Influence of spontaneous otoacoustic emissions (SOAE) on acoustic distortion product input/output functions: does the medial efferent system act differently in the vicinity of an SOAE?

The Ins and Outs of Distortion Product Otoacoustic Emission Growth: A Review

Otoacoustic emissions (OAEs) are low-level signals generated from active processes related to outer hair cell transduction in the cochlea. In current clinical applications, OAEs are typically used to detect the presence or absence of hearing loss. However, their potential extends far beyond hearing screenings. Dr. Glenis Long realized this unfulfilled potential decades ago. She subsequently devoted a large portion of her storied scientific career to understanding OAEs and cochlear mechanics, particularly at the intersection of OAEs and perceptual measures. One specific application of OAEs that has yet to be translated from research laboratories to the clinic is using them to non-invasively characterize cochlear nonlinearity-a hallmark feature of a healthy cochlea-across a wide dynamic range. This can be done by measuring OAEs across input levels to obtain an OAE growth, or input-output (I/O), function. In this review, we describe distortion product OAE (DPOAE) growth and its relation to cochlear nonlinearity and mechanics. We then review biological and measurement factors that are known to influence OAE growth and finish with a discussion of potential applications. Throughout the review, we emphasize Dr. Long's many contributions to the field.

Musicianship enhances ipsilateral and contralateral efferent gain control to the cochlea

Human hearing sensitivity is easily compromised with overexposure to excessively loud sounds, leading to permanent hearing damage. Consequently, finding activities and/or experiential factors that distinguish "tender" from "tough" ears (i.e., acoustic vulnerability) would be important for identifying people at higher risk for hearing damage. To regulate sound transmission and protect the inner ear against acoustic trauma, the auditory system modulates gain control to the cochlea via biological feedback of the medial olivocochlear (MOC) efferents, a neuronal pathway linking the lower brainstem and cochlear outer hair cells. We hypothesized that a salient form of auditory experience shown to have pervasive neuroplastic benefits, namely musical training, might act to fortify hearing through tonic engagement of these reflexive pathways. By measuring MOC efferent feedback via otoacoustic emissions (cochlear emitted sounds), we show that dynamic ipsilateral and contralateral cochlear gain control is enhanced in musically-trained individuals. Across all participants, MOC strength was correlated with the years of listeners' training suggested that efferent gain control is experience dependent. Our data provide new evidence that intensive listening experience(s) (e.g., musicianship) can strengthen the ipsi/contralateral MOC efferent system and sound regulation to the inner ear. Implications for reducing acoustic vulnerability to damaging sounds are discussed.

Contralateral suppression of distortion-product otoacoustic emissions: a potential diagnostic tool to evaluate the vestibular nerve

The amplitude of distortion-product otoacoustic emission (DPOAE) is suppressed in one ear when the contralateral ear is subjected to sound stimulation. Contralateral suppression of DPOAE is the phenomenon resulted by the efferent cochlear innervation on the outer hair cells via medial olivocochlear bundle (MOCB) and inferior vestibular nerve. We assumed that DPOAE would not be suppressed by contralateral sound stimulation in patients with vestibular nerve lesion as long as the specific pathway conveying that efferent innervation is affected. To test this hypothesis, we compared the amount of DPOAE contralateral suppression in patients with vestibular neuritis and healthy controls. Twenty healthy volunteers without hearing loss and vestibulopathy, and 13 patients with vestibular neuritis were recruited. DP audiogram was measured without contralateral sound stimulation and then with contralateral sound stimulation (70 dB HL of 2 kHz narrow band noise, NBN). The suppression value of DPOAE was evaluated according to the f2 frequency and was defined as the amount of DPOAE suppression: An-Ao, where An represents the DPOAE amplitude in the presence of contralateral NBN, and Ao represents the DPOAE amplitude in the absence of NBN. Cervical vestibular evoked myogenic potential (cVEMP) was performed in some patients with vestibular neuritis. The suppression values of DPOAE were compared between groups and were analyzed according to the results of cVEMP. The amount of suppression of DPOAE during contralateral sound stimulation was significantly reduced in the patient group compared to control at the f2 frequencies of 1257, 1587, and 2002 Hz (P=0.045, P<0.001, P=0.009, respectively). However, the results of contralateral suppression of DPOAE were not consistent with the results of cVEMP in this study. Efferent cochlear innervation was affected in vestibular neuritis. Evaluation of contralateral suppression of DPOAE can be a potential diagnostic tool to evaluate the functional integrity of the vestibular nerve. Further studies are necessary to clarify this mechanism.

