THE SCIENCE AND APPLICATIONS OF ACOUSTICS - H. H. Arnold ...

10.8 Hearing in Animals 237the hearing bones. It is claimed that the fidelity at the third ear bone is improvedthrough the use of this system. Outpatient surgery is required to attach the magnetto the hearing bone. The surgery is performed under local anesthetic and requiresabout an hour.One of the advantages of the DDHS is that it eliminates the feedback problemthat often occurs with a loose earmold of a conventional hearing aid. There is nofeedback because the DDHS does not produce amplified sound. It is also claimedthat sound quality is improved by the electromagnetic system and that earmoldsdo not have to be fitted so tightly. The original study with five volunteers wassuccessful, and FDA authorized a larger study entailing 100 patients. The DDHSis initially targeted at patients with a moderate hearing loss.Drug TherapyDrug delivery systems are being integrated into newer designs of cochlear implants.The purpose of these drugs is to arrest the shriveling or demise of remaining haircells and neural structures in the cochlea and to promote the growth of neuraltentacles (called neurites) from spiral ganglion cells toward the electrodes. Theneurites help each electrode to function as an independent channel of stimulation.Recent experiment with deafened guinea pigs demonstrated that injecting brainderivedneurotrophic factor and ciliary neurotropic factor could increase the survivaland, more importantly, the sensitivity of spiral ganglion cells.A second approach is to block apoptosis, the normal process of cell death followinginjury. Self-destruct messages can be triggered by a number of events, forexample, acoustic trauma or ototoxic drugs that work through mitogen-activatedprotein kinase (MAPK) signaling pathway. The pathway can be blocked at variouspoints, thanks to a protein called c-Jun N-terminal kinase (JNK). A peptideinhibitor, developed by a multinational team at the University of Miami, targetsthis enzyme. By blocking JNK, this team headed by Jing Wang and Thomas VanDe Water, prevented hair-cell death and hearing loss following acoustic trauma oradministration to the ototoxic antibiotic neomycin (Wang et al., 2003).10.8 Hearing in AnimalsBecause their lives depend on the acuity of their hearing, many animals hear amuch wider range of frequencies than humans do (cf. Figure 10.9). The averagehearing range for humans is about 20 Hz–17 kHz, but killer whales have respondedto tones with the frequency range of approximately 0.5–125 kHz with a peak sensitivityat about 20 kHz. Odontocetes (toothed whales) can produce sounds for twooverlapping functions: communicating and navigating. Higher frequency clicksprobably function primarily in echolocation. Most sound reception, or hearing,seems to take place through the lower jaw. A killer whale may also receive soundthrough soft tissue and bone surrounding the ear.