Origin of suppression of otoacoustic emissions evoked by two-tone bursts

No Change in Medial Olivocochlear Efferent Activity during an Auditory or Visual Task: Dual Evidence from Otoacoustic Emissions and Event-Related Potentials

The medial olivocochlear (MOC) system is thought to be responsible for modulation of peripheral hearing through descending (efferent) pathways. This study investigated the connection between peripheral hearing function and conscious attention during two different modality tasks, auditory and visual. Peripheral hearing function was evaluated by analyzing the amount of suppression of otoacoustic emissions (OAEs) by contralateral acoustic stimulation (CAS), a well-known effect of the MOC. Simultaneously, attention was evaluated by event-related potentials (ERPs). Although the ERPs showed clear differences in processing of auditory and visual tasks, there were no differences in the levels of OAE suppression. We also analyzed OAEs for the highest magnitude resonant mode signal detected by the matching pursuit method, but again did not find a significant effect of task, and no difference in noise level or number of rejected trials. However, for auditory tasks, the amplitude of the P3 cognitive wave negatively correlated with the level of OAE suppression. We conclude that there seems to be no change in MOC function when performing different modality tasks, although the cortex still remains able to modulate some aspects of MOC activity.

Otoacoustic emissions from ears with spontaneous activity behave differently to those without: Stronger responses to tone bursts as well as to clicks

It has been reported that both click-evoked otoacoustic emissions (CEOAEs) and distortion product otoacoustic emissions (DPOAEs) have higher amplitudes in ears that possess spontaneous otoacoustic emissions (SOAEs). The general aim of the present study was to investigate whether the presence of spontaneous activity in the cochlea affected tone-burst evoked otoacoustic emissions (TBOAEs). As a benchmark, the study also measured growth functions of CEOAEs. Spontaneous activity in the cochlea was measured by the level of synchronized spontaneous otoacoustic emissions (SSOAEs), an emission evoked by a click but closely related to spontaneous otoacoustic emissions (SOAEs, which are detectable without any stimulus). Measurements were made on a group of 15 adults whose ears were categorized as either having recordable SSOAEs or no SSOAEs. In each ear, CEOAEs and TBOAEs were registered at frequencies of 0.5, 1, 2, and 4 kHz, and input/output functions were measured at 40, 50, 60, 70, and 80 dB SPL. Global and half-octave-band values of response level and latency were estimated. Our main finding was that in ears with spontaneous activity, TBOAEs had higher levels than in ears without. The difference was more apparent for global values, but were also seen with half-octave-band analysis. Input/output functions had similar growth rates for ears with and without SSOAEs. There were no significant differences in latencies between TBOAEs from ears with and without SSOAEs, although latencies tended to be longer for lower stimulus levels and lower stimulus frequencies. When TBOAE levels were compared to CEOAE levels, the latter showed greater differences between recordings from ears with and without SSOAEs. Although TBOAEs reflect activity from a more restricted cochlear region than CEOAEs, at all stimulus frequencies their behavior still depends on whether SSOAEs are present or not.

Comparison of 3 ABR Methods for Diagnosis of Retrocochlear Hearing Impairment

BACKGROUND The purpose of this study was to compare the effectiveness of methods for screening for retrocochlear pathologies based on auditory evoked brainstem responses (ABRs). The study compared the sensitivity, specificity, and effectiveness of these 3 techniques. MATERIAL AND METHODS The methods were: (i) standard ABR utilizing click-evoked responses, (ii) stacked ABR based on derived-band responses, and (iii) ABRs evoked by tone-pips (ABR TP). The methods were tested on patients with retrocochlear pathologies confirmed by MRI-Gd, normal-hearing subjects, and patients with cochlear hearing loss. The system and software used in the tests was NavPro AEP v.6.2.0 (BioLogic - Natus). Prior to testing, all subjects were given comprehensive audiologic and otologic examinations, including MR imaging. Sensitivity and specificity functions and predictive values of methods were determined. RESULTS The stacked ABR method as realized in the NavPro system exhibited high sensitivity but specificity was very low, due to the high variability of stacked ABR amplitudes. The standard ABR method had good specificity, but low sensitivity in cases of small tumors (below 1 cm in diameter). Best sensitivity and specificity was obtained with the ABR TP method. CONCLUSIONS The stacked ABR method allows small acoustic tumors to be detected, but produces high percentage of false positive results. The ABR TP method offers good sensitivity and specificity, and relatively high predictive value. The best option would be to use a two-stage screening, consisting of a standard ABR in the first stage and an ABR TP test in the second.

