Some ECMR properties in relation to other signals from the auditory periphery

Physiologically active cochlear micromechanics--one source of tinnitus

Spontaneous oscillatory vibrations within the cochlea exist in many normal ears and can become audible, providing one source of mild tonal tinnitus. These vibrations have been studied experimentally using an acoustic ear-canal recording technique. The spontaneous oscillations are directly related to stimulated acoustic emissions (cochlear echoes). They arise because of the development of strong cochlear resonances by a feedback process. With the aid of cochlear resonance theory the bandwidths of spontaneous cochlear mechanical activity can be used to determine intracochlear broad-band noise levels. Ways of modifying cochlear mechanical activity have been explored. Activity can be increased or decreased by changes in middle-ear mobility or by brief overstimulation. In the latter case a biphasic recovery cycle has been identified which also seems to be relevant to temporary noise-induced tinnitus and temporary threshold shift. Tinnitus due to spontaneous cochlear vibrations is here called cochlear mechanical tinnitus. Identification criteria are given for such tinnitus involving cochlear mechanical resonance. This form is mild and likely to be found in normally hearing people with tinnitus and in those with only middle-ear disorders. It is speculated that localized oscillation of mechanically isolated cochlear elements could induce much higher levels of tinnitus without producing any externally detectable vibration.

Effect of click intensity on click-evoked otoacoustic emission waveforms: implications for the origin of emissions

The rather shallow growth of click-evoked otoacoustic emissions (CEOAE) with click intensity, namely <1 dB/dB, distinguishes genuine CEOAEs from stimulus artifacts, thereby providing the rationale for the popular 'derived nonlinear recording' method. However, other CEOAE nonlinearities regarding phase or envelope dependence on stimulus intensity have been barely acknowledged so far. The present work used CEOAEs from 20 normal ears recorded in response to 50-86 dB peak equivalent SPL clicks. The phases of CEOAE spectral components varied considerably with click intensity (sometimes more than 120 degrees ), mostly in a monotonic manner and in such a way that in the majority of ears, phase lagged with increasing intensity. When present, synchronized spontaneous otoacoustic emissions exhibited the same behavior. In a few instances, conspicuous frequency shifts of CEOAE spectral peaks were seen. In contrast to CEOAE phases, envelopes were almost intensity-invariant. This behavior contrasts with that of basilar membrane motion at the place tuned to the stimulus frequency, as consistently disclosed by several recent publications, i.e., no phase shift and large envelope shift with stimulus intensity. It is thought that the phase invariance of basilar membrane motion implies that whatever they do, outer hair cells cannot alter the resonance frequency of the cochlear partition. If one elaborates along this line of reasoning, the large phase shift of CEOAEs with click intensity implies that CEOAEs at frequency f cannot come from the place tuned to f and that instead, they may be intermodulation distortion products produced by nonlinear interactions between spectral components of the click stimulus over a significant length of the basilar membrane.

Use of transient evoked otoacoustic emissions to detect and monitor cochlear damage caused by platinum-containing drugs

Transient evoked otoacoustic emissions (TEOAE) have been evaluated as a means of monitoring cochlear function in patients receiving the chemotherapeutic agents cisplatin and carboplatin (-cis-diammine, 1,1-cyclobutane dicarboxylate (2) -0,0-platinum). Patients receiving these drugs were monitored prospectively with pure tone audiometry (PTA), tympanometry and TEOAE. Data was collected on 22 subjects receiving cisplatin and nine subjects receiving carboplatin. Significant deterioration in both PTA thresholds and TEOAE energy levels (with no change in tympanometry) were detected in the cisplatin group. No significant deterioration in audiological parameters occurred in the carboplatin group. It is indicated that cisplatin has a significant ototoxic effect in the majority of patients, whereas any ototoxic effect of carboplatin was undetectable. Our findings were different from previous studies in that the measurable changes in TEOAE occurred later than changes in the pure tone audiogram for the cisplatin group.

