Olivocochlear efferents allow the central auditory system to adjust the functioning of the inner ear during active and passive listening. While many aspects of efferent anatomy, physiology and function are well established, others remain controversial. The auditory nervous system is continuously sensing and interpreting the sounds around us. Our ears operate as the sound detectors, transducing acoustic pressure into auditory nerve action potentials, and coding the characteristics of sounds appropriately for further processing by the central auditory system.

The ears, however, do not work as fixed sound receptors. Instead, the central nervous system can adjust their functioning, and thus the coding of sounds, via olivocochlear efferents. Olivocochlear efferents can be activated by selective attention and by sounds presented to either or both ears. Therefore, the functioning of the ears is changing dynamically over time, during natural active and passive listening. 

Anatomy of Olivocochlear Efferents

Olivocochlear efferent fibers originate in the left and right superior olivary complexes (SOCs), a project to the cochlea through the vestibular nerve, enter the basal turn of the cochlea along with auditory nerve afferent fibers, and terminate in the organ of Corti. They were first described by Rasmussen, who originally classified them into crossed and uncrossed types, depending on whether they originated in the contralateral or the ipsilateral SOC, respectively. 

The anatomy of olivocochlear neurons and their projections to the cochlea varies across species. A detailed comparison of differences across species is out of the scope of this review. As reviewed, many aspects. About the anatomy, physiology, and function of olivocochlear efferents have been settled over the last few decades. Many other aspects, however, remain open.

Further research is necessary to address these and other open questions. To this end, cochlear implants offer a novel and potentially useful approach. Cochlear implants restore hearing to some deaf individuals by direct electrical stimulation of the auditory nerve; that is, they effectively function as “artificial ears.” The electrical stimulation provided by a cochlear implant bypasses the OHCs, the site of action of MOC efferents, and is independent from MOC effects.

Unlike natural efferents, the MOC effects mimicked with these strategies can be turned on and off at will, which allows within-subject comparisons of auditory performance with and without MOC efferent effects. In summary, cochlear implants offer interesting possibilities to address some of the open questions regarding the roles of olivocochlear efferents in hearing.