Frequency and Loss Dependence of the Protective Effects of the Olivocochlear Pathways in Cats

1. In the previous manuscript I suggested a frequency dependency to olivocochlear bundle (OCB)-mediated protection from loud sound by showing protection for binaural compared with monaural 11-kHz exposures but not 3-kHz exposures of the same intensity and duration. To determine whether this was the case, experiments were carried out in barbiturate-anesthetized cats using the paradigm of a unilateral brain stem incision to deefferent one cochlea in each animal before presentation of a binaural loud sound exposure. With equal-intensity, equal-duration binaural exposures, in different groups protection in OCB-intact compared with OCB-cut ears was seen only for exposures at 11, 15, or 20 kHz, but not at 3 or 7 kHz, suggesting that OCB-mediated protection was found only for higher-frequency exposures. This would be consistent with the OCB-mediated protection in guinea pig studies where 10-kHz exposures were used and its absence in a study in cats where 6-kHz exposures were used. However, this conclusion had to be qualified by the fact that the lower-frequency exposures resulted in smaller threshold losses than did the higher-frequency exposures. 2. To determine whether OCB-mediated protection could be obtained for lower-frequency exposures that were made as damaging as or more damaging than the high-frequency exposures, longer-duration, lower-frequency exposures were used. OCB-mediated protection could then be obtained for exposure at 7 kHz, 100 dB SPL for 15 min but not at 3 or 5 kHz, 100 dB SPL for 20 min or at 3 kHz, 100 dB SPL for 40 min or 106 dB SPL for 20 min. Finally, when large threshold losses were produced with exposure at 3 kHz, 106 dB SPL for 40 min, OCB-mediated protection could be obtained for this low-frequency exposure too. These effects suggested that there were different "activation threshold" for OCB-mediated protection as a function of exposure frequency. To determine whether this also applied for the higher-frequency exposures (11, 15, and 20 kHz), all of which had elicited OCB-mediated protection when presented at 100 dB SPL for 10 min, these exposure frequencies were presented at 100 dB SPL for 7 min to produce low threshold losses. Now protection was found for the 11- and 15-kHz exposures but not for the 20-kHz exposure. 3. Thus the activation threshold for OCB-mediated protection varied in a frequency-dependent manner.(ABSTRACT TRUNCATED AT 400 WORDS)