Measurement of medial olivocochlear efferent activity in humans: comparison of different distortion product otoacoustic emission-based paradigms

OBJECTIVE

To assess the suitability of contralateral suppression (CS) of distortion product otoacoustic emissions (DPOAEs) for measurement of activity of the medial olivocochlear (MOC) efferents.

BACKGROUND

The MOC efferent system has been shown to be involved in sound discrimination, selective attention to tones, sound localization, and protection of the cochlea against noise. A great variety of paradigms for measurement of MOC activity by CS of OAE (MOC reflex [MOCR]), has been described. An issue of this approach is the dependence of the CS values on stimulus parameters, especially when DPOAE are used.

METHODS

Four different measurement paradigms, which used different combinations of stimulus frequencies and primary tone levels, were applied in 16 human subjects.

RESULTS

Mean absolute values of CS were in the range of 1.2 to 2.6 dB. The use of different stimulus parameters produced not only MOCR values of different size-which was expected-but, in many cases, also different relative classifications of the subjects according to their MOCR strength.

CONCLUSION

The suppression effects on DPOAE demonstrated in this study reflect MOC activity. However, the new conclusion from our data is that CS of DPOAE measurements, as they were used in this study, may not allow for a consistent quantitative classification of human subjects according to their MOCR strength. This finding concerns interpretation of previous studies using CS of DPOAE and analogous future studies. One future approach may lie in the separation of the DPOAE components to distinguish interference phenomena, which complicate interpretation of CS values.

Effects of age and background noise on processing a mistuned harmonic in an otherwise periodic complex sound

Older adults presented with short (i.e., 40 ms) harmonic complex tones show a reduced likelihood of hearing the mistuned harmonic as a separate sound. Here, we examined whether this age difference for the mistuned harmonic would generalize to a longer signal duration (i.e., 200 ms). We measured auditory evoked fields (AEFs) using magnetoencephalography while young and older adults were presented with harmonic complex tones that either had all partials of the tones in tune (single sound object) or contained a 4 or 16% mistuned harmonic (dual sound objects). The auditory stimuli were presented in isolation or embedded in low or moderate levels of continuous white noise. For each participant, we modeled the AEFs with a pair of dipoles in the superior temporal plane and examined the effects of age and noise on the amplitude and latency of the resulting source waveforms. The present study reveals similar noise-induced increases in N1m and object-related negativity in young and older adults which may be mediated via efferent feedback connections and/or changes in the temporal window of integration. We observed less age-related differences in concurrent sound segregation for stimuli that matched the duration of the temporal integration window of auditory perception (i.e., ∼200 ms) than for short duration sounds (i.e., 40 ms). Possible explanations for this duration-dependent age-related decline in concurrent sound perception are a general slowing in auditory processing and/or lengthening of the temporal integration window.

Distortion product otoacoustic emission fine structure is responsible for variability of distortion product otoacoustic emission contralateral suppression

Alteration of the distortion product otoacoustic emission (DPOAE) level by a contralateral sound, which is known as DPOAE contralateral suppression, has been attributed to the feedback mechanism of the medial olivocochlear efferents. However, the limited dynamic range and large intra- and intersubject variabilities in the outcome of the measurement restrict its application in assessing the efferent function. In this study, the 2f(1)-f(2) DPgram was measured with a high frequency resolution in six human ears, which exhibits a fine structure with the quasiperiodic appearance of peaks and dips. In the presence of contralateral noise, the DPOAE level increased, decreased, or remained unchanged depending on the frequency. At the peaks, DPOAEs were mostly suppressed with a larger change, while those at the dips had greater variance, with increased occurrence of enhancement or no change. The difference between the peak and dip frequencies in the DPOAE-level change was significant. A switch from suppression to enhancement of the DPOAE level or vice versa with a small change in frequency was noted. These results imply that the DPOAE fine structure is a main source of the variability in DPOAE contralateral suppression measurement. The study suggests that the DPOAE contralateral suppression test would be improved if it is conducted for frequencies at major peaks of the DPOAE fine structure.

