Human medial olivocochlear reflex: Contralateral activation effect on low and high frequency cochlear response

Journal Pre-proof Human medial olivocochlear reflex: Contralateral activation effect on low and high frequency cochlear response Abdullah Mohammad Jamos, Wafaa A. Kaf, Mark E. Chertoff, John A. Ferraro PII:

S0378-5955(19)30177-7

DOI:

https://doi.org/10.1016/j.heares.2020.107925

Reference:

HEARES 107925

To appear in:

Hearing Research

Received Date: 26 April 2019 Revised Date:

7 February 2020

Accepted Date: 13 February 2020

Please cite this article as: Jamos, A.M., Kaf, W.A., Chertoff, M.E., Ferraro, J.A., Human medial olivocochlear reflex: Contralateral activation effect on low and high frequency cochlear response, Hearing Research (2020), doi: https://doi.org/10.1016/j.heares.2020.107925. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier B.V.

CRediT author statement Abdullah Jamos: Conceptualization, Methodology, Formal Analysis, Investigation, Writing-Original Draft, Visualization Wafaa Kaf: Methodology, Data curation, Writing- Review & Editing, Supervision, Project Administration John Ferraro: Methodology, Writing- Review & Editing, Supervision Mark Chertoff: Methodology, Writing- Review & Editing, Supervision, Formal Analysis

Submit to: Hearing Research Journal

Human Medial Olivocochlear Reflex: Contralateral Activation Effect on Low and High Frequency Cochlear Response

Authors: 1. Abdullah Mohammad Jamos, AuD, PhD (Corresponding author) [email protected] Department of Communication Sciences and Disorders Missouri State University 901 S. National Ave Springfield, MO, USA 65897

2. Wafaa A. Kaf, MD, PhD Missouri State University 901 S. National Ave Springfield, MO, USA 65897 [email protected]

2 3. Mark E. Chertoff, PhD University of Kansas Medical Center 3901 Rainbow Blvd. / MS 3039 Kansas City, KS, USA 66160 [email protected]

4. John A. Ferraro, PhD University of Kansas Medical Center 3901 Rainbow Blvd. / MS 3039 Kansas City, KS, USA 66160 [email protected]

OHCs

Outer hair cells

MOC

Medial Olivocochlear

CR

Cochlear response

CM

Cochlear microphonic

pCM

Partly cochlear microphonic

CBBN

Contralateral broadband noise

OAEs

Otoacoustic emissions

EcochG

Electrocochleography

DPOAEs Distortion product otoacoustic emissions SFOAEs

Stimulus frequency otoacoustic emissions

CAP

Compound action potentials

ART

Acoustic reflex threshold

TB

Tone burst

FFT

Fast Fourier transform

ACh

Acetylcholine

3 Abstract The role of the medial olivocochlear (MOC) reflex has been investigated by assessing changes of cochlear responses (CR) in humans. The CR consists of pre-neural and neural potentials originating from the inner ear, and at high signal levels is dominated by cochlear microphonic (CM). The CM originates from the outer hair cells, where the MOC fibers synapse, and there is little research about using it to investigate the MOC reflex in humans. The current study aimed to investigate the effect of contralateral activation of the MOC reflex on the CR in humans. The CR was recorded in female adults (n=16) to 500 and 2000 Hz tone burst stimuli presented at 80 dB nHL with and without contralateral broadband noise (CBBN) at 40 dB SPL. Two different methods were utilized to quantify and analyze the CR data: peak amplitude and power spectrum. Results revealed enhancement of the CR amplitude with activation of the MOC reflex. Furthermore, on average, enhancement in the CR amplitude was observed to 500 Hz, but not 2000 Hz stimulus. The CR power spectrum findings revealed similar findings to the peak amplitude. These findings indicate the MOC effect is measurable when using a low frequency stimulus, but not high frequency. Moreover, the CR could be used as a potential tool to study the MOC reflex in humans.

Highlights • • •

Cochlear response enhanced with activation of medial olivocochlear reflex. Stronger medial olivocochlear effect when using a lower frequency stimulus. Cochlear response power spectrum appears to be superior to amplitude analysis.

KEYWORDS: Medial olivocochlear reflex, outer hair cell, otoacoustic emissions, cochlear microphonic, cochlea response, electrocochleography

4 1. INTRODUCTION The auditory system contains a set of complex neural circuits including both afferent and efferent pathways. While the afferent pathway is responsible for delivering the signal from the ear to the brain, the efferent pathway, specifically the olivocochlear bundle, forms a feedback circuit between the brainstem and the inner ear. The medial olivocochlear (MOC) fibers are part of the olivocochlear bundle that originates in the medial superior olive and synapses at the base of the outer hair cells (OHCs) (Liberman & Liberman, 2019). In cats and guinea pigs, approximately two-thirds of the MOC neurons are crossed, whereas the remaining one-third are uncrossed (Brashears, Morlet, Berlin, & Hood, 2003; Brown, 2014). A recent study by Liberman and Liberman (2019) shows that the MOC fibers’ innervation density in younger subjects (<40 years old) peaks at 3000-4000 Hz region, and slowly rolls off toward lower frequency (250 Hz) regions. Furthermore, the authors show middle aged group (41-66 years old) innervation density peaks at 1000 to 3000 Hz region, and slowly rolls off toward the lower frequency region. Functionally, the MOC fibers are believed to control the amount of gain of the cochlear amplifier through modulating OHC function (Ciuman, 2010; Guinan, 2006, 2018). Several studies have shown a suppressive effect of the MOC reflex with contralateral stimulation. Activation of the MOC fibers has been studied using otoacoustic emissions (OAEs) in humans, mostly because they are recorded relatively quickly and non-invasively. Several researchers have reported suppression of OAE amplitude with activation of the MOC reflex (Brashears et al., 2003; Lilaonitkul & Guinan, 2009; Moulin, Collet, & Duclaux, 1993; Tavartkiladze, Frolenkov, & Artamasov, 1996; Zhang, Boettcher, & Sun, 2007). Lilaonitkul and Guinan (2009) reported greater suppression of stimulus frequency OAEs (SFOAEs) for low stimulus and measurement frequencies (i.e. 500 and 1000 Hz) compared to high frequencies (i.e.