Estimating cochlear frequency selectivity with stimulus-frequency otoacoustic emissions in chinchillas

It has been suggested that the tuning of the cochlear filters can be derived from measures of otoacoustic emissions (OAEs). Two approaches have been proposed to estimate cochlear frequency selectivity using OAEs evoked with a single tone (stimulus-frequency (SF)) OAEs: based on SFOAE group delays (SF-GDs) and on SFOAE suppression tuning curves (SF-STCs). The aim of this study was to evaluate whether either SF-GDs or SF-STCs obtained with low probe levels (30 dB SPL) correlate with more direct measures of cochlear tuning (compound action potential suppression tuning curves (CAP-STCs)) in chinchillas. The SFOAE-based estimates of tuning covaried with CAP-STCs tuning for >3 kHz probe frequencies, indicating that these measures are related to cochlear frequency selectivity. However, the relationship may be too weak to predict tuning with either SFOAE method in an individual. The SF-GD prediction of tuning was sharper than CAP-STC tuning. On the other hand, SF-STCs were consistently broader than CAP-STCs implying that SFOAEs may have less restricted region of generation in the cochlea than CAPs. Inclusion of <3 kHz data in a statistical model resulted in no significant or borderline significant covariation among the three methods: neither SFOAE test appears to reliably estimate an individual's CAP-STC tuning at low-frequencies. At the group level, SF-GDs and CAP-STCs showed similar tuning at low frequencies, while SF-STCs were over five times broader than the CAP-STCs indicating that low-frequency SFOAE may originate over a very broad region of the cochlea extending ≥5 mm basal to the tonotopic place of the probe.

Musical ratios in sounds from the human cochlea

The physiological roots of music perception are a matter of long-lasting debate. Recently light on this problem has been shed by the study of otoacoustic emissions (OAEs), which are weak sounds generated by the inner ear following acoustic stimulation and, sometimes, even spontaneously. In the present study, a high-resolution time–frequency method called matching pursuit was applied to the OAEs recorded from the ears of 45 normal volunteers so that the component frequencies, amplitudes, latencies, and time-spans could be accurately determined. The method allowed us to find that, for each ear, the OAEs consisted of characteristic frequency patterns that we call resonant modes. Here we demonstrate that, on average, the frequency ratios of the resonant modes from all the cochleas studied possessed small integer ratios. The ratios are the same as those found by Pythagoras as being most musically pleasant and which form the basis of the Just tuning system. The statistical significance of the results was verified against a random distribution of ratios. As an explanatory model, there are attractive features in a recent theory that represents the cochlea as a surface acoustic wave resonator; in this situation the spacing between the rows of hearing receptors can create resonant cavities of defined lengths. By adjusting the geometry and the lengths of the resonant cavities, it is possible to generate the preferred frequency ratios we have found here. We conclude that musical perception might be related to specific geometrical and physiological properties of the cochlea.

Time-frequency analysis of linear and nonlinear otoacoustic emissions and removal of a short-latency stimulus artifact

Click-evoked otoacoustic emissions (CEOAEs) are commonly recorded as average responses to a repetitive click stimulus. If the click train has constant polarity, a linear average results; if it contains a sequence of clicks of differing polarity and amplitude, a nonlinear average can be calculated. The purpose of this study was to record both protocols from the same set of ears and characterize the differences between them. The major features of CEOAEs were similar under both protocols with the exception of a region spanning 0-5 ms in time and 0-2.2 kHz in frequency. It was assumed that the signal derived from the linear protocol was contaminated by stimulus artifact, and so a simple procedure was used--involving high-pass filtering and time-windowing--to remove components of this artifact. This procedure preserved the short-latency, high-frequency responses; it also produced a marked similarity in the time-frequency plots of recordings made under the two protocols. This result means it is possible to take advantage of the better signal-to-noise ratio of the linear data compared to its nonlinear counterpart. Additionally, it was shown that CEOAEs recorded under the linear protocol appear to be less dependent on the presence of spontaneous otoacoustic emissions (SOAEs).

