Richard J. Baker

AUDITORY FILTER NONLINEARITY AT 2 KHZ IN NORMAL HEARING LISTENERS

Auditory filters broaden with increasing level. Using a recently developed method of fitting filter shapes to notched-noise masking data that explicitly models the nonlinear changes in filter shape across level, results at 2 kHz from 9 listeners over a wide range of levels and notch widths are reported. Families of roex(p,w,t) filter shapes lead to models which account well for the observed data. The primary effect of level is a broadening in the tails of the filter as level increases. In all cases, models with filter parameters depending on probe level fit the data much better than masker-dependent models. Thus auditory filter shapes appear to be controlled by their output, not by their input. Notched-noise tests, if performed at a single level, should use a fixed probe level. Filter shapes derived in this way, and normalized to have equal tail gain, are highly reminiscent of measurements made directly on the basilar membrane, including the degree of compression evidenced in the input-output function.

Auditory filter nonlinearity across frequency using simultaneous notched-noise masking.

Psychoacoustic masking experiments have been widely used to investigate cochlear function in human listeners. Here we use simultaneous notched-noise masking experiments in normal hearing listeners to characterize the changes in auditory filter shape with stimulus level over the frequency range 0.25-6 kHz. At each frequency a range of fixed signal levels (30-70 dB SPL) and fixed masker levels (20-50 dB SPL spectrum level) are used in order to obtain accurate descriptions of the filter shapes in individual listeners. The notched-noise data for individual listeners are fitted with two filter shape models: a rounded exponential (roex) shape in which the filter skirt changes as a linear function of probe-tone level and the other, in which the gain of the tip filter relative to the filter tail changes as a function of signal level [Glasberg and Moore, J. Acoust. Soc. Am. 108, 2318-2328 (2000)]. The parameters for these fitted models are then described with a simple set of equations that quantify the changes in auditory filter shape across level and frequency. Both these models fitted the data equally well and both demonstrated increasing tip-tail gain as frequency increased.

Auditory filter nonlinearity in mild/moderate hearing impairment

Sensorineural hearing loss has frequently been shown to result in a loss of frequency selectivity. Less is known about its effects on the level dependence of selectivity that is so prominent a feature of normal hearing. The aim of the present study is to characterize such changes in nonlinearity as manifested in the auditory filter shapes of listeners with mild/moderate hearing impairment. Notched-noise masked thresholds at 2 kHz were measured over a range of stimulus levels in hearing-impaired listeners with losses of 20-50 dB. Growth-of-masking functions for different notch widths are more parallel for hearing-impaired than for normal-hearing listeners, indicating a more linear filter. Level-dependent filter shapes estimated from the data show relatively little change in shape across level. The loss of nonlinearity is also evident in the input/output functions derived from the fitted filter shapes. Reductions in nonlinearity are clearly evident even in a listener with only 20-dB hearing loss.

Fast method for psychophysical tuning curve measurement in school-age children

The ‘fast’ method for measuring psychophysical tuning curves (PTC) uses a masker that sweeps across frequency and a Békésy threshold tracking procedure. The fast-PTC procedure has been recommended as a technique for diagnosing cochlear dead regions in adults. The aim of this study was to evaluate the fast-PTC procedure in children. Twelve normal-hearing children (7–10 years old) and five adults were tested. The fast-PTCs were measured for 1000 and 4000 Hz signals using ascending and descending masker sweeps. Measurements were repeated on a separate day to assess test-retest variability. All children were able to perform the task; however it was possible to define the tip in only 87% of the fast-PTCs. Although the variability in tip frequency was higher for children, the mean difference between children and adults was not statistically significant. As expected, the difference on retest was higher for children. Studies investigating the use of the fast-PTC procedure with hearing-impaired children are warranted.

Failure to find asymmetry in auditory gap detection

Several previous studies have demonstrated a right ear advantage in the detection of a brief silent gap in a broadband noise, with one study indicating that such an asymmetry does not exist. If such an asymmetry reflects more efficient temporal processing of auditory stimuli in the left hemisphere of the brain, then an asymmetry may be expected to exist regardless of the experimental procedure. Three sequential studies are summarised that use both adaptive threshold measurements and yes/no procedures to assess auditory gap detection performance both with and without the presence of a dichotic masker. These studies fail to reveal any systematic bias in performance towards one ear, and it is suggested that the right ear advantage demonstrated in previous studies may not reflect auditory gap detection performance per se, but may reflect the participants’ response bias in the particular type of tasks used.

