Tone Burst-evoked Otoacoustic Emissions Research Articles

Magnitude of medial olivocochlear reflex assayed by tone-burst-evoked otoacoustic emissions: reliability and comparison with click-evoked emissions

Objective The purpose of this pilot study was to evaluate the magnitude of the medial olivocochlear reflex (MOCR) estimated by the reduction in tone-burst evoked otoacoustic emissions (TBOAEs) measured at three levels and at three frequencies in response to fixed contralateral white noise. Results were compared with commonly used click-evoked otoacoustic emissions (CEOAEs). Design TBOAEs and CEOAEs, with and without contralateral 60 dB SPL white noise, were measured in response to stimulation at 55, 65, and 75 dB peSPL. In each subject, the set of measurements was performed twice. Of particular interest were the MOCR and its repeatability. Study sample 15 normally hearing persons (13 women, average age 32.3 years, SD = 8.1). Results For both CEOAE and TBOAEs, the reliability of the MOCR was much better for broadband measurements than for half-octave-band filtered estimates. At the same time, the reliability of MOCR in half-octave bands was higher for TBOAEs than for CEOAEs, especially at 2 and 4 kHz. Conclusions For general applications where broadband MOCR is of interest, the highest magnitude and reliability is provided by CEOAEs. However, TBOAEs may be better if a particular frequency band is of interest.

KURTOZA (SPEKTRALNA) SYGNAŁU EMISJI OTOAKUSTYCZNYCH Z POMOCĄ TRANSFORMACJI HUSIMI’EGO – BADANIE PILOTAŻOWE

BackgroundTime-frequency distributions can help reveal resonant modes of OAEs. The Husimi transform is the time-frequency distribution of probability. The sound pressure probability density function for a given frequency can be derived from the Husimi transform. Using the Husimi transform as the weight function, it is possible to define the spectral kurtosis of OAEs.Material and MethodsThe Husimi transform was calculated numerically from OAE data recorded from subjects with normal hearing. We examined click-evoked OAEs (CEOAEs) and tone-burst-evoked OAEs (TBOAEs) with stimuli centered at 1, 2, and 4 kHz, and the presence of spontaneous OAEs (SOAEs) was also investigated. The aim of this study was to examine the statistical properties of otoacoustic emissions (OAEs) and relate them to resonant modes of the cochlea. Assuming that the probability of the sound pressure of an OAE at any time and frequency is given by a Husimi-type transform, we analyzed statistical features of the probability distribution, particularly spectral kurtosis.ResultsWith evoked OAEs, a minimum in kurtosis was found at frequencies close to SOAEs. With TBOAEs, three sorts of SOAEs were found: those with high positive kurtosis, those with small positive kurtosis, and those with negative kurtosis; in these cases, SOAEs appeared at the same frequency as the kurtosis minimum.ConclusionsThe kurtosis of evoked components of an OAE is strongly affected by the presence of an SOAE. The number of positive peaks and negative troughs of spectral kurtosis in a given frequency band seem to be characteristic of each subject. It is suggested that a new way of distinguishing types of OAEs may involve calculating the spectral kurtosis, and this may be diagnostically useful.

Simultaneous suppression of tone burst-evoked otoacoustic emissions: Two and three-tone burst combinations

Previous investigations have shown that components of a tone burst-evoked otoacoustic emission (TBOAE) evoked by a 1 kHz tone burst (TB1) can be suppressed by the simultaneous presence of a 2 kHz tone burst (TB2) or a pair of tone bursts at 2 and 3 kHz (TB2 and TB3 respectively). No previous study has measured this “simultaneous suppression of TBOAEs” for both TB2 alone and TB2 and TB3 from the same ears, so that the effect of the additional presence of TB3 on suppression caused by TB2 is not known. In simple terms, three outcomes are possible; suppression increases, suppression is reduced or suppression is not affected. Comparison of previously reported simultaneous suppression data suggests TB3 causes a reduction in suppression, though it is not clear if this is a genuine effect or simply reflects methodological and ear differences between studies. This issue has implications for previously proposed mechanisms of simultaneous suppression of TBOAEs and the interpretation of clinical data, and is clarified by the present study. Simultaneous suppression of TBOAEs was measured for TB1 and TB2 as well as TB1, TB2 and TB3 at 50, 60 and 70 dB p.e. SPL from nine normal human ears. Results showed no significant difference between mean suppression obtained for the two and three-tone burst combinations, indicating the reduction of suppression inferred from comparison of previous data is likely a result of methodological and ear differences rather than a genuine effect.

