Use of distortion product otoacoustic emissions to assess middle ear transducers in rhesus monkeys.

Distortion product otoacoustic emissions (DPOAEs) can provide an objective and noninvasive assessment of the peripheral cochlear function. Auditory brainstem responses measured from implanted rhesus monkeys have shown that middle ear transducers, coupled directly to the incus, are capable of delivering the signals to the central auditory system. The DPOAEs were used as a noninvasive method of assessing the frequency specificity of this mechanical transduction. In two rhesus monkeys implanted with the middle ear transducers, one primary stimulating tone (f1) was presented acoustically, and the other primary tone (f2) was presented by the transducer, which converted the signal into a mechanical motion of the probe tip attached to the body of the incus. The nonlinear characteristics of the cochlea produced the distortion product responses at the expected frequencies (2f1 - f2). This demonstrates the fidelity of the middle ear implant signal transduction in vivo. The DPOAEs also indicate minimal changes in the post-implant middle ear transmission. This study demonstrates that the DPOAEs can be used to assess the function of implanted middle ear transducers objectively and noninvasively.

Distortion product otoacoustic emissions in rhesus (Macaca mulatta) monkey ears: normative findings.

Distortion product otoacoustic emissions (DPOAEs) in rhesus monkeys were characterized and the optimal parameters for their generation were determined. Robust DPOAEs were readily measurable from the ear canals of six rhesus monkeys (n = 12 ears). The nonmonotonic behavior of the f2/f1 ratio functions in rhesus monkeys was found to be similar to other animals and humans. The optimal mean f2/f1 ratio of 1.21 and the effect of the primary frequency and level on the optimal f2/f1 ratios were also similar to human measurements. The contour of the rhesus monkey DPOAE 'audiograms' and their behavior were also comparable to human measurements with slight differences in peak frequencies. The rhesus monkey DPOAE input/output (I/O) functions were generally monotonic with a slope approaching unity with increasing frequency. Therefore, our study shows that many basic DPOAE characteristics are remarkably similar in the two species and emphasizes the appropriateness of the rhesus monkey as a model for DPOAE research. Detailed studies of the behavior of DPOAEs can be carried out in a model that is phylogenetically close to human both in hearing and in the gross structure and histology of the inner ear.

Computer-stimulated test fitting of an implantable hearing aid using implantable hearing aid using three-dimensional CT scans of the temporal bone: preliminary study.

In preparation for future implantation of the implantable middle ear transducer in patients, a method was sought for preoperatively test fitting a model of the device, using computer generated three-dimensional (3-D) temporal bone images derived from spiral computed tomography (CT) data. A 3-D model of the implantable middle ear transducer was designed using NIH Image software on a Macintosh computer. High resolution human temporal bone CT scans were obtained using a spiral CT scanner (Siemens Somatom Plus S). The 3-D transducer model was superimposed onto 3-D reconstructions of the temporal bone using ANALYZE software on a computer graphics workstation (Sun SPARCstation 10), showing the transducer "implanted" in the temporal bone. Measurements were validated using a cadaver temporal bone. This process produced images demonstrating the "fit" of the current transducer design in the mastoid region of the adult temporal bone. It permitted assessment of the proximity of surrounding structures such as the external auditory meatus, dura, or sigmoid sinus. Preliminary cadaver validation measurements confirmed the accuracy of this method. Three-dimensional CT is a feasible method for preoperative planning of the surgical implantation of devices in the temporal bone. This method of 3-D test fitting will be used in the future to determine optimum orientation and size limitations for human implantable devices.