A Clinically Viable Medial Olivocochlear Reflex Assay Using Transient-Evoked Otoacoustic Emissions.

OBJECTIVES: The contralateral medial olivocochlear reflex (MOCR) strength may indicate various auditory conditions in humans, but a clinically viable assay and equipment are needed for quick, accurate, and reliable measurements. The first experiment compared an earlier version of the assay, which used a nonlinear-mode chirp stimulus, with a new assay using a linear-mode click stimulus, designed to give reliable MOCR measurements in most normal-hearing ears. The second experiment extended the improved assay on a purpose-built binaural hardware platform that used forward-pressure level (FPL) calibration for both the stimulus and the contralateral MOCR elicitor. DESIGN: Transient-evoked otoacoustic emission (TEOAE) tests were measured with and without a 60-dB SPL MOCR-evoking contralateral broadband noise. The normalized MOCR strength (MOCR%) was derived from the TEOAE responses for each trial pair using the complex pressure difference weighted by the TEOAE magnitude. Experiment 1 compared MOCR% within-subject and across-day using two TEOAE stimuli: nonlinear-mode chirps (50 dB SPL, bandpass 1-5 kHz, 14 ms window delayed by 2 ms) and linear-mode clicks (50 dB SPL, bandpass 0.5-2.5 kHz, 13 ms window delayed by 5 ms). TEOAE responses were analyzed in the 0.5 to 2.5 kHz band. Thirty adult participants with normal hearing (30 ears) completed the study. The TEOAE stimulus was calibrated in situ using spectral flattening, and the contralateral noise was calibrated in a coupler. Twelve TEOAE trial pairs were collected for each participant and condition. Experiment 2 used a purpose-built binaural system. The TEOAE stimuli were linear-mode clicks (50 dB SPL, bandpass 1-3 kHz, 13 ms window delayed by 5 ms), analyzed in the 1 to 3 kHz band over ~12 trial pairs. After a probe refit, an additional trial pair was collected for the two early-stopping signal-to-noise ratio criteria (15 and 20 dB). They were evaluated for single-trial reliability and test time. Nineteen adult participants with normal hearing (38 ears) completed the study. The TEOAE clicks and contralateral elicitor noise were calibrated in situ using FPL and delivered with automated timing. RESULTS: MOCR% for linear-mode clicks was distinguishable from measurement variability in 98% to 100% of participants' ears (both experiments), compared with only 73% for the nonlinear-mode chirp (experiment 1). MOCR detectability was assessed using the MOCR% across-subject/within-subject variance ratio. The ratio in experiment 1 for linear-mode clicks was higher (8.0) than for nonlinear-mode chirps (6.4). The ratio for linear-mode clicks (8.9) in experiment 2 was slightly higher than for the comparable linear-mode stimulus (8.0) in experiment 1. TEOAEs showed excellent reliability with high signal-to-noise ratios in both experiments, but reliability was higher for linear-mode clicks than nonlinear-mode chirps. MOCR reliability for the two stimuli was comparable. The FPL pressure response retest reliability derived from the SPL at the microphone was higher than the SPL retest reliability across 0.4 to 8 kHz. Stable results required 2 to 3 trial pairs for the linear-mode click (experiments 1 and 2) and three for the nonlinear-mode chirp (experiment 1), taking around 2 min on average. CONCLUSIONS: The linear-mode click assay produced measurable, reliable, and stable TEOAE and MOCR results on both hardware platforms in around 2 min per ear. The stimulus design and response window ensured that any stimulus artifact in linear mode was unlikely to confound the results. The refined assay is ready to produce high-quality data quickly for clinical and field studies to develop population norms, recognize diagnostic patterns, and determine risk profiles.

Haptic Communication of Language.