Further tests of the local nonlinear interaction-based mechanism for simultaneous suppression of tone burst-evoked otoacoustic emissions

Tone burst-evoked otoacoustic emission (TBOAE) components measured in response to a 1 kHz tone burst (TB1) are suppressed by the simultaneous presence of an additional tone burst (TB2). This “simultaneous suppression of TBOAEs” has been explained in terms of a mechanism based on local nonlinear interactions between the basilar membrane (BM) travelling waves caused by TB1 and TB2. A test of this local nonlinear interaction (LNI)-based mechanism, as a function of the frequency separation (Δf, expressed in kHz) between TB1 and TB2, has previously been reported by Killan et al. (2012) using a simple mathematical model [Killan et al., Hear. Res. 285, 58–64 (2012)]. The two experiments described in this paper add additional data on the extent to which the LNI-based mechanism can account for simultaneous suppression, by testing two further hypotheses derived from the model predictions. Experiment I tested the hypothesis that TBOAE suppression is directly linked to TBOAE amplitude nonlinearity where ears that exhibit a higher degree of amplitude nonlinearity yield greater suppression than more linear ears, and this relationship varies systematically as a function of Δf. In order to test this hypothesis simultaneous suppression at a range of values of Δf at 60 dB peak-equivalent sound pressure level (p.e. SPL) and TBOAE amplitude nonlinearity from normal human ears was measured. In Experiment II the hypothesis that suppression will also increase progressively as a function of increasing tone burst level was tested by measuring suppression for a range of Δf and tone burst levels at 40, 50, 60 and 70 dB p.e. SPL. The majority of the findings from both experiments provide support for the LNI-based mechanism being primarily responsible for simultaneous suppression. However, some data were inconsistent with this view. Specifically, a breakdown in the relationship between suppression and TBOAE amplitude nonlinearity at Δf = 1 (i.e. when TB2 was reasonably well separated from, and had a higher frequency than TB1) and unexpected level-dependence, most notably at Δf = 1, but also where Δf = −0.5, was observed. Either the LNI model is too simple or an alternative explanation, involving response components generated at basal regions of the basilar membrane, is required to account for these findings.

Tone-Burst and Click-Evoked Otoacoustic Emissions in Subjects With Hearing Loss Above 0.25, 0.5, and 1 kHz

The aim of this study was to assess the diagnostic value of click-evoked otoacoustic emissions (CEOAEs) and tone-burst evoked otoacoustic emissions (TBOAEs) in identifying the residual hearing of subjects with partial deafness (PD)--a condition in which subjects have normal thresholds at low frequencies and severe-to-profound sensorineural hearing loss at high frequencies. Otoacoustic emissions (OAEs) were measured in 23 subjects with severe-to-profound sensorineural hearing loss, 46 with PD, and 15 with normal hearing (NH). The PD subjects were divided into three subgroups according to the frequency at which hearing loss started: PD250 (NH up to 250 Hz)--20 ears; PD500 (NH to 500 Hz)--18 ears; and PD1000 (NH up to 1000 Hz)--20 ears. Standard-click stimuli, and 0.5- and 1-kHz tone bursts (average amplitude 80 ± 3 peak dB SPL, nonlinear averaging protocol), were used. The tone bursts were four cycles long with equal rise/fall times and no plateau. Recordings were performed in two acquisition windows: a standard one 20-msec wide for clicks and 1-kHz tone bursts, and one 30-msec wide for 0.5-kHz tone bursts. OAE response levels, signal-to-noise ratios, and reproducibility were examined in terms of wide-band responses and in terms of half-octave bands centered at 0.5 and 1 kHz. Receiver operator characteristic analysis was used to determine which type of stimuli best differentiates partially deaf subjects from subjects with severe-to-profound sensorineural hearing loss through the range 125 to 8000 Hz. Nearly all recordings from groups PD500 and PD1000 showed 0.5-kHz TBOAEs. By contrast, 1-kHz TBOAEs and CEOAEs were generally found only in the PD1000 group. It was also possible to detect 0.5-kHz TBOAE responses in approximately 50% of ears from the PD250 group. Receiver operator characteristic analysis demonstrated that click and 1-kHz tone bursts provide a good diagnostic measure of residual hearing even when hearing loss starts as low as 1 kHz; moreover, the 0.5-kHz TBOAE could identify residual hearing when hearing loss started just > 0.5 kHz, a situation in which clicks failed to elicit a response. In the case of partially deaf subjects, diagnosis was more accurate when OAEs were analyzed by 1/2 octave bands. Furthermore, the use of a 0.5-kHz tone burst gave responses in the highest number of subjects, even when there were hearing losses in neighboring bands. The results of this study indicate that a 0.5-kHz TBOAE is a more powerful test than the standard CEOAE when cochlear function at low frequencies is of interest. The 0.5-kHz TBOAE may be used to identify partial deafness in patients who generally fail to show a response to the commonly used clicks. In addition, use of 1/2 octave-band filtering can increase the reproducibility and power of the test.

