2f1-f2 Distortion Product Otoacoustic Emissions Research Articles

Characteristics of efferent mediated enhancement of distortion-product otoacoustic emissions

Efferent stimulation can enhance distortion-product otoacoustic emissions (DPOAEs) (Siegel and Kim, 1982), but the stimulus conditions in which enhancement occurs have not been clearly described. To clarify the efferent mediated enhancement of cochlear active process, 2f1–f2 DPOAE I/O function was measured in pigmented guinea pigs. For f2/f1=1.20, the crossed olivocochlear bundle (COCB) stimulation suppressed the DPOAE evoked by low primary levels, the rapid growth portion of an I/O function. It enhanced it in the saturating portion evoked by moderate levels, 45 to 65 dB SPL when L1−L2=0 to 20 dB, and suppressed it with further increase in stimulus level. For f2/f1=1.08, enhancement appeared only when L1=75 dB SPL and L1−L2=30 to 40 dB. For f2/f1=1.45, enhancement appeared when L1=45 to 55 dB SPL and L1−L2=-10 to -20 dB. COCB stimulation tends to alter the DPOAE I/O function toward linear growth. Both suppression and enhancement were significantly reduced by strychnine. The results suggest that effect of efferent activation on evoked OHC activity is not a simple suppression, but a modulation of the nonlinearity of its growth function. [Work supported by grants: NIH/NIDCD PO1−DC00078 and T32−DC00011.]

Modeling the temporal behavior of distortion product otoacoustic emissions.

The temporal behavior of the 2f1-f2 distortion product otoacoustic emission is theoretically investigated for the case in which the lower frequency (f1) primary tone is on continuously, and the higher frequency (f2) one is pulsed on and off [e.g., Talmadge et al., J. Acoust. Soc. Am. 105, 275-292 (1999)]. On physical grounds, this behavior is expected to be characterized by various group delays associated with the propagation of (1) the f2 cochlear primary wave between the cochlear base and the primary distortion product generation region around x2 (the f2 tonotopic place), and (2) the 2f1-f2 cochlear distortion product (DP) waves between the cochlear base, the primary generation region of the distortion product, and the region around the 2f1-f2 tonotopic place where the generated apical moving DP wave is reflected toward the cochlear base [e.g., Talmadge et al., J. Acoust. Soc. Am. 104, 1517-1543 (1998)]. An approximate analytic expression is obtained for this behavior from the analysis of the Fourier integral representation of the auditory peripheral response to the primary stimuli. This expression also approximately describes the transient build-up of the components of different latencies in terms of the damping properties of the cochlear partition. It is shown that considerable caution must be applied in attempting to relate phase derivatives of the distortion product otoacoustic emissions for steady state stimuli and the physical time delays which are associated with the temporal behavior of a distortion product emission in the case of a pulsed primary.

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.

Ototoxic Interaction Between Noise and Pheomelanin: Distortion Product Otoacoustic Emissions After Acoustical Trauma in Chloroquine-Treated Red, Black, and Albino Guinea Pigs: Interaction Ototóxics entre el Ruido y la feomelanina: Emisiones Otoacústicas por Productos de Distorsión Posteriores a Trauma Acústico en Cobayos Rojos, Negros y Albinos tratados con Cloroquina

This study provides further evidence of an ototoxic interaction between red pheomelanin pigmentation and noise-induced hearing loss. Red, black, and albino guinea pigs were treated with a low, a high, or no dose of chloroquine. The 2f1-f2 distortion product otoacoustic emission (DPOE) measurements were measured before, immediately after, and 1 month after noise exposure to a 1-kHz tone at 105 dB SPL for 72 hours. In red guinea pigs, the DPOE was severely affected by noise trauma when treated even by a low single dose of chloroquine, whereas in both albino and black guinea pigs, the chloroquine effect on the DPOE was temporary and present only when the drug was given in a high single dose. The structure most likely to be responsible for the severe loss of DPOE in chloroquine-treated red animals is the strial melanocyte. The damage may be triggered by an ototoxic noise-induced production of radical oxygen species from pheomelanin, for example, by the Fenton reaction or due to the increased variability of th...

