Deborah A. Vickers

A test for the diagnosis of dead regions in the cochlea.

Abstract Hearing impairment may sometimes be associated with complete loss of inner hair cells (IHCs) over a certain region of the basilar membrane. We call this a ‘dead region’. Amplification (using a hearing aid) over a frequency range corresponding to a dead region may not be beneficial and may even impair speech intelligibility. However, diagnosis of dead regions is not easily done from the audiogram. This paper reports the design and evaluation of a method for detecting and delimiting dead regions. A noise, called ‘threshold equalizing noise’ (TEN), was spectrally shaped so that, for normally hearing subjects, it would give equal masked thresholds for pure tone signals at all frequencies within the range 250–10 000 Hz. Its level is specified as the level in a one-ERB (132 Hz) wide band centred at 1000 Hz. Measurements obtained from 22 normal-hearing subjects and TEN levels of 30, 50 and 70 dB/ERB confirmed that the signal level at masked threshold was approximately equal to the noise level/ERB and was almost independent of signal frequency. Masked thresholds were measured for 20 ears of 14 subjects with sensorineural hearing loss, using TEN levels of 30, 50 and 70 dB/ERB. Psychophysical tuning curves (PTCs) were measured for the same subjects. When there are surviving IHCs corresponding to a frequency region with elevated absolute thresholds, a signal in that frequency region is detected via IHCs with characteristic frequencies (CFs) close to that region. In such a case, threshold in the TEN is close to that for normal-hearing listeners, provided that the noise intensity is sufficient to produce significant masking. Also, the tip of the PTC lies close to the signal frequency. When a dead region is present, the signal is detected via IHCs with CFs different from that of the signal frequency. In such a case, threshold in the TEN is markedly higher than normal, and the tip of the PTC is shifted away from the signal frequency. Generally, there was a very good correspondence between the results obtained using the TEN and the PTCs. We conclude that the measurement of masked thresholds in TEN provides a quick and simple method for the diagnosis of dead regions.

Inter-relationship between different psychoacoustic measures assumed to be related to the cochlear active mechanism.

The active mechanism in the cochlea is thought to depend on the integrity of the outer hair cells (OHCs). Cochlear hearing loss is usually associated with damage to both inner hair cells (IHCs) and OHCs, with the latter resulting in a reduction in or complete loss of the function of the active mechanism. It is believed that the active mechanism contributes to the sharpness of tuning on the basilar membrane (BM) and is also responsible for compressive input-output functions on the BM. Hence, one would expect a close relationship between measures of sharpness of tuning and measures of compression. This idea was tested by comparing three different measures of the status of the active mechanism, at center frequencies of 2, 4, and 6 kHz, using subjects with normal hearing, with unilateral or highly asymmetric cochlear hearing loss, and with bilateral loss. The first measure, HLOHC, was an indirect measure of the amount of the hearing loss attributable to OHC damage; this was based on loudness matches between the two ears of subjects with unilateral hearing loss and was derived using a loudness model. The second measure was the equivalent rectangular bandwidth (ERB) of the auditory filter, which was estimated using the notched-noise method. The third measure was based on the slopes of growth-of-masking functions obtained in forward masking. The ratio of slopes for a masker centered well below the signal frequency and a masker centered at the signal frequency gives a measure of BM compression at the place corresponding to the signal frequency; a ratio close to 1 indicates little or no compression, while ratios less than 1 indicate that compression is occurring at the signal place. Generally, the results showed the expected pattern. The ERB tended to increase with increasing HLOHC. The ratio of the forward-masking slopes increased from about 0.3 to about 1 as HLOHC increased from 0 to 55 dB. The ratio of the slopes was highly correlated with the ERB (r = 0.92), indicating that the sharpness of the auditory filter decreases as the compression on the BM decreases.