5 4000 Hz). Distortion product OAE (DPOAE) data revealed a relatively greater amount of suppression at 1000–2000 Hz compared to higher frequencies (Abdala, Mishra, & Williams, 2009; Atcherson, Martin, & Lintvedt, 2008). Activation of the MOC reflex affects the amplitude of the auditory evoked potentials. In humans, limited research has been done to investigate the effect of the MOC reflex on the auditory nerve compound action potential (CAP). Contralateral activation of the MOC reflex has been shown to result in suppression in most people, but there are occasions that some show enhancement (Lichtenhan, Wilson, Hancock, & Guinan, 2016; Najem, Ferraro, & Chertoff, 2016). However, in animals, the effect of activating the MOC reflex on the CAP and cochlear microphonic (CM) has been documented in numerous studies. At high signal levels, the CM originates mainly from the OHCs with, potentially, a small contribution from the inner hair cells, and it reflects the movement pattern of the cochlear partition basal to the traveling wave peak (Chertoff, Amani-Taleshi, Guo, & Burkard, 2002; Riazi & Ferraro, 2008; Tasaki & Fernández, 1952; Wilson, Sharp, Hansen, Kwong, & Kelly, 2007). Furthermore, several studies reported augmentation of the CM amplitude as a result of stimulating the MOC fibers (Bonfils, Remond, & Pujol, 1987; Desmedt, 1962; Elgueda, Delano, & Robles, 2011; Fex, 1959; Sohmer, 1965; Wiederhold & Peake, 1966). When a gross electrode is used near the cochlea for recording, a complex response of preneural and neural origins is measured and called the cochlear response (CR) (Chertoff, Kamerer, Peppi, & Lichtenhan, 2015; Kamerer & Chertoff, 2019; Lichtenhan, Hartsock, Gill, Guinan, & Salt, 2014). Pre-neural components of the CR include the CM and the summating potential, while neural components include the CAP (c.f. Chertoff et al., 2015; Lichtenhan et al., 2014). Furthermore, Kamerer and Chertoff (2019) have shown the CR to include responses from OHCs,

6 inner hair cells, and auditory nerve fibers when measured to very low frequencies in gerbils. However, it is not known if this is the case for humans. At low or moderate signal levels the CR has a significant neural component; however, it is dominated by a nonneural component (i.e. CM) when measured using high signal levels (i.e. >70 dB SPL) (Lichtenhan et al., 2014). The CR can be measured using tone burst (TB) stimuli at different frequencies, including low frequency stimuli (Chertoff et al., 2015; Lichtenhan et al., 2014). The current study used the CR as a method for examining the MOC reflex in humans. The CR, dominated by the CM at high signal levels, mainly originates from the OHCs where the MOC fibers synapse. The aim of this study is to investigate the effect of contralateral activation of the MOC reflex on the CR using both low and high frequency stimuli in humans. We hypothesize activation of the MOC reflex will affect the CR in humans in a similar manner to animals, i.e., it would result in augmentation of the response.

2. Methods 2.1 Participants We recruited 16 female adults (20 - 30 years old) with normal hearing from the Missouri State University campus to participate in the study. Inclusion criteria included: (a) female participants for a homogenous group and because females have shown larger OAEs suppression compared to males (Brashears et al., 2003); (b) normal otoscopic examination with clear ear canals; (c) normal middle ear function; (d) acoustic reflex threshold (ART) with contralateral broadband noise (CBBN) greater than 65 dB; (e) normal hearing defined as having thresholds better than 25 dB HL at all frequencies between 250–8000 Hz; and (f) present CR with no stimulus artifacts. Several reports show stronger MOC reflex in the right ear than the left ear (Gkoritsa et al., 2007; Khalfa & Collet, 1996; Khalfa, Morlet, Micheyl, Morgon, & Collet,

7 1997), therefore, all CR measurements were recorded from the right ear (test ear) in all participants. Measuring ART is of importance when studying the auditory efferent system. For accurate measurement of the MOC reflex, it is important to avoid simultaneous activation of the stapedius muscle (Berlin et al., 1993; Hood, Berlin, Hurley, Cecola, & Bell, 1996; Moulin et al., 1993; Velenovsky & Glattke, 2002). Margolis (1993) reports the mean ART to broadband noise stimulus in adults with normal hearing to be 70 to 75 dB HL. However, more recent research using wideband reflectance have demonstrated lower broadband noise levels can activate the stapedius muscle (Keefe, Fitzpatrick, Liu, Sanford, & Gorga, 2010). In the current study, the ART to CBBN was ≥70 dB HL in all participants (mean= 77 dB HL, SD= 4). Therefore, to activate the MOC reflex we presented CBBN at 40 dB SPL, which makes the chance to activate the stapedial muscle unlikely. Section 4.4 discusses this topic further.

2.2 Equipment A GSI TympStar middle ear analyzer was used to conduct tympanometry and ART measurements. Hearing testing was completed using the Interacoustic clinical AC-40 audiometer calibrated to meet the ANSI S3.6-2004 standards. To record the CR, we conducted electrocochleography (EcochG) using the Intelligent Hearing Systems (IHS) Smart-Evoked Potential (SmartEP), version 3.97. We recorded EcochG using a fabricated tympanic membrane electrode, tymptrode, as described by Ferraro and Durrant (2006). Stimuli for both hearing threshold and EcochG testing were delivered through ER-3A insert earphones. Calibration was completed for all equipment using a class 1 sound level meter. All tests were completed in a sound treated booth in the Auditory Research Laboratory at Missouri State University.

8

2.3 Procedures The procedures of this study were approved by the Institutional Review Board (IRB) at Missouri State University. Each participant was seated in a reclining chair and asked to relax, but not to fall asleep, to eliminate the effect of state of arousal on the MOC reflex (Froehich, Collet, Valtax, & Morgan, 1993). For the CR recording, before attaching the disposable electrodes, the area of mid-forehead and left mastoid were cleaned with an alcohol wipe and then gently scrubbed with Nuprep. The tymptrode was soaked with conductive gel for 10 minutes before the test time to ensure adequate conductivity.

