Maturation of medial efferent system function in humans

Peripheral Auditory Function in Tanzanian Children Living With HIV With Clinically Normal Hearing

IMPORTANCE

Despite normal audiometry, adults living with HIV have lower distortion product otoacoustic emissions (DPOAEs) compared with HIV-negative controls, but the degree of these differences in children living with HIV is unknown. If subclinical auditory deficits are present, results could affect developmental outcomes in children living with HIV (CLWH).

OBJECTIVE

To compare DPOAEs and auditory brainstem responses (ABR) between 2 age- and sex-matched groups of younger children with normal audiometry, 1 infected with HIV and the other uninfected.

DESIGN, SETTING, AND PARTICIPANTS

Cohort study in an infectious disease center in Dar es Salaam, Tanzania. Participants included 340 Tanzanian children aged 3 to 9 years with clinically normal hearing, type A tympanograms bilaterally, and air-conduction thresholds of 20 dB HL or less from 0.5 to 8 kHz. Participants in the cohort repeated testing approximately every 6 months (approximately 2.2 sessions per participant) for a total of 744 total observations. Data were analyzed from March 2020 to January 2022.

MAIN OUTCOMES AND MEASURES

DPOAE amplitudes from 1.5 to 8 kHz using an f2 to f1 ratio of 1.2 and L1/L2 values of 65/55 dB sound pressure level and click-evoked ABR using a slow (21.1/s) and fast (61.1/s) click rate.

RESULTS

A total of 141 CLWH (70 female participants [49.3%]; mean [SD] age, 7.24 [1.67] years) and 199 HIV-negative individuals (99 female participants [49.7%]; mean [SD] age, 7.26 [1.44] years) participated in the study. The groups did not differ significantly in age, static immittance, or air-conduction thresholds. HIV status was independently associated with approximately 1.4 dB (95% CI, -3.28 to 0.30 dB) to 3.8 dB (95% CI, 6.03 to -1.99 dB) lower DPOAE amplitudes at 6 and 8 kHz bilaterally and 0.28 μV (95% CI, 0.01 to 0.33 μV) lower ABR wave V amplitudes in the right ear.

CONCLUSIONS AND RELEVANCE

Consistent with previous findings in young adults, CLWH had slightly, but reliably, lower DPOAEs and ABR wave V amplitudes than HIV-negative controls. The magnitude of these differences was small, but results suggest an early and consistent association between HIV infection or treatment and outer hair cell and auditory brainstem responses in children as young as 3 years. These subclinical changes suggest tracking both auditory function and development outcomes in CLWH is warranted.

Reliability of Contralateral Suppression in Evoked Distortion Product Otoacoustic Emissions

Introduction  Distortion product otoacoustic emissions (DPOAE) and their suppression may be considered useful in monitoring cochlear function and the efferent auditory pathway inhibitory effect. Nonetheless, the establishment of reliable parameters of response variations is of great importance. Objectives  To verify the replicability of test and retest in the research of the inhibitory effect of the efferent pathway using contralateral suppressing stimulus during DPOAE recording for clinical applicability. Methods  Cross-sectional study with 48 volunteers, aged 18 to 30 years, with normal audiometric thresholds. The procedures included were audiometric and immittance measures to overrule any conductive or sensorineural conditions and DPOAE recordings without and with contralateral suppression with a 60 dBHL white noise. Distortion product otoacoustic emissions amplitudes were analyzed and compared in both conditions with Wilcoxon test, and the Spearman correlation test was used to assess test-retest reliability. Results  The comparative analysis showed differences between amplitudes in test and retest conditions only in 1,500 Hz for DPOAE measures with all other tested frequencies showing no differences, and no difference was observed in all recorded frequencies in the test and retest comparison for DPOAE suppression. The degree of correlation between test and retest of DPOAE amplitude was good at 6,000 Hz and strong (r > 0.880) at the other frequencies. For DPOAE with suppression, all frequencies presented strong correlation between test and retest: 1,500 Hz (r = 0.880), 2,000 Hz (r = 0.882), 3,000 Hz (r = 0.940), and 6,000 Hz (r = 0.957). Conclusions  The study found good replicability in contralateral suppression of DPOAE with potential clinical applicability, and we recommend conducting the test from 2000Hz to higher frequencies for more reliable results.

Medial olivary complex reflex in term newborns with hyperbilirubinemia

OBJECTIVE

This study aimed to compare the integrity of the efferent auditory pathways of newborns that had high hyperbilirubinemia levels and required treatment due to these and healthy newborns.

METHODS

Term-born (37 weeks or later) infants that were brought to the Newborn Polyclinic of the Başkent University Hospital were included in the study. The study included a total of 84 infants including healthy newborns (n = 42) and those that had jaundice and were receiving phototherapy (n = 42). After conducting a general otorhinolaryngology examination on all newborns included in the study, Transient Otoacoustic Emission (TEOAE) test was carried out in the absence and presence of contralateral noise. The obtained contralateral suppression values were compared between the two groups.

RESULTS

In the TEOAE test, the responses obtained at 1 kHz in the newborns receiving phototherapy were found to be lower. The difference between the groups was significant (p = 0.038). The rates of suppression presence at 2 kHz, 2.8 kHz and total OAE were found significantly higher (p < 0.05) in the group not receiving phototherapy. Among the phototherapy-receiving infants, the hyperbilirubinemia levels of the infants in whom suppression was obtained in the contralateral suppression test did not show a statistically significant difference in comparison to those in whom suppression was not obtained (p > 0.05).

CONCLUSION

Based on the obtained data, hyperbilirubinemia may have a disruptive effect on the efferent auditory system in newborns. Consequently, we are of the opinion that, in addition to hearing screening in risky newborn infants, a MOC suppression test would be useful.

Distortion product otoacoustic emissions in very preterm infants: A longitudinal study

BACKGROUND

Very preterm infants are at a greater risk of developing neurodevelopmental impairments such as neuro-motor delays, vision and hearing deficits (Roze and Breart, 2004; Saigal and Doyle, 2008) [1,2]. The hearing difficulties in preterm infants vary depending on the co-morbid conditions. However, prematurity itself is considered as a risk factor that influence the functioning of auditory system.

AIM

The current study aims to compare the DPOAEs in very preterm infants and term infants at 1 month, 3 months and 6 months of age (corrected age in preterm infants).

METHOD

DPOAEs were recorded in 72 very preterm infants and 30 term infants at 1 month, 3 months and 6 months of age. All these infants had obtained 'pass' results in newborn hearing screening using ABR. DPOAE f2 test frequency was measured at six frequencies (1500 Hz, 2000 Hz, 3000 Hz, 4500 Hz, 6000 Hz and 8000 Hz) with primary tone stimulus intensity L1 equal to 65 dBSPL and L2 equal to 55 dBSPL with primary tone f2/f1 frequency ratio of 1.2. Otoscopic examination and tympanometry was performed prior to DPOAE testing, to ascertain normal middle ear status.

RESULTS

DPOAE amplitude did not change significantly between two groups from 1 month till 6 months of age (p > 0.05). DPOAE amplitude and noise floor in very preterm infants were not different from term infants and DPOAE amplitude did not vary significantly across f2 frequencies at various time periods.

CONCLUSION

The current study findings provided evidence that prematurity did not constitute as a factor to influence the results of DPOAE in very preterm infants who passed newborn hearing screening test. Any significant reduction in DPOAE amplitude or absence of DPOAE in very preterm infants has to be considered and monitored effectively, as it may not reflect a developmental process of cochlear function; instead it could indicate the presence of inner ear or middle ear pathology.

