environment such that results obtained in different enclosures can be compared.