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 thresholds'' 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 mid 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. When the different exposures were ''titrated'' to compare conditions producing similar threshold losses, for low-damage exposures protection was obtained only for 11- and 15-kHz exposures; for moderately damaging exposures protection extended to higher but not lower frequencies; and finally, for severely damaging exposures ah exposure frequencies from 3 to 20 kHz elicited protection. Post hoc analysis revealed that this variation in activation threshold for OCB-mediated protection could be related to normal hearing sensitivity at the cochlea, as assessed by the compound action potential audiogram and to the damaging capacity of different exposure frequencies. 4. The brain stem incision used thus far did not differentiate between different components of the OCB. To determine the involvement of specific OCB subsets in protection, brain stem cuts were placed to cut the entire OCB to one cochlea and only the crossed OCB (COCB) to the other cochlea before a binaural high-frequency exposure. Data from this group showed that the COCB was critically involved in the OCB-mediated protection. 5. An inference from earlier analyses was that monaural exposures did not activate OCB-mediated protection. Testing with consecutive monaural exposure to the two ears in a group in which the OCB had been cut to only one ear explicitly confirmed that monaural exposure did not suffice to activate protection in these barbiturate-anesthetized cats. 6. Finally, although the activation threshold for protection var led with exposure frequency, for all exposure frequencies once this threshold had been exceeded there was a strong linear relationship between the amount of protection and the threshold loss that would otherwise occur. When the relationship for 11-kHz exposures in cats in this study was compared with that seen for 10-kHz exposures in previous studies in guinea pigs, there was very good similarity, showing the generality of this relationship across the two species for similar exposure frequencies.