Correlation and Reliability of Behavioral and Otoacoustic-Emission Estimates of Contralateral Medial Olivocochlear Reflex Strength in Humans

The roles of the medial olivocochlear reflex (MOCR) in human hearing have been widely investigated but remain controversial. We reason that this may be because the effects of MOCR activation on cochlear mechanical responses can be assessed only indirectly in healthy humans, and the different methods used to assess those effects possibly yield different and/or unreliable estimates. One aim of this study was to investigate the correlation between three methods often employed to assess the strength of MOCR activation by contralateral acoustic stimulation (CAS). We measured tone detection thresholds (N = 28), click-evoked otoacoustic emission (CEOAE) input/output (I/O) curves (N = 18), and distortion-product otoacoustic emission (DPOAE) I/O curves (N = 18) for various test frequencies in the presence and the absence of CAS (broadband noise of 60 dB SPL). As expected, CAS worsened tone detection thresholds, suppressed CEOAEs and DPOAEs, and horizontally shifted CEOAE and DPOAE I/O curves to higher levels. However, the CAS effect on tone detection thresholds was not correlated with the horizontal shift of CEOAE or DPOAE I/O curves, and the CAS-induced CEOAE suppression was not correlated with DPOAE suppression. Only the horizontal shifts of CEOAE and DPOAE I/O functions were correlated with each other at 1.5, 2, and 3 kHz. A second aim was to investigate which of the methods is more reliable. The test–retest variability of the CAS effect was high overall but smallest for tone detection thresholds and CEOAEs, suggesting that their use should be prioritized over the use of DPOAEs. Many factors not related with the MOCR, including the limited parametric space studied, the low resolution of the I/O curves, and the reduced numbers of observations due to data exclusion likely contributed to the weak correlations and the large test–retest variability noted. These findings can help us understand the inconsistencies among past studies and improve our understanding of the functional significance of the MOCR.

Evaluating the Function of the Medial Olivocochlear Bundle in Patients With Bilateral Tinnitus

Purpose No study has investigated the effects of contralateral noise (CN) on speech-in-noise perception (SINP) in listeners with tinnitus. The mechanisms underlying the involvement of medial olivocochlear (MOC) reflex with SINP remain to be elucidated. This study aimed to investigate the MOC function in patients with bilateral tinnitus by measuring distortion product otoacoustic emission and SINP. Method Eighteen patients with bilateral tinnitus (one male and 17 females; age: M ± SD = 45.61 ± 10.18 years) and 19 listeners without tinnitus (six males and 13 females; age: M ± SD = 34.11 ± 8.35 years) were recruited for the study. Each subject underwent distortion product otoacoustic emission measurement and the SINP test for both ears. The effects of CN on these two measurements were compared between tinnitus ears (TEs) and no-tinnitus ears (NTEs). Results The presence of CN significantly reduced distortion product (DP) amplitudes and improved SINP for TEs, and the amounts of DP suppression and SINP improvement were similar to those in NTEs. Improvement of SINP was positively correlated with DP suppression at 6185 Hz for NTEs and at 1640 Hz for TEs. Conclusions The results of this study suggest that the amounts of DP suppression and SINP improvement were similar between listeners with and without tinnitus. For both ear groups, the MOC reflex was involved with SINP at specific frequencies. Any clinical test outcomes with regard to the MOC bundle in patients with tinnitus should be interpreted with caution until further studies are conducted.

Medial olivocochlear function in children with poor speech-in-noise performance and language disorder

OBJECTIVES

Contralateral masking of transient-evoked otoacoustic emissions is a phenomenon that suggests an inhibitory effect of the olivocochlear efferent auditory pathway. Many studies have been inconclusive in demonstrating a clear connection between this system and a behavioral speech-in-noise listening skill. The purpose of this study was to investigate the activation of a medial olivocochlear (MOC) efferent in children with poor speech-in-noise (PSIN) performance and children with language impairment and PSIN (SLI + PSIN).

METHODS

Transient evoked otoacoustic emissions (TEOAEs) with and without contralateral white noise were tested in 52 children (between 6 and 12 years). These children were arranged in three groups: typical development (TD) (n = 25), PSIN (n = 14) and SLI + PSI (n = 13).

RESULTS

PSIN and SLI + PSI groups presented reduced otoacoustic emission suppression in comparison with the TD group.

CONCLUSION

Our finding suggests differences in MOC function among children with typical development and children with poor SIN and language problems.

Effects of contralateral acoustic stimulation on otoacoustic emissions induced by swept tones

The medial olivocochlear complex (MOC) is an auditory nucleus that projects efferent nerve fibers to control the behaviors of both sides of the cochlea. Otoacoutsic emissions (OAEs) are by-products the activities of the outer hair cells (OHCs) in the cochlea and could be used as a noninvasive way to study the efferent control of the MOC. However, existing results regarding the efferent control are quite controversial and often restricted to a rather limited frequency range. In this study, a new method of measuring stimulus frequency otoacoustic emissions (SFOAEs) with the presence of a contralateral acoustic stimulation (CAS) was proposed to study the efferent control over the cochlea. SFOAEs were measured with swept tones with time varying frequencies so that SFOAE spectra with and without the presence of the CAS could be compared with high frequency-resolution. The results showed that there was consistent decrease in the amplitude of the swept-tone SFOAEs across a wide frequency range from 0.5 to 8 kHz when the CAS was presented, suggesting an outstanding attenuation of OHC activities by the efferent control from the MOC. The SFOAE decrease with the presence of the CAS might provide a new approach to measure the strength of the efferent control and to evaluate the functional status of the central auditory pathway.

Auditory-neurophysiological responses to speech during early childhood: Effects of background noise

Early childhood is a critical period of auditory learning, during which children are constantly mapping sounds to meaning. But this auditory learning rarely occurs in ideal listening conditions-children are forced to listen against a relentless din. This background noise degrades the neural coding of these critical sounds, in turn interfering with auditory learning. Despite the importance of robust and reliable auditory processing during early childhood, little is known about the neurophysiology underlying speech processing in children so young. To better understand the physiological constraints these adverse listening scenarios impose on speech sound coding during early childhood, auditory-neurophysiological responses were elicited to a consonant-vowel syllable in quiet and background noise in a cohort of typically-developing preschoolers (ages 3-5 yr). Overall, responses were degraded in noise: they were smaller, less stable across trials, slower, and there was poorer coding of spectral content and the temporal envelope. These effects were exacerbated in response to the consonant transition relative to the vowel, suggesting that the neural coding of spectrotemporally-dynamic speech features is more tenuous in noise than the coding of static features-even in children this young. Neural coding of speech temporal fine structure, however, was more resilient to the addition of background noise than coding of temporal envelope information. Taken together, these results demonstrate that noise places a neurophysiological constraint on speech processing during early childhood by causing a breakdown in neural processing of speech acoustics. These results may explain why some listeners have inordinate difficulties understanding speech in noise. Speech-elicited auditory-neurophysiological responses offer objective insight into listening skills during early childhood by reflecting the integrity of neural coding in quiet and noise; this paper documents typical response properties in this age group. These normative metrics may be useful clinically to evaluate auditory processing difficulties during early childhood.

Medial efferent mechanisms in children with auditory processing disorders

Auditory processing disorder (APD) affects about 2–5% of children. However, the nature of this disorder is poorly understood. Children with APD typically have difficulties in complex listening situations. One mechanism thought to aid in listening-in-noise is the medial olivocochlear (MOC) inhibition. The purpose of this review was to critically analyze the published data on MOC inhibition in children with APD to determine whether the MOC efferents are involved in these individuals. The otoacoustic emission (OAE) methods used to assay MOC reflex were examined in the context of the current understanding of OAE generation mechanisms. Relevant literature suggests critical differences in the study population and OAE methods. Variables currently known to influence MOC reflex measurements, for example, middle-ear muscle reflexes or OAE signal-to-noise ratio, were not controlled in most studies. The use of potentially weaker OAE methods and the remarkable heterogeneity across studies does not allow for a definite conclusion whether or not the MOC reflex is altered in children with APD. Further carefully designed studies are needed to confirm the involvement of MOC efferents in APD. Knowledge of efferent functioning in children with APD would be mechanistically and clinically beneficial.

Influence of aging on medial olivocochlear system function

BACKGROUND

There is still controversy regarding the influence of aging on medial olivocochlear (MOC) system function. The main objective of this study is to measure age-related changes of MOC system function in people with normal hearing thresholds.

