Measurement of cochlear basilar membrane traveling wave velocity by derived ABR

Towards effective assessment of normal hearing function from ABR using a time-variant sweep-tone stimulus approach

Objective. The auditory brainstem response (ABR) audiometry is a means of assessing the functional status of the auditory neural pathway in the clinic. The conventional click ABR test lacks good neural synchrony and it mainly evaluates high-frequency hearing while the common tone-burst ABR test only detects hearing loss of a certain frequency at a time. Additionally, the existing chirp stimuli are designed based on average data of cochlear characteristics, ignoring individual differences amongst subjects.Approach. Therefore, this study designed a new stimulus approach based on a sweep-tone concept with a time variant and spectrum characteristics that could be customized based on an individual's cochlear characteristics. To validate the efficiency of the proposed method, we compared its performance with the click and tone-bursts using ABR recordings from 11 normal-hearing adults.Main results. Experimental results showed that the proposed sweep-tone ABR achieved a higher amplitude compared with those elicited by the click and tone-bursts. When the stimulus level or rate was varied, the sweep-tone ABR consistently elicited a larger response than the corresponding click ABR. Moreover, the sweep-tone ABR appeared earlier than the click ABR under the same conditions. Specifically, the mean wave V peak-to-peak amplitude of the sweep-tone ABR was 1.3 times that of the click ABR at 70 dB nHL (normal hearing level) and a rate of 20 s^-1, in which the former saved 40% of test time.Significance. In summary, the proposed sweep-tone approach is found to be more efficient than the traditional click and tone-burst in eliciting ABR.

PARAMETER ANALYSIS OF 2D COCHLEAR MODEL AND QUANTITATIVE RESEARCH ON THE TRAVELING WAVE PROPAGATION

The traveling wave is the most important phenomenon in cochlear macromechanics. The behaviors of the traveling wave that greatly alter the auditory discrimination, are tightly related with the mechanical properties of the basilar membrane (BM) and its surrounding lymph. As an addition to the blanks of related researches, this paper focuses on some of the key parameters that affect the cochlear response most: the BM stiffness, damping parameters and the fluid viscosity. The influence of these parameters on the traveling wave is discussed, based on our former developed 2D finite element hydrodynamic cochlear model. Moreover, the traveling wave velocity and its transmitting time are calculated based on the simulating results. Although generally a rapid fall of the velocity from the cochlear base to the characteristic frequency (CF) location is confirmed, our quantitative analysis also indicates the traveling wave velocity may be both location and frequency dependent.

Perception of across-frequency asynchrony by listeners with cochlear hearing loss

Cochlear hearing loss is often associated with broader tuning of the cochlear filters. Cochlear response latencies are dependent on the filter bandwidths, so hearing loss may affect the relationship between latencies across different characteristic frequencies. This prediction was tested by investigating the perception of synchrony between two tones exciting different regions of the cochlea in listeners with hearing loss. Subjective judgments of synchrony were compared with thresholds for asynchrony discrimination in a three-alternative forced-choice task. In contrast to earlier data from normal-hearing (NH) listeners, the synchronous-response functions obtained from the hearing-impaired (HI) listeners differed in patterns of symmetry and often had a very low peak (i.e., maximum proportion of "synchronous" responses). Also in contrast to data from NH listeners, the quantitative and qualitative correspondence between the data from the subjective and the forced-choice tasks was often poor. The results do not provide strong evidence for the influence of changes in cochlear mechanics on the perception of synchrony in HI listeners, and it remains possible that age, independent of hearing loss, plays an important role in temporal synchrony and asynchrony perception.