2.3.1 Stimulus and Recording Parameters We recorded the CR using 500 Hz TB stimulus presented at 80 dB nHL (n=14); we increased the signal level to 90 dB nHL until the CR was observed in two participants. Additionally, we used 2000 Hz TB stimulus presented at 80 dB nHL to record the CR (n=12); and we increased the signal level to 85 dB nHL (n=2) or 90 dB nHL (n=2) until the CR was observed. We recorded the CR with and without presentation of CBBN at 40 dB SPL. We used a 2-10-2 cycle envelope for both TB stimuli [500 Hz: 4 ms rise/fall and 20 ms plateau; 2000 Hz: 1 ms rise/fall and 5 ms plateau] with rarefaction stimulus polarity. Stimuli were generated using extended cosine envelope to allow for creating an extended plateau time. Each tracing is the result of averaged response of 1000 sweeps presented at a rate of 9.1/sec. Responses were amplified 100,000x and filtered using a 100-5000 Hz band-pass filter. We utilized a horizontal electrode montage with the non-inverting (+) electrode at the contralateral (left mastoid), the inverting (-) electrode resting on the right tympanic membrane, and the ground electrode at the

9 mid-forehead (Fpz). Electrode impedances were maintained below 7 kΩ for all participants. To confirm reproducibility, we repeated each trace twice for each experimental condition. A control run was completed at the beginning of the testing session with the insert earphone’s tube pinched to check for the presence of stimulus artifact (Riazi & Ferraro, 2008); examples are shown in Figures 1 and 4. The recording conditions were randomized for both stimuli used.

2.4 Data Analysis We quantified the CR by measuring amplitude and power spectrum energy using fast Fourier transform (FFT). For the CR amplitude, we calculated the average peak-to-trough amplitude of three robust and consecutive waves. For the 500 Hz CR, we picked the first wave to be the first peak identified after 10 msec to avoid any peaks associated with rise/fall times of the stimulus (see marking example on Figure 1). For the same reason, we used the first identified peak after 3 msec for the 2000 Hz CR (see marking example on Figure 4). The response FFT was measured using the spectral analysis feature of the IHS-SmartEP to determine the change in energy at the frequency region of interest (i.e. 500 and 2000 Hz). Statistical analyses were completed using IBM SPSS Statistics 24. Descriptive statistics (mean and SD) are provided for all the different conditions. To investigate the effect of activating the MOC reflex on the CR, we used a paired-samples t-test to compare the response with and without CBBN at each frequency (500 and 2000 Hz) for both dependent variables—CR amplitude and power spectrum.

10 3. Results The results of this study show the CR was recorded in all participants for both TB stimuli. We analyzed both amplitude and power spectrum of the CR, which lead to similar conclusions of response augmentation with activation of the MOC reflex.

3.1 The CR Amplitude and Power Spectrum to 500 Hz TB Stimulus Activation of the MOC fibers using CBBN resulted in an augmentation of the CR amplitude to 500 Hz TB stimulus, as illustrated in Figure 1. About 63% of participants (n= 10) demonstrated an increase in the CR amplitude with the presence of CBBN. Figure 2(a) shows the CR amplitude with CBBN (gray bars) and without CBBN (black bars) for all participants in the study. The paired samples t-test revealed a significant difference in the CR amplitude without CBBN (M= 0.4746 µV, SD= 0.2592) and with CBBN (M= 0.5996 µV, SD= 0.4012) conditions [t(15)= 2.767, p< 0.05, Cohen’s d= 0.692], illustrated in Figure 2(b). The effect size shown in the CR amplitude data is considered medium. On average, the presentation of 40 dB SPL CBBN resulted in a 26.33% increase in the CR amplitude when compared to baseline [Percentage change = (Amount of change / Amplitude with CBBN) X 100%].

…………………………. Please Insert Figure 1 about Here ………………………… …………………………. Please Insert Figure 2 about Here …………………………

The CR power spectrum revealed similar results to the amplitude. Figure 3(a) illustrates the increase in energy of the 500 Hz CR power spectrum, as well as the noise floor of the control run with the pinched tube. The paired samples t-test results revealed a significant difference in

11 the CR power spectrum without CBBN (M= 0.0078 µV, SD= 0.0052) and with CBBN (M= 0.0106 µV, SD= 0.0078) conditions [t(15)= 3.184, p< 0.01, Cohen’s d= 0.80], illustrated in Figure 3(b). The effect size of the power spectrum data is considered large. On average, the presentation of 40 dB SPL CBBN resulted in 35.9% increase in amplitude when compared to baseline.

…………………………. Please Insert Figure 3 about Here …………………………

3.2 The CR Amplitude and Power Spectrum to 2000 Hz TB Stimulus Activation of the MOC fibers using CBBN resulted in enhancement of the 2000 Hz CR amplitude as shown in Figure 4. However, the enhancement effect was observed in a smaller number of participants [≈38% of participants (n = 6)] with greater variability between subjects, compared to 500 Hz stimulus, illustrated in Figure 5(a). Results of the paired samples t-test, illustrated in Figure 5(b), revealed no significant difference in the CR amplitude without CBBN (M= 0.2152 µV, SD= 0.0732) and with CBBN (M = 0.2067 µV, SD= 0.0792) conditions [t(15)= -0.376, p= 0.71]. Furthermore, Figure 6(a) illustrates the 2000 Hz CR power spectrum extracted from the same participant’s responses in Figure 4, which shows an increase in energy at 2000 Hz, as well as the noise floor of the control run with the pinched tube. Similar to the CR amplitude results, the paired samples t-test shows no significant difference in the CR power spectrum without CBBN (M= 0.0024 µV, SD= 0.0013) and with CBBN (M= 0.0023 µV, SD= 0.0014) conditions [t(15)= -0.454, p= 0.66], as shown in Figure 6(b). …………………………. Please Insert Figure 4 about Here ………………………… …………………………. Please Insert Figure 5 about Here …………………………

12 …………………………. Please Insert Figure 6 about Here ………………………… 4. Discussion To the best of our knowledge, the present study is the first to examine the effect of activation of the MOC fibers on the CR in humans to low (500 Hz) and high (2000 Hz) frequency signals. The overall results of the current study support our hypothesis that contralateral activation of the MOC reflex results in enhancement of the CR to the 500 Hz stimulus in humans, which is in agreement with findings from animal studies. However, on average, we did not find a significant effect to the 2000 Hz stimulus.