METHOD

Bilateral assessment of the MOC effect for click-evoked otoacoustic emissions (CEOAEs; at 70±3 dB peak sound pressure level [pSPL], click at 50/second, 260 repeats, 2.5-20 millisecond window) and for distortion product otoacoustic emissions (DPOAEs; with [frequencies] f2/f1=1.22, [levels of primary tones] L1=65 dB SPL and L2=55 dB SPL; DP-grams for 2f1-f2 were collected for the f1 frequencies varying from 977 Hz to 5,164 kHz, with the resolution of four points per octave) was performed in a group of 146 (n=292 ears) healthy, right-handed subjects aged from 10-60 years with a bilateral hearing threshold from 0.25-4.0 kHz, not exceeding 20 dB hearing level; normal tympanograms; and a threshold of the contralateral stapedial reflex for broadband noise (BBN) of 75 dB SPL or higher. The MOC inhibition was assessed on the basis of changes in OAE level during BBN contralateral stimulation at 50 dB sensation level (mean, 65±3 dB SPL).

RESULTS

Comparative analysis of the MOC effect for CEOAE and DPOAE showed the weakest effect in the oldest age group (41-60 years) at almost all tested frequencies. Moreover, a weak, albeit significant, positive correlation between the level of OAE and the size of the MOC effect was documented.

CONCLUSION

On the basis of our study, we have found a decrease in the strength of the MOC system with increasing age in normally hearing subjects, as reflected by a decrease of the OAE suppression effects in older individuals and an increase of the number of CEOAE and DPOAE enhancements during contralateral acoustic stimulation in the elderly, especially in the high-frequency range.

Contralateral ear occlusion for improving the reliability of otoacoustic emission screening tests

Newborn hearing screening is an established healthcare standard in many countries and testing is feasible using otoacoustic emission (OAE) recording. It is well documented that OAEs can be suppressed by acoustic stimulation of the ear contralateral to the test ear. In clinical otoacoustic emission testing carried out in a sound attenuating booth, ambient noise levels are low such that the efferent system is not activated. However in newborn hearing screening, OAEs are often recorded in hospital or clinic environments, where ambient noise levels can be 60–70 dB SPL. Thus, results in the test ear can be influenced by ambient noise stimulating the opposite ear. Surprisingly, in hearing screening protocols there are no recommendations for avoiding contralateral suppression, that is, protecting the opposite ear from noise by blocking the ear canal. In the present study we have compared transient evoked and distortion product OAEs measured with and without contralateral ear plugging, in environmental settings with ambient noise levels <25 dB SPL, 45 dB SPL, and 55 dB SPL. We found out that without contralateral ear occlusion, ambient noise levels above 55 dB SPL can significantly attenuate OAE signals. We strongly suggest contralateral ear occlusion in OAE based hearing screening in noisy environments.

Efferent inhibitory effect observed in otoacoustic emissions and auditory brainstem response in the neonatal population

OBJECTIVE

To characterize the inhibitory effect (IE) in the otoacoustic emission (OAE) and auditory brainstem response (ABR) in newborns at high and low risk for hearing loss.

DESIGN

Cross-sectional study.

PATIENTS AND METHODS

Seventy-nine newborns at low risk for hearing loss and 46 at high risk underwent transient evoked OAE (TEOAE), distortion product OAE (DPOAE) and ABR testing with or without the presence of contralateral white noise presented at a level of 60 dB SPL.

RESULTS

For both low- and high-risk newborns, there were no significant differences in IE between the left and right ears. There was a statistically significant difference in the right-ear IE between the low- and high-risk group for DPOAE and ABR testing. There was also greater agreement of the efferent system evaluation outcomes between TEOAE and ABR.

CONCLUSIONS

ABR testing detected IE in a greater number of newborns in the low-risk, as compared to the high-risk group.

Separating the contributions of olivocochlear and middle ear muscle reflexes in modulation of distortion product otoacoustic emission levels

OBJECTIVES

Mediated by the medial olivocochlear system (MOCS), distortion product otoacoustic emission (DPOAE) levels are reduced by presentation of contralateral acoustic stimuli. Such acoustic signals can also evoke a middle ear muscle reflex (MEMR) that also attenuates recorded DPOAE levels. Our aim is to clearly differentiate these two inhibitory mechanisms and to analyze each separately, perhaps allowing the development of novel tests of hearing function.

METHODS

DPOAE were recorded in real time from chinchillas with normal auditory brainstem response thresholds and middle ear function. Amplitude reduction and its onset latency caused by contralateral presentation of intermittent narrow-band noise (NBN) were measured. Stapedius and tensor tympani muscle tendons were divided without disturbing the ossicular chain, and DPOAE testing was repeated.

RESULTS

Peak reduction of (2f1 - f2) DPOAE levels occurred when the center frequency of contralateral NBN approximated the primary tone f2, indicating an f2-frequency-specific response. For a 4.5-kHz centered NBN, DPOAE (f2 = 4.4 kHz) inhibition was 0.1 dB (p < 0.001). This response remained present after tendon division, consistent with an MOCS origin. Low-frequency NBN (center frequency: 0.5 kHz) reduced otoacoustic emission levels (0.1 dB, p < 0.001) across a wide range of DPOAE frequencies. This low-frequency response was abolished by division of the middle ear muscle tendons, clearly indicating MEMR involvement.

CONCLUSIONS

Following middle ear muscle tendon division, DPOAE inhibition by contralateral stimuli approximating the primary tone f2 persists, whereas responses evoked by lower contralateral frequencies are abolished. This distinguishes the different roles of the MOCS (f2 frequency specific) and MEMR (low frequency only) in contralateral modulation of DPOAE. This analysis helps clarify the pathways involved in an objective test that might have clinical benefit in the testing of neonates.

Effect of tinnitus on distortion product otoacoustic emissions varies with hearing loss

PURPOSE

The aim of this study was to measure the effect of tinnitus, while accounting for the effect of hearing loss and aging, on distortion product otoacoustic emissions (DPOAEs).

METHOD

DPOAEs were measured twice in both ears in 5 groups of participants: young adults with normal hearing, middle-age adults with normal hearing, adults with high-frequency sensorineural hearing loss, age-matched adults with similar hearing loss and tinnitus, and adults with normal hearing and chronic tinnitus.

RESULTS

Multivariate analysis revealed a main effect of hearing loss and age, but no effect of tinnitus, across all 5 groups. Separate tests revealed significant effects of age and tinnitus in the normal-hearing groups and hearing loss in adults with or without tinnitus, but no effect of tinnitus in those with hearing loss.

CONCLUSION

DPOAE levels in the group of adults with hearing loss and tinnitus were diminished, but those in the group with normal hearing and tinnitus were enhanced, relative to DPOAE levels in the controls. Outer hair cell function, as indexed by DPOAEs, exhibits a complex association with tinnitus, and this has implications in the use of DPOAEs as a tool both for testing for tinnitus presence and for creating a model of neural mechanisms underlying tinnitus.

Postnatal maturation of contralateral DPOAE suppression in a precocious animal model (chinchilla) of the human neonate

CONCLUSION

In the neonatal chinchilla, the degree of contralateral distortion product otoacoustic emission (DPOAE) suppression and the latency and time constants of suppression are immature for 40-60 days. This suggests that olivocochlear efferent innervation of outer hair cells is not fully mature at birth in this animal model, and this may also be the case for human neonates.

OBJECTIVES

To track postnatal changes in the dynamics of the olivocochlear efferent system in an animal model with cochlear development at birth similar to that in humans.

METHODS

Real-time measurements of contralateral DPOAE suppression were made in 79 ears of anaesthetized chinchillas, ranging in age from 1 day to 70 days. An adult control group (13 ears) was also tested. DPOAE (2f1-f2; f2 = 4.4 kHz; f2/f1 = 1.22) input/output functions were measured. Dynamics of contralateral broadband noise suppression were measured, including latency and suppression time constants.