In this review, the development of communication systems and devices that convey language tactually is examined, first from an historical perspective focusing on the communities who use the tactile modality to substitute for impairments in vision and/or hearing. Then, the more recent developments in wearable tactile communication systems for conveying text and speech to those without sensory impairments are reviewed. The performance of tactile display technology developed for these user communities is discussed in the context of the proficiency achieved by skilled users of natural methods of tactile communication. In tracing the history of tactile devices used to convey language, it is evident that technological advances in other domains, such as screen readers and speech synthesizers for the visually impaired and cochlear implants for those with hearing loss, have had a profound impact on the requirements for effective tactile language systems. For some communities, such as the Deafblind, it is essential that the tactile communication platform is bi-directional so that the user can both send and receive language. Devices developed to address such needs have yet to achieve commercial success. Recent research on wearable tactile displays has highlighted the importance of extensive training for learning and retaining languages presented tactually.

Improving Tactile Codes for Increased Speech Communication Rates in a Phonemic-Based Tactile Display.

Previous research has shown evidence of tactile speech acquisition of up to 500 English words presented as tactile phonemic patterns using a 4-by-6 tactor array worn on the forearm. This article describes modifications to some of the tactile codes encoding the 39 English phonemes, and ten additional codes as abbreviated patterns for the ten most frequent phoneme pairs in spoken English. The re-design aimed to reduce the duration of phonemes and phoneme pairs that occur most frequently, with the goal to increase tactile speech transmission rates. Code identification experiments were conducted with ten participants over three weeks using a video game. The average identification rate of the 49 modified codes (39 phonemes plus 10 phoneme pairs) was 83.3% with an average learning time of 6.2 hours. The average identification rate of the 49 codes in a retention test with 7 of the 10 participants after more than 90 days of no exposure to the tactile codes was 75.7%. An analysis using ideal transmission rates showed a 58% increase in transmission rate with the modified tactile codes as compared to the original codes, demonstrating that the improved codes can speed up tactile speech communication.

Acquisition of 500 English Words through a TActile Phonemic Sleeve (TAPS).

Recently, a phonemic-based tactile speech communication system was developed with the goal to transmit speech through the skin for people with hearing impairments and those whose auditory and visual channels are overloaded or compromised. The display, called the TActile Phonemic Sleeve (TAPS), consisted of a 4-by-6 tactor array worn on the dorsal and volar surfaces of the forearm. Earlier work showed that people were able to learn the haptic symbols for 39 English phonemes and reach a mean phoneme recognition rate of 86% correct within one to four hours of training. The current research evaluated the acquisition of up to 500 words using TAPS. A total of 51 participants were trained and tested in three studies with increasing number of phonemes and vocabulary sizes. Individual achievements varied, but the results clearly demonstrate the potential of transmitting any English word using TAPS within a reasonable period of learning. Future work will include increasing the speech transmission rate with TAPS by improving the phonemic codes and reducing the inter-phoneme intervals, addressing the reception of words and sentences composed of strings of tactile phonemes, and assessing the performance of TAPS as a speech communication system for people with severe hearing impairments.

Incidental Categorization of Vibrotactile Stimuli.

Past research has demonstrated incidental learning of task-irrelevant visual and auditory stimuli. Motivated by the possibility of similar evidence in the tactile domain and potential applications in tactile speech communication systems, we investigated incidental categorization of vibrotactile stimuli through a visuomotor task of shape identification. Two experiments were conducted where participants were exposed to position-based or movement-based vibrotactile stimuli prior to performing a speeded response to one of two targets. The two experiments differed only in the particular sets of such stimuli employed. Unbeknownst to the participants, the vibrotactile stimuli and visual targets were initially correlated perfectly to facilitate the incidental learning of their associations, briefly uncorrelated to check the cost in reaction time, and correlated again to re-establish the initial association. Finally, participants were asked to predict visual targets from novel position-based and movement-based stimuli. The results from both experiments provided evidence of incidental categorization of vibrotactile stimuli. The percent-correct scores and sensitivity indices for the overt categorization of novel stimuli from both experiments were well above chance, indicating generalization of learning. And while both experiments showed an increase in reaction time when the association between vibrotactile stimuli and visual targets was disrupted, this reaction time cost was significant only for the stimuli used in the second experiment. Our finding of incidental categorization in the tactile domain has important implications for the effective acquisition of speech in tactile speech communication systems.

A Phonemic-Based Tactile Display for Speech Communication.

