Abstract
The present study aimed at characterizing the suppressing effect of contralateral medial olivocochlear (MOC) efferents on human auditory sensitivity and mechanical cochlear responses at sound levels near behavioral thresholds. Absolute thresholds for pure tones of 500 and 4000 Hz with durations between 10–500 ms were measured in the presence and in the absence of a contralateral broadband noise. The intensity of the noise was fixed at 60 dB SPL to evoke the contralateral MOC reflex without evoking the middle-ear muscle reflex. In agreement with previously reported findings, thresholds measured without the contralateral noise decreased with increasing tone duration, and the rate of decrease was faster at 500 than at 4000 Hz. Contralateral stimulation increased thresholds by 1.07 and 1.72 dB at 500 and 4000 Hz, respectively. The mean increase (1.4 dB) just missed statistical significance (p = 0.08). Importantly, the across-frequency mean threshold increase was significantly greater for long than for short probes. This effect was more obvious at 4000 Hz than at 500 Hz. Assuming that thresholds depend on the MOC-dependent cochlear mechanical response followed by an MOC-independent, post-mechanical detection mechanism, the present results at 4000 Hz suggest that MOC efferent activation suppresses cochlear mechanical responses more at lower than at higher intensities across the range of intensities near threshold, while the results at 500 Hz suggest comparable mechanical suppression across the threshold intensity range. The results are discussed in the context of central masking and of auditory models of efferent suppression of cochlear mechanical responses.
Highlights
Physiological studies in non-human mammals have shown that activation of olivocochlear efferents suppresses cochlear mechanical responses
Assuming that thresholds depend on the medial olivocochlear (MOC)-dependent cochlear mechanical response followed by an MOC-independent, post-mechanical detection mechanism, the present results at 4000 Hz suggest that MOC efferent activation suppresses cochlear mechanical responses more at lower than at higher intensities across the range of intensities near threshold, while the results at 500 Hz suggest comparable mechanical suppression across the threshold intensity range
A first question is whether the contralateral broadband noise (CBN) increases absolute thresholds, as would be expected (e.g., Figure 1), and whether the magnitude of the increase is different at the two probe frequencies or for different probe durations
Summary
Physiological studies in non-human mammals have shown that activation of olivocochlear efferents suppresses cochlear mechanical responses. Otoacoustic emissions (OAEs) are a byproduct of cochlear mechanical responses (Kemp, 1978, 2002) For these reasons, researchers have often looked at the suppression of OAEs by contralateral stimulation as a way to physiologically characterize the suppressing effects of the contralateral medial olivocochlear reflex (MOCR) in humans (Guinan et al, 2003; Atcherson et al, 2008; Sun, 2008; Lilaonitkul and Guinan, 2009a,b; Francis and Guinan, 2010). Few studies have looked at the suppressing effects of the contralateral MOCR on human auditory sensitivity.
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