Latency of tone-burst-evoked auditory brain stem responses and otoacoustic emissions: level, frequency, and rise-time effects

Aging effects on the neural representation and perception of consonant transition cues

Older listeners have difficulty processing temporal cues that are important for word discrimination, and deficient processing may limit their ability to benefit from these cues. Here, we investigated aging effects on perception and neural representation of the consonant transition and the factors that contribute to successful perception. To further understand the neural mechanisms underlying the changes in processing from brainstem to cortex, we also examined the factors that contribute to exaggerated amplitudes in cortex. We enrolled 30 younger normal-hearing and 30 older normal-hearing participants who met the criteria of clinically normal hearing. Perceptual identification functions were obtained for the words BEAT and WHEAT on a 7-step continuum of consonant-transition duration. Auditory brainstem responses (ABRs) were recorded to click stimuli and frequency-following responses (FFRs) and cortical auditory-evoked potentials were recorded to the endpoints of the BEAT-WHEAT continuum. Perceptual performance for identification of BEAT vs. WHEAT did not differ between younger and older listeners. However, both subcortical and cortical measures of neural representation showed age group differences, such that FFR phase locking was lower but cortical amplitudes (P1 and N1) were higher in older compared to younger listeners. ABR Wave I amplitude and FFR phase locking, but not audiometric thresholds, predicted early cortical amplitudes. Phase locking to the transition region and early cortical peak amplitudes (P1) predicted performance on the perceptual identification function. Overall, results suggest that the neural representation of transition durations and cortical overcompensation may contribute to the ability to perceive transition duration contrasts. Cortical overcompensation appears to be a maladaptive response to decreased neural firing/synchrony.

Neural Adaptation at Stimulus Onset and Speed of Neural Processing as Critical Contributors to Speech Comprehension Independent of Hearing Threshold or Age

Background: It is assumed that speech comprehension deficits in background noise are caused by age-related or acquired hearing loss. Methods: We examined young, middle-aged, and older individuals with and without hearing threshold loss using pure-tone (PT) audiometry, short-pulsed distortion-product otoacoustic emissions (pDPOAEs), auditory brainstem responses (ABRs), auditory steady-state responses (ASSRs), speech comprehension (OLSA), and syllable discrimination in quiet and noise. Results: A noticeable decline of hearing sensitivity in extended high-frequency regions and its influence on low-frequency-induced ABRs was striking. When testing for differences in OLSA thresholds normalized for PT thresholds (PTTs), marked differences in speech comprehension ability exist not only in noise, but also in quiet, and they exist throughout the whole age range investigated. Listeners with poor speech comprehension in quiet exhibited a relatively lower pDPOAE and, thus, cochlear amplifier performance independent of PTT, smaller and delayed ABRs, and lower performance in vowel-phoneme discrimination below phase-locking limits (/o/-/u/). When OLSA was tested in noise, listeners with poor speech comprehension independent of PTT had larger pDPOAEs and, thus, cochlear amplifier performance, larger ASSR amplitudes, and higher uncomfortable loudness levels, all linked with lower performance of vowel-phoneme discrimination above the phase-locking limit (/i/-/y/). Conslusions: This study indicates that listening in noise in humans has a sizable disadvantage in envelope coding when basilar-membrane compression is compromised. Clearly, and in contrast to previous assumptions, both good and poor speech comprehension can exist independently of differences in PTTs and age, a phenomenon that urgently requires improved techniques to diagnose sound processing at stimulus onset in the clinical routine.

Duplex perception reveals brainstem auditory representations are modulated by listeners' ongoing percept for speech

So-called duplex speech stimuli with perceptually ambiguous spectral cues to one ear and isolated low- versus high-frequency third formant "chirp" to the opposite ear yield a coherent percept supporting their phonetic categorization. Critically, such dichotic sounds are only perceived categorically upon binaural integration. Here, we used frequency-following responses (FFRs), scalp-recorded potentials reflecting phase-locked subcortical activity, to investigate brainstem responses to fused speech percepts and to determine whether FFRs reflect binaurally integrated category-level representations. We recorded FFRs to diotic and dichotic stop-consonants (/da/, /ga/) that either did or did not require binaural fusion to properly label along with perceptually ambiguous sounds without clear phonetic identity. Behaviorally, listeners showed clear categorization of dichotic speech tokens confirming they were heard with a fused, phonetic percept. Neurally, we found FFRs were stronger for categorically perceived speech relative to category-ambiguous tokens but also differentiated phonetic categories for both diotically and dichotically presented speech sounds. Correlations between neural and behavioral data further showed FFR latency predicted the degree to which listeners labeled tokens as "da" versus "ga." The presence of binaurally integrated, category-level information in FFRs suggests human brainstem processing reflects a surprisingly abstract level of the speech code typically circumscribed to much later cortical processing.