RESULTS

DPOAE amplitude input/output functions are immature until 20-30 days postnatally. The maturation period for contralateral suppression amplitude is about 30 days. Latency of onset suppression was 40 ms at birth reducing to adult values (23 ms) at 40 days. The DPOAE suppression time constant was about 350 ms at birth and mature (230 ms) at 60 days.

Maturation of the human medial efferent reflex revisited

Past work applying otoacoustic emissions to gauge maturational status of the medial olivocochlear (MOC) reflex in human newborns has produced mixed results. The present study revisits the question while considering the dual nature of the 2f(1) - f(2) distortion product otoacoustic emission (DPOAE) and expanding measures of medial efferent function. Subjects included premature and term-born neonates, 6-month-old infants and young adults. The MOC reflex was elicited with contralateral acoustic stimulation (CAS) while shifts in amplitude and phase of the DPOAE, and its distortion and reflection components, were monitored. Overall, CAS-elicited reductions in DPOAE level did not differ among age groups. For all ages, the MOC reflex was strongest at frequencies below 1.5 kHz, and the reflection component of the DPOAE was most affected, showing maximally reduced amplitude and shallower phase slope when contralateral noise was presented. Results suggest that the MOC reflex likely reaches maturation prior to full-term birth. However, prematurely born neonates show markedly more episodes of CAS-induced DPOAE level enhancement. This may be due to more intrusive component mixing in this age group or disruptions in the formation of the MOC pathway or synapse in the most premature neonates.

The MOC reflex during active listening to speech

PURPOSE

The purpose of this study was to test the hypothesis that active listening to speech would increase medial olivocochlear (MOC) efferent activity for the right vs. the left ear.

METHOD

Click-evoked otoacoustic emissions (CEOAEs) were evoked by 60-dB p.e. SPL clicks in 13 normally hearing adults in 4 test conditions for each ear: (a) in quiet; (b) with 60-dB SPL contralateral broadband noise; (c) with words embedded (at -3-dB signal-to-noise ratio [SNR]) in 60-dB SPL contralateral noise during which listeners directed attention to the words; and (d) for the same SNR as in the 3rd condition, with words played backwards.

RESULTS

There was greater suppression during active listening compared with passive listening that was apparent in the latency range of 6- to 18-ms poststimulus onset. Ear differences in CEOAE amplitude were observed in all conditions, with right-ear amplitudes larger than those for the left. The absolute difference between CEOAE amplitude in quiet and with contralateral noise, a metric of suppression, was equivalent for right and left ears. When the amplitude differences were normalized, suppression was greater for noise presented to the right and the effect measured for a probe in the left ear.

CONCLUSION

The findings support the theory that cortical mechanisms involved in listening to speech affect cochlear function through the MOC efferent system.

The relationship between MOC reflex and masked threshold

Otoacoustic emission (OAE) amplitude can be reduced by acoustic stimulation. This effect is produced by the medial olivocochlear (MOC) reflex. Past studies have shown that the MOC reflex is related to listening in noise and attention. In the present study, the relationship between strength of the contralateral MOC reflex and masked threshold was investigated in 19 adults. Detection thresholds were determined for 1000-Hz, 300-ms tone presented simultaneously with one repetition of a 300-ms masker in an ongoing train of masker bursts. Three masking conditions were tested: 1) broadband noise 2) a fixed-frequency 4-tone complex masker and 3) a random-frequency 4-tone complex masker. Broadband noise was expected to produce energetic masking and the tonal maskers were expected to produce informational masking in some listeners. DPOAEs were recorded at fine frequency intervals from 500 to 4000 Hz, with and without contralateral acoustic stimulation. MOC reflex strength was estimated as a reduction in baseline level and a shift in frequency of DPOAE fine-structure maxima near 1000-Hz. MOC reflex and psychophysical testing were completed in separate sessions. Individuals with poorer thresholds in broadband noise and in random-frequency maskers were found to have stronger MOC reflexes.

Frequency shifts with age in click-evoked otoacoustic emissions of preterm infants

A previous study [Brienesse et al. (1997). Pediatr. Res. 42, 478-483] demonstrated a positive shift with increasing postmenstrual age (PMA) in the frequencies of synchronized spontaneous otoacoustic emissions (SSOAEs) in preterm infants. We used a mixed model approach to describe a shift with PMA in the spectra of click-evoked otoacoustic emissions (CEOAEs) measured in a group of 22 preterm infants. The rate in shift in CEOAE spectral components was found to be frequency dependent, with a mean estimate of 10 Hz/week for frequencies around 2 kHz and 30 Hz/week for frequencies around 4.25 kHz. This rate decreased with increasing PMA. Because SSOAEs are often part of the CEOAE response, a comparison was made between the shifts in SSOAEs and CEOAEs in a sub-group of 16 preterm infants. The results indicate that the shifts found for both types of OAE are similar, which supports a common mechanism for this change in OAE-characteristic. At present it is not clear to what extent developmental processes in the cochlea and the middle ear can account for these frequency shifts in the spectra of CEOAEs and SSOAEs during the preterm period.

Changes in amplitude and phase of distortion-product otoacoustic emission fine-structure and separated components during efferent activation

Medial olivocochlear (MOC) efferent fibers synapse directly on the outer hair cells (OHCs). Efferent activation evoked by contralateral acoustic stimulation (CAS) will affect OHC amplification and subsequent measures of distortion-product otoacoustic emissions (DPOAEs). The aim of this study was to investigate measures of total and separated DPOAEs during efferent activation. Efferent activation produces both suppression and enhancement of the total DPOAE level. Level enhancements occurred near fine-structure minima and were associated with consistent MOC evoked upward shifts in DPOAE fine-structure frequency. Examination of the phase of the separated components revealed that frequency shifts stemmed from increasing phase leads of the reflection component during CAS, while the generator component phase was nearly invariant. Separation of the two DPOAE components responsible for the fine-structure revealed more consistent reduction of the levels of both components. Using vector subtraction (which takes into account both level and phase) to estimate the changes in the unseparated DPOAE provided consistent evidence of DPOAE suppression. Including phase information provided a more sensitive, valid and consistent estimate of CAS function even if one does not know the position of the DPOAE in the fine-structure.

Distortion product otoacoustic emission phase and component analysis in human newborns

Apical distortion product otoacoustic emissions (DPOAEs) are comprised of at least two components, as evidenced by the interference pattern of alternating maxima and minima known as fine structure. DPOAE fine structure is produced by the shifting phase relationship in the ear canal, between the generator and characteristic frequency (CF) component of the response. Each component arises from a different cochlear region and, according to theory, reflects a distinct generation mechanism. The analysis of DPOAE components and phase in newborns may provide a window into targeted aspects of cochlear physiology during development. 2f(1)-f(2) DPOAE fine structure was recorded from 15 adults and 14 newborns using a swept-tone technique. DPOAE group delay, as well as magnitude and phase of each component, was compared between age groups. Results show narrower fine structure spacing, a longer group delay (steeper phase gradient) in low frequencies, and a stronger relative contribution from the CF component in newborns. The prolonged group delay for low-frequency DPOAEs could indicate immature basilar membrane motion in the apex of the cochlea and warrants further investigation. The enhanced contribution from the CF component may have implications for clinical practice as well as for theories of cochlear maturation.

The human auditory system: A timeline of development

This review traces the structural maturation of the human auditory system, and compares the timeline of anatomical development with cotemporaneous physiological and behavioral events. During the embryonic period, there is formation of basic structure at all levels of the system, i.e. the inner ear, the brainstem pathway, and the cortex. The second trimester is a time of rapid growth and development, and by the end of this period, the cochlea has acquired a very adult-like configuration. During the perinatal period, the brainstem reaches a mature state, and brainstem activity is reflected in behavioral responses to sound, including phonetic discrimination, and in evoked brainstem and early middle latency responses. The perinatal period is also the time of peak development of brainstem input to the cortex through the marginal layer, and of the long latency cortical potentials, the N(2) and mismatch negativity. In early childhood, from the sixth post-natal month to age five, there is progressive maturation of the thalamic projections to the cortex and of the longer latency Pa and P(1) evoked potentials. Later childhood, from six to twelve years, is the time of maturation of the superficial cortical layers and their intracortical connections, accompanied by appearance of the N(1) potential and improved linguistic discriminative abilities. Some consideration is given to the potential negative effects of deafness-induced sound deprivation during the perinatal period and childhood.