OTOACOUSTIC EMISSIONS FOR EVALUATING THE LOW-FREQUENCY HEARING OF PATIENTS CONSIDERED FOR PARTIAL DEAFNESS TREATMENT

BackgroundIn the present study otoacoustic emissions (OAEs) were used to evaluate the hearing of partial deafness subjects who had normal hearing up to 0.5 kHz and severe hearing loss above that frequency. The purpose of the study was to detect OAEs at 0.5 kHz and determine whether broad-band or band-limited values of OAE parameters were better for distinguishing ears with OAEs from those without.Material and MethodsThe study group consisted of 15 subjects with high-frequency hearing loss above 0.5 kHz. For reference, a group of 15 subjects with severe to profound sensorineural hearing loss over the whole of the 0.125–8 kHz range was used. In both groups, click-evoked OAEs (CEOAEs) and tone burst-evoked OAEs (TBOAEs) of 1 or 0.5 kHz were evaluated using a reproducibility parameter, taking into consideration broad-band, octave, and half-octave band values.ResultsIn the study group the best results were obtained by using 0.5 kHz TBOAEs. Clicks and 1 kHz tone bursts generally did not produce a response at 0.5 kHz. TBOAEs at 0.5 kHz enabled the separation of partially deaf ears from ears with severe hearing impairment. Half-octave band filtering yielded the best results; however, it was only slightly better than broadband and octave band filteringConclusionsThe results of the study indicate that the 0.5 kHz TBOAE is a better stimulus than CEOAE and 1 kHz TBOAE when cochlear function at low frequency needs to be tested. The half-octave band seems the best choice in order to get frequency-specific information. However the results should be treated with caution since the studied groups were relatively small.

The Evaluation of Patients with Partial Deafness by Transiently Evoked Otoacoustic Emissions

Otoacoustic emissions (OAEs) are well established in clinical practice and are a valuable tool in basic research (Probst et al., 1991). The most commonly used OAEs are click-evoked OAEs (CEOAEs) and distortion product OAEs (DPOAEs), which are evoked by two tonal stimuli. OAEs are believed to be good predictors of hearing status, particularly in the 1–4 kHz range. However, while DPOAEs perform more satisfactorily at higher frequencies, CEOAEs are better indicators of cochlear function at lower frequencies. OAEs are known to be frequency-specific. In most cases, OAEs do not occur when hearing loss is greater than 35 dB HL. However, if hearing is preserved at a certain frequency, there is then the possibility of recording OAEs at this specific frequency (Kemp et al., 1986). It is the basal region of the cochlea that is most prone to damage. That is reflected in OAEs that tend to have a lower amplitude or even disappear first at the higher frequencies when hearing loss is present. Transiently evoked OAEs can also be elicited by so-called ‘tone bursts’, which are short stimuli centred at a certain frequency. Several studies have shown that the spectral content of a sum of tone burst-evoked OAEs (TBOAEs) centred in the 1–4 kHz corresponds to CEOAE. It is nevertheless the case that, for tone bursts, the same level of stimulus as the equivalent click gives a higher level of OAE. Therefore those stimuli can give an advantage over standard clicks in some difficult cases. The OAE is absent in a majority of patients who undergo standard cochlear implantation. However, as it will be shown below, by using tone bursts as a stimuli it is possible to measure OAEs in some of the partial deafness subjects.

