Behavioral studies of the olivocochlear efferent system - Learning to listen in noise

Background: Olivocochlear (OC) neurons make up an efferent, descending auditory system that returns sound representations to the inner ear soon after they have entered the brain. Efferent inputs into the cochlea modulate outer hair cell activity to improve the neural encoding of auditory signals in background noise. Based on this physiological evidence, loss of efferent feedback is expected to degrade perception in noise. Attempts to confirm this prediction with long-term audiological assessments have met with mixed results. Objective: To isolate procedural factors that may diminish the demonstration of long-term OC deficits in listening tasks. Design: Operant conditioning procedures were used to train domestic cats to signal a change in the location of an auditory stimulus by responding on a lever. The smallest detectable change in location was measured by manipulating the distance between speakers under quiet conditions and in the presence of background noise. Functional consequences of efferent feedback were evaluated by comparing the sound localization thresholds of OC-lesioned cats with normal controls. Results: As predicted by the hypothesized function of OC feedback systems, the lesioned cats exhibited significantly elevated thresholds only when tested in background noise. This initially poor performance returned to normal values after long-term exposure to the testing procedure. Conclusions: The results of our animal studies support the OC enhancement of sound localization behavior in background noise. Also, our behavioral observations suggest the acquisition of alternate listening strategies that allowed lesioned cats to minimize the functional consequences of their auditory deficits by attending more closely to remaining directional cues. These learned compensatory behaviors were encouraged by our present experimental design, which incorporated long-term training under consistent stimulus conditions. These findings point out the potential limitations of the highly routine audiological procedures that have been used to assess the impact of OC feedback on human hearing.