Medial Olivocochlear Efferent Reflex in Humans: Otoacoustic Emission (OAE) Measurement Issues and the Advantages of Stimulus Frequency OAEs

Abstract

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.

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

“…Otoacoustic emissions (OAEs) are low-level sounds produced in healthy cochleae that can be measured noninvasively in humans with a sensitive microphone in the ear canal. SFOAEs were used because (1) they are the most frequency specific of any OAE and (2) SFOAEs can be evoked by a single low-level (40 dB SPL) probe tone which, by itself, elicits little MOC activity (Guinan et al 2003). MOC effects were quantified by the change produced in SFOAEs, ΔSFOAE.…”

“…The SFOAE produced by the probe tone alone, referred to as the "baseline SFOAE", was measured by the suppression method (Guinan 1990;Kalluri and Shera 2007). With this method, a second tone was presented at a frequency near the probe tone and a level 20 dB, or more, above the probe-tone level.…”

“…Multiple data blocks were averaged until the SNR criterion was reached and at least N≥12 stimulus repetitions were included. The averaged waveforms were then heterodyned (heterodyning is equivalent to sending the signal through a lock-in amplifier; for more details, see Guinan et al 2003) to obtain P(t), the magnitude and phase of the sound pressure at the probe frequency as functions of time over one repetition period (note: the magnitude and phase are expressed by the single complex time function P(t)).…”

“…An MOC elicitor, or a suppressor, can induce a change in SFOAE(t) and thereby change P(t). Changes in the SFOAE(t), ΔSFOAE(t), were calculated from P(t) by subtracting P baseline from P(t), where P baseline is the vector average of P(t) between 50 and 450 ms, i.e., the average P(t) before the onset of the suppressor or MOC elicitor (see Guinan et al 2003 for more detail). The MOC-induced change was then expressed as the normalized ΔSFOAE(t), ΔSFOAEn(t), i.e., as the change in the SFOAE normalized by the magnitude of the SFOAE.…”

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

“…MOC neurons can be activated by acoustic stimuli (Robertson and Gummer 1985), and there is a wealth of evidence that contralateral stimulation reduces the levels of human otoacoustic emissions, a physiological indicator of cochlear function (Collet et al 1990;Souter 1995;Guinan et al 2003;Atcherson et al 2008;Sun 2008;Lilaonitkul and Guinan 2009a, b;Francis and Guinan 2010). Psychoacoustical tuning curves (PTCs) (Small 1959) are regarded as the behavioral correlate of physiological cochlear tuning curves (Evans 2001;Shera et al 2002).…”