Medial olivocochlear functioning and speech perception in noise in older adults

The purposes of this study were to: (1) compare medial olivocochlear system (MOCS) functioning and speech perception in noise in young and older adults and (2) to quantify the correlation between MOCS functioning and speech perception in noise. Measurements were taken in 20 young (mean 26.3 +/- 2.1 years) and 20 older adults (mean 55.2 +/- 2.8 years). Contralateral distortion product otoacoustic emission (DPOAE) suppression was measured to assess MOCS functioning. Speech perception in noise was evaluated using the Hearing in Noise Test in noise-ipsilateral, noise-front and noise-contralateral test conditions. The results revealed that the older group had a significantly lower high-frequency (3-8 kHz) contralateral DPOAE suppression, and performed more poorly in the noise-ipsilateral condition than the younger group. However, there was no correlation between contralateral DPOAE suppression and speech perception in noise. This study suggests that poor speech perception performance in noise experienced by older adults might be due to a decline in medial olivocochlear functioning, among other factors.

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.

A Clinical Study of the Efferent Auditory System in Patients With Normal Hearing Who Have Acute Tinnitus

OBJECTIVE

Etiological diagnosis and treatment of tinnitus still remain challenging in clinical practice. The aim of this study was to determine the potential contribution of a defective cochlear efferent innervation to the onset of tinnitus in patients with normal hearing.

STUDY DESIGN

Prospective randomized controlled study.

SETTING

Otorhinolaryngology department of a general hospital.

PATIENTS

The patient group consisted of 18 normal-hearing adults (7 men, 11 women) with acute tinnitus (bilateral in 3 patients).

INTERVENTIONS

Tympanogram, stapedial muscle reflex, pure tone audiometry, tinnitus pitch matching, spontaneous otoacoustic emissions, and distortion product otoacoustic emissions (DPOAEs) in the absence and presence of contralateral suppression by white noise.

MAIN OUTCOME MEASURE

DPOAEs suppression amplitudes recorded from tinnitus and nontinnitus ears of the patients' group were compared with each other and with a control group.

RESULTS

The contralateral application of white noise induced the enhancement of DPOAE amplitudes in some patients. The suppression of DPOAE amplitudes by contralateral white noise did not reach statistically significant levels in either ear (with or without tinnitus). On the contrary, under the same conditions, our control group demonstrated statistically significant reduction of DPOAE amplitudes at all frequencies.

CONCLUSION

Patients with normal hearing acuity who have acute tinnitus seem to have a less effective functioning of the cochlear efferent system because the application of contralateral noise enhanced the DPOAEs or suppressed them less intensely than it did in a control group. Further studies may establish the clinical applications for the diagnosis of changes in efferent function, in the subjective evaluation, patient etiological grouping, treatment, or prognosis of tinnitus.

Olivocochlear reflex effect on human distortion product otoacoustic emissions is largest at frequencies with distinct fine structure dips

Activity of the medial olivocochlear efferents can be inferred by measuring the change of the level of distortion product otoacoustic emissions (DPOAE) during ipsilateral or contralateral acoustic stimulation, the so-called medial olivocochlear reflex (MOCR). A limitation of this measurement strategy, however, is the distinct variability of MOCR values depending on DPOAE primary tone levels and frequency, which makes selection of the stimulus parameters difficult. The objective of this study was to evaluate the dependence of MOCR values on DPOAE fine structure in humans. MOCR during contralateral acoustic stimulation was measured at frequencies with distinct non-monotonicity ("dip") in the DPOAE fine structure, and in frequencies with flat fine structure. One hundred and twenty one different primary tone level combinations were used (L(1)=50-60dB SPL, L(2)=35-45dB SPL, 1dB steps). The measurement was repeated on another day. The major findings were: (1) Largest MOCR effects can be found in frequencies which exhibit a distinct dip in DPOAE fine structure. (2) Primary tone levels have a critical influence on the magnitude of the MOCR effect. MOCR changes of up to 23dB following a L(1) change of only 1dB were observed. Averages of the maximum MOCR change per 1dB step were in the 3-5dB-range. Both findings can be interpreted in the light of the DPOAE two-generator model [Heitmann, J., Waldmann, B., Schnitzler, H.U., Plinkert, P.K., Zenner, H.P. 1998. Suppression of distortion product otoacoustic emissions (DPOAE) near 2f1-f2 removes DP-gram fine structure - evidence for a secondary generator. Journal of the Acoustical Society of America 103, 1527-1531]. According to the present results we propose, that assessing MOCR specifically at frequencies with a distinct dip in the DPOAE fine structure, in combination with fine variation of the stimulus tone levels, allows for a more targeted search for maximum MOCR effects. Future studies must show if this approach can contribute to the further clarification of the physiological roles of the olivocochlear efferents.

