Hearing research
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A systematic study of the encoding properties of 146 auditory nerve fibers in the Tokay gecko (Gekko gecko, L) was conducted with respect to pure tones and two-tone rate suppression. Our aim was a comprehensive understanding of the peripheral encoding of simple tonal stimuli and their representation by temporal synchronization and spike rate codes as a prelude to subsequent studies of more complex signals. Auditory nerve fibers in the Tokay gecko have asymmetrical, V-shaped excitatory tuning curves with best excitatory frequencies that range from 200-5100 Hz and thresholds between 4-35 dB SPL. ⋯ These complimentary representations within a tuning curve raise fundamental issues which need to be addressed in interpreting how more complex, bioacoustic communication signals are represented in the peripheral and central auditory system. And since auditory nerve fibers in the Tokay gecko exhibit tonal sensitivity, selective frequency tuning, and iso-intensity and iso-frequency contours that seem comparable to similar measures in birds and mammals, these issues likely apply to most higher vertebrates in general. The simpler wiring diagram of the reptilian auditory system, coupled with the Tokay gecko's remarkable vocalizations, make this animal a good evolutionary model in which to experimentally explore the encoding of more complex sounds of communicative significance.
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The ototoxicity of carboplatin, a second generation anti-cancer agent, was examined using the chinchilla as an animal model. In animals treated with a clinical therapeutic dose (400 mg/m2), the dominant degenerative change is to inner hair cells (IHCs). This is in sharp contrast to most other ototoxic agents, which damage primarily the outer hair cells (OHCs). ⋯ In contrast, CM amplitudes and 'thresholds' remained close to normal in most cases, reflecting the preservation of OHCs in the basal turn. These results indicate a high degree of independence between the inner and outer hair cell systems in the cochlear transduction mechanism. We suggest that this species-specific preparation with selective IHC loss will provide a valuable tool for studying, separately, the role of OHCs in both afferent and efferent cochlear function.
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The effects of chronic intracochlear electrical stimulation on the cochlear nucleus (CN) were studied in eight cats that were neonatally deafened by daily intramuscular injections of neomycin. Profound hearing loss was confirmed in each animal by auditory brainstem response (ABR) and frequency following response (500 Hz) testing. Five of the kittens were implanted unilaterally with a scala tympani electrode array at ages 8-16 weeks. ⋯ The CN in animals that received electrical stimulation showed significant bilateral degenerative changes in all three measured parameters. Total nuclear volume was reduced by 35-36%, spherical cell size was reduced by 20-26%, and spherical cell density decreased by 36-42%, as compared to the normal cat CN. Comparisons were also made in the stimulated animals between CN ipsilateral to the stimulated cochlea and the contralateral, unstimulated CN.(ABSTRACT TRUNCATED AT 250 WORDS)
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This paper describes the course and morphology of efferent fibres in an avian cochlea. Horseradish peroxidase stained efferent fibres in the pigeon papilla basilaris were identified by Nomarski optics and camera lucida drawings. ⋯ Small thin fibres contact short or intermediate hair cells over the free basilar membrane or tall hair cells over the neural limbus. A physiological consequence of the findings is that efferent activity will concomitantly lead to a contraction of hyaline cells and a hyperpolarization of hair cells.
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Electrical stimulation of the auditory nerve: the effect of electrode position on neural excitation.
Histological studies have shown that the Melbourne/Cochlear electrode array lies along the outer wall of the scala tympani and is therefore some distance from the residual VIIIth nerve elements. In order to investigate the influence of electrode position on neural excitation we systematically varied the position of the electrode array within the cat scala tympani while recording electrically evoked auditory brainstem responses (EABRs). Using both normal hearing and long-term deafened animals, we observed significant reductions in EABR thresholds as the electrode array was moved from the outer wall towards the modiolus. ⋯ An array lying adjacent to the modiolus would be a safe alternative while ensuring a significant reduction in threshold compared with the existing site (outer wall). This placement should result in more localized neural excitation patterns, an increase in the number of bipolar electrodes available, together with an increase in their dynamic range. These changes may lead to further improvements in speech perception among cochlear implant patients.