4.1 Effect of Presenting CBBN on the CR Our findings revealed the activation of the MOC reflex results in enhancement of the CR amplitude and power spectrum, but only at 500 Hz. Several animal studies reported enhancement of the CM amplitude with activation of the MOC fibers using electric or acoustic stimulation (Desmedt, 1962; Elgueda et al., 2011; Fex, 1967; Gifford & Guinan, 1987; Sohmer, 1966). Gifford and Gunian (1987) reported the increase in CM amplitude with electrical stimulation is equivalent to 10 dB increase at high stimulation levels in cats. Similarly, Patuzzi and Rajan (1990) demonstrated electrical activation of the MOC reflex in the guinea pig results in up to 40% increase in the CM amplitude. Similar results of the CM enhancement have been shown by Elgueda et al. (2011). Those studies report 20 to 40% augmentation of the CM amplitude in comparison to the response without MOC stimulation, which is comparable to our findings at 500 Hz. Physiologically, the MOC fibers release acetylcholine (ACh) when activated; this results in an increased potassium (K+) conductance in the OHC base due to an increased influx of calcium (Ca+2) to the OHCs which leads to an increase of the OHC apical influx of K+

13 augmenting the current that produces the CM (Dallos et al., 1997; Elgoyhen et al., 2001; Gisselsson & Orebro, 1960; Housley & Ashmore, 1991; Murugasu & Russell, 1996; Sridhar, Liberman, Brown, & Sewell, 1995). Because the CR is dominated by the CM at high intensity

signals, augmentation of the CR is potentially explained by the increased current through the OHCs. The potential involvement of neural responses in our data cannot be ruled out. We have used the term CR because we cannot be certain that the measured CR response is purely from the OHCs without the presence of other neural potentials. It is worth noting that if our data had a significant neural component, the pattern in which the MOC reflex affects the CR might be different. Human data show that activation of the MOC reflex with CBBN results in suppression of neural responses (i.e. CAP amplitude) (Lichtenhan et al., 2016; Najem et al., 2016), which is the opposite to what is expected to occur for the CM as described above. Our data demonstrates augmentation of the 500 Hz CR response, which is the expected change to the CM, not CAP. This is not to say that we rule out the presence of neural responses in our measured CR, but to indicate that, at the level we used in our study, the CR is dominated by non-neural potentials, and therefore the MOC reflex effect on the CR is expected to follow the MOC reflex effect on the CM.

4.2 MOC Reflex Relationship with Stimulus Frequency In the current study, we used 500 and 2000 Hz TB stimuli to study the effect of activating the MOC fibers. Our results showed activation of the MOC reflex causes a significant enhancement in the CR amplitude and power spectrum for 500 Hz TB stimulus, but not for the 2000 Hz TB. These results agree with published CM data in animals, which report a larger effect

14 to low frequency stimuli compared to higher frequency stimuli (Elgueda et al., 2011; Kittrell & Dalland, 1969; Konishi & Slepian, 1971; Sohmer, 1965; Sohmer, 1966; Teas, Konishi, & Nielsen, 1972). Sohmer (1965, 1966) and Kittrell and Dalland (1969) have investigated the effect of activating the MOC fibers on the CM in cats using a gross electrode placed at the round window using high signal levels (>70 dB SPL), and their results show greater augmentation of the CM amplitude to lower frequency stimuli (i.e. 500 – 1000 Hz). Furthermore, Konishi and Slepian (1971) and Teas et al. (1972) studied the effect of the MOC reflex in guinea pigs using differential electrodes in the scala media of the different cochlear turns. Both studies used a range of stimulus levels from low to high and reported larger enhancement of the CM amplitude to stimuli between 250 and 1000 Hz than the rest of the frequencies tested (i.e. 2000 to 10000 Hz). More recently, Elgueda et al. (2011) recorded the CM enhancement using a round window electrode in chinchillas and reported greater effect using low to mid frequency stimuli (1000– 2000 Hz) compared to higher frequency stimuli (10000–12000 Hz). We believe a combination of factors reported in the literature might explain the present effect to the 500 Hz TB, but not the 2000 Hz TB. The stimulus frequency used to record the CR and the MOC reflex has a complex, but interesting, relationship. Research has shown the CM measured with an electrode placed near the round window predominantly measures signals from the basal part of the cochlea (Tasaki & Fernández, 1952). However, recent studies have shown this type of measurement results in recording other pre-neural and neural responses mixed with the CM, hence the use of the term CR (Chertoff et al., 2015; Lichtenhan et al., 2014). These factors highlight the complexity of interpreting the CR (or the CM) results. One explanation is the in-phase displacement of a low frequency signal along the cochlear partition is greater than a high frequency signal, as understood by the traveling wave theory, especially at high signal

15 levels (Kittrell & Dalland, 1969; Sohmer, 1966). Consequently, more hair cells will be excited, which might result in a greater amount of current shunting through the OHCs (Kittrell & Dalland, 1969; Sohmer, 1966), and, hence, a larger CM. Another contributor to the frequency effect could be the MOC fiber innervation density (Elgueda et al., 2011; Maison, Adams, & Liberman, 2003). The MOC fibers’ innervation density peaks at the cochlea’s upper basal turn in

humans, however, the innervation density drops slowly toward the cochlear apex (Liberman & Liberman, 2019). Therefore, the use of broadband stimulus would excite all the MOC fibers which innervate OHCs along the cochlear partition and is largest, perhaps, in the region where the 500 Hz signal is producing the CR, thus resulting in the measured effect. One might contemplate that this theory would result in a measuring similar effect on the 2000 Hz CR. However, our results do not show an MOC reflex effect on the 2000 Hz CR. The small 2000 Hz CR potentially makes measuring the MOC reflex effect more difficult. We speculate the absence of the MOC reflex effect on 2000 Hz CR is due to the small response amplitude because of the OHCs’ basolateral wall. Due to the resistance-capacitance characteristics of the basolateral wall of the OHCs, it functions as a low-pass filter (Housley & Ashmore, 1992; Johnson, Beurg, Marcotti, & Fettiplace, 2011). Housley and Ashmore (1992) report the basal OHCs have a cut-off frequency of 400 Hz in guinea-pigs. However, other researchers report the OHCs’ cut-off frequency to be around 1000 Hz based on a model developed from different animal species data (Lu, Zhak, Dallos, & Sarpeshkar, 2006). More recently, Vavakou, Cooper, and van der Heijden (2019) reported the basal OHCs’ cut-off to be around 3000 Hz in gerbils. Thus, stimulating the OHCs with a high frequency stimulus (i.e. above the OHCs cut-off frequency) results in reduction of the receptor potential of the OHCs and small current going through the OHCs, which makes capturing the MOC reflex effect on high

16 frequencies more difficult. Therefore, an increase in the basolateral conductance driven by the MOC reflex might be measurable with a low frequency (i.e. 500 Hz) stimulus but not with a high frequency (i.e. 2000 Hz) stimulus due to the fixed capacitance and that 2000 Hz is above the cutoff frequency. Furthermore, this could potentially explain Konishi and Slepian’s (1971) data when the researchers placed a differential electrode inside the cochlea near the region of 6000 Hz, yet could not measure an MOC reflex effect at the 6000 Hz CM except to very high intensity. It is worth noting this speculation is based on animal models of the OHCs’ cut-off frequency, because the OHCs’ cutoff frequency for humans is not yet known and could potentially be less than what has been reported in gerbils.