Effect of loudness recruitment on the perception of amplitude modulation

People with hearing loss of cochlear origin usually display loudness recruitment; the rate of growth of loudness level with increasing sound level is greater than for a normally hearing person. Loudness recruitment has usually been studied with steady sounds of relatively long duration. The present study examines how recruitment affects the perception of dynamically varying sounds, namely amplitude modulated sinusoids. The modulation rates used (4, 8, 16, and 32 Hz) were chosen to span the range of the most prominent modulations present in the envelope of speech. Three subjects with unilateral cochlear hearing loss were used. In experiment 1, subjects were required to make loudness matches between 1‐kHz tones presented alternately to the two ears. This was done over a wide range of sound levels. Experiment 2 used 1‐kHz carriers that were amplitude modulated. The modulation was sinusoidal on a dB scale. The modulated tones were presented alternately to the two ears and were approximately equally loud in the two ears. The modulation depth was fixed in one ear, and the subject was required to adjust the modulation depth in the other ear so that the modulation depth appeared equal in the two ears. This was done for a range of modulation depths and with the fixed tone presented to both the normal and the impaired ear. A given modulation depth in the impaired ear was matched by a greater modulation depth in the normal ear. To a first approximation, the modulation‐matching functions were independent of modulation rate. Furthermore, the functions could be predicted reasonably well from the loudness‐matching results of experiment 1, obtained with steady tones. The results are consistent with the idea that loudness recruitment results from the loss of a fast‐acting compressive nonlinearity that operates in the normal peripheral auditory system. Possible implications of the results for the use of fast‐acting compression in hearing aids are discussed.

Short-term temporal integration: evidence for the influence of peripheral compression.

Thresholds for a 6.5-kHz sinusoidal signal, temporally centered in a 400-ms broadband-noise masker, were measured as a function of signal duration for normally hearing listeners and listeners with cochlear hearing loss over a range of masker levels. For the normally hearing listeners, the slope of the function relating signal threshold to signal duration (integration function) was steeper at medium masker levels than at low or high levels by a factor of nearly 2, for signal durations between 2 and 10 ms, while no significant effect of level was found for signal durations of 20 ms and more. No effect of stimulus level was found for the hearing-impaired listeners at any signal duration. For signal durations greater than 10 ms, consistent with many previous studies, the slope of the integration function was shallower for the hearing-impaired listeners than for the normally hearing listeners. However, for shorter durations, there was no significant difference in slope between the results from the hearing-impaired listeners and those from the normally hearing listeners in the high- and low-level masker conditions. A model incorporating a compressive nonlinearity, representing the effect of basilar-membrane (BM) compression, and a short-term temporal integrator, postulated to be a more central process, can account well for changes in the short-term integration function with level, if it is assumed that the compression is greater at medium levels than at low or high levels by a factor of about 4. This is in reasonable agreement with physiological measurements of BM compression, and with previous psychophysical estimates.

Simulation of the effects of loudness recruitment on the intelligibility of speech in noise.

This experiment simulated the threshold elevation and loudness recruitment associated with three different types of hearing loss: moderate flat (condition R2), severe flat (condition R3), and moderate-to-severe sloping (condition RX). This was done to allow an examination of the effects of these factors on the intelligibility of speech, in isolation from other factors that are normally associated with cochlear hearing loss, such as reduced frequency selectivity. The speech was presented at a fixed input level of 65 dB SPL, against a background of a noise whose spectrum was shaped to match the long-term average spectrum of the speech. The level of the background noise varied from 65 to 74 dB SPL. The simulation was performed by splitting the input signal into 13 frequency bands, and processing the envelope in each band so as to create loudness sensations in a normal ear that would resemble those produced in an impaired ear with recruitment. The bands were then recombined. All tests were performed using subjects with normal hearing. The simulation of hearing loss produced decrements in performance. The speech in condition R3 was inaudible. For conditions R2 and RX, the speech-to-noise ratios had to be up to 6 dB higher than in the control condition (R1, unprocessed stimuli) to achieve similar levels of performance. When linear amplification according to the NAL prescription was applied before the simulation, performance improved markedly for conditions R2 and RX, and did not differ significantly from that for R1. For condition R3, performance with simulated NAL amplification remained below that for condition R1; the decrement in performance was equivalent to about a 1 dB change in speech-to-noise ratio. The results of the present experiment show much smaller decrements in performance than those of an earlier experiment using a single talker as the interfering sound (Moore and Glasberg, 1993). It appears that loudness recruitment and threshold elevation have larger effects for a fluctuating background sound than for a steady background sound, and linear amplification is more effective in the latter case.

Factors affecting the loudness of modulated sounds.