No Impact Detected of Fetal Asphyxia on 2f1-f2 Distortion Product Otoacoustic Emissions and Brainstem Auditory Evoked Response in the Adult Rat

No Impact Detected of Fetal Asphyxia on 2f1-f2 Distortion Product Otoacoustic Emissions and Brainstem Auditory Evoked Response in the Adult Rat

Sex differences in auditory sensitivity of chinchillas before and after exposure to impulse noise.

To determine if chinchillas exhibit sex differences in 1) basic auditory sensitivity and 2) susceptibility to cochlear damage and hearing loss from high-level impulse noise. The auditory sensitivity of 73 chinchillas was assessed by measuring evoked potentials from electrodes implanted in the inferior colliculus (IC-EVPs) and cubic (2f1-f2) distortion product otoacoustic emissions (CDPs). A subgroup of 16 chinchillas were retested after exposure to simulated M16 rifle fire (150 dB pSPL impulse noise). Thresholds and postexposure temporary and permanent threshold shifts were compared as a function of sex and frequency using analysis of variance procedures. Cochleograms, showing the percent of hair cells missing as a function of location on the basilar membrane, were constructed to show inner hair cell (IHC) and outer hair cell (OHC) losses for each group. Female chinchillas had slightly lower high-frequency thresholds, and slightly higher low-frequency thresholds than male chinchillas, but similar IC-EVP and CDP amplitude functions. Significant sex differences were observed after exposure to high-level impulse noise. Overall, female chinchillas developed approximately 10 dB more high-frequency hearing loss, but approximately 5 dB less low-frequency hearing loss than males. Hair cell losses, particularly IHC losses, were substantially less for females as compared with males. The results point to close similarities between chinchillas and humans with regard to sex/gender differences in basic auditory sensitivity before noise exposure, suggesting that the chinchilla may be a good model for exploring the anatomical and physiological bases of these differences. In addition, the results show significant sex differences in the physiological and anatomical response of the chinchilla cochlea to high-level noise. Similar differences in humans could have important implications with regard to military assignments and hearing conservation programs.

The maximum permissible ambient noise and frequency-specific averaging time on the measurement of distortion product otoacoustic emissions.

The effects of ambient noise and averaging time on distortion product otoacoustic emission (DPOAE) measurements were systematically examined in 20 normally hearing adults. For each frequency from 0.7 to 6 kHz, 2f1-f2 DPOAEs were measured at nine intensity levels of ambient noise for each of four different averaging times. The dependent variable of interest was the DPOAE:noise ratio (D:N)--the relationship between DPOAE amplitude and noise level at 2f1-f2. Findings indicated that, relative to the baseline condition (25 dBA), ambient noise levels of 40 dBA or greater affected the D:N considerably in the lower frequencies. However, noise levels of 55-65 dBA did not affect the D:N in the higher frequencies. Results also indicated that, to achieve detectable DPOAEs, longer averaging times were required as ambient noise increased and as frequency decreased. Based on the results, the maximum permissible ambient noise levels (MPANLs) were estimated in relation to averaging time.

Sensitivity of transient evoked and distortion product otoacoustic emissions to the direct effects of noise on the human cochlea.

Measurement of otoacoustic emissions (OAEs) has been proposed as a sensitive test to reliably assess the effects of noise exposure. The present study in humans was designed to evaluate the sensitivity and applicability of transient evoked OAEs (TEOAEs) and 2f1-f2 distortion product OAEs (DPOAEs) as quantitative indices of the functional integrity of the outer hair cells (OHC) during growth of and recovery from temporary threshold shift (TTS). This was examined in two different groups of volunteers by measuring the per- and post-stimulatory effects of a one hour BBN and an on-site five hour exposure to loud music from a discotheque. The results of both experiments show consistent growth and recovery patterns for both DPOAEs and TEOAEs. For TEOAEs, both the reproducibility scores and signal to noise ratio values for the 4 kHz frequency band exhibited the greatest sensitivity. The DPOAEs, on the other hand, showed the greatest sensitivity between 2 and 5.5 kHz. Thus, both the TEOAEs and DPOAEs have a great potential in the detection of TTS after noise exposure.

