Hearing in mammals has considerably greater sensitivity than other vertebrates because of the ‘cochlear amplifier’, which uses outer hair cell electromotility to enhance sound transduction at the inner hair cells and hence drive auditory neurotransmission. The cochlear amplifier is controlled by negative neural feedback from the medial olivocochlear (MOC) efferent neurons located bilaterally in the medial superior olivary complex.
The efferent feedback is important for binaural hearing, and hence for sound localization and masking of noise for improved speech recognition. It also plays a protective role during acoustic overstimulation. The MOC reflex is evident as contralateral suppression, where sound in one ear causes a loss of hearing sensitivity in the opposite ear.  Here we describe studies using the peripherin knockout mouse model that show that the sensory drive for the MOC reflex stems from the type II spiral ganglion afferent innervation of the outer hair cells.  This represents a separate auditory coding channel that drives closed-loop negative feedback control of the cochlear amplifier.