Loudness matches were obtained between unmodulated carriers and carriers that were amplitude modulated either periodically (rates between 2 and 32 Hz, modulation sinusoidal either on a linear amplitude scale or on a dB scale; the latter is called dB modulation) or with the envelope of the speech of a single talker. The carrier was a 4-kHz sinusoid, white noise, or speech-shaped noise. Both normally hearing subjects and subjects with cochlear hearing loss were tested. Results were expressed as the root-mean-square (rms) level of the modulated carrier minus the level of the unmodulated carrier at the point of equal loudness. If this difference is positive, this indicates that the modulated carrier has a higher rms level at the point of equal loudness. For normally hearing subjects, the results show: (1) For a 4000-Hz sinusoidal carrier, the difference was slightly positive (averaging about 0.7 dB). There was no significant effect of modulation rate or level over the range 20-80 dB SL. (2) For a speech-shaped noise or white noise carrier, the difference was close to zero, although for large modulation depths it tended to be negative. There was no clear effect of level (over the range 35-75 dB SPL) or modulation rate. For the hearing-impaired subjects, the differences were small, but tended to be slightly negative for both the 4000-Hz carrier and the noise carriers, when the modulation rate was above 2 Hz. Again, there was no clear effect of overall level. However, for dB modulation, the differences became more negative with increasing modulation depth. For modulation rates in the range 4-32 Hz, the results could be fitted reasonably well using the assumption that the loudness of modulated sounds is based on the rms value of the time-varying intensity of the response of the basilar membrane (taking into account the compression that occurs in the normal cochlea). The implications of the results for the fitting of multi-band compression hearing aids and for the design of loudness meters are discussed.

The effects of age on temporal fine structure sensitivity in monaural and binaural conditions

Abstract Objective: To extend the study of Hopkins and Moore (2011) by examining the effect of age in the medium age range on sensitivity to temporal fine structure (TFS), which is assumed to be represented in the patterns of phase locking in the auditory nerve. Design: Monaural TFS sensitivity was assessed using the TFS1 test (Moore & Sek, 2009) at centre frequencies of 850 and 2000 Hz, and binaural TFS sensitivity was assessed using the TFS-LF test (Hopkins & Moore, 2010a) at centre frequencies of 500 and 850 Hz, using a sensation level of 30 dB. Study sample: Thirty-five newly recruited normal-hearing subjects (thresholds better than 20 dB HL from 250 to 6000 Hz) were tested. Their ages ranged from 22 to 61 years. Results: There was a significant correlation between age and TFS sensitivity at all frequencies for both TFS tests. For the single centre frequency (850 Hz) that was used for both tests, scores for the two tests were modestly but significantly correlated. Conclusions: Sensitivity to TFS decreases with increasing age. The monaural and binaural TFS tests appear to reflect at least somewhat distinct auditory processes.

Bilateral cochlear implantation for hearing-impaired children: criterion of candidacy derived from an observational study.

Objectives: Policy-makers have struggled to define the minimum degree of hearing impairment at which children should be offered cochlear implants rather than the less invasive alternative of acoustic hearing aids. This study compared outcomes for children with bilateral cochlear implants and children with bilateral hearing aids, to determine a criterion of candidacy for pediatric bilateral cochlear implantation. Design: This observational study measured the listening skills of children who received routine audiological care in the United Kingdom. Participants were recruited from hospitals, educational services, and charities. Eligibility criteria included a diagnosis of hearing impairment before 31 months of age and pure-tone thresholds greater than or equal to 50 dB HL at 2 and 4 kHz bilaterally. Seventy-one children participated, aged 46 to 86 months (mean 64 months). Twenty-eight children used bilateral implants provided in a simultaneous surgery; 43 used bilateral digital hearing aids. The two groups of children were demographically similar in variables that predict outcomes for children with hearing impairment. Children’s ability to understand speech was measured using closed-set tests of word discrimination in three conditions: in quiet, in pink noise, and in two-talker babble. For each listening test, an actuarial method was used to compare the distribution of scores from children with cochlear implants and children with hearing aids. The aim was to calculate the unaided pure-tone average (PTA) hearing level at which a child has odds of 4:1 of a better outcome with implants than with hearing aids. The PTA associated with odds of 4:1 has been used previously to define criteria of candidacy for implantation. The main analyses used a four-frequency PTA (mean of unaided thresholds at 0.5, 1, 2, and 4 kHz in the better-hearing ear). Additional analyses used a three-frequency PTA (0.5, 1, and 2 kHz) and two-frequency PTA (2 and 4 kHz). Results: Odds of 4:1 of a better outcome with implants were associated with a four-frequency PTA of 79, 86, and 76 dB HL for tests of word discrimination in quiet, noise, and babble, respectively. The mean of these three estimates is 80 dB HL. It can be difficult to measure a four-frequency PTA in young children, but a two-frequency PTA typically can be measured. Odds of 4:1 were associated with a two-frequency PTA of 83, 92, and 80 dB HL for tests of word discrimination in quiet, noise, and babble, respectively. The mean of these three estimates is 85 dB HL. Conclusions: Children with an unaided four-frequency PTA of 80 dB HL or poorer in both ears should be considered candidates for bilateral cochlear implantation. In cases where a four-frequency PTA cannot be measured, the criterion of candidacy should be a two-frequency PTA of 85 dB HL or poorer in both ears. If adopted by policy-makers, these recommendations would expand the provision of cochlear implants among children in England and Wales.