Despite a long history of research, the development of synthetic tactual aids to support the communication of speech has proven to be a difficult task. The current paper describes a new tactile speech device based on the presentation of phonemic-based tactile codes. The device consists of 24 tactors under independent control for stimulation at the forearm. Using properties that include frequency and waveform of stimulation, amplitude, spatial location, and movement characteristics, unique tactile codes were designed for 39 consonant and vowel phonemes of the English language. The strategy for mapping the phonemes to tactile symbols is described, and properties of the individual phonemic codes are provided. Results are reported for an exploratory study of the ability of 10 young adults to identify the tactile symbols. The participants were trained to identify sets of consonants and vowels, before being tested on the full set of 39 tactile codes. The results indicate a mean recognition rate of 86 percent correct within one to four hours of training across participants. Thus, these results support the viability of a phonemic-based approach for conveying speech information through the tactile sense.

Pure-Tone Audiometry With Forward Pressure Level Calibration Leads to Clinically-Relevant Improvements in Test-Retest Reliability.

OBJECTIVES: Clinical pure-tone audiometry is conducted using stimuli delivered through supra-aural headphones or insert earphones. The stimuli are calibrated in an acoustic (average ear) coupler. Deviations in individual-ear acoustics from the coupler acoustics affect test validity, and variations in probe insertion and headphone placement affect both test validity and test-retest reliability. Using an insert earphone designed for otoacoustic emission testing, which contains a microphone and loudspeaker, an individualized in-the-ear calibration can be calculated from the ear-canal sound pressure measured at the microphone. However, the total sound pressure level (SPL) measured at the microphone may be affected by standing-wave nulls at higher frequencies, producing errors in stimulus level of up to 20 dB. An alternative is to calibrate using the forward pressure level (FPL) component, which is derived from the total SPL using a wideband acoustic immittance measurement, and represents the pressure wave incident on the eardrum. The objective of this study is to establish test-retest reliability for FPL calibration of pure-tone audiometry stimuli, compared with in-the-ear and coupler sound pressure calibrations. DESIGN: The authors compared standard audiometry using a modern clinical audiometer with TDH-39P supra-aural headphones calibrated in a coupler to a prototype audiometer with an ER10C earphone calibrated three ways: (1) in-the-ear using the total SPL at the microphone, (2) in-the-ear using the FPL at the microphone, and (3) in a coupler (all three are derived from the same measurement). The test procedure was similar to that commonly used in hearing-conservation programs, using pulsed-tone test frequencies at 0.5, 1, 2, 3, 4, 6, and 8 kHz, and an automated modified Hughson-Westlake audiometric procedure. Fifteen adult human participants with normal to mildly-impaired hearing were selected, and one ear from each was tested. Participants completed 10 audiograms on each system, with test-order randomly varied and with headphones and earphones refitted by the tester between tests. RESULTS: Fourteen of 15 ears had standing-wave nulls present between 4 and 8 kHz. The mean intrasubject SD at 6 and 8 kHz was lowest for the FPL calibration, and was comparable with the low-frequency reliability across calibration methods. This decrease in variability translates to statistically-derived significant threshold shift criteria indicating that 15 dB shifts in hearing can be reliably detected at 6 and 8 kHz using FPL-calibrated ER10C earphones, compared with 20 to 25 dB shifts using standard TDH-39P headphones with a coupler calibration. CONCLUSIONS: These results indicate that reliability is better with insert earphones, especially with in-the-ear FPL calibration, compared with a standard clinical audiometer with supra-aural headphones. However, in-the-ear SPL calibration should not be used due to its sensitivity to standing waves. The improvement in reliability is clinically meaningful, potentially allowing hearing-conservation programs to more confidently determine significant threshold shifts at 6 kHz-a key frequency for the early detection of noise-induced hearing loss.

Masking release for hearing-impaired listeners: The effect of increased audibility through reduction of amplitude variability.