Considering distortion product otoacoustic emission fine structure in measurements of the medial olivocochlear reflex

In humans, when the medial olivocochlear (MOC) pathway is activated by noise in the opposite ear, changes in distortion product otoacoustic emission (DPOAE) level, i.e., the MOC reflex, can be recorded in the test ear. Recent evidence suggests that DPOAE frequency influences the direction (suppression/enhancement) of the reflex. In this study, DPOAEs were recorded at fine frequency intervals from 500 to 2500 Hz, with and without contralateral acoustic stimulation (CAS) in a group of 15 adults. The MOC reflex was calculated only at DPOAE frequencies corresponding to peaks in the fine structure. Additionally, inverse fast-Fourier transform was conducted to evaluate MOC effects on individual DPOAE components. Results show the following: (1) When considering peaks only, the mean MOC reflex was -2.05 dB and 97% of observations reflected suppression, (2) CAS reduced distortion characteristic frequency component levels more than overlap component levels, and (3) CAS produced an upward shift in fine structure peak frequency. Results indicate that when the MOC reflex is recorded at DPOAE frequencies corresponding to fine structure maxima (i.e., when DPOAE components are constructive and in phase), suppression is reliably observed and level enhancement, which probably reflects component mixing in the ear canal rather than strength of the MOC reflex, is eliminated.

TEOAE suppression in adults with learning disabilities

The presentation of contralateral noise during the recording of transient evoked otoacoustic emissions (TEOAEs) reduces the amplitude of the TEOAE in normally-hearing adults. This is known as TEOAE suppression. The present study investigated TEOAE suppression in 18 adults with learning disabilities (LDs) compared to 18 adults without LDs. TEOAEs were elicited by 60 dB p.e. SPL clicks and were suppressed by the presentation of 60 dB SPL contralateral broadband noise. Suppression was measured as a change in the overall TEOAE response amplitude, and also analysed in 2-ms epochs representing different TEOAE frequency-response bands. A significant interaction was evident between group type and ear tested. Participants in the control group had right ear dominance for the suppression effect, whereas the left ear was found to be dominant for the LD group. These findings suggest a mechanism of the medial olivary cochlear bundle and efferent auditory pathway that differs in those with LD compared to those with typical learning abilities.

Changes in the DP-gram during the preterm and early postnatal period

OBJECTIVES

To describe and define changes in the infant DP-gram during an age continuum from the preterm period through the first 6 mo of postnatal life. This information provides normative guidelines for audiologists or hearing screeners using DPOAEs to monitor infant hearing status.

DESIGN

In this retrospective study, 2f1 - f2 DP-grams (DPOAE level x f2) were recorded with primary tones at 65/55 dB SPL, f2/f1 = 1.2, and f2 frequencies ranging from 1500 to 9000 Hz. Results from one ear of 290 healthy infants ranging in age from 31 wks postconceptional age to 6-mo-old were examined. Data were collected using both longitudinal design (repeated tests on the same infant over time) and cross-sectional methodology (a different group of subjects representing each age category). Subjects were divided into three groups according to age and experimental design. The effects of age and frequency on DPOAE level were analyzed in the three groups separately.

RESULTS

The combined results from the three databases indicate that (1) DPOAE level increased for mid-frequencies throughout the preterm period, from 31 to 33 wks until the time period associated with term birth. This change was significant for 4500 and 6000 Hz; (2) DPOAE level decreased as f2 frequency increased. In many infants, a shallow trough was observed with peak amplitude at 1500 Hz, a reduction in response amplitude through 4500 Hz, and a second peak around 6000 Hz; (3) during the postnatal period from birth through 6 mo, DPOAE level did not change significantly as a function of age and the DP-gram was relatively flat across f2 frequency; and (4) infants showed mean DPOAE levels that were 4 to 12 dB higher than adult levels.

CONCLUSIONS

The results indicate a frequency-dependent increase in DPOAE level during the preterm period in human infants. After birth, there is little change in amplitude through 6 mo. The infant DPOAE remains larger than adult amplitude at all ages tested, as shown in other reports, well into childhood, suggesting continued changes in DPOAE level during the first decade of life. Recent research suggests that immaturities of the conductive pathway may account for infant-adult differences in DPOAE level; however, it is not yet clear whether other sources contribute.

Contralateral suppression of distortion product otoacoustic emissions: effect of the primary frequency in Dpgrams

The amplitude of the 2f1-f2 distortion product otoacoustic emission (DPOAE) can be suppressed by presenting contralateral acoustic stimulation. To test the hypothesis that DPOAE contralateral suppression is influenced by the primary frequency in DPgrams, DPgrams were recorded at resolutions of 1, 8, and 17 pts/octave, in the absence and presence of contralateral broadband noise (BBN). Participants were 20 normal-hearing human adults. In DPgrams with higher frequency resolutions, DPOAE suppression at amplitude peaks in DPgrams (8 pts/octave: Mean = - 0.92 dB, SD = 0.71 for BBN at 60 dB SPL; 17 pts/octave: Mean = - 0.25 to -1.44 dB, SD = 0.51 to 0.86 for BBN at 40 to 70 dB SPL, respectively) was larger than the suppression at the dips in DPgrams (8 pts/octave: Mean = - 0.13 dB, SD = 1.00; 17 pts/octave: Mean = - 0.03 to -0.73 dB, SD = 0.55 to 0.91). A larger intersubject variability in DPOAE contralateral suppression was observed at the dips. The results suggest that measuring DPOAE contralateral suppression at the primary frequencies corresponding to the peaks in DPgrams with higher frequency resolutions may improve the assessment of the efferent system function.

Distortion product otoacoustic emission suppression tuning and acoustic admittance in human infants: birth through 6 months

Previous work has reported non-adultlike distortion product otoacoustic emission (DPOAE) suppression in human newborns at f2=6000 Hz, indicating an immaturity in peripheral auditory function. In this study, DPOAE suppression tuning curves (STCs) were recorded as a measure of cochlear function and acoustic admittance/reflectance (YR) in the ear canal recorded as a measure of middle-ear function, in the same 20 infants at birth and through 6 months of age. DPOAE STCs changed little from birth through 6 months, showing excessively narrow and sharp tuning throughout the test period. In contrast, several middle-ear indices at corresponding frequencies shifted systematically with increasing age, although they also remained non-adultlike at 6 months. Linear correlations were conducted between YR and DPOAE suppression features. Only two correlations out of 76 were significant, and all but three YR variables accounted for <10% of the variance in DPOAE suppression tuning. The strongest correlation was noted between admittance phase at 5700 Hz and STC tip-to-tail (R=0.49). The association between middle-ear variables and DPOAE suppression may be stronger during other developmental time periods. Study of older infants and children is needed to fully define postnatal immaturity of human peripheral auditory function.

Olivocochlear efferents: anatomy, physiology, function, and the measurement of efferent effects in humans

This review covers the basic anatomy and physiology of the olivocochlear reflexes and the use of otoacoustic emissions (OAEs) in humans to monitor the effects of one group, the medial olivocochlear (MOC) efferents. MOC fibers synapse on outer hair cells (OHCs), and activation of these fibers inhibits basilar membrane responses to low-level sounds. This MOC-induced decrease in the gain of the cochlear amplifier is reflected in changes in OAEs. Any OAE can be used to monitor MOC effects on the cochlear amplifier. Each OAE type has its own advantages and disadvantages. The most straightforward technique for monitoring MOC effects is to elicit MOC activity with an elicitor sound contralateral to the OAE test ear. MOC effects can also be monitored using an ipsilateral elicitor of MOC activity, but the ipsilateral elicitor brings additional problems caused by suppression and cochlear slow intrinsic effects. To measure MOC effects accurately, one must ensure that there are no middle-ear-muscle contractions. Although standard clinical middle-ear-muscle tests are not adequate for this, adequate tests can usually be done with OAE-measuring instruments. An additional complication is that most probe sounds also elicit MOC activity, although this does not prevent the probe from showing MOC effects elicited by contralateral sound. A variety of data indicate that MOC efferents help to reduce acoustic trauma and lessen the masking of transients by background noise; for instance, they aid in speech comprehension in noise. However, much remains to be learned about the role of efferents in auditory function. Monitoring MOC effects in humans using OAEs should continue to provide valuable insights into the role of MOC efferents and may also provide clinical benefits.

