Age-related changes in auditory evoked potentials of gerbils. III. Low-frequency responses and repetition rate effects.

The auditory brainstem response (ABR) was recorded non-invasively from Mongolian gerbils ranging in age from 6 to 36 months. The ABR was elicited using gaussian tone bursts at octave intervals from 1 to 16 kHz. Responses were bandpass filtered from 30 to 300 Hz (LF-ABR; low-frequency component) and from 300 to 3000 Hz (HF-ABR; high-frequency component). In Experiment A, the thresholds of the two components (HF- and LF-ABR) were compared in 6- and 36-month subjects. The LF-ABR varied more with age than did the HF-ABR, particularly at stimulus frequencies of 2 kHz and above. As shown previously for the HF-ABR, the latencies of the LF-ABR increased as a function of hearing loss in aged gerbils whereas amplitudes of the LF-ABR were reduced in all aged gerbils, regardless of age-related threshold elevation. In Experiment B, tone bursts were presented at rates of 11-91/s to gerbils aged 6, 18, 30, and 36 months. Increased repetition rate resulted in an increase in the latency of both the HF- and LF-ABR, but to the same degree in each age group. Similarly, the interpeak intervals of the HF-ABR increased as a function of repetition rate in all subjects to the same degree. Increased age and increased repetition rate both resulted in significant reductions in ABR amplitudes, but rate did not interact with age. The data suggest that (1) the LF-ABR may be more sensitive to aging than is the HF-ABR and (2) there are no age-related changes in the HF- or LF-ABR which are dependent upon the repetition rate.