Considering distortion product otoacoustic emission fine structure in measurements of the medial olivocochlear reflex

Abstract

In humans, when the medial olivocochlear ͑MOC͒ pathway is activated by noise in the opposite ear, changes in distortion product otoacoustic emission ͑DPOAE͒ level, i.e., the MOC reflex, can be recorded in the test ear. Recent evidence suggests that DPOAE frequency influences the direction ͑suppression/enhancement͒ of the reflex. In this study, DPOAEs were recorded at fine frequency intervals from 500 to 2500 Hz, with and without contralateral acoustic stimulation ͑CAS͒ in a group of 15 adults. The MOC reflex was calculated only at DPOAE frequencies corresponding to peaks in the fine structure. Additionally, inverse fast-Fourier transform was conducted to evaluate MOC effects on individual DPOAE components. Results show the following: ͑1͒ When considering peaks only, the mean MOC reflex was −2.05 dB and 97% of observations reflected suppression, ͑2͒ CAS reduced distortion characteristic frequency component levels more than overlap component levels, and ͑3͒ CAS produced an upward shift in fine structure peak frequency. Results indicate that when the MOC reflex is recorded at DPOAE frequencies corresponding to fine structure maxima ͑i.e., when DPOAE components are constructive and in phase͒, suppression is reliably observed and level enhancement, which probably reflects component mixing in the ear canal rather than strength of the MOC reflex, is eliminated.

Changes in Distortion Product Otoacoustic Emission Caused by Contralateral Broadband Noise

“…The majority of investigators have reported DPOAE suppression during the presentation of contralateral noise in humans and amplitude level reduction in f DP within the range from 1 to 4 dB, depending on the frequency tested ( As shown in literature, DPgram frequency resolution clearly effects changes in DPOAE level at selected points of maxima and minima. For high frequency resolution, suppression occurs more often and is larger at fine-structure maxima, whereas enhancement of level is more likely to occur near the minima (Sun, 2008;Abdala et al, 2009). Frequency dependence of the DPOAE suppression is a consequence of the MOC effects providing frequency-specific inhibitory feedback in the wide frequency region.…”

“…Such interaction, which has such a strong effect, depends on frequency and leads to the ripple shapes of DPOAE fine structure, and accordingly to a large inter-and intra-subject variability of DPOAEs. This mechanism contributes to the complex differences in the DPOAE levels measured with or without CAS which may manifest themselves in the form of suppression or enhancement (Abdala et al, 2009).…”

“…It is also not clear how to apply DPOAE fine structure to measure the DPOAE contralateral effect since the optimal frequency resolution is not known. Abdala et al (2009) suggested recording the MOC reflex only at fine structure peak frequencies. One can expect that the higher the resolution, the more rippled and complex the DPgram would be.…”

“…For sound level ranges, where OAEs monotonically increased, MOC-induced suppression of the OAEs was observed. However, for the OAEs measured at DPOAE dips, MOC effects often show enhancement of the OAEs for some frequencies Muller et al, 2005;Abdala et al, 2009). The reason for this is that if the contralateral acoustic stimulation (CAS) induced effects on DPOAEs are measured at dips (where DPOAE components are out of phase), CAS (activating the MOC) can suppress one of the components more than the other and produce a release of cancellation; this in turn produces some sort of enhancement (Abdala et al, 2009;Deeter et al, 2009).…”

Estimation of Round-Trip Outer-Middle Ear Gain Using DPOAEs

“…Efferent activation affects the outer hair cells amplification and therefore the DPOAE level; hence, it might impact the OMEG estimates. It has been shown that the impact of efferents is minimal on the generator components in comparison with their effects on the composite DPOAEs (Abdala et al 2009;Henin et al 2011Henin et al , 2014. Furthermore, the efferents impact on the generator components is reduction of the level by upto 1.5 dB, while the efferent effect on the composite DPOAE is less systematic and can lead to enhancement or reduction of the level by about 5 dB (Henin et al 2011).…”