Using Personal Response Systems to Assess Speech Perception Within the Classroom: An Approach to Determine the Efficacy of Sound Field Amplification in Primary School Classrooms.

Objectives: The assessment of the combined effect of classroom acoustics and sound field amplification (SFA) on children’s speech perception within the “live” classroom poses a challenge to researchers. The goals of this study were to determine: (1) Whether personal response system (PRS) hand-held voting cards, together with a closed-set speech perception test (Chear Auditory Perception Test [CAPT]), provide an appropriate method for evaluating speech perception in the classroom; (2) Whether SFA provides better access to the teacher’s speech than without SFA for children, taking into account vocabulary age, middle ear dysfunction or ear-canal wax, and home language. Design: Forty-four children from two school-year groups, year 2 (aged 6 years 11 months to 7 years 10 months) and year 3 (aged 7 years 11 months to 8 years 10 months) were tested in two classrooms, using a shortened version of the four-alternative consonant discrimination section of the CAPT. All children used a PRS to register their chosen response, which they selected from four options displayed on the interactive whiteboard. The classrooms were located in a 19th-century school in central London, United Kingdom. Each child sat at their usual position in the room while target speech stimuli were presented either in quiet or in noise. The target speech was presented from the front of the classroom at 65 dBA (calibrated at 1 m) and the presented noise level was 46 dBA measured at the center of the classroom. The older children had an additional noise condition with a noise level of 52 dBA. All conditions were presented twice, once with SFA and once without SFA and the order of testing was randomized. White noise from the teacher’s right-hand side of the classroom and International Speech Test Signal from the teacher’s left-hand side were used, and the noises were matched at the center point of the classroom (10sec averaging [A-weighted]). Each child’s expressive vocabulary age and middle ear status were measured individually and each child’s home language and any special educational needs were recorded. Results: All children were able to use the PRS handsets, and the CAPT speech perception test was sufficiently sensitive to highlight differences in perception in the different listening conditions. Scores were higher in quiet than in any noise condition. Results showed that group performance was significantly better with SFA than without it. The main demographic predictor of performance was expressive vocabulary age. SFA gave more benefit to the poorer performers in the group. There were no significant effects on performance relating to middle ear status or home language; however, the size of the population was too small to be able to fully explore these aspects in greater detail. Conclusion: PRS together with the CAPT provides a sensitive measure for in situ speech perception testing within the classroom. Vocabulary age has a large effect on a child’s ability to perceive the speech signal. SFA leads to improved speech perception, when the speech signal has been degraded because of poor acoustics or background noise and has a particularly large effect for children with lower vocabulary ages.

Relative importance of different spectral bands to consonant identification: Relevance for frequency transposition in hearing aids

Listeners with high-frequency dead regions (DRs) benefit from amplification of frequencies up to 1.7 times the edge frequency, fe, of the DR. Better consonant identification might be achieved by replacing the band from fe to 1.7fe with a higher spectral band. We aimed to identify the optimal band, using simulations with normal-hearing listeners. In experiment 1, nonsense syllables were lowpass filtered to simulate DRs with fe of 0.5, 0.75, and 1.0 kHz. Identification was measured for each of these base bands alone and with a bandpass-filtered band added (but not transposed). The added band either extended from fe to 1.7fe or its center frequency was increased, keeping bandwidth fixed in ERBN-number. Performance improved with increasing center frequency and then reached an asymptote or declined. Experiment 2 used a mid-frequency base band, and a lower-frequency added band. The results also showed a beneficial effect of frequency separation of the added and base bands. Experiment 3 resembled experiment 1, but with bandwidth fixed in Hertz. For higher-frequency added bands, the benefit was lower than for experiment 1.