Predictors of TTS after pure-tone exposure in the cat

Abstract

Anesthetized cats were exposed to 4-kHz pure tones, of varying level and duration. The N1 auditory-nerve responses to filtered clicks, at 2, 4, 6, 8, 10, and 15 kHz, as well as 2f1 − f2 distortion product (DP) signals in the ear canal, were recorded before and after exposure. The f1, f2 pairs were 4.0 and 5.2 kHz for all animals and, additionally, 1.6 and 2.1 kHz and 4.6 and 6.0 kHz in a subset of animals. In all cases, L1 = L2 + 10 dB. Multiple polynomial regression analysis, relating pre-exposure 10-μV N1 thresholds at 4 and 6 kHz, −10-dB SPL DP thresholds for 4.0- and 5.2- and 4.6- and 6.0-kHz primaries and noise dose (total energy) to 6-kHz N1 threshold shift (TS) and DP TS (4.0- and 5.2-kHz primaries), as outcomes. The TS for both N1's and DPs displayed considerable variability between animals, even with the same exposure. Pre-exposure 4- and 6-kHz thresholds were the best predictors of 6-kHz TS (r2 = 0.57 and 0.58, respectively, p = 0.0001) and 6-kHz TS was the outcome that could be predicted with the most certainty. Addition of DP threshold did slightly increase the predictive ability of the model. Pre-exposure N1 threshold only weakly predicted DP TS. The DP TS assessed 6-kHz N1 TS weakly (r2 = 0.27) but significantly (p = 0.014). The poorer ability of DP threshold and DP TS to predict N1, TS, compared to pre-exposure N1 thresholds, is consistent with the variability between animals in DP levels, which are susceptible to abnormalities in forward conduction, sensory function, and reverse conduction of DP signals into the ear canal. [Work supported by Veterans Administration Medical Research Funds.]