4.3 The CR Amplitude vs. Power Spectrum and the MOC Reflex We used two different methods to quantify and analyze the CR in our study. Data was analyzed through measuring amplitude and power spectrum for each CR recording. Our results for both methods were very similar and led to similar conclusions. For the purposes of this study, we used the CR amplitude as the common way for measuring evoked potential response. However, using the CR amplitude could be challenging due to its variability and vulnerability to contamination with ambient noise. Moreover, after examining the amplitude of each cycle within one CR recording (see Figures 2 and 6), we could see slight variability of the amplitude. For this reason, we used an average of the amplitude of three consecutive cycles to minimize variability. To minimize any additional measurement biases, we used the same three cycles for all traces of each subject. This procedure could be slightly challenging and time consuming. To combat this issue, we attempted to use the second method—power spectrum energy—to analyze the CR. The use of power spectrum analysis when studying the CR could provide us with several advantages,

17 which would help overcome some of the shortcomings of using the amplitude data. Such advantages include: (1) it limits the amplitude variability issue within the same CR recording, (2) it allows for the examination of the frequency component of each CR recording—including response distortions, and (3) it appears to show greater effect of the MOC reflex to 500 Hz TB stimulus (i.e. ≈36% enhancement) compared to the amplitude data (i.e. ≈26% enhancement).

4.4 MOC Reflex and Stapedial Muscle Reflex The involvement of the stapedius muscle has been always questioned in the auditory efferent system literature to confirm the effect measured is resulting from the MOC reflex, not the stapedial muscle reflex. In animals, the stapedius muscle can be severed (Desmedt, 1962; Gifford & Guinan, 1987; Kosnish, & Slepian, 1970; Sohmer, 1965; 1966), however, this type of control is not possible in humans. Instead, we have used two ways to limit the stapedius muscle activation. First, it has been recommended to use stimulus levels that are not high enough to elicit a stapedial muscle reflex (Berlin et al., 1995; Henin, Long, & Thompson, 2014; Lilaonitkul & Guinan, 2009; Tavartkiladze et al., 1996). In the current study, CBBN was presented at a low level (i.e. 40 dB SPL) that would have a very slim chance of activating the stapedius muscle. Keefe et al. (2010) used wideband reflectance to measure ART in adults and neonates, and their results show the median ART for broadband noise in adults was 64 dB SPL, and in some participants was measured as low as 54 dB SPL. Similar results have been reported by Henin et al. (2014). It is worth highlighting once again that all participants in the current study are young adults, and the lowest ART measured was 70 dB (subject #3). Second, we argue the activation of the stapedius muscle would influence the pattern of change of the CR response. Activation of the stapedial muscle increases the ossicular chain stiffness, which would affect the stimulus

18 intensity, especially to low frequency stimuli (Feeney & Schairer, 2015; Henin et al., 2014). Such effect would be through increasing the middle ear impedance which would oppose transmission of low frequencies causing attenuation of the stimulus. As a result, the CR amplitude would have been expected to decrease, especially to the 500 Hz stimulus. However, our results show presentation of 40 dB SPL CBBN results in augmentation of the 500 Hz CR, not suppression, which is similar to CM data from animals. Therefore, the careful selection of stimulus level and the nature of the effect make the involvement of the stapedius muscle unlikely.

4.5 Study Limitation This is the first study to examine the effect of activating the MOC fibers on the CR in humans. Therefore, findings from the present study cannot be directly discussed because there is no available literature about the effect of activation of MOC reflex on the CR in humans. Because of that, our findings were compared to published data from animal studies. In addition, the majority of research on the MOC reflex in humans is focused on OAEs, which originate from the OHCs like the CM, yet they are physiologically different. Therefore, it would not be accurate to conduct this type of comparison. Furthermore, while the CR behaved similarly to what is expected from the CM response with MOC reflex activation, it is difficult to ignore the potential presence of neural component in the CR and how it might be affected by the MOC reflex. Finally, because this study evaluated only females, generalization to males would not be appropriate.

5. Conclusions

19 The current study was conducted to provide better understanding of the MOC reflex function in humans. We investigated the effect of activating the MOC reflex with and without CBBN on the CR. Our results revealed enhancement of the CR at 500 Hz with stimulation of the MOC fibers. Additionally, activating the MOC reflex revealed stronger effect when a lower frequency stimulus was used compared to higher frequency stimuli. The CR has the potential to be used to study the MOC reflex function in humans because it reflects changes in cochlear potentials, especially to a low frequency stimulus. Finally, using the frequency domain to analyze the CR appears to be promising in studying and evaluating the MOC reflex function.

Funding: This research was partly funded by the Graduate College at Missouri State University. This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.

20 References Abdala, C., Mishra, S. K., & Williams, T. L. (2009). Considering distortion product otoacoustic emission fine structure in measurements of the medial olivocochlear reflex. Journal of the Acoustical Society of America, 125(3), 1584-1594. doi: 10.1121/1.3068442 Atcherson, S. R., Martin, M. J., & Lintvedt, R. (2008). Contralateral noise has possible asymmetric frequency-sensitive effect on the 2F1-F2 otoacoustic emission in humans. Neuroscience Letters, 438(1), 107-110. doi: 10.1016/j.neulet.2008.04.050 Berlin, C. I., Hood, L. J., Hurley, A. E., Wen, H., & Kemp, D. T. (1995). Binaural noise suppresses linear click-evoked otoacoustic emissions more than ipsilateral or contralateral noise. Hearing Research, 87(1-2), 96-103. doi:10.1016/03785955(95)00082-F Berlin, C. I., Hood, L. J., Wen, H., Szabo, P., Cecola, R. P., Rigby, P., Jackson, D. F. (1993). Contralateral suppression of non-linear click-evoked otoacoustic emissions. Hearing Research, 71(1-2), 1-11. doi: https://doi.org/10.1016/0378-5955(93)90015-S Bonfils, P., Remond, M., & Pujol, R. (1987). Variations of cochlear microphonic potential after sectioning efferent fibers to the cochlea. Hearing Research, 30(2-3), 267-272. doi: https://doi.org/10.1016/0378-5955(87)90142-0 Brashears, S. M., Morlet, T. G., Berlin, C. I., & Hood, L. J. (2003). Olivocochlear efferent suppression in classical musicians. Journal of the American Academy of Audiology, 14(6), 314-324. Brown, M.C. (2014). Single-unit labeling of medial Olivocochlear neurons: the cochlear frequency map for efferent axons. Journal of Neurophysiology, 111(11), 2177-2186. doi:10.1152/jn.00045.2014 Chertoff, M. E., Amani-Taleshi, D., Guo, Y., & Burkard, R. (2002). The influence of inner hair cell loss on the instantaneous frequency of the cochlear microphonic. Hearing Research, 174(1-2), 93-100. doi: https://doi.org/10.1016/S0378-5955(02)00642-1 Chertoff, M. E., Kamerer, A. M., Peppi, M., & Lichtenhan, J. T. (2015). An analysis of cochlear response harmonics: Contribution of neural excitation. The Journal of the Acoustical Society of America, 138(5), 2957-2963. doi: http://dx.doi.org/10.1121/1.4934556 Ciuman, R. R. (2010). The efferent system or olivocochlear function bundle – fine regulator and protector of hearing perception. International Journal of Biomedical Science, 6(4), 276288. Dallos, P., He, D.Z.Z., Lin, X., Sziklai, I., Mehta, S., & Evans, B.N. (1997). Acetylcholine, outer hair cell electromotility, and the cochlear amplifier. The Journal of Neuroscience, 17(6), 2212-2226. doi: https://doi.org/10.1523/JNEUROSCI.17-06-02212.1997 Desmedt, J. E. (1962). Auditory-evoked potentials from cochlea to cortex as influenced by activation of the efferent olivo-cochlear bundle. The Journal of the Acoustical Society of America, 34(8), 1478-1496. doi: https://doi.org/10.1121/1.1918374 Elgoyhen, A.B., Vetter, D.E., Katz, E., Rothlin, C.V., Heinemann, S.F., & Boulter, J. (2001). α10: A determinant of nicotinic cholinergic receptor function in mammalian vestibular

