Scandinavian audiology. Supplementum
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Auditory neuropathy is often characterized by normal thresholds, present otoacoustic emissions, poor speech discrimination, absent acoustic reflexes, absent or abnormal auditory brainstem response waveform, but normal late cortical potential. This paper describes an animal model that has many characteristics of auditory neuropathy. Chinchillas can be deprived of a significant portion of the neural inputs to the central auditory system by administering carboplatin, an antineoplastic agent that selectively destroys inner hair cells (IHCs) and type I auditory nerve fibers. ⋯ Acoustically responsive auditory nerve fibers in ears with mild to moderate IHC loss have normal thresholds and tuning curves with narrowly tuned tips. Although the central auditory pathway is deprived of much of its sensory inputs, the amplitude of the local field potential in the auditory cortex was normal or enhanced, while those from the inferior colliculus were slightly reduced. The results are related to those of a patient with auditory neuropathy.
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To better understand the nature of mechanical changes following exposure to high-level impact noise, normal cochleas and cochleas from chinchillas exposed to either 125 or 131 dB SPL noise were stained with phalloidin for F-actin and examined using confocal microscopy. As seen in previous experiments, 125 dB exposures produced much more variable results than 131 dB exposures. ⋯ After 30 days, Deiters cells were disarrayed and cups were separated from OHC neural poles. Exposure to noise at a level of 131 dB SPL produced less variable results than 125 dB exposure, and damage was generally more widespread and severe.
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Four noise reduction methods for use in sensory aids for hearing impairment were evaluated. These include a two-microphone adaptive noise canceller, short-term Wiener filtering, a transformed spectrum subtraction technique, and sinusoidal modelling. ⋯ The transformed spectrum-subtraction technique failed to improve performance as the front-end of a hearing aid, but yielded improvements in performance as a preprocessor for the Nucleus Cochlear Implant. Sinusoidal modelling resulted in significant improvements in signal-to-noise ratio, but without a corresponding improvement in speech intelligibility.
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Toluene is a widely used organic solvent causing loss of auditory sensitivity in rats and presumably in humans. Also, the hearing loss in humans occupationally exposed to noise has been reported to be aggravated by simultaneous exposure to solvents. The aim of the present investigation was to study the effects of toluene, alone or in combination with other factors, on auditory sensitivity. ⋯ The OHCs are particularly susceptible to toluene exposure. The auditory sensitivity loss induced by toluene can be enhanced by ASA and by ensuing noise exposure. Also, toluene exposure can influence the rate of progress of age-related hereditary loss of auditory sensitivity.
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Review
Genetic influences on susceptibility of the auditory system to aging and environmental factors.
Auditory susceptibility to aging and environmental factors (noise and toluene) was investigated in two genotypes of mice, CBA/Ca (showing a moderate hearing loss with onset late in life) and C57BL/6J (undergoing spontaneous auditory degeneration with onset during young adulthood). Both strains of mice were often used together as a model of human age-related hearing loss, with a primary decline in auditory high-frequency sensitivity. Auditory function was analysed by recording auditory brainstem response (ABR). ⋯ First an additivity and then, at total threshold shifts of 30-50 dB, a blocking-like interaction were observed. The rate of progression in post-noise hearing loss did not exceed the spontaneous rate of age-related degeneration. The difference between exposed and non-exposed groups decreased with advancing age for both strains of mice.(ABSTRACT TRUNCATED AT 400 WORDS)