MEASURES OF AUDITORY BRAIN-STEM RESPONSES, DISTORTION-PRODUCT OTOACOUSTIC EMISSIONS, HAIR CELL LOSS, AND FORWARD MASKED TUNING CURVES IN THE WALTZING GUINEA-PIG

Measures of the auditory brain-stem response (ABR), distortion product otoacoustic emission (2f1-f2), hair cell loss, and forward masked tun ing curves were obtained from waltzing guinea pigs and their age-match ed controls at postnatal day 2, 9, 15, and 30. A mild ABR threshold sh ift (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 o f the instrumentation, could not be elicited. The mean distortion prod uct otoacoustic emission (DPOE) amplitude as a function of f1 amplitud e for the postnatal day 2, 9, and 15 waltzing guinea pigs were only be tween 3 and 8 dB below the control values for stimulus levels below 65 dB SPL. The DPOE audiogram constructed for the waltzing guinea pigs s hows 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 am plitude 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 6 5 dB SPL, DPOEs could be recorded yet these responses were depressed f rom 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 t he prominent alteration occurs on the third row outer hair cells. To a lesser extent, the second row outer hair cells, and then the first ro w outer hair cells are affected while the inner hair cells appear norm al. In contrast, the organ of Corti from the postnatal day 30 waltzing guinea pig showed a more extensive outer hair cell loss among all thr ee rows as well as a considerable degree of inner hair cell loss. Forw ard masked auditory brain-stem response tuning curves were generated a t 8, 4, and 1 kHz for control and waltzing guinea pigs between 2 and 1 2 days of age. All tuning curves obtained from waltzing guinea pigs sh owed progressive decreases in sensitivity with increasing age. The Q 1 0 dB values of the 1- and 4-kHz tuning curves were not different betwe en the controls and the waltzing guinea pigs at any age. However, comp ared to the controls, the Q 10-dB values of the 8-kHz tuning curves we re reduced by 50% for the waltzing guinea pigs.