Time-course of the human medial olivocochlear reflex

Abstract

The time-course of the human medial olivocochlear reflex (MOCR) was measured via its suppression of stimulus-frequency otoacoustic emissions (SFOAEs) in nine ears. MOCR effects were elicited by contralateral, ipsilateral or bilateral wideband acoustic stimulation. As a first approximation, MOCR effects increased like a saturating exponential with a time constant of 277+/-62 ms, and decayed exponentially with a time constant of 159+/-54 ms. However, in ears with the highest signal-to-noise ratios (4/9), onset time constants could be separated into "fast," tau= approximately 70 ms, "medium," tau = approximately 330 ms, and "slow," tau = approximately 25 s components, and there was an overshoot in the decay like an under-damped sinusoid. Both the buildup and decay could be modeled as a second order differential equation and the differences between the buildup and decay could be accounted for by decreasing one coefficient by a factor of 2. The reflex onset and offset delays were both approximately 25 ms. Although changing elicitor level over a 20 dB SPL range produced a consistent systematic change in response amplitude, the time course did not show a consistent dependence on elictor level, nor did the time-courses of ipsilaterally, contralaterally, and bilaterally activated MOCR responses differ significantly. Given the MOCR's time-course, it is best suited to operate on acoustic changes that persist for 100's of milliseconds.

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

“…Under natural listening conditions, the MOCR may be elicited by sound through both ipsilateral and contralateral pathways (Guinan 2006). Thus, the total amount of gain reduction by the reflex depends on the combination of MOC activity in both pathways.…”

“…In the environment, the MOCR in one ear would typically be evoked by sound coming into that ear (ipsilateral stimulation) and also sound coming into the other ear (contralateral stimulation). In cats, the ipsilateral reflex is two to three times stronger than the contralateral reflex, although in humans, they may be closer to equal (Guinan 2006). Although the magnitude of the ipsilateral effect due to shocking the MOC bundle has been measured, it is methodologically more difficult to evaluate quantitatively the ipsilateral effect using sound as an elicitor.…”

“…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.…”

“…In the study, the continuous ipsilateral noise likely elicited the MOCR itself; thus, presenting the contralateral noise elicited an increase in MOCR strength relative to the baseline set by the ipsilateral noise. This contralateral increase may underestimate the magnitude of both the ipsilateral and the combined MOCR, since in cats the ipsilateral reflex is two to three times stronger than the contralateral reflex (Guinan 2006).…”

“…For example, shocking the MOC bundle has been shown to decrease the gain of the basilar membrane (BM) input-output (I/O) function and to raise the tip of the BM tuning curve (Murugasu and Russell 1996). Other studies have elicited the MOCR by presenting contralateral sound, which takes advantage of the fact that the MOCR is a bilateral reflex and has been used to study potential perceptual benefits of the MOCR (Guinan 2006). For example, the MOCR has been reported to produce an anti-masking effect in physiological responses when short signals are presented in noise (Nieder and Nieder 1970;Dolan and Nuttall 1988; but also see Scharf et al 1997).…”