Neonatal Hearing Screening: Evaluation of Tone-Burst and Click-Evoked Otoacoustic Emission Test Criteria

Click-evoked otoacoustic emissions (CEOAEs) are widely used in universal neonatal hearing screening programs. A common finding in many such programs is a relatively high false-positive rate. This is often due to infant physiological noise adversely affecting the emission recording, leading to a "refer" screening outcome. In an attempt to reduce false-positive screening outcomes related to the effects of noise on otoacoustic emission response detection, tone-burst-evoked otoacoustic emissions (TBOAEs) were included in a neonatal hearing screening program because TBOAEs may elicit a greater signal-to-noise ratio than CEOAEs. The research project compared the pass/refer rate for a CEOAE-based test using established pass/refer criteria with the pass/refer rate for screening criteria that were based on TBOAE results alone or on combined CEOAE and TEOAE results. Neonates were recruited at the Hong Kong Adventist Hospital, and both CEOAEs and TBOAEs were performed. Six passing criteria were used in this study, based on CEOAEs only; CEOAEs plus 1 kHz TBOAEs; CEOAEs plus 2 kHz TBOAEs; CEOAEs plus 3 kHz TBOAEs; CEOAEs plus 1, 2, and 3 kHz TBOAEs; and TBOAEs only. Data from 298 neonates (546 ears) were obtained. Criteria set 1, using CEOAEs only, demonstrated a pass rate of 79.1%, and 114 ears were referred. Criteria set 2, using CEOAEs together with TBOAEs recorded at 1 kHz, passed 39 more ears than Protocol 1, and the pass rate was 86.3%. Hence, the overall referral rate for total number of screened ears decreased by 7.2 percentage points. Criteria set 3, using CEOAEs together with TBOAEs recorded at 2 kHz, and Criteria set 4, using CEOAEs in conjunction with TBOAEs recorded at 3 kHz, gave pass rates similar to Criteria set 1. Criteria set 5, using TBOAE information at frequencies where CEOAEs were not rated as "pass," raised the pass rate from 79.1 to 87.6%, reducing the overall referral rate by 8.5 percentage points. Criteria set 6, in which neonates were screened with TBOAEs recorded at 1, 2, and 3 kHz, gave a pass rate of 78.4%, similar to results for the CEOAE-only procedure. Both Criteria sets 2 and 5, which combined CEOAE and TBOAE recordings, gave significantly higher pass rates than Criteria sets 1, 3, 4, and 6. The results suggest that the introduction of combined CEOAE and TBOAE protocols may assist in the reduction of refer outcomes, and hence the false-positive rates, of neonatal hearing screening programs.

Simultaneous suppression of tone burst-evoked otoacoustic emissions – Effect of level and presentation paradigm

There is conflict in the literature over whether individual frequency components of a transient-evoked otoacoustic emission (TEOAE) are generated within relatively independent “channels” along the basilar membrane (BM), or whether each component may be generated by widespread areas of the BM. Two previous studies on TEOAE suppression are consistent with generation within largely independent channels, but with a degree of interaction between nearby channels. However, both these studies reported significant suppression only at high stimulus levels, at which the “nonlinear” presentation paradigm was used. The present study clarifies the separate influences of stimulus level and presentation paradigm on this type of suppression. TEOAEs were recorded using stimulus tone bursts at 1, 2 and 3 kHz and a complex stimulus consisting of a digital addition of the three tone bursts, over a range of stimulus levels and both “linear” and “nonlinear” presentation paradigms. Responses to the individual tone bursts were combined offline and compared with responses to the complex stimuli. Results clearly demonstrate that TEOAE suppression under these conditions is dependent upon stimulus level, and not upon presentation paradigm. It is further argued that the data support the “local” rather than “widespread” model of TEOAE generation, subject to nonlinear interactions between nearby generation channels.

Suppression of tone burst evoked otoacoustic emissions in relation to frequency separation

Tone burst evoked otoacoustic emissions (TBEOAEs) were measured for two tone bursts presented separately and as a two-tone burst complex to examine the linearity of TBEOAE generators for different frequency separations of the stimuli. The stimuli were: (a) tone bursts of 5-ms duration and center frequencies of 1, 1.5, 2 and 3 kHz; (b) complex stimuli with the 1-kHz tone burst combined digitally with each of the other specified tone bursts. Signals were delivered at 70 dB SPL using a non-linear processing method and at 60 dB SPL using a linear method to 21 ears of normally hearing adults. Spectra of TBEOAEs obtained with single-tone bursts were superimposed (composite) and compared to those of the two-tone burst complex. A close correspondence between the composite and complex spectra was present in all ears. However, the components on the higher-frequency slope of the 1-kHz spectral peak were reduced in the complex spectra obtained with a frequency separation of 0.5 kHz when compared to the corresponding composite spectra. The reduction was greater at a stimulus level of 70 dB SPL than with 60 dB SPL. The effect was smaller for a frequency separation of 1 kHz, and almost absent for the tone burst separation of 2 kHz. Thus, suppression leads to weak non-linear frequency superposition for higher-level, closely spaced stimuli.