Diagnosing cochlear dead regions in children

Objective: A dead region (DR) is defined as a region in the cochlea where inner hair cells and/or neurons are functioning so poorly that a tone producing peak vibration in this region is detected by off-frequency listening, i.e., via a place on the basilar membrane with a characteristic frequency different from that of the tone. The presence of a DR can have a significant effect on the perception of speech. People with and without DRs may differ in the benefit obtained from amplification and require different hearing aid settings. The Threshold Equalizing Noise (TEN) test and psychophysical tuning curves (PTCs) are two procedures used to identify a DR in adults. Because diagnosing a DR involves measuring masked thresholds, and there are reports in the literature that young children perform poorly compared with adults in background noise, it may be possible that the criteria used with adults may not be appropriate when testing children. Therefore, the aim of this study was to evaluate the consistency of the fast-PTC and TEN tests in diagnosing a DR in hearing-impaired children. In addition, the masked thresholds for normal-hearing children were measured with different TEN levels to assess whether any age-related effect in children compared with adults may occur. Design: Participants were divided into two groups: eight normal-hearing children (16 ears) and 12 hearing-impaired children (21 ears), aged 7 to 13 yr. TEN is based on measuring masked threshold in TEN. For normal-hearing participants, the masked thresholds were measured for five levels of noise (30, 40, 50, 60, and 70 dB per averaged equivalent rectangular bandwidth). For hearing-impaired participants, the level of the TEN was selected separately for each ear based on the highest acceptable level minus 5 dB. The TEN test results in hearing-impaired children were further validated by measuring fast-PTCs. The fast-PTC technique involves measuring the level of the narrowband noise masker needed to mask the signal. The center frequency of the masker sweeps across the required frequency range. Results: The masked thresholds in TEN measured for normal-hearing children were usually below and never higher than 5 dB above TEN level per averaged equivalent rectangular bandwidth. This suggests that no age-related effect on masked threshold in children compared with adults was observed. All hearing-impaired children were able to perform the TEN test and fast-PTCs. The results of the two tests were consistent in 17 of 21 ears (81%): eight ears did not show evidence of a DR and nine ears did. In three ears, the criteria for a DR were met on the TEN test, but there was no evidence of a DR on the fast-PTC test. In one ear, the TEN test did not show evidence of DRs at two frequencies, whereas fast-PTCs did. Conclusions: The results of this study suggest that DRs can be detected in children using the fast-PTC technique and the TEN test interpreted with the adult criteria, which are the most appropriate in terms of specificity and sensitivity. However, in cases in which the masked threshold is 10 to 15 dB above the TEN level, it is recommended to confirm DR diagnosis with fast-PTC measurement.

The effect of low-pass filtering on identification of nonsense syllables in quiet by school-age children with and without cochlear dead regions

Objectives: The presence of cochlear dead regions (DRs) can have a significant effect on speech perception. Some studies have reported that adults do not benefit from amplification of frequencies well inside an extensive DR. However, the importance of high-frequency amplification for hearing-impaired children has been emphasized by many researchers. This study investigates the benefit of high-frequency amplification for children with various degrees of high-frequency hearing impairment, with and without DRs. Design: The children, aged 8 to 13 years, were divided into two groups according to the severity of their hearing impairment. Group MS had moderate to severe impairment (9 ears without DRs and 3 ears with restricted DRs). Group SP had severe to profound hearing impairment (7 ears with DRs and 1 ear without a DR). The vowel-consonant-vowel stimuli were subjected to the frequency-gain characteristics prescribed by the desired sensation level fitting method and presented via headphones broadband and under various low-pass filtering conditions. Results: Group MS benefited from high-frequency amplification whether or not a restricted DR was present. In contrast, ears in group SP with continuous extensive DRs showed limited benefit from high-frequency amplification. For the latter, performance improved with increasing cutoff frequency up to approximately 1 octave above the edge frequency of the DR and generally stayed the same, or deteriorated, with further increases in bandwidth. In one case of severe to profound hearing impairment without evidence of DRs, performance increased with increasing cutoff frequency up to 2 kHz and remained almost constant with further increases in bandwidth. Conclusions: For children with severe to profound hearing impairment and continuous high-frequency DRs commencing from approximately 1 kHz, applying amplification only for frequencies up to approximately 1 octave above the edge frequency of the DR may be of benefit. Tests with more participants are needed to confirm this finding.