Onset of basilar membrane non-linearity reflected in cubic distortion tone input-output functions

The basilar membrane (BM) input output (I/O) function is a non-linear compressive function over much of its operating range. A low level non-compressive region with a break-point or compression threshold between 20 and 40 dB SPL has been identified. To date, no similar compression threshold in cubic distortion tone otoacoustic emission (CDT) data, which would illustrate the dependence of the CDT on BM growth, has been demonstrated. A Taylor series expansion of the outer hair cell gating function yields an amplitude term for 2f1-f2 of p.A1(2).A2, where A1 and A2 are the displacement amplitudes of the BM for two pure tone input stimuli of levels L1 and L2, p a constant. By selectively varying either L1 or L2 with f2/f1 appropriately chosen to reduce suppression effects, the CDT I/O function can be examined for deviation from the power law. In particular, if the amplitude of the CDT were dependent on BM displacement amplitude, then it should be possible by an appropriate choice of parameters to measure compression threshold. We have examined CDT I/O functions for an f2 of 8 kHz in the guinea pig and found them to be consistent with the expected power law. With L1 held constant, L2 varied and f2/f1 = 1.6, a low level region with a slope of one and a compressive region with a slope of 0.14-0.27 corresponding to the analogous regions of the BM I/O function was identified, with a break-point or compression threshold of 22-33 dB SPL.

Otoacoustic emissions measured with a physically open recording system

Otoacoustic emissions have historically been measured with an acoustical probe assembly hermetically sealed in the ear canal, imposing in most cases a limited stimulus bandwidth. A physically open recording system should afford the possibility of a greater stimulus bandwidth but the change in acoustical load may affect the magnitude of otoacoustic emissions obtained. Here it is reported that the authors have measured in the guinea pig transient-evoked otoacoustic emissions extending in frequency to 20 kHz and cubic distortion tone otoacoustic emissions for f2 = 4737 and 8096 Hz with a physically open sound system. To address the effect of acoustical load provided by a physically open versus hermetically sealed system, the authors compared the amplitude of electrically evoked otoacoustic emissions recorded from a guinea pig in each case. The change in acoustical load in the ear canal introduced by the change in recording setup did not appear to make a substantial difference to the magnitude of otoacoustic emissions measured. A physically open recording system provides a good alternative to traditional acoustical probe assemblies sealed in the ear canal for the laboratory measurement of acoustically evoked otoacoustic emissions, with the advantage of permitting a greater stimulus bandwidth.

Time-frequency distribution of evoked otoacoustic emissions

Otoacoustic emissions (OAE) are non-stationary signals that vary in time depending on the characteristics of the stimulus. Traditional spectral analysis using Fourier methods ignores the effects of time and can miss important temporal information. Therefore, a better form of spectral analysis requires the use of time-frequency distribution methods. Traditionally, short time Fourier transforms (STFT), commonly known as spectrograms, are used to provide such time-frequency representations. STFT however, suffer from poor resolution and do not provide enough detail about the characteristics of the emissions. In this study, recently developed time-frequency distributions, the Wigner Distribution (WD) and the Choi-Williams Distribution (CWD) are investigated to provide high resolution representations of transient evoked OAEs. Although WD has excellent properties for time-frequency analysis, it suffers from cross-term artefacts generated when multiple sinusoids are present. CWD provides a solution to this problem at the expense of poor time and frequency support. In this study, we use both distributions to estimate the cross-products and provide a relatively artefact-free time-frequency distribution of OAEs. This method is applied to both click and tone burst evoked OAE and shows a more detailed time-frequency representation with as many crests and valleys as different latencies.