The masking release (i.e., better speech recognition in fluctuating compared to continuous noise backgrounds) observed for normal-hearing (NH) listeners is generally reduced or absent in hearing-impaired (HI) listeners. One explanation for this lies in the effects of reduced audibility: elevated thresholds may prevent HI listeners from taking advantage of signals available to NH listeners during the dips of temporally fluctuating noise where the interference is relatively weak. This hypothesis was addressed through the development of a signal-processing technique designed to increase the audibility of speech during dips in interrupted noise. This technique acts to (i) compare short-term and long-term estimates of energy, (ii) increase the level of short-term segments whose energy is below the average energy, and (iii) normalize the overall energy of the processed signal to be equivalent to that of the original long-term estimate. Evaluations of this energy-equalizing (EEQ) technique included consonant identification and sentence reception in backgrounds of continuous and regularly interrupted noise. For HI listeners, performance was generally similar for processed and unprocessed signals in continuous noise; however, superior performance for EEQ processing was observed in certain regularly interrupted noise backgrounds.

Effect of Energy Equalization on the Intelligibility of Speech in Fluctuating Background Interference for Listeners With Hearing Impairment.

The masking release (MR; i.e., better speech recognition in fluctuating compared with continuous noise backgrounds) that is evident for listeners with normal hearing (NH) is generally reduced or absent for listeners with sensorineural hearing impairment (HI). In this study, a real-time signal-processing technique was developed to improve MR in listeners with HI and offer insight into the mechanisms influencing the size of MR. This technique compares short-term and long-term estimates of energy, increases the level of short-term segments whose energy is below the average energy, and normalizes the overall energy of the processed signal to be equivalent to that of the original long-term estimate. This signal-processing algorithm was used to create two types of energy-equalized (EEQ) signals: EEQ1, which operated on the wideband speech plus noise signal, and EEQ4, which operated independently on each of four bands with equal logarithmic width. Consonant identification was tested in backgrounds of continuous and various types of fluctuating speech-shaped Gaussian noise including those with both regularly and irregularly spaced temporal fluctuations. Listeners with HI achieved similar scores for EEQ and the original (unprocessed) stimuli in continuous-noise backgrounds, while superior performance was obtained for the EEQ signals in fluctuating background noises that had regular temporal gaps but not for those with irregularly spaced fluctuations. Thus, in noise backgrounds with regularly spaced temporal fluctuations, the energy-normalized signals led to larger values of MR and higher intelligibility than obtained with unprocessed signals.

Level variations in speech: Effect on masking release in hearing-impaired listeners.

Acoustic speech is marked by time-varying changes in the amplitude envelope that may pose difficulties for hearing-impaired listeners. Removal of these variations (e.g., by the Hilbert transform) could improve speech reception for such listeners, particularly in fluctuating interference. Léger, Reed, Desloge, Swaminathan, and Braida [(2015b). J. Acoust. Soc. Am. 138, 389-403] observed that a normalized measure of masking release obtained for hearing-impaired listeners using speech processed to preserve temporal fine-structure (TFS) cues was larger than that for unprocessed or envelope-based speech. This study measured masking release for two other speech signals in which level variations were minimal: peak clipping and TFS processing of an envelope signal. Consonant identification was measured for hearing-impaired listeners in backgrounds of continuous and fluctuating speech-shaped noise. The normalized masking release obtained using speech with normal variations in overall level was substantially less than that observed using speech processed to achieve highly restricted level variations. These results suggest that the performance of hearing-impaired listeners in fluctuating noise may be improved by signal processing that leads to a decrease in stimulus level variations.

Auditory-Tactile integration: Effects of Phase of Sinusoidal Stimulation at 50 and 250 Hz.

The perceptual integration of 50- and 250-Hz, 500-ms vibrotactile and auditory tones was studied in detection experiments as a function of the relative phase (0°, 72°, 144°, 216°, and 288°) of the tone pulses. Vibrotactile stimuli were delivered through a single-channel vibrator to the left middle fingertip and auditory stimuli were presented diotically through headphones in a background of 50 dB SPL broadband noise. The observers were four adults with normal hearing. The vibrotactile and auditory stimulus levels used each yielded 63-77%-Correct unimodal detection performance in a two-interval two-alternative forced-choice task. Scores for the auditory-alone and tactile-alone conditions averaged roughly 70%-Correct. Mean scores for the auditory plus tactile conditions averaged across different phases were 77.1%-Correct at 50 Hz and 79.6%-Correct at 250 Hz. At 50 Hz, no differences in performance were observed as a function of the relative phase at which the combined auditory and tactile signals were presented. At 250 Hz, significantly higher scores were observed for one phase combination (72°) compared to two of the other four relative phases. Performance on the auditory plus tactile conditions resulted in significant integrative effects and was generally more consistent with a "Pythagorean Sum" model than with either an "Algebraic Sum" or an "Optimum Single Cannel" model of perceptual integration.