Time course of the medial olivocochlear efferent effect on otoacoustic emissions in humans.

The time course of the medial olivocochlear efferent system has been studied in humans, using the suppressive effect of a contralateral broad-band noise (CBBN) on 2f1-f2 distortion product otoacoustic emissions (DPOAE) amplitude. DPOAE were recorded with F2 at 1.5 kHz, with a temporal resolution of 2.6 s, in the presence and absence of a 40 dB SL continuous CBBN, whose duration ranged between 30 s and 20 min. The CBBN suppressive effect on DPOAE amplitude was greatest from the first 2.6 s, and was sustained for 20 min. At the CBBN offset, when the CBBN duration was > or = 2 min, DPOAE amplitude continued to increase for > 1 min, showing an efferent effect outlasting CBBN stimulation.

A developmental study of distortion product otoacoustic emission (2f1-f2) suppression in humans

Suppression of the 2f1-f2 distortion product otoacoustic emission (DPOAE) provides an effective paradigm for the study of functional cochlear maturation in humans. DPOAE iso-suppression tuning curves (STCs) represent some aspect of peripheral filtering, probably related to the boundaries of distortion generation. Studies conducted thus far suggest that the cochlear tuning assessed by this technique is adult-like in humans by term birth (Abdala et al., Hear. Res. 98 (1996) 38–53; Abdala and Sininger, Ear Hear. 17 (1996) 374–385). However, there have been no studies of cochlear tuning in premature human neonates. DPOAE STCs and suppression growth functions were measured from 14 normal-hearing adults, 33 term and 85 premature neonates to investigate the developmental time course of cochlear frequency resolution and non-linearity. Premature neonates showed non-adult-like DPOAE suppression at f2 of 1500 and 6000 Hz: (1) STCs were narrower in width (Q10) and steeper in slope on the low-frequency flank of the tuning curve; (2) suppressor tones lower in frequency than f2 produced atypically shallow growth of DPOAE suppression. The influence of immature conductive pathways cannot be entirely ruled out as a factor contributing to these results. However, findings may indicate that an immaturity exists in cochlear frequency resolution and non-linearity just prior to term birth. The bases of this immaturity are hypothesized to be outer hair cell in origin.

Evaluating noise induced hearing loss with distortion product otoacoustic emissions.

This study assessed the clinical efficacy of screening for noise induced hearing loss (NIHL) with distortion product otoacoustic emissions (DPOAE). DPOAEs were recorded from 76 military personnel (137 ears) aged between 17 and 41 years in response to equilevel 70 dB SPL primary stimulating tones. The 2f1-f2 DPOAE levels were correlated with audiometric thresholds at frequencies close to f2. Ears with normal audiograms, but with a history of military noise exposure, had DPOAEs that were significantly decreased in amplitude as compared to the ears of normal hearing non-exposed to noise subjects. These ears also had an increased absence of DPOAEs as compared with the ears of the normal hearing non-exposed to noise subjects. Although, in general, the DPOAE amplitudes and spectral frequency ranges reflected the audiometric NIHL configurations, in a number of cases DPOAEs were present for hearing losses up to 75 dB HL. As a consequence, DPOAEs correlated moderately and negatively with the audiometric thresholds. Applying test criteria designed to logically reflect NIHL, DPOAE sensitivity and specificity levels ranged between 0.51-0.90 and 0.63-0.25, respectively. These findings indicate that DPOAEs, recorded and analysed as described, are not sufficiently sensitive to serve as a single test to identify NIHL.

Characteristics of the Acoustic Distortion Product 2f2-f1 from the Normal Human Ear

The aim of this study was to determine the basic properties of the 2f2-f1 distortion product otoacoustic emissions (DPOEs) in 36 normally hearing subjects and to compare their characteristics with those of the 2f1-f2 DPOEs. The influence of stimulation parameters on 2f2-f1 DPOE behaviour was examined: i) stimulation levels varied from 40 to 70 dB sound pressure level in 10 dB steps; and ii) two values of the f2/f1 ratio were used (1.12 and 1.22). The prevalence and the level of the 2f2-f1 DPOEs depended on three parameters: f2 frequency, stimulation level and f2/f1 ratio. The results showed many differences between the characteristics of 2f1-f2 and 2f2-f1 DPOEs, probably due to their being generated by different mechanisms. Thus, the 2f2-f1 DPOEs may provide different information about the cochlear status and could be of clinical interest.

