Abstract
The active cochlear mechanism amplifies responses to low-intensity sounds, compresses the range of input sound intensities to a smaller output range, and increases cochlear frequency selectivity. The gain of the active mechanism can be modulated by the medial olivocochlear (MOC) efferent system, creating the possibility of top-down control at the earliest level of auditory processing. In humans, MOC function has mostly been measured by the suppression of otoacoustic emissions (OAEs), typically as a result of MOC activation by a contralateral elicitor sound. The exact relationship between OAE suppression and cochlear gain reduction, however, remains unclear. Here, we measured the effect of a contralateral MOC elicitor on perceptual estimates of cochlear gain and compression, obtained using the established temporal masking curve (TMC) method. The measurements were taken at a signal frequency of 2 kHz and compared with measurements of click-evoked OAE suppression. The elicitor was a broadband noise, set to a sound pressure level of 54 dB to avoid triggering the middle ear muscle reflex. Despite its low level, the elicitor had a significant effect on the TMCs, consistent with a reduction in cochlear gain. The amount of gain reduction was estimated as 4.4 dB on average, corresponding to around 18 % of the without-elicitor gain. As a result, the compression exponent increased from 0.18 to 0.27.
Highlights
Animal data suggest that the medial olivocochlear (MOC) efferent reflex can exert powerful modulation of peripheral auditory responses, reducing the gain of the active cochlear amplifier (provided by the outer hair cells (OHCs)) by as much as 20–30 dB (Murugasu and Russell 1996)
Studies that have measured MOC-induced suppression of otoacoustic emissions (OAEs) in humans have yielded results that seem to contrast with the animal results, suggesting that the human MOC reflex mainly affects low frequencies, similar to those affected by the middle ear muscle (MEM) reflex (Lilaonitkul and Guinan 2009a, 2012; Zhao and Dhar 2012), and is largely unspecific in frequency, at least when elicited by contralateral sounds
In the upcoming modeling section (BCochlear IO Function Model^ section), we present an analysis suggesting that these effects are caused by different physiological factors: (i) the average temporal masking curve (TMC) slope is determined by the decay rate, μ, of the masker effect over time; (ii) the relative positions along the ordinate of the on- and off-frequency TMCs are determined by the passive attenuation, P, of the off-frequency masker tail response (Fig. 5F); and (iii) the relative slopes of the on- and off-frequency TMCs are determined by the cochlear gain, Gmax, and compression exponent, c, of the on-frequency masker tip response (Fig. 5A, B)
Summary
Animal data suggest that the medial olivocochlear (MOC) efferent reflex can exert powerful modulation of peripheral auditory responses, reducing the gain of the active cochlear amplifier (provided by the outer hair cells (OHCs)) by as much as 20–30 dB (Murugasu and Russell 1996). Animal research has suggested that the MOC reflex is frequency-specific (i.e., can only be elicited by a limited range of frequencies around the probe frequency; Warren and Liberman 1989) and primarily affects frequencies above those affected by the middle ear muscle (MEM) reflex (Guinan and Gifford 1988). This suggests that the MOC reflex may complement the MEM reflex to form a complete barrier against noise insult (Liberman and Guinan 1998). Ipsilateral elicitor effects on OAEs have been found to be more frequency-specific (Lilaonitkul and Guinan 2009a, b, 2012), but it remains unclear, to what degree these effects are caused by intrinsic cochlear (nonefferent) mechanisms (Guinan et al 2003)
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