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

Effects of a preceding sound on medial olivocochlear bundle reflex
as a function of the preceding time interval
Sho OTSUKA1,2,3,4; Seiji NAKAGAWA1,2,3; Shigeto FURUKAWA4
1Dept. of Medical Eng., Graduate School of Sci. & Eng., Chiba University, Japan
2Center for Frontier Medical Eng., Chiba University, Japan

34Med-Tech Link Center, Chiba University Hospital, JapanNTT Communication Science Laboratories, NTT corporation, JapanABSTRACT
Our previous study reported that a preceding sound, regardless of whether it induced a medial olivocochlear
bundle reflex (MOCR) by itself, expedites MOCR, and a 60-dB-SPL preceding-noise, which also induces anMOCR, that enhances, in addition to expediting, the MOCR. The current study compares the dependency ofthe enhancing and expediting effects on the preceding time interval. The MOCR is induced by sounds andexerts an inhibitory effect on the outer hair cells. The suppressive effect was assessed non-invasively in termsof the suppression of otoacoustic emissions (OAEs) induced by a contralateral acoustic stimulation, referred
to as MOCR elicitor. A 60-dB-SPL noise was used as a preceding sound, and the inter-stimulus interval (ISI)
between the preceding sound and the MOCR elicitor was varied from 0.5 to 2 s. The MOCR strength
decreased as the ISI became larger and reached the same level as without a preceding sound within 2 s of ISI.
In contrast, the onset delay of the MOCR did not depend on the ISI and was still significantly smaller than
without a preceding sound even at 2 s of ISI. The difference in the dependency of the preceding time intervalimplicates that the preceding-sound-induced enhancing and expediting effects are underpinned by different
mechanisms.
Effects of a preceding sound on medial olivocohlear bundle reflex as a function of the preceding time interval

Keywords: medial olivocochlear reflex, preceding sound, otoacoustic emissions

1. INTRODUCTION
The medial olivocochlear reflex (MOCR) is a feedback response that controls the input to theauditory system. Outer hair cells (OHCs) are innervated by the medial part of the superior olivary
complex via the medial olivocochlear (MOC) bundle. These MOC fibers are activated by acoustic
stimulation and exert an inhibitory effect on OHC motility. This suppressive effect has been termed
the MOCR, and is thought to play an important role in improving the detection of signals in thepresence of background noise (1, 2), in selectively attending to target signals (3), and in protecting
the auditory periphery from damage caused by acoustic overexposure (4, 5).
Most previous studies of the MOCR have measured the responses induced by single acoustic stimuli,
such as a single short-duration or continuous contralateral white noise (5, 6, 7, 8), and it is not clear
whether or how the MOC system changes its behavior based on the history of time-varying acoustic
inputs. In contrast, in a previously published study, we reported that a preceding sound, regardlesswhether it induced an MOCR by itself, expedites MOCR, and a 60-dB-SPL preceding-noise, whichitself induces an MOCR, and not only expedites but also enhances the MOCR (9). The result implies
that MOCR dynamically changes depending on the history acoustic inputs. However, it is not clearwhether the preceding-sound-induced enhancing and expediting effects are underpinned by the same
16626

otsuka.s@chiba-u.jp

mechanism. To clarify this, the present study compares the dependency of the enhancing andexpediting effects on the inter-stimulus interval (ISI) between the preceding sound and the MOCR
elicitor. Hypothetically, if both effects were caused by the same mechanism, their ISI dependency
would be similar.

2. METHODS
2.1 Participants
Twelve volunteers (two males and ten females) aged 21–32 years (mean=23.8, standarddeviation=3.7) participated in the study. Ears from all volunteers had normal pure-tone audiometricthresholds (HL< 20dB), ranging from 0.25 to 8kHz. The experiments were approved by the Research
Ethics Committee of NTT Communication Science Laboratories and Chiba University.
2.2 Stimuli and procedure
We measured the contralateral MOCR in terms of the suppression of click-evoked otoacousticemissions (CEOAEs) induced by noise presented to the contralateral ear. Click trains were played intothe right ear canal to generate CEOAEs from within the cochlea. Each click had a duration of 100 μs
and was presented at a 55-dB peak-equivalent sound pressure level (SPL). Clicks were presented at arate of 50 times per second. The MOCR was elicited by a noise presented to the left ear, referred toas the MOCR elicitor, during the CEOAE recording. The noise was band-pass filtered between 100
and 10,000 Hz, with a duration of 0.5 s, including a 10-ms raised-cosine ramp. The noise waspresented to the left ear at 60-dB SPL. As a preceding sound, the same noise was presented prior theMOCR elicitor. To evaluate the effect of the preceding time interval, the inter-stimulus interval (ISI)
between the preceding sound and MOCR elicitor was varied in 0.5, 1, 1.5 and 2 s. A MOCR without
any preceding sound was also measured.
2.3 Recording and data analysis
Ear-canal sound pressure was recorded with an Etymotic Research ER-10B low-noise microphonesystem inserted in each ear. The recorded signals were band-pass filtered between 1 and 4 kHz, forwhich the largest MOCR-related CEOAE suppression was observed. An epoch with duration of 2.5 s,including 1.5 s pre-onset and 0.5 s post-offset of the MOCR elicitor, was extracted from the filteredsignals for every presentation of an MOCR elicitor. The extracted epochs were averaged across thetrials for each condition, and a time series composed of 125 CEOAE waveform samples was obtained
from the averaged epoch. To smooth fluctuations included in the time series, ten adjacent CEOAE
waveform samples were averaged for each time point. The CEOAE level (in dB SPL) was calculated
as an RMS value for each waveform sample in the 8–18-ms region of the waveform. Lastly, a timecourse of MOCR was obtained by subtracting the baseline level from the time series of CEOAE levels.
The baseline level was defined as the average CEOAE level in a period of 1 sec prior to the onset ofthe first stimulus in a series. The strength of the MOCR for each time course was defined as the mean
suppression between time points 0.25 and 0.75 s after the onset of the preceding sound. The onset
delay was defined as the time at which the OAE level decreased by 0.25 dB from the baseline.
2.4 Equipment
Stimuli were digitally synthesized with sampling rates of 48 kHz and converted to analog signals
using an Fireface UCX (16 bits). The analog signals were amplified by a headphone buffer andpresented through Etymotic Research ER-3A earphones connected to an ER-10B low-noise
microphone system. The two outputs from the ER-2A were calibrated using a DB2012 accessory(external ear simulator) of a Bruel and Kjaer Type 4257 ear simulator (IEC 711). Ear canal soundpressure was recorded using an Etymotic Research ER-10B low-noise microphone system inserted ineach ear. All measurements were conducted in a double-wall sound-attenuating room.

