Characterizing auditory processing and perception in individual listeners with sensorineural hearing loss

This study considered consequences of sensorineural hearing loss in ten listeners. The characterization of individual hearing loss was based on psychoacoustic data addressing audiometric pure-tone sensitivity, cochlear compression, frequency selectivity, temporal resolution, and intensity discrimination. In the experiments it was found that listeners with comparable audiograms can show very different results in the supra-threshold measures. In an attempt to account for the observed individual data, a model of auditory signal processing and perception [Jepsen et al., J. Acoust. Soc. Am. 124, 422-438 (2008)] was used as a framework. The parameters of the cochlear processing stage of the model were adjusted to account for behaviorally estimated individual basilar-membrane input-output functions and the audiogram, from which the amounts of inner hair-cell and outer hair-cell losses were estimated as a function of frequency. All other model parameters were left unchanged. The predictions showed a reasonably good agreement with the measured individual data in the frequency selectivity and forward masking conditions while the variation of intensity discrimination thresholds across listeners was underestimated by the model. The model and the associated parameters for individual hearing-impaired listeners might be useful for investigating effects of individual hearing impairment in more complex conditions, such as speech intelligibility in noise.     

Psychometric functions for discrimination of two-component complex tones in listeners with normal hearing and listeners with hearing loss

This study compared the ability of 5 listeners with normal hearing and 12 listeners with moderate to moderately severe sensorineural hearing loss to discriminate complementary two-component complex tones (TCCTs). The TCCTs consist of two pure tone components (f1 and f2) which differ in frequency by delta f (Hz) and in level by delta L (dB). In one of the complementary tones, the level of the component f1 is greater than the level of component f2 by the increment delta L; in the other tone, the level of component f2 exceeds that of component f1 by delta L. Five stimulus conditions were included in this study: fc = 1000 Hz, delta L = 3 dB; fc = 1000 Hz, delta L = 1 dB; fc = 2000 Hz, delta L = 3 dB; fc = 2000 Hz, delta L = 1 dB; and fc = 4000 Hz, delta L = 3 dB. In listeners with normal hearing, discrimination of complementary TCCTs (with a fixed delta L and a variable delta f) is described by an inverted U-shaped psychometric function in which discrimination improves as delta f increases, is (nearly) perfect for a range of delta f's, and then decreases again as delta f increases. In contrast, group psychometric functions for listeners with hearing loss are shifted to the right such that above chance performance occurs at larger values of delta f than in listeners with normal hearing. Group psychometric functions for listeners with hearing loss do not show a decrease in performance at the largest values of delta f included in this study. Decreased TCCT discrimination is evident when listeners with hearing loss are compared to listeners with normal hearing at both equal SPLs and at equal sensation levels. In both groups of listeners, TCCT discrimination is significantly worse at high center frequencies. Results from normal-hearing listeners are generally consistent with a temporal model of TCCT discrimination. Listeners with hearing loss may have deficits in using phase locking in the TCCT discrimination task and so may rely more on place cues in TCCT discrimination.     

Localization in speech mixtures by listeners with hearing loss

The ability of listeners with bilateral sensorineural hearing loss to localize a speech source in a multitalker mixture was measured. Five simultaneous words spoken by different talkers were presented over loudspeakers in a small room, and listeners localized one target word. Errors were significantly larger in this group compared to a control group with normal hearing. Localization of the target presented alone was not different between groups. The results suggest that hearing loss does not impair spatial hearing per se, but degrades the spatial representation of multiple simultaneous sounds.     

Perception of roughness by listeners with sensorineural hearing loss

The perception of auditory roughness presumably results from imperfect spectral or temporal resolution. Sensorineural hearing loss, by affecting spectral resolution, may therefore alter roughness perception. In this study, normal-hearing and hearing-impaired listeners estimated the roughness of amplitude-modulated tones varying in carrier frequency, modulation rate, and modulation depth. Their judgments were expected to reflect effects of impaired spectral resolution. Instead, their judgments were similar, in most respects, to those of normally-hearing listeners, except at very slow modulation rates. Results suggest that mild-to-moderate sensorineural hearing loss increases the roughness of slowly fluctuating signals.     