Patterns of Change in Growth Function of Distortion Product Otoacoustic Emissions in Meniere's Disease

The patterns of change in the growth function of distortion product otoacoustic emissions (DPOAEs) associated with hearing improvement in six patients with Meniere's disease were investigated. The growth functions of 2F1-F2 DPOAEs at F2 frequencies of 1001, 2002 and 4004 Hz (DP-1, DP-2 and DP-4, respectively) were measured with F2 intensities ranging from 70 to 37 dBSPL in 3-dB steps. The F1 intensity was maintained 10 dB higher than the F2 intensity. The growth function was paired with the hearing threshold at the corresponding F2 frequency, and the relationships between changes in DP-1, DP-2 and DP-4 and those in hearing thresholds at 1, 2 and 4 kHz, respectively, were also investigated. The patterns of change in the growth function associated with hearing improvement could be classified into five types. In the first type, the DPOAE growth function became detectable, while the remaining four types were distinguished by combinations of changes in DPOAE amplitudes for lower and higher primary intensities. Multiple parameters, such as maximum amplitude and detection threshold of the growth function, appeared to be required for simple detection and discrimination of these patterns of change. It was also found that the DPOAE growth functions clearly changed in some cases even though the hearing thresholds did not change significantly at the corresponding F2 frequencies. This finding suggests that DPOAE growth function measurement can detect small changes in cochlear function which do not lead to changes in hearing threshold, and has higher sensitivity than pure tone audiometry in monitoring of cochlear function. In conclusion, our findings suggest that measurement of the DPOAE growth function is useful for monitoring cochlear function, and that information on its patterns of change is clinically important and useful.

Evidence that inner hair cells are the major source of cochlear summating potentials

The role of the inner hair cells (IHCs) in generating the cochlear summating potentials (SP) was assessed by measuring SP, cochlear nerve action potentials (CAP), cochlear microphonics (CM) and 2f1-f2 distortion product otoacoustic emissions (DPOAEs) in 15 chinchillas with either acute chemical de-afferentation, accomplished by applying kainic acid to the round window, or surgical de-afferentation and basal IHC loss, which developed within two months after sectioning the auditory nerve. In the auditory nerve sectioned ears, type I ganglion cells disappeared whereas most, if not all, type II ganglion cells were still present. Histological analysis of surface preparations and sections through the modiolus verified the de-afferentation in both models and showed a large IHC loss at the base of the cochlea in the ears with the auditory nerve sectioned while other structures of the cochlea remained intact. Unoperated (left) ears of 9 animals served as controls. CM and DPOAEs were normal in all ears whereas the CAP was substantially depressed in de-afferented ears. Comparisons among the SP input-output functions suggest that (1) the IHCs are the major generator of SP recorded from the round window in chinchilla, in particular at low to moderate stimulus levels, (2) the SP recorded from the round window largely reflects the responses from hair cells at the base of the cochlea, and (3) kainic acid results in an increase of SP amplitude to high-level stimuli whereas the SP to low- to moderate-level stimuli remains in the normal range.

The influence of the cochlear efferent system on chronic acoustic trauma

The role of the olivocochlear bundle (OCB) in modulating noise-induced permanent injury to the auditory periphery was studied by completely sectioning the OCB fibers in chinchillas and exposing the animals while awake to a broad-band noise at 105 dB SPL for 6 h. Outer hair cell (OHC) function was assessed by measuring 2f1–f2 distortion product otoacoustic emissions (DPOAE) at frequencies from 1.2 to 9.6 kHz and cochlear microphonics (CM) at frequencies from 1 to 8 kHz. As a result of de-efferentation, the CM was decreased but the DPOAEs were unchanged in de-efferented ears as compared with efferented control and sham-operated ears. Following noise exposure, the ears that were de-efferented showed significantly more depression of DPOAE input/output functions and greater decrement of CM amplitude. The differences between de-efferented and efferent-innervated ears were evident across all the frequencies. The cochlear lesions of the OHCs reflected by traditional cytocochleograms, however, were minimal in both efferented and de-efferented ears. The results indicate that cochlear de-efferentation decreases the CM in chinchilla and increases the ear's susceptibility to noise-induced permanent hearing damage. More importantly, de-efferentation increases susceptibility at low frequencies as well as high frequencies.

