Interaural attenuation versus frequency for guinea pig and chinchilla CM response.

Abstract

Interaural attenuation was estimated for the guinea pig and the chinchilla by determining isopotential curves for the cochlear microphonic (CM) response produced by contralateral and by ipsilateral stimulation at frequencies from 300 Hz to 14.3 kHz. Intracochlear electrodes were used to record the CM from the basal turn. For ipsilateral stimulation, bulla-sealed to bulla-open sound pressure ratios showed effects similar to those reported by other investigators: 10–12-dB loss for chinchilla and 20–22-dB loss for the guinea pig for the bulla-sealed condition. The variability of the sound pressure measures, considered across animals, was larger in chinchillas than in guinea pigs, and the variance was greatest at frequencies near those at which the external auditory canals and bulla cavities show resonant peaks. There appears to be additional low-frequency resonances associated with contralateral stimulation for the closed bulla in the chinchilla, largest at 1.1 kHz for frequencies sampled in this report. The interaural attenuation for the chinchilla is largest at low frequencies and is at a minimum of about 34 dB for the open bulla, near 3.0 kHz; in the guinea pig, the minimum interaural attenuation occurs for the closed bulla, about 44 dB at 300 Hz. Subject Classification: 65.42, 65.40, 65.75.