Auditory thresholds in rats of different age and strain. A behavioral and electrophysiological study.

The auditory sensitivity of albino rats was determined behaviorally by conditioned suppression of licking and electrophysiologically by the auditory brain stem response (ABR). The behavioral thresholds were obtained with 10-Hz frequency-modulated tones, and the ABR with 1/1- or 1/3-octave filtered sine waves. Individual variability, reproducibility of responses and the influence of sex, age and strain were investigated. The behavioral and electrophysiological techniques were compared in animals with noise-induced high-frequency hearing losses. The results showed the highest degree of normal auditory sensitivity to be around 12-24 kHz. The variability in 20 rats was about 15 dB. In young animals, strain and sex had no influence. A deterioration in high-frequency hearing sensitivity was observed in aged hypertensive rats, whereas normotensive ones showed only minimal changes. The thresholds obtained with the electrophysiological (ABR) technique were 10-20 dB higher than those obtained behaviorally. Both techniques (behavioral and 1/3-octave ABR) assess high-frequency hearing loss equally well. It was concluded that the ABR (1/3-octave filtered sine waves) is suitable for the determination of hearing thresholds and the assessment of hearing loss, at least in the high-frequency range. The ABR technique is especially useful in long-term experiments during which thresholds are determined repeatedly.