Differences in Auditory Sensitivity Between Guinea Pig and Chinchilla

Abstract

Species differences in (a) acoustic impedance at the tympanic membrane and (b) the ratio of sound pressure at the tympanic membrane to sound pressure in the field (Pd/Pf) contribute significantly to species differences in (1) behavioral auditory thresholds and (2) sensitivity to noise-induced cochlear fatigue. Chinchillas exposed continuously for two or more days to an octave band of noise centered at 0.5 kHz at 105 dB SPL or centered at 4.0 kHz at 80 dB SPL show shifts in behavioral auditory thresholds of more than 50 dB at appropriate frequencies and corresponding loss of sensitivity for cochlear microphonic responses. Guinea pigs show only trivial loss of CM under the same exposure conditions. CM measurements indicate that in the chinchilla the transmission ratio, bulla closed/bulla open, is approximately 0 dB, but at low frequencies in the guinea pig transmission ratios of about −18 dB corresponding to impedance changes described by Zwislocki [J. Acoust. Soc. Amer. 35, 1034 (1963)] are confirmed. Also, in the guinea pig resonances for Pd/Pf occur at 7.5 to 10 kHz in contrast to the resonances at 2.5 to 5 kHz in the chinchilla [G. von Bismarck, M. S. Thesis, MIT, Cambridge, Mass. (1967)].