Influence of primary frequencies ratio on distortion product otoacoustic emissions amplitude. II. Interrelations between multicomponent DPOAEs, tone-burst-evoked OAEs, and spontaneous OAEs.

Distortion product otoacoustic emissions (DPOAEs) are used widely in humans to assess cochlear function. It is well known that 2f1-f2 DPOAE amplitude increases as the f2/f1 ratio increases from 1.0 to about 1.20, and then decreases as the f2/f1 ratio increases above 1.20, showing an amplitude ratio function, which is thought to be related to cochlear filtering properties. Different lower sideband DPOAEs are believed to show the same amplitude ratio functions as the 2f1-f2 DPOAE, with a magnitude peak situated at a constant DPOAE frequency relative to f2. More recently, several studies have suggested the involvement of a DPOAE component coming from its own distortion product place as well as the DPOAE component coming from the f2 place. To investigate DPOAE generation sites and the importance of the DPOAE frequency place, amplitude ratio functions of 2f1-f2, 3f1-2f2, 4f1-3f2 and 2f2-f1, 3f2-2f1, 4f2-3f1 DPOAE components have been systematically studied in 18 normally hearing subjects, using an f2 fixed, f1 sweep method, and an f1 fixed, f2 sweep method, at ten different f2 frequencies. Results show a dependency of the distortion magnitude peak on f2 frequency for each lower sideband DPOAE, and a small frequency shift of the distortion peak for the high order lower sideband DPOAE components. Strong correlation between the different lower sideband DPOAE amplitude were obtained, whether they were recorded with the same f1 (and a different f2) or with the same f2 (and a different f1), suggesting that lower side-band DPOAE amplitude does not depend on small variations in the f2 frequency. Moreover, correlations between DPOAE amplitude and tone-burst evoked otoacoustic emissions (TBOAEs) are highly significant for TBOAEs centered at the f2 frequency and at 1/2 octave below the f2 frequency, suggesting some degree of importance of the cochlear status at frequencies below f2 in DPOAE amplitude. Subjects presenting spontaneous otoacoustic emissions showed a greater lower sideband DPOAE amplitude recorded for low f2/f1 ratios, and a distortion magnitude peak shifted towards higher frequencies. The best correlation between upper sideband DPOAE amplitude and lower sideband DPOAE amplitude occurred for lower sideband DPOAEs generated by an f2 frequency 1/2 octave to 1 octave below the primaries used to generate upper sideband DPOAEs, suggesting a site of generation basal to f2 for the upper sideband DPOAEs. Correlations between TBOAE amplitude and upper sideband DPOAE amplitude agreed with a site of upper sideband DPOAE generation basal to f2, and which would move with the DPOAE frequency itself.

Test-Retest Reliability of Tone-Burst-Evoked Otoacoustic Emissions

In this study, the short- and long-term test-retest reliabilities of tone-burst-evoked otoacoustic emissions (TBOAEs) with 12 different tone-burst stimuli (4 frequencies [1, 1.5, 2 and 3 kHz] at 3 stimulus levels [approximately 76, approximately 67 and approximately 55 dB pcSPL]) were examined in 30 normal hearing subjects. Click-evoked and spontaneous OAEs were recorded in parallel with TBOAEs to facilitate cross-comparisons and the generalization of results. Findings for click-evoked and spontaneous OAEs were comparable with most literature data. High reliability for TBOAEs was established for high and mid stimulus levels at all frequencies tested with reference to test-retest prevalence rate, test retest occurrence, intra-subject test retest difference and correlation coefficient. Derived half-octave band analysis at the frequency corresponding to the stimulus was found to reflect real TBOAE performance more reliably than broadband analysis. No significant difference between short- and long-term reliabilities was noted from all results. Similar test-retest reliabilities for high-level TBOAEs and click-evoked OAEs was obtained, suggesting that TBOAEs could potentially contribute to clinical assessment.