Exposing distinct subcortical components of the auditory brainstem response evoked by continuous naturalistic speech

Speech processing is built upon encoding by the auditory nerve and brainstem, yet we know very little about how these processes unfold in specific subcortical structures. These structures are deep and respond quickly, making them difficult to study during ongoing speech. Recent techniques have begun to address this problem, but yield temporally broad responses with consequently ambiguous neural origins. Here, we describe a method that pairs re-synthesized ‘peaky’ speech with deconvolution analysis of electroencephalography recordings. We show that in adults with normal hearing the method quickly yields robust responses whose component waves reflect activity from distinct subcortical structures spanning auditory nerve to rostral brainstem. We further demonstrate the versatility of peaky speech by simultaneously measuring bilateral and ear-specific responses across different frequency bands and discuss the important practical considerations such as talker choice. The peaky speech method holds promise as a tool for investigating speech encoding and processing, and for clinical applications.

Subcortical neural generators of the envelope-following response in sleeping children: A transfer function analysis

Multiple auditory structures, from cochlea to cortex, phase-lock to the envelope of complex stimuli. The relative contributions of these structures to the human surface-recorded envelope-following response (EFR) are still uncertain. Identification of the active contributor(s) is complicated by the fact that even the simplest two-tone (f₁&f₂) stimulus, targeting its (f₂-f₁) envelope, evokes additional linear (f₁&f₂) and non-linear (2f₁-f₂) phase-locked components as well as a transient auditory brainstem response (ABR). Here, we took advantage of the generalized primary tone phase variation method to isolate each predictable component in the time domain, allowing direct measurements of onset latency, duration and phase discontinuity values from which the involved generators were inferred. Targeting several envelope frequencies (0.22-1 kHz), we derived the EFR transfer functions along a vertical vertex-to-neck and a horizontal earlobe-to-earlobe recording channels, yielding respectively EFR-V and EFR-H waveforms. Subjects (N= 30) were sleeping children with normal electrophysiological thresholds and normal oto-acoustic emissions. Both EFR-H and EFR-V phase-locking values (PLV) transfer functions had a low-pass profile, EFR-V showing a lower cut-off frequency than EFR-H. We also computed the frequency-latency relationships of both EFRs onset latencies. EFR-H data fitted a power-law function incorporating a frequency-dependent traveling wave delay and a fixed one amounting to 1.2 ms. The fitted function nicely fell within five published estimations of the latency-frequency function of the ABR wave-I, thus pointing to a cochlear nerve origin. The absence of phase discontinuity and overall response durations that were equal to that of the stimulus indicated no contribution from a later generator. The recording of an entirely similar EFR-H response in a patient who had severe brainstem encephalitis with a normal, isolated, ABR wave-I but complete absence of later waves, further substantiated a cochlear nerve origin. Modeling of the EFR-V latency-frequency functions indicated a fixed transport time of 2 ms with respect to EFR-H onset, suggesting a cochlear nucleus (CN) origin, here also, without indication for multiple generators. Other features of the EFR-V response pointing to the CN were, at least for the EFR frequency below the cut-off values of the transfer functions, higher PLVs coupled with increased harmonic distortion. Such a behavior has been described in the so-called highly-synchronized neurons of the ventral cochlear nucleus (VCN). The present study compellingly demonstrated the advantage of isolating the EFR in the temporal domain so as to extract detailed spectro-temporal parameters that, combined with orthogonal recording channels, shed new light on the involved neural generators.

