Study of the medial olivocochlear efferent system through otoacoustic emissions

The purpose of this study was to investigate the efferent medial olivocochlear system through the use of otoacoustic emissions and especially the modifications that occur at this level with age. The activity of outer hair cells (OHCs) (peripheral) and of the efferent medial olivocochlear (MOC) system (neural and central) was examined by means of distortion product otoacoustic emissions (DPOAEs). For comparison, overall hearing capability was captured by means of pure-tone audiometry. In 75 subjects (10-82 years), high-resolution (∆f2 = 47 Hz) pure-tone threshold and DPOAE fine structure was recorded between 3 and 6 kHz using the same ear probe. DPOAE thresholds were estimated by extrapolation of DPOAE input/output functions. Efferent MOC reflex strength was determined by means of DPOAEs at selected frequencies with and without contralateral acoustic stimulation. DPOAE levels and thresholds decreased with age whereas the decrease in efferent reflex strength was less evident exhibiting also large MOC reflexes in elder subjects. There was a good correlation between DPOAE thresholds and pure-tone thresholds also in the elderly. These findings suggest that the aging process predominantly occur at peripheral stages of the auditory pathway. The MOC system originates from neurons in medial and ventral regions of the superior olivary complex and contains myelinated fibres that almost exclusively synapse on OHCs and thus is able to control OHC motility. The MOC system consists of the crossed MOC fibers and the uncrossed MOC fibers, which can be stimulated by a sound ipsilaterally respectively contralaterally. In the present study DPOAE fine structure, CAS DPOAE, and pure-tone threshold fine structure were measured in humans of different age in order to find out whether more peripheral or central processes are involved in presbycusis. Measurement of the fine structure was done for two purposes. First, high resolution measures are more able to reveal minute changes in sound processing of their respective stages on the auditory pathways. Thus, a comparison of the fine-structures of both measures should allow a better differentiation between peripheral and central processes involved in presbycusis. Second, DPOAE fine-structure was used to determine the specific frequency at which CAS DPOAE are known to yield large enhancement/suppression effects. When comparing DPOAEs (which are capable of evaluating peripheral hearing performance), to pure-tone thresholds (which comprise both peripheral and central hearing capability), sensory and central presbycusis may be differentiated (Gates et al., 2002). If sensory processes were involved both pure-tone threshold and DPOAE level would decline in the same way. If exclusively central disorders were the cause for presbycusis, only pure-tone thresholds would be affected. In this case a distinct hearing loss but nearly normal DPOAEs would appear. Animal studies have shown that DPOAEs were less affected than neural thresholds in cases where the endocochlear potential decreased. Thus if metabolic processes were the reason for presbycusis a larger decline in pure-tone hearing thresholds would appear. (Mills et al., 1993). Age related disorders of the afferent and efferent auditory pathway, if present, may be assessed by determining the reflex strenght of the medial olivocohlear (MOC) system DPOAEs were measured between f2 = 3 and 6 kHz with a frequency resolution of f2 = 47 Hz and plotted in form of the DPOAE gram (see example in Fig. 1b). The primary tone level L2 was set to 60, 50, 40, 30 and 20 dB SPL, whereas L1 was set according to the equation L1 = 0.4 L2 + 39 dB SPL (Janssen et al., 1995; Kummer et al., 2000). This level setting was used to account for the nonlinear interaction of the two primaries at the DPOAE generation site at the f2 place (Kummer et al., 2000). The measuring duration for one ear amounted on average to about 30 minutes. For assessing the reflex strength of the efferent MOC system, DPOAEs were measured with and without contralateral acoustic stimulation (CAS) at two specific frequencies. The first frequency, f2,dip, was located at a distinct dip in the DPOAE fine structure, while the second frequency, f2,flat, was at a location, where there was no major change of Ldp across frequency, i.e. in a flat region of the DPOAE fine structure. This was done, since contralateral effects were found to be different in magnitude in dips and flat regions of the DPOAE fine structure (Müller et al., 2005; Wagner et al., 2007