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Central components of the temporary threshold shift.
Effects of noise on hearing. Henderson D, Hamernik RP, Dosanjh DS, Mills JH, eds. New York: Raven Press, 1976 Jan; :247-262
Low level and short duration susceptibility to auditory fatigue during asymptotic temporary threshold shift (TTS) were studied. Sound exposure did not appear to affect the direct current endocochlear potential in chinchillas, but cochlear microphonic (CM) potential showed a loss of sensitivity and a reduction of maximum voltage. The primary dysfunction for TTS was in the hair cells as indicated by CM loss in sensitivity and auditory evoked response (AER) threshold shift. Neurons with characteristic frequencies (CF) between 3.5 and 20.0 kilohertz (kHz) had thresholds 20 to 40 decibels (dB) higher in noise exposed compared to control animals. Asymptotic TTS exposure also caused 20 to 40dB threshold shift for frequencies near CF. Chinchillas exposed to between 3 and 6kHz octave noise band at 86dB sound pressure level (SPL) had thresholds 30 to 60dB higher at the midfrequencies 1 to 5 hours postexposure. The excitatory region of the tuning curve showed a greater loss in sensitivity near CF than below CF. CM, AER, and single units from the cochlear nucleus and inferior colliculus showed a loss in sensitivity which was similar to behavioral TTS data. In cats, threshold shifts up to 32dB in the frequency following response were observed after a mild TTS exposure. TTS exposure of chinchillas at frequencies of 0.4 to 8.0kHz for 8 minutes at 95dB SPL produced no effect on visual detection level for the eighth nerve action potential. The median threshold shift for all cochlear nucleus units was 6.8dB and for the colliculus neurons it was 13dBs. Most frequently, TTS exposure displaced the function to the right, but it did not appear to significantly alter the firing pattern of units in the cochlear nucleus or the inferior colliculus. The author concludes that low level short duration exposures may primarily fatigue retrocochlear structures and thus reduce the sensitivity of central auditory neurons.
NIOSH-Grant; Inner-ear; Auditory-system; Audiofrequency; Noise-exposure; Neurophysiological-effects; Laboratory-animals; Acoustics; Electrophysiology
Henderson-D; Hamernik-RP; Dosanjh-DS; Mills-JH
Effects of noise on hearing
Upstate Medical Center, Syracuse, New York
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