Multiparametric assessment of the impact of opsin expression and anesthesia on striatal cholinergic neurons and auditory brainstem activity

Recent developments in genetic engineering have established murine models that permit the selective control of cholinergic neurons via optical stimulation. Despite copious benefits granted by these experimental advances, the sensory physiognomy of these organisms has remained poorly understood. Therefore, the present study evaluates sensory and neuronal response properties of animal models developed for the study of optically induced acetylcholine release regulation. Auditory brainstem responses, fluorescence imaging, and patch clamp recording techniques were used to assess the impact of viral infection, sex, age, and anesthetic agents across the ascending auditory pathway of ChAT-Cre and ChAT-ChR2(Ai32) mice. Data analyses revealed that neither genetic configuration nor adeno-associated viral infection alters the early stages of auditory processing or the cellular response properties of cholinergic neurons. However, anesthetic agent and dosage amount profoundly modulate the response properties of brainstem neurons. Last, analyses of age-related hearing loss in virally infected ChAT-Cre mice did not differ from those reported in wild type animals. This investigation demonstrates that ChAT-Cre and ChAT-ChR2(Ai32) mice are viable models for the study of cholinergic modulation in auditory processing, and it emphasizes the need for prudence in the selection of anesthetic procedures.

A simple algorithm for objective threshold determination of auditory brainstem responses

The auditory brainstem response (ABR) is a sound-evoked neural response commonly used to assess auditory function in humans and laboratory animals. ABR thresholds are typically chosen by visual inspection, leaving the procedure susceptible to user bias. We sought to develop an algorithm to automate determination of ABR thresholds to eliminate such biases and to standardize approaches across investigators and laboratories. Two datasets of mouse ABR waveforms obtained from previously published studies of normal ears as well as ears with varying degrees of cochlear-based threshold elevations (Maison et al., 2013; Sergeyenko et al., 2013) were reanalyzed using an algorithm based on normalized cross-covariation of adjacent level presentations. Correlation-coefficient vs. level data for each ABR level series were fit with both a sigmoidal and two-term power function. From these fits, threshold was interpolated at different criterion values of correlation-coefficient ranging from 0 to 0.5. The criterion value of 0.35 was selected by comparing visual thresholds to computed thresholds across all frequencies tested. With such a criterion, the mean algorithm-computed thresholds were comparable to the visual thresholds noted by two independent observers for each data set. The success of the algorithm was also qualitatively assessed by comparing averaged waveforms at the thresholds determined by the two methods, and quantitatively assessed by comparing peak 1 amplitude growth functions expressed as dB re each of the two threshold measures. Application of a cross-covariance analysis to ABR waveforms can emulate visual thresholding decisions made by highly trained observers. Unlike previous applications of similar methodologies using template matching, our algorithm performs only intrinsic comparisons within ABR sets, and therefore is more robust to equipment and investigator differences in assessing waveforms, as evidenced by similar results across the two datasets.

Between-ear sound frequency disparity modulates a brain stem biomarker of binaural hearing

The auditory brain stem response (ABR) is an evoked potential that indexes a cascade of neural events elicited by sound. In the present study we evaluated the influence of sound frequency on a derived component of the ABR known as the binaural interaction component (BIC). Specifically, we evaluated the effect of acoustic interaural (between-ear) frequency mismatch on BIC amplitude. Goals were to 1) increase basic understanding of sound features that influence this long-studied auditory potential and 2) gain insight about the persistence of the BIC with interaural electrode mismatch in human users of bilateral cochlear implants, presently a limitation on the prospective utility of the BIC in audiological settings. Data were collected in an animal model that is audiometrically similar to humans, the chinchilla (Chinchilla lanigera; 6 females). Frequency disparities and amplitudes of acoustic stimuli were varied over broad ranges, and associated variation of BIC amplitude was quantified. Subsequently, responses were simulated with the use of established models of the brain stem pathway thought to underlie the BIC. Collectively, the data demonstrate that at high sound intensities (≥85 dB SPL), the acoustically elicited BIC persisted with interaurally disparate stimulation (click frequencies ≥1.5 octaves apart). However, sharper tuning emerged at moderate sound intensities (65 dB SPL), with the largest BIC occurring for stimulus frequencies within ~0.8 octaves, equivalent to ±1 mm in cochlear place. Such responses were consistent with simulated responses of the presumed brain stem generator of the BIC, the lateral superior olive. The data suggest that leveraging focused electrical stimulation strategies could improve BIC-based bilateral cochlear implant fitting outcomes.NEW & NOTEWORTHY Traditional hearing tests evaluate each ear independently. Diagnosis and treatment of binaural hearing dysfunction remains a basic challenge for hearing clinicians. We demonstrate in an animal model that the prospective utility of a noninvasive electrophysiological signature of binaural function, the binaural interaction component (BIC), depends strongly on the intensity of auditory stimulation. Data suggest that more informative BIC measurements could be obtained with clinical protocols leveraging stimuli restricted in effective bandwidth.