Measures of Cochlear Travelling Wave Delay in Humans: I. Comparison of Three Techniques in Subjects with Normal Hearing

This study was designed to estimate and compare measures of cochlear travelling wave delay and travelling wave velocity in normally-hearing adults. Travelling wave delay and velocity measures were estimated in 23 normally-hearing adults using three different test techniques: i) derived auditory brainstem responses (ABR); ii) derived frequency-following responses (FFR); and iii) tone-burst-evoked otoacoustic emissions (TBEOAE). Estimates from ABR and TBEOAE were comparable to each other in terms of both averaged group values and associated standard deviations. Furthermore, mean cochlear travelling wave velocity estimated from ABR and TBEOAE were comparable to published estimates. Average cochlear delays obtained using the derived FFR were significantly shorter than those obtained from the other two techniques, possibly due to the effect of cochlear microphonic contamination. Among the dependent variables investigated, measures of delay are to be preferred over velocity since the latter are based on uncertain values of cochlear distance.

Tone burst-evoked otoacoustic emissions in cats with acoustic overstimulation and anoxia

Transient evoked otoacoustic emissions (TEOAE) produced by a 2 kHz tone burst could be detected in 30 out of 37 ears (81% detectability) in 21 cats. The amplitude of tone burst-evoked TEOAE was saturated at a stimulus level between 45 and 50 dB SPL and the latency time of peak amplitude was 6.23 ms on average (5.53–7.28 ms). The effects of pure tone overstimulation and short-term anoxia on the tone burst-evoked TEOAE in cats were evaluated. A permanent detection threshold shift of the TEOAE was confirmed at 24 h and 1 week after the overstimulation at 125 dB SPL. In these cases, damaged first row outer hair cells and inner hair cells were observed over an average length of 3.3 mm (16% of the entire cochlear length) by scanning electron microscopy. These findings suggested that the TEOAE can detect localized cochlear hair cell damage. A temporary detection threshold shift of the TEOAE was observed after the overstimulation at 105 dB SPL, and the threshold shift recovered in 107.5 min on average. In the short-term anoxia trial, the TEOAE amplitude started to decrease 45–90 s after the anoxia and recovered completely when the duration of anoxia was under 1 min. However, the TEOAE amplitude did not recover pre-anoxia values (it remained below 80% of its initial value) after 5 min when the anoxia was over 2 min. These findings demonstrated that the detection threshold and amplitude of the TEOAE were also affected by metabolic changes of the cochlear hair cells. Tone burst-evoked TEOAE are useful for the evaluation of localized histological and functional damage of the cochlear hair cells.

Evoked otoacoustic emissions in guinea pig: Basic characteristics

Different types of evoked otoacoustic emissions (EOAEs) such as stimulus frequency emissions, tone burst and click EOAEs, were investigated in the guinea pig. Their correlates on cochlear microphonic potential were also recorded. Although it was confirmed that click EOAEs are difficult to detect in the guinea pig, partly because their delay (2 to 3 ms, measured on tone burst EOAEs) is much shorter than in man, other types of OAE were found in the interval (1.5–5 kHz) for almost every normal animal. Many of their properties were quite different from man, for instance their small number, low level, and the wide frequency range of some of them (up to 500 Hz), suggesting that they represent a sort of continuum. It is proposed that these particularities may not arise from different generating mechanisms but should be correlated with the well-known differences in hair cells patterns.

Spontaneous, click-, and toneburst-evoked otoacoustic emissions from normal ears

Evoked and spontaneous otoacoustic emissions were recorded bilaterally in a group of normal subjects (n =14) using clicks and tonebursts at four frequencies (0.5, 1, 1.5, and 3 kHz). All ears (n = 28) demonstrated evoked emissions, but not to every stimulus type. The 0.5-kHz toneburst evoked emissions in only 10 (36%) ears, the 1.5-kHz toneburst in all ears, and the remaining stimuli in at least 80%of ears. Two distinct patterns of evoked emissions were identified. Five (18%) ears showed short, broadband click-evoked emissions lasting less than 20 ms after stimulus onset. In these ears, toneburst-evoked emissions were often more prominent than click-evoked emissions and no spontaneous emissions were detected. Twenty-three (82%) ears showed click-evoked emissions lasting longer than 20 ms poststimulus onset. Spectral analysis of these emissions demonstrated several (2–10) narrow frequency peaks. Highly similar peaks were present in the spectra of toneburst-evoked emissions within the range of toneburst spectra. Spontaneous emissions were recorded in 12 of the 23 ears. In these ears, at the frequencies of spontaneous emissions, prominent peaks in both click- and toneburst-evoked emission spectra were always present. Otoacoustic emission characteristics correlated significantly between the ears of individual subjects inferring that a symmetrical cochlear mechanism generates otoacoustic emissions.