Effects of middle-ear immaturity on distortion product otoacoustic emission suppression tuning in infant ears

Distortion product otoacoustic emission (DPOAE) measures of cochlear function, including DPOAE suppression tuning curves and input/output (I/O) functions, are not adultlike in human infants. These findings suggest the cochlear amplifier might be functionally immature in newborns. However, many noncochlear factors influence DPOAEs and must be considered. This study examines whether age differences in DPOAE I/O functions recorded from infant and adult ears reflect maturation of ear-canal/middle-ear function or cochlear mechanics. A model based on linear middle-ear transmission and nonlinear cochlear generation was developed to fit the adult DPOAE I/O data. By varying only those model parameters related to middle-ear transmission (and holding cochlear parameters at adult values), the model successfully fitted I/O data from infants at birth through age 6 months. This suggests that cochlear mechanics are mature at birth. The model predicted an attenuation of stimulus energy through the immature ear canal and middle ear, and evaluated whether immaturities in forward transmission could explain the differences consistently observed between infant and adult DPOAE suppression. Results show that once the immaturity was compensated for by providing infants with a relative increase in primary tone level, DPOAE suppression tuning at f2= 6000 Hz was similar in adults and infants.

A retrospective study of the spectral probability of spontaneous otoacoustic emissions: rise of octave shifted second mode after infancy

The recording of otoacoustic emissions (OAEs) is today a widely used tool in medical diagnosis. The mechanisms of OAE generation in the cochlea and their transmission to the external ear canal, however, are not well understood and a matter of long-standing debates. Here, the frequency distribution of 1660 spontaneous otoacoustic emissions (SOAEs) from three surveys, covering 296 human subjects, is analyzed. Neonates show a monomodal distribution with a peak at 4 kHz, but both children (5-11 y) and adults show an identical bimodal distribution, with two peaks in an octave distance (1.5 and 3 kHz). For the combined children and adult data, distribution density at the two peak tops is 2.8 and 2.7 times as high as at the low between them. Mean SOAE amplitudes are unrelated to the two peaks, but show a significant narrow-band dip precisely at the low in between at 2140 Hz (P<0.008). External ear canal resonance can explain the single 4 kHz mode at birth and the 3 kHz mode in children and adults. The octave shifted 1.5 kHz mode remains without mechanical explanation. It may reflect descending neural influence from central octave band processing. The results are relevant for the interpretation of OAE levels in medical diagnosis.

Transient otoacoustic emissions in the detection of olivocochlear bundle maturation

OBJECTIVE

The medial olivocochlear bundle, forms a neural pathway connecting the superior olivary complex to the outer hair cells of mainly the contralateral cochlea. Although the biological significance of the bundle remains ambiguous, it is clear that activation of medial olivocochlear fibers alters cochlear output and this can be experimentally evident through a series of changes on otoacoustic emissions, called otoacoustic emission suppression. The aim of this study is to observe the maturation of the medial olivocochlear bundle by measuring the suppression effects on transient otoacoustic emissions.

METHODS

The study population consisted of 27 premature babies (53 ears) from the P&A Kyriakou Neonatal Intensive Care Unit (conceptional age: 31-36 weeks) and 43 fullterm babies (61 ears) (conceptional age: 37-42 weeks) from the same unit. The ILO-92 system is used to deliver both the linear clicks to the ear examined (producing the emissions) and the white noise to the contralateral ear. The mode used is based on the alternating on and off presence of the white noise in the contralateral ear.

RESULTS

12/53 (22%) ears of preterm babies presented suppression > or =1 dB. 32/61 (52.4%) ears of fullterm babies presented suppression > or =1 dB. The mean suppression for the ears of premature infants was 0.52 dB, (+/-0.1 S.E.M.). The mean suppression for the ears of fullterm infants was 0.90 dB, (+/-0.09 S.E.M.). At the second half of the time window, suppression of fullterms rises above 1 dB (a pattern similar to that of adults), whereas it remains below 1 dB for preterms.

CONCLUSIONS

Suppression of otoacoustic emissions helps us to study the function of the medial olivocochlear bundle in different populations. It appears in premature babies but becomes stronger as the postconceptional age advances.

Lateral asymmetry in the ABR of neonates: Evidence and mechanisms

Lateralized processing of auditory stimuli occurs at the level of the auditory cortex but differences in function between the left and right sides are not clear at lower levels of the auditory system. The current study is designed to (1) investigate asymmetric auditory function at the ear and brainstem in human infants and (2) investigate possible mechanisms for asymmetry at these levels. Study 1 evaluated auditory brainstem responses (ABRs) in response to high and low-level clicks presented to the right and left ears of neonates. Wave V was significantly larger in amplitude and waves III and V were shorter in latency when the ABR was generated in the right ear. Study 2 investigated two possible mechanisms of such asymmetry by (a) using contralateral white noise masking to activate the medial olivocochlear system and (b) increasing stimulus rate to reveal neural conduction and synaptic mechanisms. ABR wave V, evoked by clicks to the left ear, showed a greater reduction in amplitude with contralateral noise than the response evoked from the right ear. No systematic asymmetries in ABR latencies or amplitudes were found with increased stimulus rate. We conclude that (1) the click-evoked ABR in neonates demonstrates asymmetric auditory function with a small but significant right ear advantage and (2) asymmetric activation of the medial olivocochlear system, specifically greater contralateral suppression of ABR produced by the left ear, is a possible mechanism for asymmetry.

Early maturation of evoked otoacoustic emissions and medial olivocochlear reflex in preterm neonates

The present study was designed to investigate the early maturation of the brainstem regulation of the cochlear function in preterm neonates. Evoked otoacoustic emissions (EOAE) and their regulation via the medial olivocochlear efferent (MOC) reflex were investigated in a large population of preterm neonates and compared with full-term neonates and young babies from birth to 4 y and school-aged children. In 28-wk preterm neonates, EOAE were seen in the mid-frequency range. These responses extended both to the low (down to 1025 Hz) and high (up to 6152 Hz) frequency ranges at 38 wk of gestational age and remained stable up to 4 mo. At this stage, the amplitude of EOAE overlapped adult values. EOAE amplitudes then decreased to reach adult values at 3 y of age. Maturation of MOC efferents innervating the outer hair cells was investigated by studying the suppressive effect of contralateral sound on the EOAE amplitudes (MOC reflex). The first MOC responses were recorded in preterm neonates of 32-33 wk of gestational age, reaching adult-like values at 37 wk of gestational age. The maximum effect of MOC efferent activation occurred between 2000 and 4000 Hz. These results suggest that, in humans, MOC efferents mature in utero. Thus, testing the MOC reflex may have a clinical relevance to detect an abnormal development of the auditory pathways, particularly of a brainstem circuitry not explored through conventional testing.

Evaluation of distortion products produced by the human auditory system

During the simultaneous monaural presentation of two primary tones, distortion products can be measured acoustically in the ear canal (DPOAEs) and electrically as auditory evoked potentials (DPAEPs). The purpose of this investigation was to elucidate the sources of nonlinearity within the human auditory system responsible for generating quadratic (QDT) and cubic (CDT) difference tones. Measurements of DPOAEs and DPAEPs were obtained from 24 normal-hearing adults (12 male) in conditions with and without presentation of a 60 dB SPL contralateral noise. The effects of primary-tone signal duration and mode of presentation on measurements of DPAEPs were also examined. Results indicated that overall, both acoustic and electric distortion products were suppressed during presentation of a contralateral noise. Increases in the duration of the primary tones caused increases in DPAEP amplitudes. A greater proportion of individuals exhibited DPAEPs with monotic compared to dichotic presentation of the primary tones. The findings of the investigation supported the conjecture that a cochlear nonlinearity produced CDT acoustic and electric distortion products. Evidence concerning the origin of the QDT DPAEP was inconclusive, and contributions from both cochlear and neural nonlinear sources could not be ruled out.