Using Thresholds in Noise to Identify Hidden Hearing Loss in Humans

OBJECTIVES

Recent animal studies suggest that noise-induced synaptopathy may underlie a phenomenon that has been labeled hidden hearing loss (HHL). Noise exposure preferentially damages low spontaneous-rate auditory nerve fibers, which are involved in the processing of moderate- to high-level sounds and are more resistant to masking by background noise. Therefore, the effect of synaptopathy may be more evident in suprathreshold measures of auditory function, especially in the presence of background noise. The purpose of this study was to develop a statistical model for estimating HHL in humans using thresholds in noise as the outcome variable and measures that reflect the integrity of sites along the auditory pathway as explanatory variables. Our working hypothesis is that HHL is evident in the portion of the variance observed in thresholds in noise that is not dependent on thresholds in quiet, because this residual variance retains statistical dependence on other measures of suprathreshold function.

DESIGN

Study participants included 13 adults with normal hearing (≤15 dB HL) and 20 adults with normal hearing at 1 kHz and sensorineural hearing loss at 4 kHz (>15 dB HL). Thresholds in noise were measured, and the residual of the correlation between thresholds in noise and thresholds in quiet, which we refer to as thresholds-in-noise residual, was used as the outcome measure for the model. Explanatory measures were as follows: (1) auditory brainstem response (ABR) waves I and V amplitudes; (2) electrocochleographic action potential and summating potential amplitudes; (3) distortion product otoacoustic emissions level; and (4) categorical loudness scaling. All measurements were made at two frequencies (1 and 4 kHz). ABR and electrocochleographic measurements were made at 80 and 100 dB peak equivalent sound pressure level, while wider ranges of levels were tested during distortion product otoacoustic emission and categorical loudness scaling measurements. A model relating the thresholds-in-noise residual and the explanatory measures was created using multiple linear regression analysis.

RESULTS

Predictions of thresholds-in-noise residual using the model accounted for 61% (p < 0.01) and 48% (p < 0.01) of the variance in the measured thresholds-in-noise residual at 1 and 4 kHz, respectively.

CONCLUSIONS

Measures of thresholds in noise, the summating potential to action potential ratio, and ABR waves I and V amplitudes may be useful for the prediction of HHL in humans. With further development, our approach of quantifying HHL by the variance that remains in suprathreshold measures of auditory function after removing the variance due to thresholds in quiet, together with our statistical modeling, may provide a quantifiable and verifiable estimate of HHL in humans with normal hearing and with hearing loss. The current results are consistent with the view that inner hair cell and auditory nerve pathology may underlie suprathreshold auditory performance.

Perceptual Encoding in Auditory Brainstem Responses: Effects of Stimulus Frequency

PURPOSE

A central question about auditory perception concerns how acoustic information is represented at different stages of processing. The auditory brainstem response (ABR) provides a potentially useful index of the earliest stages of this process. However, it is unclear how basic acoustic characteristics (e.g., differences in tones spanning a wide range of frequencies) are indexed by ABR components. This study addresses this by investigating how ABR amplitude and latency track stimulus frequency for tones ranging from 250 to 8000 Hz.

METHOD

In a repeated-measures experimental design, listeners were presented with brief tones (250, 500, 1000, 2000, 4000, and 8000 Hz) in random order while electroencephalography was recorded. ABR latencies and amplitudes for Wave V (6-9 ms) and in the time window following the Wave V peak (labeled as Wave VI; 9-12 ms) were measured.

RESULTS

Wave V latency decreased with increasing frequency, replicating previous work. In addition, Waves V and VI amplitudes tracked differences in tone frequency, with a nonlinear response from 250 to 8000 Hz and a clear log-linear response to tones from 500 to 8000 Hz.

