Age-related changes in auditory evoked potentials of gerbils. I. Response amplitudes.

Auditory brainstem responses (ABRs) were recorded in young (6-10 month) and aged (36 month) Mongolian gerbils. For each subject, ABR thresholds and response amplitudes were measured at octave intervals from 1 through 16 kHz. Data from the young animals served as the baselines for comparison to aged animals which were categorized on the basis of auditory brainstem response (ABR) thresholds. The aged groups included subjects with thresholds (a) at the mean of a pool of 50 aged gerbils, (b) one standard deviation (SD) lower than the mean, (c) one sd higher than the mean, and (d) near normal for young gerbils. The amplitudes of ABR waveforms for the aged gerbils were reduced compared to the young subjects, particularly at high sound pressure levels. This was true even for aged subjects with thresholds similar to those for younger subjects. The slopes of the amplitude-intensity (I/O) functions were shallower in all aged subjects compared to young subjects. The results suggest that ABR amplitudes and I/O slopes decrease as a function of age and that the decreases are not a direct result of loss of auditory sensitivity. The reductions in ABR amplitudes from aged gerbils presumably reflect age-related pathology in the auditory periphery, as previous studies have shown reductions in amplitudes of the compound action potential of aged gerbils.