21 and cochlear mechanosensory hair cells. Proceedings of the National Academy of Science of the United States of America, 98(6), 3501-3506. doi:10.1073/pnas.051622798 Elgueda, D., Delano, P.H., & Robles, L. (2011). Effects of electrical stimulation of olivocochlear fibers in cochlear potentials in the chinchilla. Journal of the Association for Research in Otolaryngology, 12, 317-327. doi: 10.1007/s10162-011-0260-9 Feeney, M.P., & Schairer, K.S. (2015). Acoustic stapedial reflex measurements. In J. Katz (7th Ed.), Handbook of clinical audiology (pp. 165-186). Philadelphia, PA: Lippincott Williams & Wilkins. Ferraro, J. A., & Durrant, J. D. (2006). Electrocochleography in the evaluation of patients with Ménière's disease/endolymphatic hydrops. American Journal of Audiology, 17(1), 4568. doi: https://doi.org/10.3766/jaaa.17.1.6 Fex, J. (1959). Augmentation of cochlear microphomic by stimulation of efferent fibers to the cochlea. Acta Oto-laryngologica, 50(3-6), 540-541. doi: https://doi.org/10.3109/00016485909129230 Fex, J. (1967). Efferent inhibition in the cochlea related to hair-cell dc activity: study of postsynaptic activity of the crossed olivocochlear fibers in the cat. The Journal of the Acoustical Society of America, 41(3), 666-675. doi: https://doi.org/10.1121/1.1910395 Froehich, P., Collet, L., Valtax, J.L., & Morgan, A. (1993). Sleep and active cochlear micromechanical properties in human subjects. Hearing Research, 66(1), 1-7 doi: https://doi.org/10.1016/0378-5955(93)90254-X Gifford, M.L., & Guinan Jr., J.J. (1987). Effects of electrical stimulation of medial olivocochlear neurons on ipsilateral and contralateral cochlear responses. Hearing Research, 29(2-3), 179-194. doi: https://doi.org/10.1016/0378-5955(87)90166-3 Gisselsson, L., & Orebro, O. (1960). Effect on microphonics of acetylcholine injected into the endolymphatic space. Acta Oto-laryngologica, 51(5), 636-638. doi: https://doi.org/10.3109/00016486009124540 Gkoritsa, E., Korres, S., Segas, I., Xenelis, I., Apostolopoulos, N., & Ferekidis, E. (2007). Maturation of the auditory system: 2. Transient otoacoustic emission suppression as an index of the medial olivocochlear bundle maturation. International Journal of Audiology, 46(6), 277-286. doi: https://doi.org/10.1080/14992020701261405 Guinan, J.J. (2006). Olivocochlear efferents: Anatomy, physiology, function, and the measurement of efferent effects in humans. Ear and Hearing, 27(6), 589-607. doi:10.1097/01.aud.0000240507.83072.e7 Guinan, J.J. (2018). Olivocochlear efferents: Their action, effects, measurements and uses, and the impact of the new conception of cochlear mechanical responses. Hearing Research, 362, 38-47. doi: https://doi.org/10.1016/j.heares.2017.12.012 Henin, S., Long, G.R., & Thompson, S. (2014). Wideband detection of middle ear muscle activation using swept-tone distortion product otoacoustic emissions. The Journal of the Acoustical Society of America, 136(1), 272-283. doi: 10.1121/1.4883361