Consonant identification in noise using Hilbert-transform temporal fine-structure speech and recovered-envelope speech for listeners with normal and impaired hearing.

Consonant-identification ability was examined in normal-hearing (NH) and hearing-impaired (HI) listeners in the presence of steady-state and 10-Hz square-wave interrupted speech-shaped noise. The Hilbert transform was used to process speech stimuli (16 consonants in a-C-a syllables) to present envelope cues, temporal fine-structure (TFS) cues, or envelope cues recovered from TFS speech. The performance of the HI listeners was inferior to that of the NH listeners both in terms of lower levels of performance in the baseline condition and in the need for higher signal-to-noise ratio to yield a given level of performance. For NH listeners, scores were higher in interrupted noise than in steady-state noise for all speech types (indicating substantial masking release). For HI listeners, masking release was typically observed for TFS and recovered-envelope speech but not for unprocessed and envelope speech. For both groups of listeners, TFS and recovered-envelope speech yielded similar levels of performance and consonant confusion patterns. The masking release observed for TFS and recovered-envelope speech may be related to level effects associated with the manner in which the TFS processing interacts with the interrupted noise signal, rather than to the contributions of TFS cues per se.

Otoacoustic-emission-based medial-olivocochlear reflex assays for humans.

Otoacoustic emission (OAE) tests of the medial-olivocochlear reflex (MOCR) in humans were assessed for viability as clinical assays. Two reflection-source OAEs [TEOAEs: transient-evoked otoacoustic emissions evoked by a 47 dB sound pressure level (SPL) chirp; and discrete-tone SFOAEs: stimulus-frequency otoacoustic emissions evoked by 40 dB SPL tones, and assessed with a 60 dB SPL suppressor] were compared in 27 normal-hearing adults. The MOCR elicitor was a 60 dB SPL contralateral broadband noise. An estimate of MOCR strength, MOCR%, was defined as the vector difference between OAEs measured with and without the elicitor, normalized by OAE magnitude (without elicitor). An MOCR was reliably detected in most ears. Within subjects, MOCR strength was correlated across frequency bands and across OAE type. The ratio of across-subject variability to within-subject variability ranged from 2 to 15, with wideband TEOAEs and averaged SFOAEs giving the highest ratios. MOCR strength in individual ears was reliably classified into low, normal, and high groups. SFOAEs using 1.5 to 2 kHz tones and TEOAEs in the 0.5 to 2.5 kHz band gave the best statistical results. TEOAEs had more clinical advantages. Both assays could be made faster for clinical applications, such as screening for individual susceptibility to acoustic trauma in a hearing-conservation program.

Auditory and tactile gap discrimination by observers with normal and impaired hearing.

Temporal processing ability for the senses of hearing and touch was examined through the measurement of gap-duration discrimination thresholds (GDDTs) employing the same low-frequency sinusoidal stimuli in both modalities. GDDTs were measured in three groups of observers (normal-hearing, hearing-impaired, and normal-hearing with simulated hearing loss) covering an age range of 21-69 yr. GDDTs for a baseline gap of 6 ms were measured for four different combinations of 100-ms leading and trailing markers (250-250, 250-400, 400-250, and 400-400 Hz). Auditory measurements were obtained for monaural presentation over headphones and tactile measurements were obtained using sinusoidal vibrations presented to the left middle finger. The auditory GDDTs of the hearing-impaired listeners, which were larger than those of the normal-hearing observers, were well-reproduced in the listeners with simulated loss. The magnitude of the GDDT was generally independent of modality and showed effects of age in both modalities. The use of different-frequency compared to same-frequency markers led to a greater deterioration in auditory GDDTs compared to tactile GDDTs and may reflect differences in bandwidth properties between the two sensory systems.

Consonant identification using temporal fine structure and recovered envelope cues.