Outer and middle ear status and distortion product otoacoustic emissions in children with sickle cell disease

The purpose of this study was to investigate distortion product otoacoustic emissions (DPOAEs) and outer/middle ear status in 12 African American children with normal hearing and homozygous sickle cell disease (SCD) and age-, gender-, and ear-matched African American controls. C. R. Downs, A. Stuart, & D. Holbert (2000) reported that DPOAE amplitudes were significantly larger for children with SCD. Because the integrity of the middle ear system directly influences OAE characteristics, it was felt that concurrent investigation of DPOAE amplitudes and outer/middle ear function in children with SCD was warranted. DPOAEs were evoked by 13 primary-tone pairs with f2 frequencies ranging from 1000 to 4500 Hz. Outer/middle ear status was assessed with tympanometry through indices of peak compensated static acoustic admittance, tympanometric width, tympanometric peak pressure, ear canal volume, and middle ear resonance frequency. Tympanograms were recorded with probe-tone frequencies of 226 and 678 Hz. DPOAE amplitudes were significantly larger for children with SCD (p < .05). There were no group differences in any of the middle ear indices (p > .05). These findings suggest that increased DPOAE amplitudes for children with SCD cannot be attributed to differences in outer/middle ear function as assessed with tympanometry.

Input-output functions for stimulus-frequency otoacoustic emissions in normal-hearing adult ears

Input-output (I/O) functions for stimulus-frequency (SFOAE) and distortion-product (DPOAE) otoacoustic emissions were recorded in 30 normal-hearing adult ears using a nonlinear residual method. SFOAEs were recorded at half octaves from 500-8000 Hz in an L1=L2 paradigm with L2=0 to 85 dB SPL, and in a paradigm with L1 fixed and L2 varied. DPOAEs were elicited with primary levels of Kummer et al. [J. Acoust. Soc. Am. 103, 3431-3444 (1998)] at f2 frequencies of 2000 and 4000 Hz. Interpretable SFOAE responses were obtained from 1000-6000 Hz in the equal-level paradigm. SFOAE levels were larger than DPOAEs levels, signal-to-noise ratios were smaller, and I/O functions were less compressive. A two-slope model of SFOAE I/O functions predicted the low-level round-trip attenuation, the breakpoint between linearity and compression, and compressive slope. In ear but not coupler recordings, the noise at the SFOAE frequency increased with increasing level (above 60 dB SPL), whereas noise at adjacent frequencies did not. This suggests the existence of a source of signal-dependent noise producing cochlear variability, which is predicted to influence basilar-membrane motion and neural responses. A repeatable pattern of notched SFOAE I/O functions was present in some ears, and explained using a two-source mechanism of SFOAE generation.

Influence of contralateral stimulation by two-tone complexes, narrow-band and broad-band noise signals on the 2f1-f2 distortion product otoacoustic emission levels in humans

In order to test the frequency specificity of the efferent suppressive effect on otoacoustic emissions, changes in the 2f1-f2 distortion product otoacoustic emission (DPOAE) levels induced by contralateral stimuli of different spectra were measured in 10 normally hearing adults. Three types of contralateral stimuli were used: (i) a set of 6 pairs of pure tones with the same frequencies as used for DPOAE stimulation; (ii) 6 narrow-band noise signals with cut-off frequencies equal to the frequencies of the primary tones used for DPOAE stimulation; and (iii) broad-band noise with a bandwidth of 840-6,000 Hz. A small suppressive effect was observed mainly in the mid-frequency region. Broad-band noise was more effective at suppressing DPOAEs than narrow-band noises and two-tone complexes. Occasionally, small enhancements in DPOAE amplitudes were observed. Based on the results of this study, it is concluded that DPOAE changes induced by contralateral stimuli are not frequency-specific, and are too small to have routine clinical value.

Effects of spontaneous otoacoustic emissions on distortion product otoacoustic emission

OBJECTIVE

It has been reported that spontaneous otoacoustic emission (SOAE) can prolong the responses or increase the echo power of transiently evoked otoacoustic emission (TEOAE), yet the effects of SOAE on distortion product otoacoustic emission (DPOAE) have been studied less thoroughly. As most of the previous studies have not paid attention to the patient's age, sex and hearing level, they have not reflected possible effects of those factors. We studied the effects of SOAE specifically on DPOAE in the following subjects.