CONCLUSIONS

Results demonstrate that the ABR provides a useful measure of early perceptual encoding for stimuli varying in frequency and that the tonotopic organization of the auditory system is preserved at this stage of processing for stimuli from 500 to 8000 Hz. Such a measure may serve as a useful clinical tool for evaluating a listener's ability to encode specific frequencies in sounds.

SUPPLEMENTAL MATERIAL

https://doi.org/10.23641/asha.6987422.

The Effect of Interaural Mismatches on Contralateral Unmasking With Single-Sided Vocoders

OBJECTIVES

Cochlear-implant (CI) users with single-sided deafness (SSD)-that is, one normal-hearing (NH) ear and one CI ear-can obtain some unmasking benefits when a mixture of target and masking voices is presented to the NH ear and a copy of just the masking voices is presented to the CI ear. NH listeners show similar benefits in a simulation of SSD-CI listening, whereby a mixture of target and masking voices is presented to one ear and a vocoded copy of the masking voices is presented to the opposite ear. However, the magnitude of the benefit for SSD-CI listeners is highly variable across individuals and is on average less than for NH listeners presented with vocoded stimuli. One possible explanation for the limited benefit observed for some SSD-CI users is that temporal and spectral discrepancies between the acoustic and electric ears might interfere with contralateral unmasking. The present study presented vocoder simulations to NH participants to examine the effects of interaural temporal and spectral mismatches on contralateral unmasking.

DESIGN

Speech-reception performance was measured in a competing-talker paradigm for NH listeners presented with vocoder simulations of SSD-CI listening. In the monaural condition, listeners identified target speech masked by two same-gender interferers, presented to the left ear. In the bilateral condition, the same stimuli were presented to the left ear, but the right ear was presented with a noise-vocoded copy of the interfering voices. This paradigm tested whether listeners could integrate the interfering voices across the ears to better hear the monaural target. Three common distortions inherent in CI processing were introduced to the vocoder processing: spectral shifts, temporal delays, and reduced frequency selectivity.

RESULTS

In experiment 1, contralateral unmasking (i.e., the benefit from adding the vocoded maskers to the second ear) was impaired by spectral mismatches of four equivalent rectangular bandwidths or greater. This is equivalent to roughly a 3.6-mm mismatch between the cochlear places stimulated in the electric and acoustic ears, which is on the low end of the average expected mismatch for SSD-CI listeners. In experiment 2, performance was negatively affected by a temporal mismatch of 24 ms or greater, but not for mismatches in the 0 to 12 ms range expected for SSD-CI listeners. Experiment 3 showed an interaction between spectral shift and spectral resolution, with less effect of interaural spectral mismatches when the number of vocoder channels was reduced. Experiment 4 applied interaural spectral and temporal mismatches in combination. Performance was best when both frequency and timing were aligned, but in cases where a mismatch was present in one dimension (either frequency or latency), the addition of mismatch in the second dimension did not further disrupt performance.

CONCLUSIONS

These results emphasize the need for interaural alignment-in timing and especially in frequency-to maximize contralateral unmasking for NH listeners presented with vocoder simulations of SSD-CI listening. Improved processing strategies that reduce mismatch between the electric and acoustic ears of SSD-CI listeners might improve their ability to obtain binaural benefits in multitalker environments.

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.

Modeling signal propagation in the human cochlea

The level-dependent component of the latency of human auditory brainstem responses (ABR) to tonebursts decreases by about 38% for every 20-dB increase in stimulus level over a wide range of both frequency and level [Neely, Norton, Gorga, and Jesteadt (1998). J. Acoust. Soc. Am. 31, 87-97]. This level-dependence has now been simulated in an active, nonlinear, transmission-line model of cochlear mechanics combined with an adaptation stage. The micromechanics in this model are similar to previous models except that a dual role is proposed for the tectorial membrane (TM): (1) passive sharpening the tuning of sensory-cell inputs (relative to basilar-membrane vibrations) and (2) providing an optimal phase shift (relative to basilar-membrane vibrations) of outer-hair-cell feedback forces, so that amplification is restricted to a limited range of frequencies. The adaptation stage, which represents synaptic adaptation of neural signals, contributes to the latency level-dependence more at low frequencies than at high frequencies. Compression in this model spans the range of audible sound levels with a compression ratio of about 2:1. With further development, the proposed model of cochlear micromechanics could be useful both (1) as a front-end to functional models of the auditory system and (2) as a foundation for understanding the physiological basis of cochlear amplification.