Cochlear maturation and otoacoustic emissions in preterm infants: a time–frequency approach

Click-evoked otoacoustic emissions (CEOAEs) from preterm infants were analyzed to characterize developmental changes of cochlear active mechanisms. Due to their strong time-varying properties, CEOAEs were studied with a time-frequency approach--the wavelet transform (WT). By means of the WT, CEOAEs were decomposed into 12 frequency bands, spanning the 0.25-6.25 kHz range. For each band, the root-mean-square (RMS) level and latency were studied as functions of both frequency and age. Because CEOAEs were averaged using the non-linear mode of acquisition, the developmental changes in observed in this study are related to the non-linear component (which is actually the most predominant component of the active cochlear response) of CEOAEs, the linear one being mostly canceled out by non-linear averaging. In our study, there was evidence that properties of CEOAE non-linear components are related to the post-conception age (PCA) in that the levels and latency of CEOAE frequency components changed until the age of about 38 weeks post-conception, whereas after 38 weeks, CEOAE features were very similar to those of term newborns. In particular, the CEOAE levels increased and latency decreased with age. The observed changes in CEOAE properties seem to reveal a development of cochlear active mechanisms, although contributions from outer and middle ear development cannot be excluded. Also, in agreement with previous physiological and behavioral findings, our results revealed that the development of CEOAE properties was not the same for all the frequencies, being greater for frequencies 4 kHz, and resembled the development of the cochlear partition, which proceeds from base to apex.

Development of f2/f1 ratio functions in humans

Otoacoustic emissions (OAEs) presumably represent active processes within the cochlea fundamental to frequency-selectivity in peripheral auditory function. Maturation of the cochlear amplifier, vis-a-vis frequency encoding or selectivity, has yet to be fully characterized in humans. The purpose of this study was to further investigate the maturation of features of the f2/f1 frequency ratio (Distortion Product OAE amplitude X f2/f1 ratio) presumed to reflect cochlear frequency selectivity. A cross-sectional, multivariate study was completed comparing three age groups: pre-term infants, term infants and young adult subjects. Frequency ratio functions were analyzed at three f2 frequencies--2000, 4000 and 6000 Hz. An analysis included an estimation of the optimal ratio (OR) and a bandwidth-like measure (Q3). Analysis revealed significant interactions of age x frequency x gender for optimal ratio and a significant interaction of age x frequency for Q3. Consistent and statistically significant differences for both OR and Q3 were found in female subjects and when f2 = 2 or 6 kHz. This supports research by others [Abdala, J. Acoust. Soc. Am. 114, 3239-3250 (2003)] suggesting that the development of cochlear active mechanisms may still be somewhat in flux at least through term birth. Furthermore, OAEs appear to demonstrate gender differences in the course of such maturational changes.

Assessment of medial olivocochlear system function in pre-term and full-term newborns using a rapid test of transient otoacoustic emissions

This study was conducted to investigate maturation of the medial olivocochlear efferent system (MOCS) in pre- and full-term neonates using Quickscreen (Otodynamics Ltd) and to confirm previous findings on transient otoacoustic emission (TEOAE) suppression in neonates. MOCS maturation was investigated in 46 neonates born at the Chaim Sheba Medical Center, Tel Hashomer, Israel, using Quickscreen. All neonates were normal with no family history of general or auditory disease and no risk factors for hearing impairment. MOCS function appears gradually in human pre-term neonates and is considered to reach maturity shortly after term birth. The clinical value of MOCS testing in specific populations of newborns at risk for hearing and/or brainstem function can be legitimately raised as activation of the MOCS clearly alters cochlear output. The present results can be interpreted to support the testing of infants at risk of developing abnormal MOCS function using a commercially available rapid TEOAE measurement system.

A longitudinal study of distortion product otoacoustic emission ipsilateral suppression and input/output characteristics in human neonates

Past work has shown that distortion product otoacoustic emission (DPOAE) (2f1-f2) ipsilateral suppression and input/output (I/O) characteristics are not adult-like in prematurely born neonates [Abdala, J. Acoust. Soc. Am. 110, 1465-1476 (2001)]. These age differences are most pronounced at f2 = 6000 Hz and have been interpreted to indicate a subtle immaturity in human cochlear function prior to term birth. It is still not clear, however, whether term-born neonates are completely adult-like in cochlear function. To study this question, DPOAE suppression and I/O functions for f2 = 6000 Hz were measured in a group of prematurely born neonates at weekly intervals over a period of 7-8-weeks, a group of normal-hearing adults, and during a one-time test session in a group of term-born neonates. Results show that there was no significant change in suppression tuning, suppression growth, and various I/O characteristics across test session for premature neonates, but there was an age-group effect; even once prematurely born neonates reached the equivalence of term-like status (38-40-weeks postconceptional weeks), they continued to show narrower suppression tuning than adults, shallower suppression growth for low-frequency side suppressor tones, and an elevated amplitude saturation plateau on the I/O function. Term-born neonates showed DPOAE results that were comparable to those measured from premature neonates and unlike adult findings. These results suggest that a subtle immaturity in cochlear function persists into the postnatal period.

Contralateral suppression of distortion-product otoacoustic emissions declines with age: a comparison of findings in CBA mice with human listeners

OBJECTIVES/HYPOTHESIS

The auditory efferent system plays presumed roles in enhancing signals in noise, maintaining the cochlea for optimal acoustic signal processing, and may have a protective role in preserving auditory function in the face of ototoxic events. The objective of the study was to measure age-related changes of the medial olivocochlear efferent system in mice by comparing distortion-product otoacoustic emissions generated with and without contralateral white noise stimulation. Consistent with prior work, distortion-product otoacoustic emissions were typically reduced in magnitude when white noise was presented to the contralateral ear. This contralateral suppression is attributed to activation of the medial olivocochlear efferent system, which has an inhibitory effect on the cochlear hair cell system. By studying contralateral suppression on cochlear output in subjects of different ages, it is possible to describe aging effects on the medial olivocochlear system.

STUDY DESIGN

CBA mice were divided into three age groups: young adult, middle-aged, and old-aged animals (21, 13, and 22 animals per group, respectively), and auditory brainstem responses were obtained before distortion-product otoacoustic emission testing to assess overall hearing abilities.

METHODS

2f1-f2 distortion-product otoacoustic emission recordings were obtained from individual subjects (anesthetized with ketamine/xylazine) in each age group under two conditions: 1) in quiet and 2) in the presence of a contralaterally applied wideband noise.

RESULTS

Principal findings were that distortion-product otoacoustic emission levels decreased with age for mice in a way similar to humans, when correcting for the absolute difference in life spans. In addition, contralateral suppression declined in middle-aged and old-aged groups relative to the young adults for mice in a manner similar to humans. The contralateral suppression decline at low frequencies preceded that of the decline in distortion-product otoacoustic emissions with age.

CONCLUSION

Functional decline of the medial olivocochlear efferent system with age precedes outer hair cell degeneration. Loss of medial olivocochlear suppressive function may play a role in the development of presbycusis in both clinical cases and animal models.

