Otoacoustic emissions (OAEs) are useful for studying medial olivocochlear (MOC) efferents, but several unresolved methodological issues cloud the interpretation of the data they produce. Most efferent assays use a ''probe stimulus'' to produce an OAE and an ''elicitor stimulus'' to evoke efferent activity and thereby change the OAE. However, little attention has been given to whether the probe stimulus itself elicits efferent activity. In addition, most studies use only contralateral (re the probe) elicitors and do not include measurements to rule out middle-ear muscle (MEM) contractions. Here we describe methods to deal with these problems and present a new efferent assay based on stimulus frequency OAEs (SFOAEs) that incorporates these methods. By using a postelicitor window, we make measurements in individual subjects of efferent effects from contralateral, ipsilateral, and bilateral elicitors. Using our SFOAE assay, we demonstrate that commonly used probe sounds (clicks, tone pips, and tone pairs) elicit efferent activity, by themselves. Thus, results of efferent assays using these probe stimuli can be confounded by unwanted efferent activation. In contrast, the single 40 dB SPL tone used as the probe sound for SFOAEbased measurements evoked little or no efferent activity. Since they evoke efferent activation, clicks, tone pips, and tone pairs can be used in an adaptation efferent assay, but such paradigms are limited in measurement scope compared to paradigms that separate probe and elicitor stimuli. Finally, we describe tests to distinguish middle-ear muscle (MEM) effects from MOC effects for a number of OAE assays and show results from SFOAE-based tests. The SFOAE assay used in this study provides a sensitive, flexible, frequency-specific assay of medial efferent activation that uses a low-level probe sound that elicits little or no efferent activity, and thus provides results that can be interpreted without the confound of unintended efferent activation.

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.