Human Medial Olivocochlear Reflex: Effects as Functions of Contralateral, Ipsilateral, and Bilateral Elicitor Bandwidths

Abstract

Animal studies have led to the view that the acoustic medial olivocochlear (MOC) efferent reflex provides sharply tuned frequency-specific feedback that inhibits cochlear amplification. To determine if MOC activation is indeed narrow band, we measured the MOC effects in humans elicited by 60-dB sound pressure level (SPL) contralateral, ipsilateral, and bilateral noise bands as a function of noise bandwidth from 1/2 to 6.7 octaves. MOC effects were quantified by the change in stimulus frequency otoacoustic emissions from 40 dB SPL probe tones near 0.5, 1, and 4 kHz. In a second experiment, the noise bands were centered 2 octaves below probe frequencies near 1 and 4 kHz. In all cases, the MOC effects increased as elicitor bandwidth increased, with the effect saturating at about 4 octaves. Generally, the MOC effects increased as the probe frequency decreased, opposite expectations based on MOC innervation density in the cochlea. Bilateral-elicitor effects were always the largest. The ratio of ipsilateral/contralateral effects depended on elicitor bandwidth; the ratio was large for narrow-band probe-centered elicitors and approximately unity for wide-band elicitors. In another experiment, the MOC effects from increasing elicitor bandwidths were calculated from measurements of the MOC effects from adjacent half-octave noise bands. The predicted bandwidth function agreed well with the measured bandwidth function for contralateral elicitors, but overestimated it for ipsilateral and bilateral elicitors. Overall, the results indicate that (1) the MOC reflexes integrate excitation from almost the entire cochlear length, (2) as elicitor bandwidth is increased, the excitation from newly stimulated cochlear regions more than overcomes the reduced excitation at frequencies in the center of the elicitor band, and (3) contralateral, ipsilateral, and bilateral elicitors show MOC reflex spatial summation over most of the cochlea, but ipsilateral spatial summation is less additive than contralateral.

Frequency Tuning of the Efferent Effect on Cochlear Gain in Humans

“…The tuning of the efferent effect filter (0.5-0.7 octaves above and below the signal frequency) is within the range obtained in humans using OAEs (Lilaonitkul and Guinan 2009;Zhao and Dhar 2012). 4.…”

“…The frequency range of the efferent effect found in the present study is similar to that observed with the measurement of otoacoustic emissions in humans, using simultaneous or contralateral elicitors. The tuning of the efferent effect can extend 0.5-2 octaves above and below the frequency range of interest (spontaneous OAEs) or probe frequency (stimulus-frequency OAEs), as measured using simultaneous ipsilateral/contralateral tones (Lilaonitkul and Guinan 2009;Zhao and Dhar 2012). In most cases the tone/narrowband noise eliciting the efferent effect has maximal effect above a presentation level of 60 dB SPL, and in some cases the efferent effect has been shown to be more pronounced if the elicitor/precursor sound is about 0.5-1 octave below the probe frequency (e.g., Mott et al 1989;Harrison and Burns 1993).…”

Contralateral Efferent Reflex Effects on Threshold and Suprathreshold Psychoacoustical Tuning Curves at Low and High Frequencies

“…Even if it did, its effects would be identical for PTCs obtained with and without CWN and would cancel out in the comparison [this is unless the effects of ipsiand contralateral MOCR elicitors add up nonlinearly, which is unlikely the case as suggested by Fig. 3C of Lilaonitkul and Guinan (2009a)]. Therefore, the present approach may still be reasonably used to characterize the effect of contralateral sounds on cochlear responses.…”

“…Electrical shocks on the midline olivo-cochlear bundle may have activated both the ipsi-and contralateral MOCR, hence exaggerating the effects. A further piece of evidence in support of contralateral MOCR effects at 4 kHz is that Lilaonitkul and Guinan (2009a) showed that contralateral broadband noise reduced the level of human SFOAE both at 500 Hz and 4 kHz. The magnitude of the reduction was about 2.4 times larger at 500 Hz than at 4 kHz (e.g., their Fig.…”

Modeling the Anti-masking Effects of the Olivocochlear Reflex in Auditory Nerve Responses to Tones in Sustained Noise

“…For example, the ipsilateral noise may have (1) elicited the ipsilateral MOCR, (2) produced an excitatory response in AN fibers, which decreased the change in discharge rate in response to the tone, and (3) suppressed the tone. Although several studies have separated the elicitor from the signal either in time or in frequency in order to be able to compare ipsilateral and contralateral effects Maison and Liberman 2000;Lilaonitkul and Guinan 2009), these approaches based on otoacoustic emissions (OAEs) do not provide a direct estimate of the decrease in OHC gain produced by the ipsilateral MOC pathway when elicited by sound Guinan 2006). examined the effects of the MOCR by presenting a sound contralateral to the measurement ear.…”