Comparison of Distortion Product OAE Generation between a Patient Group Requiring Frequent Gentamicin Therapy and Control Subjects

Although the animal models used to characterize aminoglycoside ototoxicity are well developed, the initial stages of the ototoxic process of this important group of antibiotics in humans are less well understood. A group that receives frequent aminoglycoside therapy are cystic fibrosis (CF) patients, who may receive cumulative doses of gentamicin over 200 g in their lifetime. Consequently they represent a group in which it is particularly appropriate to monitor regularly auditory function. In this preliminary study, 15 young (aged 9-18 years) CF patients had their pure tone thresholds measured over 0.25-12 kHz. Their distortion product otoacoustic emission (DPOAE) growth functions were also measured at f2 = 2, 4 and 6 kHz with f2/f1 = 1.22. These results were compared with those obtained from 36 control volunteers of similar age. Fourteen of the CF patients had normal hearing (pure tone audiogram (PTA) thresholds < or = 10 dB HL over 0.25-8 kHz). In this group, there was a significant elevation of the stimulus levels required to generate a 2f1-f2 DPOAE < or = 10 dB SPL at 4 kHz. This elevation may represent one of the earliest changes in outer hair cell performance caused by gentamicin, although it may also be due to the CF condition itself. Whilst this measurement alone cannot be taken to indicate any serious cochlear dysfunction, it may have some clinical use as an early indicator or marker of functional deficit in the cochlea.

2f1-f2 distortion product otoacoustic emission latency: changes with frequency and level of primaries.

Distortion product otoacoustic emissions (DPOAEs) provide a non-invasive and relatively direct method for evaluating cochlear travel time in humans. In the present study, the 2f1-f2 DPOAE latency was deduced from DPOAE phase shift according to f2 frequency shift, with f1 being fixed, using the ILO92 system. Latencies of 2f1-f2 DPOAEs were recorded at various primary frequencies and levels. Results were not wholly consistent with previous latency estimates. This study showed that the functions describing both 2f1-f2 DPOAE latency decrease with primary frequency increase, at several primary-tone levels, and 2f1-f2 DPOAE latency decrease with primary-level increase, at various frequencies, are best fit by exponential equations. The results of the evaluation of the latency-frequency-level relationship indicated the possibility of linking these three variables by the following expression: Latency = 68.30 exp (-0.027I -0.000434 f)+1.13.

Distortion product otoacoustic emission test of sensorineural hearing loss in humans: comparison of unequal- and equal-level stimuli.

Distortion product otoacoustic emissions (DPOEs) at the frequency of 2f1 -f2 (f1 < f2) were measured in 77 human adult ears with normal hearing or sensorineural hearing loss. The purpose of this study was to compare the performances of DPOE tests conducted with two sets of stimuli: 1) L1 = 65, L2 = 50 dB sound pressure level (SPL) re 20 microPa ("65/50"), and 2) L1 = L2 = 65 dB SPL ("65/65"). Half-octave DPOE root-mean-square levels at 1,000, 2,000, 4,000, and 6,000 Hz were computed from the initial DPOEs measured at 0.25-octave intervals. Correlation coefficient and decision-theory analyses were applied to evaluate the DPOE test performance. For both stimuli, DPOE level exhibited significant correlation with pure tone hearing threshold. When the criterion DPOE level distinguishing normal from impaired hearing was adjusted, the curves of sensitivity and specificity crossed, and the values at the crossing were higher than 80% at frequencies of 2,000 to 6,000 Hz for both stimuli. The area under the receiver operating characteristic (ROC) curve, which provides an overall evaluation of the test performance independent of the criterion DPOE level, was .90 or higher at 2,000 to 6,000 Hz for both stimuli. At 2,000 and 4,000 Hz, all measures of test performance were higher for the 65/50 stimulus than the 65/65 stimulus: area under the ROC curve (.96 to .97 versus .90 to .91, statistically significant, p < .001, Wilcoxon test), sensitivity/specificity (90% to 93% versus 80% to 85%), and correlation coefficient (.78 to .87 versus .66 to .79). At 1,000 and 6,000 Hz, the performances of the DPOE tests were similar for the two stimuli. These results support the conclusion that a DPOE test with L1 = 65 and L2 = 50 dB SPL provides a better performance than that with L1 = L2 = 65 dB SPL and recommend the use of stimuli with L1 being higher than L2 by about 15 dB. These results also support a growing view that 2f1-f2 DPOEs can be utilized clinically as a reliable method of testing human sensorineural hearing loss.