6627

3. RESULT AND DISCUSSION
Consistent with the previous study (9), it took several hundred ms for the MOCR to build up andthe strength of suppression ranged from 2.5 to 3 dB. The MOCR strength decreased as the ISI became
larger and reached the same level as without a preceding sound within 2 s of ISI. In contrast, the onset
delay of the MOCR did not depend on the ISI and was still significantly smaller than without apreceding sound even at 2 s of ISI. We compared the strengths and onset delay of the MOCR induced
by a preceding sound across the conditions using a repeated-measures analysis of variance (ANOVA),
with the ISI as the within-subjects factor. The ANOVA revealed a significant effect of ISI on thestrength of the MOCR (F3, 33 = 10.5, p<0.001). A post hoc comparison using Ryan’s method showedthat MOCR with ISI of 2 s was significantly larger than that with ISI of 1 s (T = 3.0, p =0.0047<nominal level calculated by Ryan’s method; Fig. 1) and 1.5 s (T = 5.1, p<0.0001<nominal
level calculated by Ryan’s method; Fig. 1). In contrast, there was no significant effect of ISI on theonset delay of the MOCR (F3, 33 = 0.48, p = 0.70).
Together with the previous results that a 40-dB preceding tone, which itself does not induce MOCR,
did not enhance MOCR (Otsuka et al. 2018), the enhancement may reflect an adaptation induced bya preceding sound. A possible mechanism is the slow component of the MOCR. Cooper and Guinan
reported that repeated electrical shocks to the MOC bundle increase MOCR strength on a time scaleof tens of seconds, referring to this as the “slow effect” (10). Guinan and coworkers attributed theslow effect to a gradual decrease in OHC stiffness due to prolonged release of ACh, which is theprimary neurotransmitter at MOC terminals (11). The decrease in OHC stiffness should recover withtime after the stimulation, and the monotonic decrease of MOCR enhancement as a function of ISI
may reflect the recovery process. In contrast, the expediting effect lasts at least two seconds. The
difference in the dependency of the preceding time interval implicates that the expediting effect cannot
be attributed easily to the adaptation in the cellular processes, and the preceding-sound-induced
enhancing and expediting effects are underpinned by different mechanisms.

Figure 1 Comparison of the effects of ISI between a preceding noise and MOCR elicitor on thestrength and onset delay of MOCR. Error bars represent standard error of the mean. **p<0.01,***p<0.0001 (Corrected for multiple comparisons with Ryan’s method). ISI, inter-stimulus interval;
MOCR, medial olivocochlear reflex.

6628

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

REFERENCES
1. Kawase T, Delgutte B, Liberman MC. Antimasking effects of the olivocochlear reflex. II. Enhancementof auditory-nerve response to masked tones. J Neurophysiol. 1993;70(6):2533–49.
2. Otsuka S, Nakagawa S, Furukawa S. The relationship between characteristics of medial olivocochlear
reflex and speech-in-noise-reception performance. Acoust Sci Technol (in press).
3. Maison S, Micheyl C, Collet L. Influence of focused auditory attention on cochlear activity in humans.
Psychophysiology. 2001;38(1):35–40.
4. Maison SF, Liberman MC. Predicting Vulnerability to Acoustic Injury with a Noninvasive Assay ofOlivocochlear Reflex Strength. J Neurosci. 2000;20(12):4701–7.
5. Otsuka S, Tsuzaki M, Sonoda J, Tanaka S, Furukawa S. A Role of Medial Olivocochlear Reflex as a
Protection Mechanism from Noise-Induced Hearing Loss Revealed in Short-Practicing Violinists. PLOS
ONE. 2016;11(1):e0146751.
6. Backus BC, Guinan JJ. Time-course of the human medial olivocochlear reflex. J Acoust Soc Am.
2006;119:2889-904.
7. Bassim MK, Miller RL, Buss E, Smith DW. Rapid adaptation of the 2f1-f2 DPOAE in humans: binauraland contralateral stimulation effects. Hear Res. 2003;182(1–2):140–52.
8. Kim DO, Dorn PA, Neely ST, Gorga MP. Adaptation of Distortion Product Otoacoustic Emission inHumans. J Assoc Res Otolaryngol. 2001;2(1):31–40.
9. Otsuka S, Nakagawa S, Furukawa S. A Preceding Sound Expedites Medial Olivocochlear Reflex. Acta
Acustica united with Acustica. 2018;104:804 – 8.
10.Cooper NP, Guinan JJ. Separate mechanical processes underlie fast and slow effects of medial
olivocochlear efferent activity. J Physiol. 2003;548(1):307–12.
11.Guinan JJ. Olivocochlear Efferents: Anatomy, Physiology, Function, and the Measurement of Efferent
Effects in Humans. Ear Hear. 2006;27(6):589–607.

6629