PROCEEDINGS of the23rd International Congress on Acoustics
9 to 13 September 2019 in Aachen, Germany

Correlative changes of medial olivocochlear reflex andelectroencephalographic activity during sleep
Yuto SUZUKI1; Sho OTSUKA1,2,3; Seiji NAKAGAWA1,2,3
1

Dept. of Medical Eng., Graduate School of Sci. & Eng., Chiba University, Japan
2

Center for Frontier Medical Eng., Chiba University, Japan

3

Med-Tech Link Center, Chiba University Hospital, JapanABSTRACT
There is a projection of efferent nerve fibers called the olivocochlear bundle connecting the brainstem to theouter hair cells (OHCs). It is said that the olivocochlear system protects the inner ear by responding to sounds
and suppressing the OHC amplification. Therefore, measuring the reaction (medial olivocochlear reflex;MOCR) strength is expected to be an applicable method of evaluating the possibility of noise-induced hearing
impairment in advance. Since the MOCR strength fluctuates with each measurement, there is a problem that
it is difficult to evaluate the function of the MOC system stably. In this study, we examined the influence offluctuation of awareness during sleeping on MOCR. The awareness was evaluated objectively using
electroencephalogram, and MOCR strength was evaluated by measuring otoacoustic emissions, which reflect
the OHC amplification. There was a significant positive correlation between MOCR strength and α wave
amplitudes, which reflects awareness. This result indicates that MOCR strength, and hence the inner ear
protective function become weaker as awareness decreases. Therefore, exposure to loud sounds during adecline in awareness may increase the risk of causing noise-induced hearing impairment.
Keywords: Medial olivocochlear reflex, Sleep, Electroencephalogram1. INTRODUCTION
From the brainstem to the outer hair cells (OHCs), there is a projection of efferent nerve fibers
called the olivocochlear bundle. It acts to suppress the amplification of OHCs in response to acoustic
stimulation, and this reaction is called medial olivocochlear reflex (MOCR) (1). MOCR is thought toprotect the inner ear from loud noises via this suppressing the OHC amplification (2). Therefore,
measuring MOCR strength is expected to be an applicable method of evaluating the possibility ofnoise-induced hearing impairment in advance (2, 3). However, since MOCR strength fluctuates witheach measurement, there is a problem that it is difficult to evaluate the function of the MOC systemstably. For example, it has been reported that the MOCR becomes weaker during sleep (4,5). Therefore,
it is possible that MOCR strength depends on awareness. Despite this possibility, no studies haveevaluated awareness level quantitatively and examined its relationship with MOCR. In this study, eachstage from awakening to sleep was objectively evaluated using electroencephalogram (EEG) and theirrelationship with MOCR strength was investigated.

2. METHOD
The following measurements were taken during two hours of sleep in seven participants withnormal hearing (one female, six males, average age 22.3 years, standard deviation 1.23 years).
2.1 MOCR MEASUREMENT
Otoacoustic emissions (OAE) is a weak sound generated on the basement membrane and is
1s-nakagawa@chiba-u.jp6616

considered to reflect the amplification function of OHCs. It is known that when noise is presented tothe contralateral ear on the OAE measurement side, the amplitude of the OAE is suppressed (1, 3).
The amount of suppression is considered to reflect the MOCR strength (3). In this study, we measured
the OAE evoked by a click in the right ear, when the contralateral noise for inducing the MOCR waspresented to the left ear. Ear canal sound pressure was recorded using an Etymotic Research ER -10B
low-noise microphone system inserted in each ear. The click had a duration of 100 μs and waspresented at 60-dB peak-equivalent sound pressure level. The noise was band-pass filtered between
100 and 1000 Hz and had a duration of 0.5 seconds including a 10-ms raised-cosine ramp. The noise
was presented at sound pressure level of 60 dB. During the two-hours measurement session, the click
and the noise were presented at intervals of 25 ms and 5s, respectively. The OAE waveforms measured
from one minute before and one minute after each time point were extracted, and then the waveforms
measured in the interval with and without contralateral noise were averaged respectively. The pressurelevels of the OAEs for each condition were defined as RMS values in the region of 8–18 ms and are
denoted as P with and P without (in Pa). MOCR strength was defined as 20log10(P with/P without ).
2.2 ELECTROENCEPHALOGRAM (EEG) MEASUREMENT
The dominant component of EEG changes depending on sleep stage: β waves (13-30 Hz) at restwith opened eyes, α waves (8-13 Hz) at rest with closed eyes, θ waves (4-8 Hz) in light sleep, and δwaves (0.5-4 Hz) in deep sleep. To assess sleep stage objectively, the amplitude of each EEG
component was monitored during the two-hours measurement session. The spectrum of EEG at eachtime point was calculated by applying Fast Fourier Transform (FFT) for a duration of 2 minutes at 1minute intervals within the 2-hours EEG measurement session. The strength of each EEG component
at each time point was calculated by averaging the spectral amplitude in the frequency range
corresponding to each EEG band. EEG was recorded using an amplifier (Nihon Kohden Corp. MEG2208). The reference electrode was attached to the left earlobe (A1), the different electrode to theright parietal region (C4), and the ground electrode was set to intercostal space (Fz).

3. RESULTFigure 1 shows the averaged correlation coefficient between the time variation of OAE suppression(MOCR strength) and the time variation of each EEG component. The correlation coefficient between
the time variation of the amount of OAE suppression and the α-wave activity was statisticallysignificantly greater than 0 (p = 0.0488), but the correlation coefficient between other EEG
components was not significantly different from 0 (p > 0.05). Furthermore, the time variation of OAE
amplitude did not have a significant correlation with the α-wave activity.

Figure 1 – Average value of correlation coefficient between β wave, α wave, θ wave, δ wave and theamount of OAE suppression. Error bars represent standard variation of the mean. *p < 0.05.

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4. DISCUSSION
The amount of OAE suppression (MOCR strength) had a statistically significant positive
correlation with the time variation of α-wave activity (appearing mainly from awakening to sleep
onset). This result indicates that MOCR strength becomes weaker at a relatively early stage of sleep
onset, which has a large change in awareness level, regardless of the depth of sleep. No correlation
was found between the OAE amplitude and the α-wave activity. This result indicates that the inner ear
function itself is not affected by the awareness level, but the activity of the MOC activity is affectedby awareness level. Consequently, it is inferred that the inner ear protection function provided by
MOCR also decreases when awareness level decreases. Therefore, being exposed to loud noise whileawareness levels are decrease might increase the risk of causing noise-induced deafness.

ACKNOWLEDGEMENTS
This work was supported by JSPS KAKENHI Grant number 18K18066 and a research grant fromHealth Science Center Foundation.

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