Multicomponent acoustic distortion product otoacoustic emission phase in humans. II. Implications for distortion product otoacoustic emissions generation

Phase characteristics and latency of 2f1-f2, 3f1-2f2, and 2f2-f1 acoustic distortion product otoacoustic emissions (DPOAEs) recorded in normally hearing adults have been described in a companion paper [Moulin and Kemp, "Multicomponent acoustic distortion product otoacoustic emission phase in humans. I. General characteristics," J. Acoust. Soc. Am. 100, 1617-1639 (1996)]. Phase-versus-frequency functions (PFF) have been recorded using two methods, depending whether f1 or f2 was swept in frequency, defining, respectively, an "f1 sweep" and an "f2 sweep" method. The present study compares 2f1-f2, 3f1-2f2, 4f1-3f2, and 2f2-f1 DPOAEs phase characteristics, and latencies obtained by these two methods. For lower sideband DPOAEs, the f2 sweep gave consistently longer latencies than the f1 sweep method, whereas no difference was obtained for the 2f2-f1 DPOAE. This suggests that the lower sideband DPOAEs and the 2f2-f1 DPOAE do not come from the same aspect of the traveling wave. The longer delay obtained with an f2 sweep is suggested to be due to an additional delay added by the proximity of the peak of the f2 traveling wave. The place-fixed and the wave-fixed models of DPOAE generation are discussed, and an "intermediate" model of DPOAE generation is introduced to explain such additional delay and the existence of phase irregularities which were observed in the PFF. The ratio of the latencies obtained by the two sweep methods is frequency dependent and a relation to cochlear frequency selectivity is suggested. Finally, analysis of the differences in latency of the different lower sideband DPOAE components allowed an estimation of the return latency of DPOAEs.

The ipsilaterally evoked olivocochlear reflex causes rapid adaptation of the 2 f1−f2 distortion product otoacoustic emission

The onset behavior of the distortion product otoacoustic emission (DPOAE) at 2f1-f2 in anesthetized cats was measured with temporal resolution finer than 70 ms. The amplitude of the DPOAE adapts after onset of the primary tones by as much as 6 dB for monaural stimulation and 10 dB when the primaries are presented binaurally. DPOAE adaptation consists of a large, rapid component, with a time constant of roughly 100 ms, and a small, slower component with a time constant of roughly 1000 ms. The rapid component disappears when only the crossed olivocochlear bundle (OCB) is cut, whereas the slow adaptation persists after complete OCB section. The loss of rapid adaptation upon OC section is accompanied by a concomitant increase in the steady-state amplitude of the DPOAE. Thus an intact OC reflex can significantly alter DPOAEs obtained during routine measurement. Rapid adaptation of the monaurally evoked 2f1-f2 DPOAE is probably mediated by reflex activity in ipsilaterally responsive OC neurons innervating outer hair cells. The effects of this ipsilateral reflex on DPOAE amplitudes are typically twice as large as those of the contralateral reflex, presumably because there are twice as many ipsilaterally responsive OC neurons. Tests for the ipsilateral OC reflex based on the phenomenon of rapid adaptation should be both feasible and useful in human subjects.