Frequency difference limens in normal and sensorineural hearing impaired chinchillas

This study assessed normal frequency discrimination ability in the chinchilla and determined how this ability changes as a function of an experimentally induced sensorineural hearing loss. Four chinchillas were trained by the methods of positive reinforcement to report absolute thresholds and frequency difference limens (FDLs). Subjects were then treated with the aminoglycosidic antibiotic amikacin until a 30-dB hearing loss was measured at 10.0 kHz. Absolute and frequency difference thresholds were determined during and after drug treatment. When post-drug thresholds had stabilized, subjects were sacrificed and their cochleas stained, embedded in plastic, microdissected, and viewed with phase contrast microscopy to permit examination of the cochlear tissue. Post-drug data suggest that frequency discrimination at a high frequency is unaffected by a 40- to 45-dB sensorineural hearing loss, considerable hair cell damage, and the resultant disruption of the cochlear micromechanics. The data, in concert with previously published reports, suggest that FDLs may be less affected by a high-frequency sensorineural hearing loss than by a low-frequency sensorineural hearing loss.