Effects of age on contralateral suppression of distortion product otoacoustic emissions in human listeners with normal hearing

The auditory efferent system presumably plays a role in enhancing signals in noise and, in particular, speech perception in background noise. This study measured the age-related changes of the medial olivocochlear (MOC) system by comparing distortion product otoacoustic emissions (DPOAEs) with and without contralateral white noise stimulation. Otoacoustic emissions were typically reduced in level (magnitude) when white noise was presented to the contralateral ear. This contralateral suppression (CS) is attributed to activation of the MOC system, which has an inhibitory effect on the outer hair cell (OHC) system. By studying CS on cochlear output in human listeners of different ages, it is possible to describe aging effects on the MOC system. Human subjects were young adult, middle aged and old (n = 10/group). All subjects had normal hearing and middle-ear function based upon standard audiometric criteria. The present study recorded 2f(1)-f(2) DPOAE-grams in response to moderate primary tones (L1 = 75, L2 = 65 dB SPL), from 1 to 6.3 kHz. The principal findings were that DPOAE levels were smaller in the old compared to the young group and that CS declined with age for the middle-aged and old groups. In addition, CS in the 1- to 2-kHz range was greater than in the 4- to 6-kHz range for all ages, but especially for the old group. These findings suggest that a functional decline of the MOC system with age precedes OHC degeneration. Moreover, the MOC system maintains better function in the 1- to 2-kHz range than in the 4- to 6-kHz range as a function of age.

Contralateral suppression of DPOAE measured in real time

The aim of this study was to measure contralateral suppression of distortion product otoacoustic emissions (DPOAE) in real time. A total of 10 human subjects were studied with a novel device to record DPOAE without signal time averaging, using digital narrow band pass filtering. Real time DPOAE levels were recorded at 2f1-f2 using primary tone settings of f2/f1 = 1.22 and L1 = 70 dB SPL, L2 = 65 dB SPL, at five values of f2 between 2.2 and 7.7 kHz. An acoustic stimulus was applied intermittently to the contralateral ear to cause DPOAE suppression. Characteristic features of contralateral suppression were identified and distinguished from small spontaneous variations in the real time DPOAE signal. Magnitude of suppression increased with contralateral stimulus intensity. Onset latency of suppression was around 43 ms (31-95 ms). Potential clinical applications are discussed in the light of these findings, including a role in improving the specificity of neonatal hearing screening.

DPOAE suppression tuning: cochlear immaturity in premature neonates or auditory aging in normal-hearing adults?

Previous work has shown that distortion product otoacoustic emission (DPOAE) suppression tuning curves (STCs) recorded from premature neonates are narrower than adult STCs at both low and high frequencies. This has been interpreted to indicate an immaturity in cochlear function prior to term birth. However, an alternative explanation for this finding is that adult DPOAE STCs are broadened and reflect cochlear hair cell loss in normal-hearing adults due to aging, and natural exposure to noise and ototoxins. This alternative hypothesis can be tested by studying suppression tuning in normal-hearing school-aged children. If normal-hearing children, who have not aged significantly or been exposed to noise/ototoxins, have DPOAE suppression tuning similar to adults, the auditory aging hypothesis can be ruled out. However, if children have tuning similar to premature neonates and dissimilar from adults, it implicates aging or other factors intrinsic to the adult cochlea. DPOAE STCs were recorded at 1500, 3000, and 6000 Hz using optimal parameters in normal-hearing children and adults. DPOAE STCs collected previously from premature neonates were used for age comparisons. In general, results indicate that tuning curves from children are comparable to adult STCs and significantly different from neonatal STCS at 1500 and 6000 Hz. Only the growth of suppression was not adultlike in children and only at 6000 Hz. These findings do not strongly support the auditory aging hypothesis as a primary explanation for previously observed neonatal-adult differences in DPOAE suppression tuning. It suggests that these age differences are most likely due to immaturities in the neonatal cochlea. However, nonadultlike suppression growth observed in children at 6000 Hz warrants further attention and may be indicative of subtle alternations in the adult cochlea at high frequencies.

Maturation of the human cochlear amplifier: distortion product otoacoustic emission suppression tuning curves recorded at low and high primary tone levels

The cochlear amplifier shows level-dependent function and works optimally at low levels. For this reason, manipulation of stimulus level is a route through which the human cochlear amplifier can be investigated in a noninvasive manner. Distortion product otoacoustic emissions (DPOAEs) evoked as a function of stimulus level provide a tool for exploration of human cochlear amplifier function and, when applied to neonates, for investigation of cochlear maturation. The current experiment generated 2f1-f2 DPOAE ipsilateral suppression tuning curves (STCs) at three primary tone levels and five f2 frequencies in a large group of premature and term neonates and adults. The differences between tuning generated with low- and high-level primary tones was measured to provide a gross estimate of the "tuning enhancement effect" attributed to the cochlear amplifier. Other features of the DPOAE suppression tuning curves were measured as well. Consistent with previous reports, at 1500 and 6000 Hz, STCs were narrower, with a steeper slope on the low-frequency flank of the tuning curve in premature neonates versus adults. Additionally, only DPOAE STCs from adults and term neonates became markedly broader and more shallow when recorded with high-level primary tones. It has been hypothesized that the excessive narrowness of suppression tuning and the absence of a level effect on DPOAE STCs recorded in premature neonates reflects a subtle immaturity in cochlear amplifier function just prior to term birth.

Estimating the acoustic reflex threshold from wideband measures of reflectance, admittance, and power

OBJECTIVE

A method was developed to estimate the contralateral acoustic reflex threshold using shifts in wideband energy reflectance, admittance magnitude and power.

DESIGN

In the first experiment contralateral reflex thresholds for a noise activator were estimated on three adult participants using reflectance, admittance and power measurements at frequencies from 250 to 8000 Hz. The reflex threshold was defined using a magnitude and a correlation technique, both having the property of examining the pattern of the reflex-induced shift across a fairly broad frequency range (250 to 2000 Hz). In the second experiment, the magnitude method was modified to include an F test for the comparison of the magnitude of reflex-induced shifts in reflectance, admittance and power relative to response differences in a no-activator baseline condition. Data from four additional participants then were analyzed across a broader frequency range using a method that combined magnitude and correlation methods of estimating reflex thresholds.

RESULTS

Acoustic reflex thresholds were obtained using reflectance, admittance and power-level measures in all subjects in both experiments. Individual reflex threshold estimates were as much as 24 dB lower than with the clinical system, with an average of approximately 14 dB lower for the three participants in the first experiment, and approximately 18 dB lower for the four participants in the second experiment.

CONCLUSIONS

Wideband measures of reflectance, admittance and power were successfully used to estimate acoustic reflex thresholds in seven participants. A reflex threshold test was devised based on the magnitude of the response shift in the presence of a contralateral activator, and the similarity of the response shift spectra across frequency between successive activator levels. Across all participants in the study, the new test yielded a more sensitive measure of the acoustic reflex threshold than the clinical method. This finding has both clinical and theoretical implications for the study of the acoustic reflex.

Distortion product otoacoustic emission suppression in subjects with auditory neuropathy

OBJECTIVE

The objective of this experiment was to address: 1) whether normal efferent system function is required for normal cochlear tuning as measured by distortion product otoacoustic emission (DPOAE) suppression in humans and 2) whether cochlear function, assessed by DPOAE suppression tuning, is normal in a small group of patients with auditory neuropathy.

DESIGN

DPOAE suppression tuning curves (STCs) are similar to other physiologic measures of tuning. They are generated by evoking a DPOAE with two simultaneously presented pure tones and then suppressing the distortion product with a third tone of varying frequency and level. In this study, DPOAE STCs were generated with f2 frequencies of 1500, 3000, and 6000 Hz in 15 normal-hearing adults and four subjects with documented auditory neuropathy. Tuning curve width, slope and tip characteristics, as well as rate of suppression growth were measured in each group. Contralateral suppression of otoacoustic emissions (OAEs) was also recorded as an index of medial efferent function.

RESULTS

Results show that the four subjects with auditory neuropathy lacked efferent suppression of OAEs. However, these four subjects showed normal estimates of cochlear tuning as measured by DPOAE suppression results.

CONCLUSIONS

This finding suggests that normal efferent system function is not required at the time of test for normal DPOAE suppression tuning. It also suggests that cochlear function as evaluated by detailed measures of DPOAE suppression, is normal in these "typical" patients with auditory neuropathy.