A mechanism for simultaneous suppression of tone burst-evoked otoacoustic emissions

Tone burst-evoked otoacoustic emission (TBOAE) components in response to a 1 kHz tone burst are suppressed by the simultaneous presence of tone bursts at higher frequencies. To date, the underlying cause of this "simultaneous suppression" of TBOAEs is unclear. This paper describes a potential mechanism based on local nonlinear interactions between basilar membrane (BM) travelling waves, and tests the extent to which it is able to account for this specific suppression phenomenon. A simple mathematical model based on local nonlinear interactions was developed, and its predictions for a range of tone burst pairs were compared to corresponding TBOAE suppression data recorded from fourteen normally hearing human ears at a level of 60 dB p.e. SPL. Model predictions and mean TBOAE suppression data showed close agreement for all pairs of tone bursts. These results suggest that simultaneous suppression of TBOAEs can be explained solely in terms of the local nonlinear interaction-based mechanism. However, the involvement of other mechanisms, involving components generated at places basal to their characteristic place along the BM, cannot be excluded.

Transient evoked otoacoustic emissions in superior canal dehiscence syndrome

Superior semicircular canal dehiscence syndrome (SCDS) is a clinical disorder that is characterized by vertigo and oscillopsia induced by loud sounds. Transient evoked otoacoustic emissions (TEOAEs) allow to noninvasively check the integrity of the cochlea. The present study aimed at identifying cochlear stress as the result of micro alterations of the cochlear functionality due to anatomic anomaly. 11 SCDS and 10 normal individuals as control group were submitted to history taking, otological examination, basic audiologic evaluation and TEOAEs analysis using the standard wideband protocol and moving time window analysis. Although TEOAEs test results showed no statistically significant difference using the standard protocol, off-line analysis of the waveforms' "effective duration" was statistically significantly shortened (p < 0.0001) when compared to normal ears. In conclusion, dehiscence of bone overlying the superior canal has been shown to have effects on inner ear function in terms of a third mobile window theory, thus altering pressure across cochlear partition with decrease in inner ear impedance.

Use of the matching pursuit algorithm with a dictionary of asymmetric waveforms in the analysis of transient evoked otoacoustic emissions

Transiently evoked otoacoustic emissions (TEOAEs) are normally modeled as the sum of asymmetric waveforms. However, some previous studies of TEOAEs used time-frequency (TF) methods to decompose the signals into symmetric waveforms. This approach was justified mainly as a means to reduce the complexity of the calculations. The present study extended the dictionary of numeric functions to incorporate asymmetric waveforms into the analysis. The necessary calculations were carried out using an adaptive approximation algorithm based on the matching pursuit (MP) numerical technique. The classic MP dictionary uses Gabor functions and consists of waveforms described by five parameters, namely, frequency, latency, time span, amplitude, and phase. In the present investigation, a sixth parameter, the degree of asymmetry, was added in order to enhance the flexibility of this approach. The effects of expanding the available functions were evaluated by means of both simulations using synthetic signals and authentic TEOAEs. The resulting analyses showed that the contributions of asymmetric components in the OAE signal are appreciable. In short, the expanded analysis method brought about important improvements in identifying TEOAE components including the correct detection of components with long decays, which are often related to spontaneous OAE activity, the elimination of a "dark energy" effect in TF distributions, and more reliable estimates of latency-frequency relationships. The latter feature is especially important for correct estimation of latency-frequency data, which is a crucial factor in investigations of OAE-generation mechanisms.

Otoacoustic emissions evoked by 0.5 kHz tone bursts

The aim of this research is to extend previous studies of the time-frequency features of otoacoustic emissions (OAEs) using information about the properties of the signals at low frequencies. Responses to 0.5 kHz tone bursts were compared to OAEs that were evoked by click stimuli and by 1, 2, and 4 kHz tone burst stimuli. The OAEs were measured using 20 and 30 ms intervals between stimuli. The analysis revealed no differences in the time-frequency properties of 1, 2, and 4 kHz bursts measured using these two different acquisition windows. However, at 0.5 kHz the latency of the response was affected significantly if a shorter time window was used. This was caused by the fact that the response reached a maximum after an average time of 15.4 ms, and lasted a few milliseconds longer. Therefore, for this particular stimulus, the use of a 30 ms time window seems more appropriate. In addition, as an example of the possible application of low-frequency OAEs, signals were measured in patients suffering from partial deafness, characterized by steep audiograms with normal thresholds up to 0.5 kHz and almost total deafness above this frequency. Although no response to clicks was observed in these subjects, the use of 0.5 kHz tone bursts did produce OAEs.