Comparison of Transient-Evoked Otoacoustic Emission Waveforms and Latencies Between Nonlinear Measurement Techniques

The nonlinear differential technique is commonly used to remove stimulus artifact when measuring transient-evoked otoacoustic emissions (TEOAE). However, to ensure removal of stimulus artifact, the initial 2.5-ms of the sound pressure recording must be discarded. Discarding this portion of the response precludes measurement of TEOAE energy above approximately 5 kHz and may limit measurement of shorter-latency TEOAE components below 5 kHz. The contribution from short-latency components influences the overall latency of the emission, including its dependence on frequency and stimulus level. The double source, double-evoked technique provides an alternative means to eliminate stimulus energy from the TEOAE and permits retention of the entire response. This study describes the effect of measurement technique on TEOAE waveforms and latencies. TEOAEs were measured in 26 normal hearing subjects using the nonlinear differential and double source, double-evoked techniques. The nonlinear differential technique limited measurement of short-latency TEOAE components at frequencies as low as ~3 kHz. Loss of these components biased TEOAE latencies to later moments in time and reduced the dependence of latency on stimulus level and frequency. In studies investigating TEOAE latency, the double source, double-evoked technique is recommended as it permits measurement of the both long- and short-latency components of the TEOAE.

Tone-burst auditory brainstem response wave V latencies in normal-hearing and hearing-impaired ears

The metric used to equate stimulus level [sound pressure level (SPL) or sensation level (SL)] between ears with normal hearing (NH) and ears with hearing loss (HL) in comparisons of auditory function can influence interpretation of results. When stimulus level is equated in dB SL, higher SPLs are presented to ears with HL due to their reduced sensitivity. As a result, it may be difficult to determine if differences between ears with NH and ears with HL are due to cochlear pathology or level-dependent changes in cochlear mechanics. To the extent that level-dependent changes in cochlear mechanics contribute to auditory brainstem response latencies, comparisons between normal and pathologic ears may depend on the stimulus levels at which comparisons are made. To test this hypothesis, wave V latencies were measured in 16 NH ears and 15 ears with mild-to-moderate HL. When stimulus levels were equated in SL, latencies were shorter in HL ears. However, latencies were similar for NH and HL ears when stimulus levels were equated in SPL. These observations demonstrate that the effect of stimulus level on wave V latency is large relative to the effect of HL, at least in cases of mild-to-moderate HL.

Functional modeling of the human auditory brainstem response to broadband stimulation

Population responses such as the auditory brainstem response (ABR) are commonly used for hearing screening, but the relationship between single-unit physiology and scalp-recorded population responses are not well understood. Computational models that integrate physiologically realistic models of single-unit auditory-nerve (AN), cochlear nucleus (CN) and inferior colliculus (IC) cells with models of broadband peripheral excitation can be used to simulate ABRs and thereby link detailed knowledge of animal physiology to human applications. Existing functional ABR models fail to capture the empirically observed 1.2-2 ms ABR wave-V latency-vs-intensity decrease that is thought to arise from level-dependent changes in cochlear excitation and firing synchrony across different tonotopic sections. This paper proposes an approach where level-dependent cochlear excitation patterns, which reflect human cochlear filter tuning parameters, drive AN fibers to yield realistic level-dependent properties of the ABR wave-V. The number of free model parameters is minimal, producing a model in which various sources of hearing-impairment can easily be simulated on an individualized and frequency-dependent basis. The model fits latency-vs-intensity functions observed in human ABRs and otoacoustic emissions while maintaining rate-level and threshold characteristics of single-unit AN fibers. The simulations help to reveal which tonotopic regions dominate ABR waveform peaks at different stimulus intensities.