Distortion product otoacoustic emissions in the C57BL/6J mouse model of age-related hearing loss

One of the earliest histopathological changes associated with age-related hearing loss appears to be the disruption of outer hair cells (OHCs). To evaluate age-related changes in OHC function, distortion product otoacoustic emissions (DPOAEs) were recorded in the young and aging C57BL/6J mouse. Starting in young adulthood, the C57 mouse displays age-related elevation of auditory brainstem response thresholds, beginning in the high frequencies and progressing toward lower frequencies. The 2f1-f2 DPOAEs of mice between 2 and 20 months of age were examined for f2s between 8 and 16 kHz. In this octave region, the features of 2f1-f2 DPOAEs in the 2-month-old C57 mouse were comparable to those described for non-murine rodents in the literature in terms of optimum f2/f1 ratio, optimum primary level difference, input/output (I/O) function features and microstructure. It was determined that f2/f1 = 1.2 and L1-L2 = 20 dB were optimal stimulus parameters for investigation of the effects of age on C57 DPOAEs. Age-related changes in DPOAE I/O functions consisted of a right shift (i.e. increased DPOAE detection thresholds), disappearance of 'notches' and shallowing of the slopes after 8 months of age. As DPOAE I/O functions continued to shift to the right and DPOAE levels decreased with age, the appearance of I/O functions became complex to include regions of steep or shallow slopes and plateaus. The present results suggest that the age-related elevation of auditory thresholds in the C57 mice is associated with substantial progressive changes in OHC function.

Effect of interstimulus interval on evoked otoacoustic emissions

In order to explore extensively the effect of interstimulus interval, including very short interstimulus intervals, on evoked otoacoustic emissions (EOAEs), several EOAE recordings were carried out using pairs of clicks: a suppressor click preceded the stimulus click generating an EOAE, with various intervals between the two clicks. EOAEs elicited by two clicks separated by intervals under 8-9 ms had significantly smaller amplitudes than EOAEs evoked by the stimulus alone. The amplitude decay correlated with the interclick interval, and was about 40% when the interclick interval decreased from 12 to 1 ms. This phenomenon has been noted before but not precisely quantified. It might reflect an adaptive mechanism within the outer hair cells, which has been previously described, or else mechanical interactions on the basilar membrane. The delay in EOAE decrease is of the same order as the first phase of neural adaptation, known as 'rapid adaptation', and these thus may prove to be correlated.

Wavelet analysis of real ear and synthesized click evoked otoacoustic emissions

Wavelet analysis was performed to obtain time-frequency analyses of click evoked otoacoustic emissions from normal ears and one ear with a high-frequency hearing loss; mainly to introduce this relatively new method and to show its potentials for emission analysis. The same analysis was then used to obtain time-frequency decompositions of synthesized emissions. It was found that the introduction of a slightly irregular frequency to place relation for the inner ear yielded synthetic results that were remarkably similar to those obtained from real ears.

Otoacoustic emissions: a new method to diagnose hearing impairment in children

Otoacoustic emissions (OAEs) are epiphenomena of sensitive, amplifying processes during hearing which can be detected in persons with normal inner ear function. They originate from the cochlea and are interpreted as an energy leakage of cochlear processes, perhaps resulting from active outer hair cell movements. OAEs travel from the cochlea through the middle ear to the external auditory canal where they can be detected using sensitive miniature microphones. Transient evoked (TEOAE) tests allow to otoacoustic emissions non-invasively check the integrity of the cochlea. In the neonatal period, registration of OAEs can be accomplished during natural sleep. In infants and neonates TEOAEs can be used as screening test with a screening level at 30 dB HL in paediatric audiology. They are less time consuming and elaborate than auditory brainstem responses (ABR) and they are more sensitive than behavioral tests. TEOAEs are constant over long periods of time and they are reduced or absent due to various adverse influences in the inner ear. These latter characteristics may allow monitoring of the inner ear function over time e.g. during disease and/or during ototoxic therapeutic interventions. Limitations of this new method are due to the fact that TEOAEs are absent in patients with a more than 30 dB HL hearing loss. Thus a hearing threshold cannot be determined. Diseases of the inner ear which are common in early childhood (like otitis media) reduce the transfer of TEOAEs and may wrongly indicate a cochlear hearing disorder. New methods for evaluation and interpretation of TEOAE test results are currently developed which may allow to circumvent this problem.(ABSTRACT TRUNCATED AT 250 WORDS)

