**5. New technology**

32 Hearing Loss

Despite decades of investigation since the discovery of the olivocochlear pathway, understanding of its purpose remains somewhat speculative (Rasmussen, 1946). Proposed roles include protection against noise-induced hearing loss, enhancement of discrimination of sound in noise, or a role predominantly during development of the auditory pathway (Micheyl, Khalfa, Perrot, & Collet, 1997; Rajan & Johnstone, 1988; Walsh, McGee, McFadden,

There are a few studies of inter-cochlear interaction in humans which are consistent with MOCS functioning to reduce sensitivity of the cochlea to auditory stimuli. For example, contralateral pure tone stimulation causes a reduction of compound action potentials (Folsom & Owsley, 1987). Contralateral narrow band noise causes a 'negativation' of the summating potential response to ipsilateral tone bursts (i.e. the negative amplitude of summating potential increases) (Innitzer & Ehrenberger, 1977). There are indications that cortical function (e.g. visual or auditory attention tasks) influences olivocochlear activity via descending neural pathways (Froehlich, Collet, & Morgon, 1993; Maison, Durrant,

Much more information on olivocochlear function has come from electrophysiological studies in animal models. Various investigations have supported the conclusion that MOCS activity turns down the gain of the cochlear amplifier (Siegel & Kim, 1982). The cochlear amplifier is an active process within the cochlea in which motor activity of OHCs increases sensitivity of the cochlea, by amplification of the basilar membrane motion induced by acoustic energy. With electrical stimulation of the olivocochlear bundle (OCB) in the floor of 4th ventricle, the amplitude of the compound action potential of the auditory nerve induced by auditory stimuli is reduced (Galambos, 1956; Nieder & Nieder, 1970; Wiederhold & Peake, 1966). In this way, the threshold of the auditory nerve can be increased by as much as 25dB an effect referred to as the 'level shift' (Galambos, 1956). By using focal simulation near the cell bodies of olivocochlear fibers, it has been shown that MOCS mediates this effect (i.e., via action on OHCs), rather than LOCS (Gifford & Guinan Jr, 1987). Electrical stimulation of the OCB increases the cochlear microphonic and causes a decrease in the electrical impedance of scala media of the guinea pig (Mountain, Daniel Geisler, & Hubbard, 1980). These changes are considered to be due to hyperpolarization of outer hair cells (Art, Fettiplace, & Fuchs, 1984; Mountain et al., 1980). Thus electrical stimulation of MOCS suppresses OHC activity so dampening basilar membrane motion and reducing cochlear amplification. This has an indirect effect on IHC activity, as demonstrated by the

Contralateral acoustic stimulation (CAS) has been found to elicit similar effects to electrical stimulation of the MOCS. This was first reported by Fex, who found that CAS increased the cochlear microphonic (Fex, 1962). Recording from the round window in cats, Liberman showed that the compound action potential generated by ipsilateral tone pips was suppressed by contralateral noise or tones. Sectioning of the olivocochlear bundle in the floor of 4th ventricle or in the inferior vestibular nerve abolished this contralateral suppression effect (M. C. Liberman, 1989; Warren III & Liberman, 1989b). Such studies clearly show that the MOCS is stimulated by ascending signals from the auditory

**4.2 Physiology of the olivocochlear pathway** 

& Liberman, 1998).

level shift.

pathway.

Gallineau, Micheyl, & Collet, 2001).