Tuning of SFOAEs Evoked by Low-Frequency Tones Is Not Compatible with Localized Emission Generation

Stimulus-frequency otoacoustic emissions (SFOAEs) appear to be well suited for assessing frequency selectivity because, at least on theoretical grounds, they originate over a restricted region of the cochlea near the characteristic place of the evoking tone. In support of this view, we previously found good agreement between SFOAE suppression tuning curves (SF-STCs) and a control measure of frequency selectivity (compound action potential suppression tuning curves (CAP-STC)) for frequencies above 3 kHz in chinchillas. For lower frequencies, however, SF-STCs and were over five times broader than the CAP-STCs and demonstrated more high-pass rather than narrow band-pass filter characteristics. Here, we test the hypothesis that the broad tuning of low-frequency SF-STCs is because emissions originate over a broad region of the cochlea extending basal to the characteristic place of the evoking tone. We removed contributions of the hypothesized basally located SFOAE sources by either pre-suppressing them with a high-frequency interference tone (IT; 4.2, 6.2, or 9.2 kHz at 75 dB sound pressure level (SPL)) or by inducing acoustic trauma at high frequencies (exposures to 8, 5, and lastly 3-kHz tones at 110-115 dB SPL). The 1-kHz SF-STCs and CAP-STCs were measured for baseline, IT present and following the acoustic trauma conditions in anesthetized chinchillas. The IT and acoustic trauma affected SF-STCs in an almost indistinguishable way. The SF-STCs changed progressively from a broad high-pass to narrow band-pass shape as the frequency of the IT was lowered and for subsequent exposures to lower-frequency tones. Both results were in agreement with the "basal sources" hypothesis. In contrast, CAP-STCs were not changed by either manipulation, indicating that neither the IT nor acoustic trauma affected the 1-kHz characteristic place. Thus, unlike CAPs, SFOAEs cannot be considered as a place-specific measure of cochlear function at low frequencies, at least in chinchillas.

Relating the Variability of Tone-Burst Otoacoustic Emission and Auditory Brainstem Response Latencies to the Underlying Cochlear Mechanics

Forward and reverse cochlear latency and its relation to the frequency tuning of the auditory filters can be assessed using tone bursts (TBs). Otoacoustic emissions (TBOAEs) estimate the cochlear roundtrip time, while auditory brainstem responses (ABRs) to the same stimuli aim at measuring the auditory filter buildup time. Latency ratios are generally close to two and controversy exists about the relationship of this ratio to cochlear mechanics. We explored why the two methods provide different estimates of filter buildup time, and ratios with large inter-subject variability, using a time-domain model for OAEs and ABRs. We compared latencies for twenty models, in which all parameters but the cochlear irregularities responsible for reflection-source OAEs were identical, and found that TBOAE latencies were much more variable than ABR latencies. Multiple reflection-sources generated within the evoking stimulus bandwidth were found to shape the TBOAE envelope and complicate the interpretation of TBOAE latency and TBOAE/ABR ratios in terms of auditory filter tuning.

Basal contributions to short-latency transient-evoked otoacoustic emission components

The presence of short-latency (SL), less compressive-growing components in bandpass-filtered transient-evoked otoacoustic emission (TEOAE) waveforms may implicate contributions from cochlear regions basal to the tonotopic place. Recent empirical work suggests a region of SL generation between ∼1/5 and 1/10-octave basal to the TEOAE frequency's tonotopic place. However, this estimate may be biased to regions closer to the tonotopic place as the TEOAE extraction technique precluded measurement of components with latencies shorter than ∼5 ms. Using a variant of the non-linear, double-evoked extraction paradigm that permitted extraction of components with latencies as early as 1 ms, the current study empirically estimated the spatial-extent of the cochlear region contributing to 2 kHz SL TEOAE components. TEOAEs were evoked during simultaneous presentation of a suppressor stimulus, in order to suppress contributions to the TEOAE from different places along the cochlear partition. Three or four different-latency components of similar frequency content (∼2 kHz) were identified for most subjects. Component latencies ranged from 1.4 to 9.6 ms; latency was predictive of the component's growth rate and the suppressor frequency to which the component's magnitude was most sensitive to change. As component latency decreased, growth became less compressive and suppressor-frequency sensitivity shifted to higher frequencies. The shortest-latency components were most sensitive to suppressors approximately 3/5-octave higher than their nominal frequency of 2 kHz. These results are consistent with a distributed region of generation extending to approximately 3/5-octave basal to the TEOAE frequency's tonotopic place. The empirical estimates of TEOAE generation are similar to model-based estimates where generation of the different-latency components occurs through linear reflection from impedance discontinuities distributed across the cochlear partition.

