Simultaneous Measurement of Noise-Activated Middle-Ear Muscle Reflex and Stimulus Frequency Otoacoustic Emissions

Abstract

Otoacoustic emissions serve as a noninvasive probe of the medial olivocochlear (MOC) reflex. Stimulus frequency otoacoustic emissions (SFOAEs) elicited by a low-level probe tone may be the optimal type of emission for studying MOC effects because at low levels, the probe itself does not elicit the MOC reflex J. Assoc. Res. Otolaryngol. 4:521]. Based on anatomical considerations, the MOC reflex activated by ipsilateral acoustic stimulation (mediated by the crossed olivocochlear bundle) is predicted to be stronger than the reflex to contralateral stimulation. Broadband noise is an effective activator of the MOC reflex; however, it is also an effective activator of the middle-ear muscle (MEM) reflex, which can make results difficult to interpret. The MEM reflex may be activated at lower levels than measured clinically, and most previous human studies have not explicitly included measurements to rule out MEM reflex contamination. The current study addressed these issues using a higherfrequency SFOAE probe tone to test for cochlear changes mediated by the MOC reflex, while simultaneously monitoring the MEM reflex using a lowfrequency probe tone. Broadband notched noise was presented ipsilaterally at various levels to elicit probetone shifts. Measurements are reported for 15 normal-hearing subjects. With the higher-frequency probe near 1.5 kHz, only 20% of subjects showed shifts consistent with an MOC reflex in the absence of an MEM-induced shift. With the higher-frequency probe near 3.5 kHz, up to 40% of subjects showed shifts in the absence of an MEM-induced shift. However, these responses had longer time courses than expected for MOC-induced shifts, and may have been dominated by other cochlear processes, rather than MOC reflex. These results suggest caution in the interpretation of effects observed using ipsilaterally presented acoustic activators intended to excite the MOC reflex.

The Effect of Contralateral Acoustic Stimulation on Spontaneous Otoacoustic Emissions

“…It was assumed that the 226 Hz probe tone is only sensitive to the acoustic impedance change of the middle ear induced by the MEM reflex because SFOAEs are extremely difficult to measure at this low-frequency range (Goodman and Keefe 2006). The choice of the 602-Hz probe tone was based on Feeney et al (2003) which showed that the greatest change in reflectance due to the MEM reflex was approximately around this frequency.…”

“…These results were consistently lower than those obtained with the commercial middle-ear analyzer (Interacoustics Audio Traveller AA220). Two previous papers have developed SFOAE-based techniques to measure the MEM reflex threshold Goodman and Keefe 2006). In both of these publications, the lowest elicitor level shown to evoke the MEM reflex was between 55 and 60 dB SPL.…”

“…The range of temporal characteristics of the MOC and MEM reflexes, and the overlap between them makes it difficult to isolate their influence on OAEs (Goodman and Keefe 2006). As has been discussed above, the relative contribution of the MEM reflex to OAE shifts has been found to be species-dependent in laboratory animals.…”

“…First, the clinical MEM reflex thresholds vary with the sensitivity of the instrumentation and the testing procedures (Gelfand 1984). Moreover, thresholds measured using clinical instruments have been consistently found to be substantially higher than those measured using a variety of experimental techniques (Neumann et al 1996;Feeney et al 2003;Guinan et al 2003;Muller-Wehlau et al 2005;Goodman and Keefe 2006). However, most studies using contralateral noise elicitors have not controlled for the MEM reflex stringently, leaving their results difficult to interpret.…”

Loudness functions with air and bone conduction stimulation in normal-hearing subjects using a categorical loudness scaling procedure

“…Also, the sound transmission through the outer and middle ear was expected to be linear (Merchant et al, 1996) with the exception of the acoustic reflex that may have affected the transmission at the higher levels for the low-frequency sound (Neumann et al, 1996;Zhao et al, 2010). It has been shown that the acoustic reflex may be elicited at stimulation levels down to 50 to 60 dB SPL (Goodman et al, 2006;Zhao et al, 2010). Consequently, the acoustic reflex may influence the sound transmission for the low-frequency sound at levels above 50 dB SPL.…”