Click-evoked oto-acoustic emissions in 1036 ears of healthy newborns

Click-evoked oto-acoustic emissions (EOAEs) were recorded in 1036 ears of healthy newborns and in 71 normal-hearing adult ears. Newborns aged between 3 and 238 h were examined in a separate but not silent room of the obstetric ward. The adults were tested in a quiet but not sound-treated room. The recordings were more difficult in the newborn than in the adult, which was mirrored in recording parameters such as the time required for measurement (up to 7 min in newborns vs. 1-2 min in adult ears). Recording was always successful in adults, while retests were necessary in 4% of newborns. Also the artefact-rejection level and the stimulus stability were more favourable in adults. Still, EOAE recording for screening purposes in newborns seems feasible. Response levels in newborns (range 1.6-38.6; mean 20.2 dB SPL) appear to be higher than in adults (range 2.7-20.6; mean 12.8 dB SPL). The overall prevalence of EOAEs in newborns amounted to 93.4% and appeared to be age related. It rises from 78% in ears from newborns younger than 36 h to 99% in ears of newborns older than 108 h. This rise may be related to the middle ear clearance of amniotic fluid in the first days post partum. The prevalence in newborns older than 3-4 days is comparable with the prevalence of 97.2% in adults. Therefore, newborns should not be screened before the age of 4 days. In search of an objective EOAE detection variable, the prevalence of EOAEs for different age groups was calculated for various criterion values of reproducibility. These prevalences were compared to subjectively scored EOAE prevalences in the same age groups. A reproducibility criterion of about 50% appears to be useful for mass screening in newborns.

Distortion-product otoacoustic emissions in normal and impaired ears: insight into generation processes

Otoacoustic emissions can be used to study cochlear function in an objective, non-invasive and rapid manner. These and other desirable features of emissions have inspired a significant amount of investigation into the practicalities of utilizing evoked emissions as clinical tests of hearing. Variables which affect the measurement of emissions can be sorted into two major categories consisting of factors affecting either emission generation or expression. The present report consolidates and summarizes recent findings of a series of experiments in our laboratory which address both the generation and expression of transiently evoked and distortion-product otoacoustic emissions. Because these two emission types have the greatest promise of becoming clinically useful, a complete understanding of the factors responsible for their measured properties is particularly important.

Evoked otoacoustic emissions as cochlear Bragg reflections

The reflection of cochlear waves is mathematically described for various assumptions about the dependence of the echo-generating mechanism on place and frequency, without requiring knowledge of the physical details of the mechanism. Any 'wave-related' reflections, caused by intrinsic features of the resonance or active-feedback mechanism, cannot explain long group delays in an approximately shift-similar basilar membrane (BM). Only 'BM related' scattering due to inhomogenities is a possible explanation then. Reflection at a fixed point of the BM could cause only narrow-band evoked otoacoustic emissions (EOAEs). Long group delays in a wider frequency band can be obtained by assuming Bragg reflection at an approximately periodic inhomogenity of BM parameters, possibly due to spatial variations of active undamping. This explains the long delays, about inversely proportional to frequency, the decreasing instantaneous frequency, the often found multi-packet or modulated structure of echoes (vanishing at higher SPL), the spectra of simultaneous and delayed EOAEs and their relation to the threshold of hearing. These phenomena do not necessarily require multiple reflection due to middle/inner-ear mismatch.