The effect of stimulus bandwidth on the nonlinear-derived tone-burst-evoked otoacoustic emission

Intermodulation distortion has been hypothesized as a mechanism contributing to the generation of short-latency (SL) components in the transient-evoked otoacoustic emission (TEOAE). Presumably, nonlinear interactions between the frequency components within the evoking stimulus induce cochlear distortion products, which mix in the cochlea and ear canal with reflected energy from each stimulus-frequency's tonotopic place. The mixing of these different components is evidenced in the bandpass-filtered emission waveform as a series of different latency peaks. The current study tested the hypothesis that intermodulation distortion, induced within the spectral bandwidth of the evoking stimulus, is the primary mechanism through which the SL components are generated. The nonlinear-derived tone-burst-evoked OAE (TBOAEnl) was evoked using 2-kHz tone bursts with durations of 3, 6, 12, and 24 cycles. As tone burst duration doubled, the spectral bandwidth was halved. It was hypothesized that contributions to the TBOAEnl from SL components would decrease as tone burst duration increased and spectral bandwidth decreased, if the SL components were generated through intermodulation distortion. Despite differences in spectral bandwidth between the evoking stimuli, the latencies and magnitudes of the different latency components between the 3- and 6-cycle TBOAEnl were comparable. The 12- and 24-cycle TBOAEnl envelopes were characteristic of destructive phase interactions between different latency components overlapping in time. The different latency components in the 3- and 6-cycle TBOAEnl introduced a characteristic level dependency to TBOAEnl magnitude and latency when analyzed across a broad time window spanning the different components. A similar dependency described the 12- and 24-cycle TBOAEnl input/output and latency-intensity functions, suggesting that the SL components evident in the shorter-duration TBOAEnl equally contributed to the longer-duration TBOAEnl, despite reductions in spectral bandwidth. The similarity between the different TBOAEnl suggests that they share a common generation mechanism and casts doubt on intermodulation distortion as the generation mechanism of SL TEOAE components in humans.

Reliability and clinical test performance of cochlear reflectance

OBJECTIVE

Cochlear reflectance (CR) is the cochlear contribution to ear-canal reflectance. CR is equivalent to an otoacoustic emission (OAE) deconvolved by forward pressure in the ear canal. Similar to other OAE measures, CR level is related to cochlear status. When measured using wideband noise stimuli, potential advantages of CR over other types of OAEs include (1) the capability to cover a wider frequency range more efficiently by requiring fewer measurements, (2) minimal influence on the recorded emission from the measurement system and middle ear, (3) lack of entrainment of spontaneous OAEs, and (4) easier interpretation because of the existence of an equivalent linear model, which validates the application of linear systems theory. The purposes of this study were to evaluate the reliability, assess the accuracy in a clinical screening paradigm, and determine the relation of CR to audiometric thresholds. Thus, this study represents an initial assessment of the clinical utility of CR.

DESIGN

Data were collected from 32 normal-hearing and 58 hearing-impaired participants. A wideband noise stimulus presented at seven stimulus levels (10 to 70 dB SPL, 10 dB steps) was used to elicit the CR. Reliability of CR was assessed using Cronbach's α, standard error of measurement, and absolute differences between CR data from three separate test sessions. Test performance was evaluated using clinical decision theory. The ability of CR to predict audiometric thresholds was evaluated using regression analysis.

RESULTS

CR repeatability across test sessions was similar to that of other clinical measurements. However, both the accuracy with which CR distinguished normal-hearing from hearing-impaired ears and the accuracy with which CR predicted audiometric thresholds were less than those reported in previous studies using distortion-product OAE measurements.

CONCLUSIONS

CR measurements are repeatable between test sessions, can be used to predict auditory status, and are related to audiometric thresholds. However, under current conditions, CR does not perform as well as other OAE measurements. Further developments in CR measurement and analysis methods may improve performance. CR has theoretical advantages for cochlear modeling, which may lead to improved interpretation of cochlear status.