Stop-consonant recognition for normal-hearing listeners and listeners with high-frequency hearing loss. II: Articulation index predictions

Articulation index (AI) theory was used to evaluate stop-consonant recognition of normal-hearing listeners and listeners with high-frequency hearing loss. From results reported in a companion article [Dubno et al., J. Acoust. Soc. Am. 85, 347-354 (1989)], a transfer function relating the AI to stop-consonant recognition was established, and a frequency importance function was determined for the nine stop-consonant-vowel syllables used as test stimuli. The calculations included the rms and peak levels of the speech that had been measured in 1/3 octave bands; the internal noise was estimated from the thresholds for each subject. The AI model was then used to predict performance for the hearing-impaired listeners. A majority of the AI predictions for the hearing-impaired subjects fell within +/- 2 standard deviations of the normal-hearing listeners' results. However, as observed in previous data, the AI tended to overestimate performance of the hearing-impaired listeners. The accuracy of the predictions decreased with the magnitude of high-frequency hearing loss. Thus, with the exception of performance for listeners with severe high-frequency hearing loss, the results suggest that poorer speech recognition among hearing-impaired listeners results from reduced audibility within critical spectral regions of the speech stimuli.     

Proportional frequency compression of speech for listeners with sensorineural hearing loss.

This study examined proportional frequency compression as a strategy for improving speech recognition in listeners with high-frequency sensorineural hearing loss. This method of frequency compression preserved the ratios between the frequencies of the components of natural speech, as well as the temporal envelope of the unprocessed speech stimuli. Nonsense syllables spoken by a female and a male talker were used as the speech materials. Both frequency-compressed speech and the control condition of unprocessed speech were presented with high-pass amplification. For the materials spoken by the female talker, significant increases in speech recognition were observed in slightly less than one-half of the listeners with hearing impairment. For the male-talker materials, one-fifth of the hearing-impaired listeners showed significant recognition improvements. The increases in speech recognition due solely to frequency compression were generally smaller than those solely due to high-pass amplification. The results indicate that while high-pass amplification is still the most effective approach for improving speech recognition of listeners with high-frequency hearing loss, proportional frequency compression can offer significant improvements in addition to those provided by amplification for some patients.     

Temporal gap resolution in listeners with high-frequency sensorineural hearing loss

Temporal gap resolution was measured in five normal-hearing listeners and five cochlear-impaired listeners, whose sensitivity losses were restricted to the frequency regions above 1000 Hz. The stimuli included a broadband noise and three octave band noises centered at 0.5, 1.0, and 4.0 kHz. Results for the normal-hearing subjects agree with previous findings and reveal that gap resolution improves progressively with an increase in signal frequency. Gap resolution in the impaired listeners was significantly poorer than normal for all signals including those that stimulated frequency regions with normal pure-tone sensitivity. Smallest gap thresholds for the impaired listeners were observed with the broadband signal at high levels. This result agrees with data from other experiments and confirms the importance of high-frequency signal audibility in gap detection. The octave band data reveal that resolution deficits can be quite large within restricted frequency regions, even those with minimal sensitivity loss.     

Excess masking among listeners with a sensorineural hearing loss

Three experiments were conducted to determine whether listeners with a sensorineural hearing loss exhibited greater than normal amounts of masking at frequencies above the frequency of the masker. Excess masking was defined as the difference (in dB) between the masked thresholds actually obtained from a hearing-impaired listener and the expected thresholds calculated for the same individual. The expected thresholds were the power sum of the listener's thresholds in quiet and the average masked thresholds obtained from a group of normal-hearing subjects at the test frequency. Hearing-impaired listeners, with thresholds in quiet ranging from approximately 35-70 dB SPL (at test frequencies between 500-3000 Hz), displayed approximately 12-15 dB of maximum excess masking. The maximum amount of excess masking occurred in the region where the threshold in quiet of the hearing-impaired listener and the average normal masked threshold were equal. These findings indicate that listeners with a sensorineural hearing loss display one form of reduced frequency selectivity (i.e., abnormal upward spread of masking) even when their thresholds in quiet are taken into account.     

Discrimination of interaural temporal disparities by normal-hearing listeners and listeners with high-frequency sensorineural hearing loss

Thresholds of ongoing interaural time difference (ITD) were obtained from normal-hearing and hearing-impaired listeners who had high-frequency, sensorineural hearing loss. Several stimuli (a 500-Hz sinusoid, a narrow-band noise centered at 500 Hz, a sinusoidally amplitude-modulated 4000-Hz tone, and a narrow-band noise centered at 4000 Hz) and two criteria [equal sound-pressure level (Eq SPL) and equal sensation level (Eq SL)] for determining the level of stimuli presented to each listener were employed. The ITD thresholds and slopes of the psychometric functions were elevated for hearing-impaired listeners for the two high-frequency stimuli in comparison to: the listener's own low-frequency thresholds; and data obtained from normal-hearing listeners for stimuli presented with Eq SPL interaurally. The two groups of listeners required similar ITDs to reach threshold when stimuli were presented at Eq SLs to each ear. For low-frequency stimuli, the ITD thresholds of the hearing-impaired listener were generally slightly greater than those obtained from the normal-hearing listeners. Whether these stimuli were presented at either Eq SPL or Eq SL did not differentially affect the ITD thresholds across groups.     