Spontaneous otoacoustic emissions in a nonhuman primate. II. Cochlear anatomy

Both cochleas of a rhesus monkey exhibiting stable spontaneous and stimulus-frequency emissions were evaluated histologically using surface-preparation methods to determine if certain features of these emissions could be related to structural properties of the organ of Corti (OC). The comprehensive assessment included preparation of routine cytocochleograms and a detailed study of the arrangement of cochlear sensory cells, best revealed by the precise positional relationships between stereocilia bundles, in selected areas representing low-, medium-, and high-frequencies. Several additional measurements were made in an area extending from about 25-60% distance from the apex, which was estimated to encompass the cochlear region where emissions were generated. These quantifications included measures, in both micrometers and Hertz, of the distances between irregularities in the lateral border of the OC due to a sporadically occurring fourth row of outer hair cells (OHCs). Measures, in micrometers, of the changes in the radial extent of the corresponding OC in the presence or absence of this extra fourth row of OHCs were also made. A final measure within low-, medium-, and high-frequency OC regions consisted of describing the angles that the tips of the stereocilia bundles were displaced from an axis parallel to the tunnel of Corti. For comparative purposes, similar plots were made in comparable regions of the OC in the normal and experimental cochleas of three additional rhesus monkeys in which one ear had been systematically exposed to noise. In the emitting-monkey cochlea, there was a mild loss of sensory cells scattered throughout the OC which was generally greater for the OHCs. No evidence of small circumscribed lesions, defined as a loss of more than four adjacent hair cells, was found. The most striking observation which varied in degree across the three other monkeys was a generalized irregularity in the cellular organization of the OHC region which was most pronounced in the low- and midfrequency regions of the OC. The notable cellular disorganization specific to the apical half of the cochlea was reflected by an increased variance in the distribution of deviation angles measured for corresponding stereocilia bundles. Outer hair cells in the remaining basal region of the OC were arranged in three regular rows with the usual stereocilia orientation.(ABSTRACT TRUNCATED AT 400 WORDS)

Spontaneous otoacoustic emissions in a nonhuman primate. I. Basic features and relations to other emissions

Otoacoustic emissions in both ears of a rhesus monkey exhibiting stable spontaneous emissions (SOEs) were monitored over a 1-year period. The amplitudes and frequencies of both SOEs and stimulus-frequency emissions (SFEs) were routinely recorded, while transiently evoked (EOE) and distortion-product emissions (DPEs), at the frequency 2f1-f2, were occasionally examined. Between evaluation sessions, both the frequencies and amplitudes of SFEs remained relatively stable in both ears, while the frequencies and amplitudes of SOEs were less constant. Isosuppression contours for SOEs, plotted as a function of frequency and level of tonal maskers, revealed sharp tuning consistent with normal frequency selectivity. Detailed analyses of long-term measurements showed that SOEs occurred most frequently at the peaks of the SFE response. A regular frequency spacing between neighboring amplitude maxima and minima of the SFEs was consistent with the notion that this particular emitted response may result from a periodic disruption of the orderly pattern of sensory cells along the organ of Corti. Intramuscular administration of aspirin abolished SOE and SFE responses, while DPEs remained relatively unchanged suggesting the involvement of separate mechanisms in the generation of different emissions.

Nonlinear phenomena in click- and tone-burst-evoked otoacoustic emissions from human ears

Otoacoustic emissions have been recorded from normally hearing subjects in response to clicks and 1-kHz tone bursts. Input-output relationships for response magnitudes and wave delays are presented. For the response magnitudes, two main effects are seen: (i) nonlinearities are maximal at moderate to high intensity levels (saturation), while deviations from linearity are minimal at the lowest levels (around the psychoacoustic threshold); (ii) the nonlinear behaviour is different at different time intervals (after stimulation): deviations from linearity are maximal for the latest parts of the response. Level- and time-dependent phenomena are also observed in the delay of identifiable response waves.

Adaptation in hearing-impaired ears: effects of intermediate duration stimuli

Recovery from adaptation was measured in acoustically-traumatized ears of cats for pure-tone adapters 1-60 s in duration at levels of 60-100 dB SPL. Adaptation was assessed by measuring the whole nerve action potentials in response to pure-tone stimuli. Effects of changing probe frequency and of changing adapter duration were measured. Impaired ears showed less adaptation than normal ears for adapters of similar SPL, but they showed relatively normal spread of adaptation across probe frequency. The effect was larger for long duration stimuli than for short duration stimuli.