SUBJECTS AND METHODS

The subjects were all females ranging in age from 19 to 24 (average: 21.4) and the 78 ears had a hearing threshold under 15 dB for 1.2, and 4 kHz on pure-tone hearing test. IL088 (Otodynamics) was used for measurement of SOAE and IL092 (Otodynamics) for DPOAE. SOAEs were measured by time-averaging over 100 of the responses, of which those showing a clear peak 3 dB above the noise floor and being reproducible were considered as SOAE-positive. In all the ears. DPOAE responses were measured at L1 = L2 = 70 dB, and in 42 ears also at L1 = L2 = 60 dB and L1/L2 = 60/50 dB. The subjected ears were grouped into two by the presence or the absence of SOAE, and DPOAE amplitudes of 1, 2, and 4 kHz were compared. respectively.

RESULTS

Of the total, 39 ears were SOAE-positive and 39 SOAE-negative. Statistically no significant difference was observed in the average hearing level between the SOAE-positive and SOAE-negative groups. The hearing levels did not significantly differ in the frequencies of 1, 2. and 4 kHz, respectively, indicating that influence of the hearing level on DPOAE could be excludable. DPOAE amplitudes at L1 = L2 = 70 dB in the frequencies of 1, 2, and 4 kHz were higher in the SOAE-positive group than in the SOAE-negative group. And DPOAE amplitudes were also higher in SOAE-positive group at L1 = L2 = 60 dB and L1/L2 = 60/50 dB in the frequency of 1.2, and 4 kHz, but significant differences were observed only in the frequencies of 4 kHz. By grouping the ears by the number of SOAE. we revealed the tendency that the larger the number of SOAE, the higher the DPOAE amplitudes.

CONCLUSIONS

We evidenced that SOAE has significant effects on DPOAE responses. In clinical application of DPOAE measurement, therefore, the effects should be seriously taken into account.

Otoacoustic Emissions in Patients with Retrocochlear Dysfunction: A Report of Five Cases

This article describes the results of otoacoustic emissions testing in five patients with sensorineural hearing loss. We conducted an audiologic evaluation and performed image and cerebral function studies on each patient. Our investigation revealed that all five had spontaneous otoacoustic emissions and/or distortion product otoacoustic emissions. Pharmacologic treatment with anxiolytics, antidepressants, and anticonvulsant drugs was successful in alleviating symptoms in three patients. An organic and a central functional cause of symptoms was found in the remaining two patients. Before pharmacologic therapy, the spontaneous otoacoustic emissions were of high intensity, and the distortion product emissions were minimal. Following drug therapy, the former disappeared and the latter appeared.

[Precocious maturation of auditory evoked potentials in prematures: influence of gestational age and sex]

In humans, the onset of the auditory function occurs at the beginning of the third trimester of pregnancy. Brainstem auditory evoked potentials (BAEPs) can be recorded in preterm neonates as early as 25-26 weeks of gestational age. The latency of BAEP waves show a significant decrease according to increasing age to achieve adult values at the end of the first month after birth for wave I and near 3 to 5 years old for waves III and V. Auditory evoked responses are influenced by gender, notably with significantly higher wave latencies in males than in females. These gender differences in auditory function appear early in humans, some being observed as soon as 34 weeks of gestational age.

Effect of contralateral masking on the latency of otoacoustic emissions elicited by acoustic distortion products

Otoacoustic emissions (OAE) are sound products generated by the outer hair cells (OHC) in the inner ear. The OHC are capable of moving spontaneously or in response to acoustic stimuli (spontaneous otoacoustic emissions and evoked otoacoustic emissions), these movements are known as electromotility. Electromotility is affected when contralateral acoustic stimulation is introduced to the ear. Different types of stimuli may produce this response. Clicks, pure tones, and white masking noise have been used as contralateral stimulation. This effect appears to be mediated by the medial efferent olivocochlear bundle. Contralateral masking produces suppression of OAE, especially on the amplitude. However, the effect of contralateral masking on the latency of distortion product otoacoustic emissions (DPOAE) has not been studied. The purpose of this paper is to investigate whether contralateral masking, with wide band masking noise, may produce a significant change on the latency of the DPOAE. Three different latency measurements of DPOAE measurements were made on low, middle and high frequencies of fl including 574 Hz, 2454 Hz and 4919 Hz. Each one of these frequencies was measured with and without contralateral masking. Twenty-eight ears of 15 subjects were studied. Non-significant differences (P > 0.05) between masked and unmasked conditions were found in all cases. It is concluded that contralateral masking does not appear to affect latency of DPOAE.