Providing low- and mid-frequency speech information to listeners with sensorineural hearing loss.

The present study examined the benefits of providing amplified speech to the low- and mid-frequency regions of listeners with various degrees of sensorineural hearing loss. Nonsense syllables were low-pass filtered at various cutoff frequencies and consonant recognition was measured as the bandwidth of the signal was increased. In addition, error patterns were analyzed to determine the types of speech cues that were, or were not, transmitted to the listeners. For speech frequencies of 2800 Hz and below, a positive benefit of amplified speech was observed in every case, although the benefit provided was very often less than that observed in normal-hearing listeners who received the same increase in speech audibility. There was no dependence of this benefit upon the degree of hearing loss. Error patterns suggested that the primary difficulty that hearing-impaired individuals have in using amplified speech is due to their poor ability to perceive the place of articulation of consonants, followed by a reduced ability to perceive manner information.     

Perceptual coherence in listeners having longstanding childhood hearing losses, listeners with adult-onset hearing losses, and listeners with normal hearing

Perceptual coherence, the process by which the individual elements of complex sounds are bound together, was examined in adult listeners with longstanding childhood hearing losses, listeners with adult-onset hearing losses, and listeners with normal hearing. It was hypothesized that perceptual coherence would vary in strength between the groups due to their substantial differences in hearing history. Bisyllabic words produced by three talkers as well as comodulated three-tone complexes served as stimuli. In the first task, the second formant of each word was isolated and presented for recognition. In the second task, an isolated formant was paired with an intact word and listeners indicated whether or not the isolated second formant was a component of the intact word. In the third task, the middle component of the three-tone complex was presented in the same manner. For the speech stimuli, results indicate normal perceptual coherence in the listeners with adult-onset hearing loss but significantly weaker coherence in the listeners with childhood hearing losses. No differences were observed across groups for the nonspeech stimuli. These results suggest that perceptual coherence is relatively unaffected by hearing loss acquired during adulthood but appears to be impaired when hearing loss is present in early childhood.     

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.     

Sensory and nonsensory influences on children's performance of dichotic pitch perception tasks

Dichotic pitch perception reflects the auditory system's use of binaural cues to perceptually separate different sound sources and to determine the spatial location of sounds. Several studies were conducted to identify factors that influence children's dichotic pitch perception thresholds. An initial study of school children revealed an age-related improvement in thresholds for lateralizing dichotic pitch tones. In subsequent studies potential sensory and nonsensory limitations on young children's performance of dichotic pitch lateralization tasks were examined. A training study showed that with sufficient practice, young children lateralize dichotic pitch stimuli as well as adults, indicating an age difference in perceptual learning of the lateralization task. Changing the task requirements so that young children made a judgment about the pitch of dichotic pitch tones, rather than the spatial location of the tones, also resulted in significantly better thresholds. These findings indicate that nonsensory factors limit young children's performance of dichotic pitch tasks.     

Stop-consonant recognition for normal-hearing listeners and listeners with high-frequency hearing loss. I: The contribution of selected frequency regions

The purpose of this study is to specify the contribution of certain frequency regions to consonant place perception for normal-hearing listeners and listeners with high-frequency hearing loss, and to characterize the differences in stop-consonant place perception among these listeners. Stop-consonant recognition and error patterns were examined at various speech-presentation levels and under conditions of low- and high-pass filtering. Subjects included 18 normal-hearing listeners and a homogeneous group of 10 young, hearing-impaired individuals with high-frequency sensorineural hearing loss. Differential filtering effects on consonant place perception were consistent with the spectral composition of acoustic cues. Differences in consonant recognition and error patterns between normal-hearing and hearing-impaired listeners were observed when the stimulus bandwidth included regions of threshold elevation for the hearing-impaired listeners. Thus place-perception differences among listeners are, for the most part, associated with stimulus bandwidths corresponding to regions of hearing loss.     