Listening to the cochlea with high-frequency ultrasound

We have developed a high-frequency pulsed-wave Doppler ultrasound probe as a promising minimally-invasive technique for measuring intracochlear mechanics without damaging the cochlea. Using a custom high-frequency ultrasound system, we have measured dynamic motion of intracochlear structures by recording the pulsed-wave Doppler signal resulting from the vibration of the basilar and round window membranes. A 45 MHz needle-mounted Doppler probe was fabricated and placed against the round window membranes of eight different fresh human temporal bones. Pulsed-wave ultrasonic Doppler measurements were performed on the basilar membrane and round window membrane during the application of pure tones to the external ear canal. Doppler vibrational information for acoustic input frequencies ranging from 100-2000 Hz was collected and normalized to the sound pressure in the ear canal. The middle ear resonance, located at approximately 1000 Hz, could be characterized from the membrane velocities, which agreed well with literature values. The maximum normalized mean velocity of the round window and the basilar membrane were 180 μm/s/Pa and 27 μm/s/Pa at 800 Hz. The mean phase difference between the membrane displacements and the applied ear canal sound pressure showed a flat response almost up to 500 Hz where it began to accumulate. This is the first study that reports the application of high frequency pulsed wave Doppler ultrasound for measuring the vibration of basilar membrane through the round window. Since it is not required to open or damage the cochlea, this technique might be applicable for investigating cochlear dynamics, in vivo.

Lack of Diagnostic Value of High-Pass Noise Masking of Auditory Brainstem Responses in Ménière's Disease

INTRODUCTION

Different electrophysiologic tests were developed to ascertain or detect endolymphatic hydrops (ELH). Recently, Cochlear Hydrops Analysis Masking Procedure (CHAMP), a new auditory brainstem response (ABR) technique, proved to be able to separate normal controls from definite Ménière's disease (MD) with 100% sensitivity and 100% specificity.

OBJECTIVE

To evaluate the applicability and diagnostic value of CHAMP in a series of MD and non-MD patients with otovestibular complaints.

STUDY DESIGN

An observational retrospective study.

SETTING

Tertiary referral centre.

PATIENTS

Forty-five patients, of which 28 patients had MD.

INTERVENTIONS

Cochlear Hydrops Analysis Masking Procedure test was conducted, and audiometric data and clinical information were collected. All responses were blindly evaluated and divided into three categories: (1) test suggestive for cochlear/ELH, (2) test within normal ranges, and (3) test not interpretable.

MAIN OUTCOME MEASURES

Sensitivity and specificity, evaluation of diagnostic value.

RESULTS

Forty-nine percent of the tests were found to be not interpretable. Of the remaining responses, 13 tests were suggestive for ELH, and 16 tests were within normal ranges. This yields a sensitivity of 31% and a specificity of 28%. There was no significant difference between the mean latency difference (mean [SD]) for Wave V of the MD group (0.43 ms [0.37]) and the non-MD group (0.65 ms [0.44]). Using logistic regression analysis, we found that CHAMP test did not contribute to the ability to discriminate between hydropic and nonhydropic ears.

CONCLUSION

In contrast to studies assessing CHAMP performance in Ménière patients and normal controls, the present study revealed this new test offers no discriminative value in differentiating Ménière's from non-MD subjects with otovestibular symptoms.

On the frequency dependence of the otoacoustic emission latency in hypoacoustic and normal ears

Experimental measurements of the otoacoustic emission (OAE) latency of adult subjects have been obtained, as a function of frequency, by means of wavelet time-frequency analysis based on the iterative application of filter banks. The results are in agreement with previous OAE latency measurements by Tognola et al. [Hear. Res. 106, 112-122 (1997)], as regards both the latency values and the frequency dependence, and seem to be incompatible with the steep 1/f law that is predicted by scale-invariant full cochlear models. The latency-frequency relationship has been best fitted to a linear function of the cochlear physical distance, using the Greenwood map, and to an exponential function of the cochlear distance, for comparison with derived band ABR latency measurements. Two sets of ears [94 audiometrically normal and 42 impaired with high-frequency (f > 3 kHz) hearing loss] have been separately analyzed. Significantly larger average latencies were found in the impaired ears in the mid-frequency range. Theoretical implications of these findings on the transmission of the traveling wave are discussed.