The contribution of recovered envelopes (RENVs) to the utilization of temporal-fine structure (TFS) speech cues was examined in normal-hearing listeners. Consonant identification experiments used speech stimuli processed to present TFS or RENV cues. Experiment 1 examined the effects of exposure and presentation order using 16-band TFS speech and 40-band RENV speech recovered from 16-band TFS speech. Prior exposure to TFS speech aided in the reception of RENV speech. Performance on the two conditions was similar (∼50%-correct) for experienced listeners as was the pattern of consonant confusions. Experiment 2 examined the effect of varying the number of RENV bands recovered from 16-band TFS speech. Mean identification scores decreased as the number of RENV bands decreased from 40 to 8 and were only slightly above chance levels for 16 and 8 bands. Experiment 3 examined the effect of varying the number of bands in the TFS speech from which 40-band RENV speech was constructed. Performance fell from 85%- to 31%-correct as the number of TFS bands increased from 1 to 32. Overall, these results suggest that the interpretation of previous studies that have used TFS speech may have been confounded with the presence of RENVs.

Psychoacoustic and phoneme identification measures in cochlear-implant and normal-hearing listeners.

The purpose of this study is to identify precise and repeatable measures for assessing cochlear-implant (CI) hearing. The study presents psychoacoustic and phoneme identification measures in CI and normal-hearing (NH) listeners, with correlations between measures examined. Psychoacoustic measures included pitch discrimination tasks using pure tones, harmonic complexes, and tone pips; intensity perception tasks included intensity discrimination for tones and modulation detection; spectral-temporal masking tasks included gap detection, forward and backward masking, tone-on-tone masking, synthetic formant-on-formant masking, and tone in noise detection. Phoneme perception measures included vowel and consonant identification in quiet and stationary and temporally gated speech-shaped noise. Results on psychoacoustic measures illustrate the effects of broader filtering in CI hearing contributing to reduced pitch perception and increased spectral masking. Results on consonant and vowel identification measures illustrate a wide range in performance across CI listeners. They also provide further evidence that CI listeners obtain little to no release of masking in temporally gated noise compared to stationary noise. The forward and backward-masking measures had the highest correlation with the phoneme identification measures for CI listeners. No significant correlations between speech reception and psychoacoustic measures were observed for NH listeners. The superior NH performance on measures of phoneme identification, especially in the presence of background noise, is a key difference between groups.

Masking Release for Igbo and English.

In this research, we explored the effect of noise interruption rate on speech intelligibility. Specifically, we used the Hearing In Noise Test (HINT) procedure with the original HINT stimuli (English) and Igbo stimuli to assess speech reception ability in interrupted noise. For a given noise level, the HINT test provides an estimate of the signal-to-noise ratio (SNR) required for 50%-correct speech intelligibility. The SNR for 50%-correct intelligibility changes depending upon the interruption rate of the noise. This phenomenon (called Masking Release) has been studied extensively in English but not for Igbo - which is an African tonal language spoken predominantly in South Eastern Nigeria. This experiment explored and compared the phenomenon of Masking Release for (i) native English speakers listening to English, (ii) native Igbo speakers listening to English, and (iii) native Igbo speakers listening to Igbo. Since Igbo is a tonal language and English is a non-tonal language, this allowed us to compare Masking Release patterns on native speakers of tonal and non-tonal languages. Our results for native English speakers listening to English HINT show that the SNR and the masking release are orderly and consistent with other English HINT data for English speakers. Our result for Igbo speakers listening to English HINT sentences show that there is greater variability in results across the different Igbo listeners than across the English listeners. This result likely reflects different levels of ability in the English language across the Igbo listeners. The masking release values in dB are less than for English listeners. Our results for Igbo speakers listening to Igbo show that in general, the SNRs for Igbo sentences are lower than for English/English and Igbo/English. This means that the Igbo listeners could understand 50% of the Igbo sentences at SNRs less than those required for English sentences by either native or non-native listeners. This result can be explained by the fact that the perception of Igbo utterances by Igbo subjects may have been aided by the prediction of tonal and vowel harmony features existent in the Igbo language. In agreement with other studies, our results also show that in a noisy environment listeners are able to perceive their native language better than a second language. The ability of native language speakers to perceive their language better than a second language in a noisy environment may be attributed to the fact that: Native speakers are more familiar with the sounds of their language than second language speakers.One of the features of language is that it is predictable hence even in noise a native speaker may be able to predict a succeeding word that is scarcely audible. These contextual effects are facilitated by familiarity.

