Behavioral Studies of the Olivocochlear Efferent System

Abstract

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.

Mice Lacking the Alpha9 Subunit of the Nicotinic Acetylcholine Receptor Exhibit Deficits in Frequency Difference Limens and Sound Localization

“…Results also agree with studies from medial olivocochlear lesions that showed substantial across subject performance variability and recovery of normal function with repeated testing, suggesting that other pathways can compensate for perceptual deficits (May and McQuone, 1995; Hienz et al, 1998; May et al, 2004). The decrease in MAA in three out of four α9KO mice likely reflects the impairment in normal circuit reorganization that is present in the LSO of these mice as a result of abnormal spontaneous activity before hearing onset (Clause et al, 2014).…”

Auditory Efferents Facilitate Sound Localization in Noise in Humans: Figure 1.

“…They extend the results of an earlier study, in which cats with OC lesions were found to localize sounds in noise less accurately than cats with an intact OC system (May et al, 2004). While OC lesions presumably lead to the elimination— or to a drastic reduction—of OC influences on the cochlea, the current results demonstrate a relationship between OC activity and sound-localization performance in noise under physiological conditions.…”

“…Overall, the current findings lend further support to the view that an important function of the OC reflex is to help protect sound-localization ability against the detrimental effects of background noise, especially at low (i.e., unfavorable) SNRs (May et al, 2004; Smith et al, 2004; Reiss et al, 2011), and they suggest that interindividual variability in OC-reflex strength is one of the factors that can account for interindividual differences in sound-localization performance in noise in normal-hearing human listeners (Best et al, 2005). …”

Separate contributions of enhanced and suppressed sensitivity to the auditory attentional filter

“…Physiological and anatomical evidence supports the notion of a frequencyspecific action of the auditory efferent fibers on cochlear function (Robertson and Gummer, 1985;Liberman and Brown, 1986;Ota et al, 2004). Several behavioral studies in animals have also revealed that the OCB improves performance in listening-in-noise tasks, such as speech recognition (Dewson, 1968;Hienz et al, 1998), sound localization (May et al, 2004), and intensity discrimination (May and McQuone, 1995).…”