Evoked acoustic emissions and cochlear microphonics in the mustache bat, Pteronotus parnellii

In the echolocating bat, Pteronotus parnellii, otoacoustic responses at a frequency of 62 kHz are measurable in the external ear canal during continuous and after transient acoustic stimulation. These responses are interpreted to represent emissions from the cochlea. They can reach an amplitude as large as 70 dB SPL and occur in the frequency range most important for echolocation, namely on the average about 700 Hz above the constant frequency component of the orientation calls. A sharp maximum of the amplitude of cochlear microphonic potentials at about 62 kHz could be correlated with the emission frequency. In one bat an evoked otoacoustic response changed to a spontaneous otoacoustic emission. The frequency and amplitude of the evoked otoacoustic responses reversibly decreased after exposure for 1 min to continuous sounds of more than 85 dB SPL with frequencies of about 2.5-7.5 kHz above the emission frequency. Similar effects occurred during anaesthesia or cooling. A possible relation between the existence of otoacoustic emissions and morphological specializations of the cochlea is discussed.

Power supplies for hearing aids

The design of a hearing-aid system involves three disciplines of applied science--electrochemistry, electrical engineering and audio engineering. This paper is concerned with the interface between the first two. Batteries are essentially non-linear components. Optimum performance is only achievable when the electrical requirements of the hearing aid are closely matched with the voltage, rate capability and impedance of the battery. After years of optimalization, the modern '675' button cell has earned universal acceptability and is now used in most 'behind-the-ear' hearing aids. When more power is required, the larger and less specialized LR6 'penlight' cell is typically specified. Higher voltage might lead to better circuit efficiency, and there is some pressure to introduce a 3 V lithium-based product. Lithium should give superior energy density, but there are problems which remain to be solved. In the end, it is quite possible that the market might settle for an ecologically acceptable long-life lower voltage metal-air cell. If so, the recent zinc-air system may well have a future and could conceivably succeed both the mercury '675' and the alkaline 'penlight' cells.

Evoked acoustic emissions from the human ear. I. Equipment and response parameters

Using signal averaging technique, stimulated acoustic emissions can be recorded from the human ear with a probe in the external ear canal. An acoustic click stimulus was used, produced by half a sinusoid of 2 kHz with the polarity corresponding to the rarefaction mode. A number of different techniques were developed in order to evaluate the latency and configuration of the emissions objectively. Recordings from a normal-hearing subject served as an example and a clear response could be traced down and below the psychoacoustic threshold. The threshold was elevated and the response pattern altered when a sensorineural hearing loss was induced by ingestion of acetylsalicylate. No response could be recorded from a deaf ear with an intact eardrum and a mobile ossicular chain.

Acoustical responses and suppression-period patterns in guinea pigs

Acoustic responses to short sound stimuli have been measured in the external ear canal of guinea pigs. Unequivocal responses had frequencies between 1.8 and 3.2 kHz, latencies (to their peak pressures) of 2.8 to 4.5 ms and sound pressures up to -10 dB SPL. A set of criteria was developed to clearly distinguish acoustical responses from after-resonances of the stimuli. Added phase-locked, very-low-frequency tones were able under some conditions to totally suppress the acoustical responses. The suppression-period pattern thus produced strongly resemble those obtained for man and the masking-period patterns obtained with human subjects.

Properties of the generator of stimulated acoustic emissions

In Part I, the sound emitting properties of the peripheral auditory system are briefly reviewed and links with other literature are cited. (Summary of Symposium section introduction). In Part II, the stimulation of acoustic emissions by clicks is examined experimentally and analytically, in order to specify the physical qualities and parameters of the unknown generator which may be considered as driving the ear drum. Special attention is paid to absolute magnitudes to the qualitative interpretation of the generator's nonlinear properties and to the extent of inter-stimulus interactions in the time and frequency domains. New transient suppression results are presented. From the observations made, and without reference to the likely cochlear origin of the phenomenon, it is deduced that the generator is a multiple channel system, with each channel consisting of at least a steep-sloped narrow band filter followed by a fast acting compressive nonlinearity. A secondary nonlinearity seems also to be present, of a form which requires the filter itself to be nonlinear. The maximum power output of the generator is of the order 0.26 X 10(-12) W.