22 Hood, L. J., Berlin, C. I., Hurley, A., Cecola, R. P., & Bell, B. (1996). Contralateral suppression of transient-evoked otoacoustic emissions in humans: Intensity effects. Hearing research, 101(1-2), 113-118. doi: https://doi.org/10.1016/S0378-5955(96)00138-4 Housley, G. D., & Ashmore, J. F. (1991). Direct measurement of the action of acetylcholine on isolated outer hair cells of the guinea pig cochlea. Proceedings of the Royal Society B: Biological Sciences, 244(1310), 161-167. doi: 10.1098/rspb.1991.0065 Housley, G. D., Ashmore, J. F. (1992). Ionic currents of outer hair cells isolated from the guineapig cochlea. The Journal of Physiology, 448, 73-98. doi: 10.1113/jphysiol.1992.sp019030 Johnson, S. L., Beurg, M., Marcotti, W., & Fettiplace, R. (2011). Prestin-driven cochlear amplification is not limited by the outer hair cell membrane time constant. Neuron, 70(6), 1143-1154. doi: 10.1016/j.neuron.2011.04.024. Kamerer, A.M., & Chertoff, M.E. (2019). An analytic approach to identifying the sources of the low-frequency round window cochlear response. Hearing Research, 375, 53-65. https://doi.org/10.1016/j.heares.2019.02.001 Keefe, D. H., Fitzpatrick, D., Liu, W., Sanford, C. A., & Gorga, M. P. (2010). Wideband acoustic reflex test in a test battery to predict middle-ear dysfunction. Hearing Research, 263, 52-65. doi: 10.1016/j.heares.2009.09.008 Khalfa, S., & Collet, L. (1996). Functional asymmetry of medial olivocochlear system in humans. Towards a peripheral auditory lateralization. Neuroreport, 7(5), 993-996. doi: 10.1097/00001756-199604100-00008 Khalfa, S., Morlet, T., Micheyl, C., Morgon, A., & Collet, L. (1997). Evidence of peripheral hearing asymmetry in humans: Clinical implications. Acta Oto-laryngologica, 117(2), 192-196. doi: https://doi.org/10.3109/00016489709117767 Kittrell, B. J., & Dalland, J. I. (1969). Frequency dependence of cochlear microphonic augmentation produced by olivo-cochlear bundle stimulation. The Laryngoscope, 79(2), 228-238. doi: 10.1288/00005537-196902000-00004 Konishi, T., & Slepian, J. Z. (1971). Effects of the electrical stimulation of the crossed olivocochlear bundle on cochlear potentials recorded with intracochlear electrodes in guinea pigs. Journal of Acoustical Society of America, 49(6), 1762-1769. doi: https://doi.org/10.1121/1.1912579 Liberman, L.D., & Liberman, M.C. (2019). Cochlear efferent innervation is sparse in humans and decrease with age. The Journal of Neuroscience, 39(48), 9560-9569. doi: https://doi.org/10.1523/JNEUROSCI.3004-18.2019 Lichtenhan, J.T., Hartsock, J.J., Gill, R.M., Guinan, J.J., & Salt, A.N. (2014). The auditory nerve overlapped waveform (ANOW) originates in the cochlear apex. Journal of the Association for Research in Otolaryngology, 15, 395-411. doi: https://doi.org/10.1007/s10162-014-0447-y Lichtenhan, J.T., Wilson, U.S., Hancock, K.E., & Guinan, J.J. (2016). Medial Olivocochlear Efferent Reflex Inhibition of Human Cochlear Nerve Responses. Hearing Research, 333, 216-224. doi: 10.1016/j.heares.2015.09.001

23 Lilaonitkul, W., & Guinan, J. J. (2009). Human medial olivocochlear reflex: effects as functions of contralateral, ipsilateral, and bilateral elicitor bandwidths. Journal of the Association for Research in Otolayrngology, 10(3), 459-470. doi: 10.1007/s10162-009-0163-1 Lu, T. K., Zhak, S., Dallos, P., & Sarpeshkar, R. (2006). Fast cochlear amplification with slow outer hair cells. Hearing Research, 214(1-2), 45-67. doi:10.1016/j.heares.2006.01.018 Maison, S. F., Adams, J. C., & Liberman, M. C. (2003). Olivocochlear innervation in the mouse: immunocytochemical maps, crossed versus uncrossed contributions, and transmitter colocalization. The Journal of Comparative Neurology, 455(3), 406–416. doi:10.1002/cne.10490 Margolis, R. H. (1993). Detection of hearing impairment with the acoustic stapedius reflex. Ear and Hearing, 14(1), 3-10. Moulin, A., Collet, L., & Duclaux, R. (1993). Contralateral auditory stimulation alters acoustic distortion products in humans. Hearing Research, 65(1-2), 193-210. doi: https://doi.org/10.1016/0378-5955(93)90213-K Murugasu, E., & Russell, I. J. (1996). The effect of efferent stimulation on basilar membrane displacement in the basal turn of the guinea pig cochlea. The Journal of Neuroscience, 16, 325-332. doi: https://doi.org/10.1523/JNEUROSCI.16-01-00325.1996 Najem, F., Ferraro, J., & Chertoff, M. (2016). The effect of contralateral pure tone on the compound action potential in humans: Efferent tuning curves. Journal of the American Academy of Audiology, 27(2), 103-116. doi: 10.3766/jaaa.15002 Patuzzi, R., & Rajan, R. (1990). Does electrical stimulation of the crossed olivo-cochlear bundle produce movement of the organ of Corti?. Hearing Research, 45, 15-32. doi: https://doi.org/10.1016/0378-5955(90)90179-S Riazi, M., & Ferraro, J. A. (2008). Observations on mastoid versus ear canal recorded cochlear microphonic in newborns and adults. Journal of the American Academy of Audiology, 19(1), 46-55. doi: https://doi.org/10.3766/jaaa.19.1.5 Sohmer, H. (1965). The effect of contralateral olivo-cochlear bundle stimulation on the cochlear potentials evoked by acoustic stimuli of various frequencies and intensities. Acta Otolaryngologica 60, 59-70. doi: https://doi.org/10.3109/00016486509126988 Sohmer, H. (1966). A comparison of the efferent effects of the homolateral and contralateral olivo-cochlear bundles. Acta Oto-laryngologica 62, 76-87. doi: https://doi.org/10.3109/00016486609119552 Sridhar, T. S., Liberman, M. C., Brown, M. C., & Sewell, W. F. (1995). A novel cholinergic “Slow Effect” of efferent stimulation on cochlear potentials in the guinea pig. The Journal of Neuroscience, 15(5), 3667-3678. doi: https://doi.org/10.1523/JNEUROSCI.15-05-03667.1995 Tasaki, I., & Fernández, C. (1952). Modification of cochlear microphonics and action potentials by KC1 solution and by direct currents. Journal of Neurophysiology, 15(6), 497-512. doi:10.1152/jn.1952.15.6.497

