Forward-masking AP tuning curves in normal and in acoustically traumatized ears.

Compound action potential tuning curves (APTC's) were developed in a series of normal and acoustically traumatized cats. Probe frequency was set to 4 kHz ad probe level was set to 15-20 dB SL re: AP threshold for both normal and impaired ears. In addition, normal ears received the probe tone at a level of 70-75 dB SPL. Probe-elicited AP amplitude was measured in a forward-masking paradigm as a function of both masker level and frequency. Results indicated that the sharpness of the APTC's (defined by Q 10) was unaffected by acoustic trauma. The relative difference between required masker level on the tail and at the tip was reduced for the noise-damaged ears. Thus sharpness around the tip was essentially normal yet low-frequency energy was more effective at masking high-frequency region in these impaired ear. When slope of the decrement in normalized, probe-elicited AP amplitude-versus-masker level functions are calculated for different frequency maskers, normal ears showed a frequency dependence such that low-frequency maskers resulted in the steepest slopes with progressively less steep slopes as masker frequency increased. Some of the noise-damaged ears did not show this same frequency dependence. Specifically, the slopes of these functions were not as different for low-and high-frequency maskers. These slope data may suggest that nonlinearities, present in the normal system, may be reduced as a consequence of this cochlear insult.