Basic characteristics of distortion product otoacoustic emissions in infants and children

Distortion-product otoacoustic emissions at the 2 f1-f2 frequency (DPOAEs) are being advocated as a clinical tool for diagnosis of peripheral auditory pathology. Because they can be measured quickly and noninvasively, they may be an excellent method for identifying hearing loss in infants and children. However, few studies have examined the characteristics of DPOAEs in infants and children or detailed if, and how, their responses differ from those of adults. The purpose of the current study was to determine basic characteristics of DPOAEs in infants, toddlers, children, and young adults and to define any differences among age groups. An additional goal was to ensure that the presence of spontaneous otoacoustic emissions (SOAEs) did not confound any developmental effect. DPOAE input/output (I/O) functions at seven f2 frequencies and SOAEs were measured from one ear of 196 subjects. Children aged less than 1 yr had significantly higher mean DPOAE levels than older children and adults, and children aged 1-3 yr had higher mean DPOAE levels than teens and adults. These differences were dependent on frequency but were independent of f2 level and SOAE status. At every f2 frequency, groups of individuals having SOAEs had higher mean DPOAE levels than those not having SOAEs.

Otoacoustic emission amplification after inner hair cell damage

Otoacoustic emissions (OAEs) are considered to originate from active cochlear processes involving the outer hair cells (OHC). These emissions are suppressed by activity in the efferent olivocochlear bundle (OCB) and following OHC damage caused by noise exposure or ototoxic drugs. Temporary enhancement of OAEs may occur following noise exposure, and permanent enhancement of emissions has been associated with primary afferent dysfunction in the auditory system. This suggests that there are active adaptation processes in the cochlea exist that could potentially compensate for loss of afferent input. We have used the anti-cancer drug carboplatin to induce selective inner hair cell (IHC) lesions in the cochleae of chinchilla and measured the elevation of auditory thresholds that occurred using brainstem responses (ABR). Following carboplatin treatment click evoked otoacoustic emissions (CEOAEs) were amplified from cochlear frequency regions, which demonstrated extensive IHC damage but apparently normal OHCs. These results support the theory that OHCs cells are involved in the production of these cochlear emissions but also provides further evidence that active adaptation processes exist in the cochlea. It is postulated that loss of afferent input reduces the activity in the medial efferent OCB resulting in de-suppression of OHC contractility. Enhanced OHC contractility could then produce amplification of CEOAEs.

Contralateral auditory stimulation and otoacoustic emissions: a review of basic data in humans

The influence of contralateral auditory stimulation on otoacoustic emissions (OAE), spontaneous OAE, evoked OAE and acoustic distortion products, can be summarized as follows: (1) alteration (mainly a decrease) of OAE amplitude; (2) alteration of response spectrum (upward shift frequency of SOAE); (3) alteration of phase; (4) effect dependent on intensity of contralateral stimulation; (5) effect inversely dependent on intensity of ipsilateral stimulation; (6) frequency specificity of the suppressive effect. Involvement of the medial olivocochlear bundle is highly probable but one cannot exclude a double pathway including also the acoustic reflex.

Differential effects of visual attention on spontaneous and evoked otoacoustic emissions

A visual task decreases evoked otoacoustic emissions (TEOAE) to an interindividually variable degree. The aim of these two studies was to assess whether the presence or absence of spontaneous otoacoustic emissions (SOAE) is involved in this variability. The first experiment investigated spontaneous otoacoustic emission (SOAE) frequency shifts during a visual attention task with a Fast Fourier Transform analysis, and found no attention effect on SOAEs. Using a larger subject pool, the second experiment concerned the attention effect on transiently evoked otoacoustic emissions and SOAE amplitude, using different measurement methods with the same visual task. The sample was composed of two groups of subjects: group A comprising subjects presenting SOAEs and group B subjects without SOAEs. A decrease in EOAE amplitude for group B during attention was observed and seems to confirm a possible influence of attention on the cochlea via the medial olivocochlear efferent system (MOES). The TEOAE amplitude of group A did not vary during attention, showing the peculiarity of this population of subjects and suggesting a different cochlear functioning.