24 Tavartkiladze, G. A., Frolenkov, G. I., & Artamasov, S. V. (1996). Ipsilateral suppression of transient evoked otoacoustic emission: Role of the medial olivocochlear system. Acta Oto-larygologica, 116(2), 213-218. doi: https://doi.org/10.3109/00016489609137826 Teas, D. C., Konishi, T., & Nielsen, D. W. (1972). Electrophysiological studies on the spatial distribution of the crossed olivocochlear bundle along the guinea pig cochlea. The Journal of the Acoustical Society of America, 51(4), 1256-1264. doi: https://doi.org/10.1121/1.1912969 Vavakou, A., Cooper, N. P., & van der Heijden, M. (2019). The frequency limit of outer hair cell motility measured in vivo. Elife, 8, e47667. Doi: https://doi.org/10.7554/eLife.47667 Velenovsky, D. S., & Glattke, T. J. (2002). The effect of noise bandwidth on the contralateral suppression of transient evoked otoacoustic emissions. Hearing Research, 164(1-2), 3948. doi: https://doi.org/10.1016/S0378-5955(01)00393-8 Wiederhold, M. L., & Peake, W. T. (1966). Efferent inhibition of auditory-nerve responses: Dependence on acoustic-stimulus parameters. The Journal of the Acoustical Society of America, 40(6), 1427-1430. doi: https://doi.org/10.1121/1.1910243 Wilson, W. J., Sharp, K. J., Hansen, C., Kwong, P., & Kelly, A. (2007). Especially prominent cochlear microphonic activity in the auditory brainstem response. International Journal of Audiology, 46(7), 362-373. doi: 10.1080/14992020701297557 Wittekindt, A., Gaese, B.H., & Kössl, M. (2009). Influence of contralateral acoustic stimulation on the quadratic distortion product F2–F1 in humans. Hearing Research, 247, 27-33. doi: https://doi.org/10.1016/j.heares.2008.09.011 Zhang, F., Boettcher, F. A., & Sun, X. (2007). Contralateral suppression of distortion product otoacoustic emissions: Effect of the primary frequency in Dpgrams. International Journal of Audiology, 46(4), 187-195. doi: 10.1080/14992020601164162

Human Medial Olivocochlear Reflex: Contralateral Activation Effect on Low and High Frequency Cochlear Response

Figure 1. The CR to a 500 Hz TB with and without 40 dB SPL CBBN showing three traces: control run with tube pinched (top trace-light gray), the CR without CBBN = baseline (middle trace-drack gray), and the CR with CBBN (bottom trace-black). The control run indicates the recorded CR to be true—not stimulus artifact. The recording with and without CBBN show an enhancement of the CR amplitude with the presence of CBBN. The Roman numerical markings (I, II, III) represent the peaks used for analysis of the CR amplitude. Figure 2. Mean and individual CR amplitudes for 500 Hz TB stimulus. (a) Shows the CR amplitude with (gray bars) and without (black bars) CBBN. Data presented from all participants of the study. (b) Shows the mean amplitude of the CR with (gray bar) and without (black bar) CBBN, which is illustrating the significant enhancement of the CR with presenting 40 dB SPL CBBN [* p < 0.05]. Error bars represent ±1 SE. Figure 3. The CR power spectrum to 500 Hz TB stimulus with and without 40 dB SPL CBBN. (a) Shows an example of the power spectrum data extracted from the CR shown in Figure 2. Compared to baseline (black line), the power spectrum energy at 500 Hz region increased with presenting 40 dB SPL CBBN (grey line); dotted line shows power spectrum of the control run with the pinched tube. (b) Shows means of the CR power spectrum for 500 Hz stimulus with and without CBBN. Power spectrum energy significantly increased with presenting 40 dB SPL CBBN (gray bar) in compassion to baseline (black bar) CBBN [** p < 0.01]. The dotted bar represents the mean of the noise floor at 500 Hz, which is extracted from the control run with pinched tube. Error bars represent ±1 SE. Figure 4. The CR to a 2000 Hz TB with and without 40 dB SPL CBBN showing three traces: control run with tube pinched (top trace-light gray), the CR without CBBN = baseline (middle trace-drack gray), and the CR with CBBN (bottom trace-black). The control run indicates the recorded CR to be true—not stimulus artifact. The recording from this subject revealed enhancement of the CR with presenting 40 dB SPL CBBN, however, this was the case in only small number of participants (n=6). The Roman numerical markings (I, II, III) represent the peaks used for analysis of the CR amplitude. Figure 5. Mean and individual CR amplitudes for 2000 Hz TB stimulus. (a) Shows the CR amplitude with (gray bars) and without (black bars) CBBN. Data presented from all participants of the study. (b) Shows the mean amplitude of the CR with (gray bar) and without (black bar) 40 dB SPL CBBN. The graph is showing no significant difference on the group data. Error bars represent ±1 SE. Figure 6. The CR power spectrum to 2000 Hz TB stimulus with and without 40 dB SPL CBBN. (a) Shows an example of the power spectrum data extracted from the CR shown in Figure 5. Compared to baseline (black line), the power spectrum energy at 2000 Hz region increased with presenting 40 dB SPL CBBN (grey line); dotted line shows power spectrum of the control run with the pinched tube. (b) Shows means of the CR power spectrum with (gray bar) and without (black bar) CBBN. The dotted bar represents the mean of the noise floor at 2000 Hz, which is extracted from the control run with pinched tube. Error bars represent ±1 SE.

a)

b)

1.8

Without CBBN

1.6

With CBBN

CR Amplitude (µV)

1.4

*

1.2 1.0 0.8 0.6 0.4 0.2 0.0 1

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4

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7

8 9 10 11 12 13 14 15 16 Participant

500 Frequency (Hz)

a) 0.014 Control (Pinched Tube)

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0.012

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0.01

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0.008 0.006 0.004 0.002

202 221 241 260 280 299 319 339 358 378 397 417 436 456 475 495 514 534 553 573 592 612 632 651 671 690 710 729 749 768 788

0

Frequency (Hz)

Power Spectrum (µV)

**

b) 0.014

Control (Pinched Tube)

0.012

Without CBBN

0.010

With CBBN

0.008 0.006 0.004 0.002 0.000 500 Frequency Hz

a)

b)

1.8

CR Amplitude (µV)

1.6 1.4

Without CBBN

1.2

With CBBN

1.0 0.8 0.6 0.4 0.2 0.0 1

2

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4

5

6

7

8 9 10 11 12 13 14 15 16 Participant

2000 Frequency (Hz)

a) 0.014

Control (Pinched Tube) Without CBBN With CBBN

0.01 0.008 0.006 0.004 0.002

1,006 1,074 1,143 1,211 1,279 1,348 1,416 1,484 1,553 1,621 1,689 1,758 1,826 1,895 1,963 2,031 2,100 2,168 2,236 2,305 2,373 2,441 2,510 2,578 2,646 2,715 2,783 2,852 2,920 2,988

0

Frequency (Hz)

b) Power Spectrum (µV)

FFT Energy (µV)

0.012

0.014

Control (Pinched Tube)

0.012

Without CBBN With CBBN

0.010 0.008 0.006 0.004 0.002 0.000 2000 Frequency Hz

Human Medial Olivocochlear Reflex: Contralateral Activation Effect on Low and High Frequency Cochlear Response

Highlights • • •

Cochlear response enhanced with activation of medial olivocochlear reflex. Stronger medial olivocochlear effect when using a lower frequency stimulus. Cochlear response power spectrum appears to be superior to amplitude analysis.