Measures of the auditory brain-stem response (ABR), distortion product otoacoustic emission (2f1-f2), hair cell loss, and forward masked tuning curves were obtained from waltzing guinea pigs and their age-matched controls at postnatal day 2, 9, 15, and 30. A mild ABR threshold shift (10-15 dB) is seen by 2 days postnatal and gradually increases to a more severe threshold shift (40-50 dB) by postnatal day 15. Already by 30 days the auditory brain-stem response, being beyond the output of the instrumentation, could not be elicited. The mean distortion product otoacoustic emission (DPOE) amplitude as a function of f1 amplitude for the postnatal day 2, 9, and 15 waltzing guinea pigs were only between 3 and 8 dB below the control values for stimulus levels below 65 dB SPL. The DPOE audiogram constructed for the waltzing guinea pigs shows no more than an 8-dB mean difference from the control values when the intensity of f1 was 50 dB SPL, and no more than 10 dB when f1 was 60 dB SPL. Analysis of the individual cases revealed that the DPOE amplitude could be greater than control values. On the contrary, when f1 stimulus levels were below 65 dB SPL, DPOEs could not be detected for the postnatal day 30 waltzing guinea pigs. At stimulus levels above 65 dB SPL, DPOEs could be recorded yet these responses were depressed from control values by 10 to 25 dB. Analysis of surface preparations of the organ of Corti from the day 15 waltzing guinea pig reveals that the prominent alteration occurs on the third row outer hair cells. To a lesser extent, the second row outer hair cells, and then the first row outer hair cells are affected while the inner hair cells appear normal. In contrast, the organ of Corti from the postnatal day 30 waltzing guinea pig showed a more extensive outer hair cell loss among all three rows as well as a considerable degree of inner hair cell loss. Forward masked auditory brain-stem response tuning curves were generated at 8, 4, and 1 kHz for control and waltzing guinea pigs between 2 and 12 days of age. All tuning curves obtained from waltzing guinea pigs showed progressive decreases in sensitivity with increasing age. The Q 10 dB values of the 1- and 4-kHz tuning curves were not different between the controls and the waltzing guinea pigs at any age.(ABSTRACT TRUNCATED AT 400 WORDS)

The acoustic stapedius reflex (ASR) threshold on non-anesthetized rabbits was compared to some measures of the single auditory nerve fiber activity of rabbits. The observations were made on normal-hearing animals, with some additional data from noise-exposed individuals. The results showed that the ASR threshold was reached at a sound level above saturation of discharged rate for individual neurons at their characteristics frequency (CF) in normal animals. It was found, on the other hand, that the ASR threshold measured across frequencies from 0.25 to 12.0 kHz were at a level similar to that of the tails of the frequency tuning curves (FTCs). Cochlear lesions-induced changes in FTC tail levels were paralleled by changes in ASR threshold levels. The raise of ASR threshold was, however, somewhat larger than the raise of the tails which might be explained by the significant relative decrease in the total number of units found in the frequency region corresponding to the lesion. There was also a decrease in the high spontaneous rate (SR) compared to the low and medium SR fibers for higher frequencies. It is concluded that the FTC tails can be a major eighth-nerve correlate to ASR activation.

Neural activity of single eighth nerve fibers was recorded with glass microelectrodes in anesthetized normal-hearing rabbits. The units had a spontaneous rate ranging from 0 to approximately 120 spikes/s. In a large number of fibers this rate was below 2 spikes/s. The frequency tuning curves (FTCs) had a tip and a tail region for fibers with a high or medium characteristic frequency (CF). For low-frequency units the FTC was more symmetrically U-shaped. The tip threshold reached the behavioral threshold and units with thresholds of up to more than 60 dB above the mean behavioral threshold were found. There was a weak negative correlation between spontaneous rate and tip threshold. Frequency selectivity, Q10, was about 2 for units with CF below 2 kHz and about 5 for those with CF above 4 kHz. The peristimulus time (PST) histogram showed an initial peak, a plateau, and poststimulus inhibition. For the majority of fibers the dynamic range was 20–30 dB. Some fibers did not reach saturation within the stimulus intensity available. The tip-to-tail distance was 50 dB for high-frequency units at one octave below CF, a matter of potential interest for further studies of animals with inner ear lesions.