Suppression tuning characteristics of 2f1-f2 distortion product otoacoustic emissions

The 2f1-f2 distortion product otoacoustic emission (DPOAE) is suppressed by a third tone following a frequency selective pattern and outlining tuning curves which are generally similar to neural and psychophysical findings. The most effective suppressor tone lies between f1 and f2; less suppressive effects are produced by an added tone that lies in the 2f1-f2 site. The slope of iso-suppression curves is much steeper on the higher flank than on the lower flank of the suppression curve (respectively 100-115 dB/octave and 25-35 dB/octave). A suppression fine structure can be observed in the region of the tip especially for higher DPs probably connected with the fine structure of distortion products and with the instability of cochlear activity. A saturation point, around 70 dB or more has been evidenced on the growth rate functions. Continuous and interrupted suppressor presentation can induce some differences in the responses. Suppression of DPOAEs can provide a wider knowledge on active non-linear mechanisms in the cochlea, and on frequency selectivity also in a clinical context.

Contralateral suppression of transiently evoked otoacoustic emissions and tinnitus

The present paper reports individual data obtained in three different patients who consulted for unilateral tinnitus in the Department of Otolaryngology. After pure tone and high-frequency audiometry, Audioscan audiometry was recorded, and tinnitus measurement which comprised a determination of pitch and loudness matches. Spontaneous, transient evoked and 2f1-f2 distortion product otoacoustic emissions at 65 dB SPL stimulus intensity were then determined. The functioning of the medial olivocochlear system (MOC) was also tested from a comparison between OAE input/output curves obtained in the presence and absence of 30 dB SL contralateral stimulation by a broadband noise: MOC global effectiveness was assessed through transient evoked emissions while those concerning distortion product emissions allowed a precise testing at the tinnitus frequency itself. The examples here displayed illustrate the diversity of results that can be found in such investigations, thereby preventing a general law to be established from the global testing of MOC functioning. On the contrary, the local testing at the precise frequency of tinnitus revealed the existence of an alteration of MOC functioning in at least one ear as shown by either a weak, null or inverse effect of contralateral stimulation. A better understanding of the sensorineural forms of tinnitus should come from the extensive investigation of MOC efficiency along the basilar membrane when routine clinical investigations lead one to suspect an involvement of this system, due to discordant damaging between inner and outer hair cells. Such studies would allow one to test the validity of hypotheses which invoke MOC-related mechanisms as an essential link for the emergence and persistence of tinnitus.

Growth of distortion product otoacoustic emissions with primary-tone level in humans

This study addressed the hypotheses that the growth of the level of distortion product otoacoustic emissions (LDP) with primary-tone level reflects the behavior of a third-order nonlinear polynomial system, and that two sources exist for these distortion products. The results indicated that the 2f1-f2 otoacoustic emission in humans can be measured over a much larger stimulus range than reported previously, even for stimuli (L1 = L2) as low as 10 dB SPL (re 20 microPa). The input/output functions are best described as a straight line with a rate of growth of about 1 dB/dB of stimulus level. For stimulus levels at which metabolically active, nonlinear cochlear processes are in operation, the system does not behave as a simple third-order nonlinear polynomial. Small plateaus and sharp discontinuities or 'notches' can occur in the functions at stimulus levels of approximately 55 dB SPL. These characteristics are consistent with the notion of two separate sources of the LDP, one at low stimulus levels, and one at high levels. An alternative explanation is that the measured otoacoustic emission does not represent only the activity at a single location along the basilar membrane but includes the effects of interactions among similar signals arising from multiple locations, or from the original source via multiple paths.

Contralateral sound suppresses distortion product otoacoustic emissions through cholinergic mechanisms

Presentation of an acoustic signal to one ear can suppress sound-evoked activity recorded at the opposite ear. The suppression appears to be mediated by medial olivocochlear (MOC) efferent neurons synapsing with outer hair cells (OHCs) and acting through the MOC neurotransmitter, acetylcholine (ACh). The purpose of the present investigation was to study the suppression of distortion product otoacoustic emissions (DPOAEs) by contralateral sound and to examine whether the suppression could be blocked by known antagonists of olivocochlear (OC) efferent activity. Urethane-anesthetized guinea pigs were used. Perilymph spaces of ipsilateral cochleae were alternately perfused with artificial perilymph and drugs at 2.5 microliters/min for 10 min. After each period of perfusion, DPOAEs were measured before, during and after contralateral wideband noise (WBN) stimulation. Pre-perfusion, contralateral WBN attenuated the ipsilateral DPOAEs between 1-3 dB. This suppression was blocked reversibly by strychnine (10 microM), curare (10 microM) and atropine (20 microM), known antagonists of OC efferent activity. These results confirm the findings of Puel and Rebillard (1990) that contralateral WBN can suppress DPOAEs in anesthetized guinea pigs. Furthermore, results suggest that this efferent control of the cochlear mechanical response can either be mediated by both nicotinic and muscarinic cholinergic receptors, or that a single receptor with as yet undescribed structure and pharmacology mediates effects seen.