Human auditory steady-state responses to amplitude-modulated tones: phase and latency measurements

Human auditory steady-state responses were recorded to four stimuli, with carrier frequencies (f(c)) of 750, 1500, 3000 and 6000 Hz, presented simultaneously at 60 dB SPL. Each carrier frequency was modulated by a specific modulation frequency (f(m)) of 80.6, 85.5, 90.3 or 95.2 Hz. By using four different recording conditions we obtained responses for all permutations of f(m) and f(c). The phase delays (P) of the responses were unwrapped and converted to latency (L) using the equation: L=P/(360xf(m)). The number of cycles of the stimulus that occurred prior to the recorded response was estimated by analyzing the effect of modulation frequency on the responses. These calculations provided latencies of 20.7, 17.7, 16.1 and 16.1 ms for carrier frequencies 750, 1500, 3000 and 6000 Hz. This latency difference of about 4.5 ms between low and high carrier frequencies remained constant over many different manipulations of the stimuli: faster modulation rates (150-190 Hz), binaural rather than monaural presentation, different intensities, stimuli presented alone or in conjunction with other stimuli, and modulation frequencies that were separated by as little as 0.24 Hz. This frequency-related delay is greater than that measured using transient evoked potentials, most likely because of differences in how transient and steady-state responses are generated and how their latencies are determined.

Effects of chronic tobramycin treatment on distortion product otoacoustic emissions

OBJECTIVE

To investigate the effects of chronic tobramycin treatment on distortion product otoacoustic emission (DPOAE) latencies and response growth detection thresholds in human subjects to determine the sensitivity of these DPOAE features to ototoxic damage.

DESIGN

Six groups of children in two different age ranges were tested: three groups in the 7 to 14 yr age range, i.e., six children with normal hearing, four cystic fibrosis (CF) patients who received no aminoglycosides, and eight CF patients who received low- to moderate-cumulative doses of tobramycin (< 1250 mg/kg) for respiratory infections; and three groups of five subjects each in the 15 to 23 yr age range, i.e., the healthy group and the CF groups that received low- (< 285 mg/kg) and moderate-(1000 to 2000 mg/kg) cumulative drug dosages. The aggregate drug dosages compiled longitudinally over the past 5 yr were used to group the drug-treated CF patients. All subjects showed normal audiometric profiles (< or = 25 dB HL in the conventional frequency region and age-appropriate thresholds as described by Osterhammel and Osterhammel [1979] in the high-frequency region) and DP-grams (absolute DPOAE and noise amplitudes being consistent with the normative data obtained with the CUBeDIS system at this institution).

RESULTS

Even though the audiometric profiles and DP-grams of all drug-treated CF groups were identical to their healthy counterparts, the DPOAE latencies and growth function thresholds showed significant changes. Whereas low and low-to-moderate doses of tobramycin were related to DPOAE latency prolongations, higher cumulative drug doses of 1000 to 2000 mg/kg produced significant reductions in DPOAE latencies. Response growth detection thresholds at high frequencies showed significant elevations in all CF patient groups treated with tobramycin, regardless of drug dosages, as compared with the control subjects.

CONCLUSIONS

DPOAE amplitudes may not reflect the earliest changes produced by chronic aminoglycoside treatment, suggesting that cochlear ototoxicity may be more effectively monitored through the assessment of latencies and response growth detection thresholds. These findings pertain at least to the early stages of ototoxicity development, specifically during chronic tobramycin treatment. In light of the small sample size, however, these outcomes must be considered as tentative.

The effects of sensory hearing loss on cochlear filter times estimated from auditory brainstem response latencies

Derived-band auditory brainstem responses (ABRs) were obtained in 43 normal-hearing and 80 cochlear hearing-impaired individuals using clicks and high-pass noise masking. The response times across the cochlea [the latency difference between wave V's of the 5.7- and 1.4-kHz center frequency (CF) derived bands] were calculated for five levels of click stimulation ranging from 53 to 93 dB p.-p.e. SPL (23 to 63 dB nHL) in 10-dB steps. Cochlear response times appeared to shorten significantly with hearing loss, especially when the average pure tone (1 to 8 kHz) hearing loss exceeded 30 dB. Examination of derived-band latencies indicates that this shortening is due to a dramatic decrease of wave V latency in the lower CF derived band. Estimates of cochlear filter times in terms of the number of periods to maximum response (Nmax) were calculated from derived-band latencies corrected for gender-dependent cochlear transport and neural conduction times. Nmax decreased as a function of hearing loss, especially for the low CF derived bands. The functions were similar for both males and females. These results are consistent with broader cochlear tuning due to peripheral hearing loss. Estimating filter response times from ABR latencies enhances objective noninvasive diagnosis and allows delineation of the differential effects of pathology on the underlying cochlear mechanisms involved in cochlear transport and filter build-up times.