Susceptibility to intraspeech spread of masking in listeners with sensorineural hearing loss

Previous research with speechlike signals has suggested that upward spread of masking from the first formant (F 1) may interfere with the identification of place of articulation information signaled by changes in the upper formants. This suggestion was tested by presenting two-formant stop consonant--vowel syllables varying along a/ba--/da/--/ga/ continuum to hearing-impaired listeners grouped according to etiological basis of the disorder. The syllables were presented monaurally at 80 dB and 100 dB SPL when formant amplitudes were equal and when F 1 amplitude was reduced by 6, 12, and 18 dB. Noise-on-tone masking patterns were also generated using narrow bands of noise at 80 and 100 dB SPL to assess the extent of upward spread of masking. Upward spread of masking could be demonstrated in both speech and nonspeech tasks, irrespective of the subject's age, audiometric configuration, or etiology of hearing impairment. Attenuation of F 1 had different effects on phonetic identification in different subject groups: While listeners with noise-induced hearing loss showed substantial improvement in identifying place of articulation, upward spread of masking did not consistently account for poor place identification in other types of sensorineural hearing impairment.     

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.     

Psychometric functions for frequency discrimination from listeners with sensorineural hearing loss

Psychometric functions for pulsed pure-tone frequency discrimination were obtained from hearing-impaired listeners at frequencies with normal hearing and at frequencies with mild or moderate hearing losses. The general form of psychometric functions at hearing-impaired frequencies was found to be the same as at normal-hearing frequencies, i.e.,d' was linear with the frequency difference between tones, in Hz. For all but one psychometric function, the addition of an intercept term to the fitting equation did not account for significantly more variance than did the slope term alone. Therefore, it was concluded that psychometric functions for frequency discrimination can be adequately described with only one parameter: the slope of the psychometric function. Deficits in discrimination at hearing-loss frequencies were manifested by more gradual slopes of psychometric functions. Procedures for normalizing psychometric functions are presented, which facilitate comparisons of normal and impaired frequency discrimination data across studies and frequencies. Comparisons of dlf's (difference limen for frequency) obtained with adaptive and fixed procedures show a bias toward larger dlf's with adaptive procedures, but only at higher frequencies. A discussion of equal-interval and equal-ratio adaptive stepping rules indicates that an equal-ratio rule may be preferable.     

Minimum stimulus levels for temporal gap resolution in listeners with sensorineural hearing loss

The minimum sensation levels required for optimal temporal gap resolution were measured in five listeners with moderately severe degrees of sensorineural hearing loss. The stimuli were three continuous octave-band noises centered at 0.5, 2.0, and 4.0 kHz. Subjects used a Békésy tracking procedure to determine the minimum signal levels needed to resolve periodic temporal gaps of fixed durations. Analysis of data across subjects and signal revealed only a weak correlation between this minimum SL and the corresponding HLs; most listeners resolved threshold gaps at minimum levels of 25-35 dB SL, independent of degree of hearing loss. The results differ from those of normal subjects with masking-induced hearing loss [Fitzgibbons, Percept. Psychophys. 35, 446-450 (1984)], which showed an inverse relationship between HL and the SLs required for gap threshold. The findings indicate that assessment of optimal gap resolution in listeners with cochlear impairment requires stimulus presentation levels of at least 25-35 dB SL. Even with sufficient stimulus intensity, each of the hearing-impaired listeners exhibited abnormal gap resolution for each octave-band signal.     

Inferred basilar-membrane response functions for listeners with mild to moderate sensorineural hearing loss

Psychophysical estimates of cochlear function suggest that normal-hearing listeners exhibit a compressive basilar-membrane (BM) response. Listeners with moderate to severe sensorineural hearing loss may exhibit a linearized BM response along with reduced gain, suggesting the loss of an active cochlear mechanism. This study investigated how the BM response changes with increasing hearing loss by comparing psychophysical measures of BM compression and gain for normal-hearing listeners with those for listeners who have mild to moderate sensorineural hearing loss. Data were collected from 16 normal-hearing listeners and 12 ears from 9 hearing-impaired listeners. The forward masker level required to mask a fixed low-level, 4000-Hz signal was measured as a function of the masker-signal interval using a masker frequency of either 2200 or 4000 Hz. These plots are known as temporal masking curves (TMCs). BM response functions derived from the TMCs showed a systematic reduction in gain with degree of hearing loss. Contrary to current thinking, however, no clear relationship was found between maximum compression and absolute threshold.