Voluntary contraction of middle ear muscles: effects on input impedance, energy reflectance and spontaneous otoacoustic emissions

Two types of measurements were performed on a subject able to voluntarily contract her middle ear muscles (MEM). First, wideband measurements (0-11 kHz) of middle ear input impedance and energy reflectance were obtained when the subject was relaxed and when she contracted her MEM. The changes in impedance observed with voluntary MEM contraction were similar to those reported in the literature for acoustically-elicited MEM contractions. The energy reflectance increased for frequencies below about 4 kHz. Second, the effects of voluntary MEM contraction on the frequencies and levels of spontaneous otoacoustic emissions (SOAEs) were measured and compared to effects evoked by contralateral acoustic stimulation. Effects on SOAEs appear to be a more sensitive indicator of MEM activity than changes in impedance, and the effects due to voluntary MEM contraction were qualitatively similar to those evoked by contralateral acoustic stimulation. These results suggest that in subjects with normally-functioning middle ears, only some effects on otoacoustic emissions caused by contralateral stimuli whose levels are below the contralateral acoustic reflex threshold can be unequivocally attributed to the action of cochlear efferents. The temporal aspects of SOAE frequency shifts caused by voluntary contraction of MEM show that voluntary contraction fatigues rapidly over a time period of tens of seconds.

Modulation of f2-f1: evidence for a GABA-ergic efferent system in apical cochlea of the guinea pig

f2-f1, but not 2f1-f2, was reduced in amplitude during continuous stimulation of the test ear with the primary tones, and with single tones near the primary frequencies. Stimulation of the contralateral ear, either with broad band noise or with single tones near the primary frequencies, also reduced f2-f1. Ipsilateral and contralateral effects were additive and were restricted to the frequency range between about 2 kHz and 7 kHz. Contralateral, but not ipsilateral suppression, was blocked after systemic administration of strychnine. Ipsilateral suppression was eliminated by perfusion of the cochlea with tetrodotoxin. Both contralateral and ipsilateral suppression were abolished after perfusion of the cochlea with bicuculline. The results are evidence for a role for a GABA-ergic efferent system in the modulation of outer hair cell mechanics in the apical cochlea.

Use of otoacoustic emissions to explore the medial olivocochlear system in humans

The association between contralateral stimulation and otoacoustic emissions makes it possible to study the medial olivocochlear system in humans. Characterization of this functional exploration and clinical applications are discussed.

Interrelations between transiently evoked otoacoustic emissions, spontaneous otoacoustic emissions and acoustic distortion products in normally hearing subjects

Active cochlear mechanisms and especially outer hair cells seem to be involved in oto-acoustic emissions (OAEs) genesis. This study sought to investigate basic characteristics of spontaneous otoacoustic emissions (SOAEs), click-evoked otoacoustic emissions (TOAEs) and interrelations between SOAEs, TOAEs and 2f1-f2 and 2f2-f1 distortion product OAEs (DPOAEs) in 135 normally hearing subjects. A gender effect was shown on TOAEs and DPOAEs amplitude, and is attributed to the higher incidence of SOAEs in women (58%) than in men (22%). Moreover, SOAEs presence seems to mask the age effect found, especially at high frequency components, on TOAEs amplitude. A general influence of SOAEs on TOAEs and DPOAEs is shown, especially at frequencies ranging from 1 kHz to 3 kHz, collecting more than 66% of the SOAEs peaks recorded. Lastly, correlations between TOAEs frequency band amplitude and 2f1-f2 DPOAEs amplitude, shows frequency specificity, at least at low frequencies (i.e., from 0.5 to 2 kHz) in agreement with previous works suggesting that the 2f1-f2 DPOAEs generation site is at the geometric mean of the primaries. The same correlations calculated with 2f2-f1 DPOAEs amplitude show frequency specificity at low frequencies i.e., at 800 Hz and 1600 Hz. 2f2-f1 DPOAEs in humans are shown to be generated near the 2f2-f1 frequency region on the cochlear partition.