Auditory-filter characteristics for listeners with real and simulated hearing impairment.

Functional simulation of sensorineural hearing impairment is an important research tool that can elucidate the nature of hearing impairments and suggest or eliminate compensatory signal-processing schemes. The objective of the current study was to evaluate the capability of an audibility-based functional simulation of hearing loss to reproduce the auditory-filter characteristics of listeners with sensorineural hearing loss. The hearing-loss simulation used either threshold-elevating noise alone or a combination of threshold-elevating noise and multiband expansion to reproduce the audibility-based characteristics of the loss (including detection thresholds, dynamic range, and loudness recruitment). The hearing losses of 10 listeners with bilateral, mild-to-severe hearing loss were simulated in 10 corresponding groups of 3 age-matched normal-hearing listeners. Frequency selectivity was measured using a notched-noise masking paradigm at five probe frequencies in the range of 250 to 4000 Hz with a fixed probe level of either 70 dB SPL or 8 dB SL (whichever was greater) and probe duration of 200 ms. The hearing-loss simulation reproduced the absolute thresholds of individual hearing-impaired listeners with an average root-mean-squared (RMS) difference of 2.2 dB and the notched-noise masked thresholds with an RMS difference of 5.6 dB. A rounded-exponential model of the notched-noise data was used to estimate equivalent rectangular bandwidths and slopes of the auditory filters. For some subjects and probe frequencies, the simulations were accurate in reproducing the auditory-filter characteristics of the hearing-impaired listeners. In other cases, however, the simulations underestimated the magnitude of the auditory bandwidths for the hearing-impaired listeners, which suggests the possibility of suprathreshold deficits.

Temporal masking functions for listeners with real and simulated hearing loss.

A functional simulation of hearing loss was evaluated in its ability to reproduce the temporal masking functions for eight listeners with mild to severe sensorineural hearing loss. Each audiometric loss was simulated in a group of age-matched normal-hearing listeners through a combination of spectrally-shaped masking noise and multi-band expansion. Temporal-masking functions were obtained in both groups of listeners using a forward-masking paradigm in which the level of a 110-ms masker required to just mask a 10-ms fixed-level probe (5-10 dB SL) was measured as a function of the time delay between the masker offset and probe onset. At each of four probe frequencies (500, 1000, 2000, and 4000 Hz), temporal-masking functions were obtained using maskers that were 0.55, 1.0, and 1.15 times the probe frequency. The slopes and y-intercepts of the masking functions were not significantly different for listeners with real and simulated hearing loss. The y-intercepts were positively correlated with level of hearing loss while the slopes were negatively correlated. The ratio of the slopes obtained with the low-frequency maskers relative to the on-frequency maskers was similar for both groups of listeners and indicated a smaller compressive effect than that observed in normal-hearing listeners.

Temporal modulation transfer functions for listeners with real and simulated hearing loss.

A functional simulation of hearing loss was evaluated in its ability to reproduce the temporal modulation transfer functions (TMTFs) for nine listeners with mild to profound sensorineural hearing loss. Each hearing loss was simulated in a group of three age-matched normal-hearing listeners through spectrally shaped masking noise or a combination of masking noise and multiband expansion. TMTFs were measured for both groups of listeners using a broadband noise carrier as a function of modulation rate in the range 2 to 1024 Hz. The TMTFs were fit with a lowpass filter function that provided estimates of overall modulation-depth sensitivity and modulation cutoff frequency. Although the simulations were capable of accurately reproducing the threshold elevations of the hearing-impaired listeners, they were not successful in reproducing the TMTFs. On average, the simulations resulted in lower sensitivity and higher cutoff frequency than were observed in the TMTFs of the hearing-impaired listeners. Discrepancies in performance between listeners with real and simulated hearing loss are possibly related to inaccuracies in the simulation of recruitment.