Auditory steady state responses in normal-hearing and hearing-impaired adults: an analysis of between-session amplitude and latency repeatability, test time, and F ratio detection paradigms.

Objectives: The aim of this study was to assess the between-session repeatability of auditory steady state response (ASSR) amplitudes and to examine F ratio response detection parameters. Design: Suprathreshold ASSRs were recorded from 20 normal-hearing and 10 hearing-impaired subjects. Amplitudes and latencies were recorded in two test sessions conducted on separate days. Analysis: The repeatability coefficients (limits of expected variation in repeat measurements) for amplitude and latency of ASSRs were calculated. The test time required for the responses to reach significance at 1%, 2%, and 5% F ratios was analyzed. The percentage false response detection rate was calculated to determine the suitability of current ASSR threshold estimation protocols for use in audiology clinics. Results: The repeatability coefficients for the amplitude of ASSRs were 29 nV for the normal-hearing subjects and 57 nV for the hearing-impaired subjects. The repeatability coefficients for the latency of ASSR were 1.10 msec for the normal-hearing subjects and 1.19 msec for the hearing-impaired subjects. High false-positive detection rates were found for detection procedures that used variable test time (“stop when significance reached” methods). Conclusions: The results of this study showed that ASSR amplitudes are highly variable between test sessions with an average estimated variability in response amplitude of ±40% for normal-hearing participants and ±97% for hearing-impaired participants. This could be a possible cause of test–retest differences in ASSR threshold measurements, as it could potentially lead to thresholds that were above the EEG noise level and significant in one test session subsequently falling below the EEG noise level in the repeat test session leading to insignificant response and thus poorer ASSR threshold.

No evidence for enhanced processing of speech that is low-pass filtered near the edge frequency of cochlear dead regions in children

Abstract Objectives: Cochlear dead regions (DRs) are regions in the cochlea where the inner hair cells and/or neurons are not functioning. Adults with extensive high-frequency DRs have enhanced abilities in processing sounds with frequencies just below the edge frequency, fedge, of the DR. It was assessed whether the same is true for children. Design: Performance was compared for children aged 8 to 13 years with: DRs (group DR), hearing impairment but without DRs (group NODR), and normal hearing (group NH). Seven ears in each group were tested. Each ear in the DR group was matched in age and low-frequency hearing with an ear in the NODR group, and in age with an ear in the NH group, giving seven “triplets”. Within each triplet, the percent correct identification of vowel-consonant-vowel stimuli was measured using stimuli that were low-pass filtered at fedge and 0.67fedge, based on the ear with a DR. For the hearing-impaired ears, stimuli were given frequency-selective amplification as prescribed by DSL 4.1. Results: No significant differences in performance were found between groups for either low-pass cut-off frequency. Conclusion: Unlike adults, the children with DRs did not show enhanced discrimination of speech stimuli with frequencies below fedge.

Perception of low-pass filtered speech in hearing-impaired children, with and without cochlear dead regions and children with normal hearing

There are reports that adults with high-frequency cochlear dead regions (CDRs) exhibit an enhanced ability to use audible low-frequency acoustic information. This results in better performance of participants with CDRs compared to those without CDRs on tasks where the speech is low-pass filtered with a cutoff frequency near the edge frequency of the CDR (Moore and Vinay 2009). This enhanced ability to use low-frequency information may be related to cortical plasticity induced by the presence of a CDR and may be stronger in children due to maximal plasticity of the central auditory pathways. The aim of this study was to determine if children (aged 7-15 years) with a high-frequency congenital hearing loss and CDRs also show enhanced ability to use low frequency information. Vowel-consonant-vowel nonsense speech stimuli were low-pass filtered at various frequencies, amplified to correct for any hearing loss and presented via headphones. The percentage of correctly identified consonants for each low-pass filt...