Auditory filter characteristics and consonant recognition for hearing-impaired listeners

To examine the association between frequency resolution and speech recognition, auditory filter parameters and stop-consonant recognition were determined for 9 normal-hearing and 24 hearing-impaired subjects. In an earlier investigation, the relationship between stop-consonant recognition and the articulation index (AI) had been established on normal-hearing listeners. Based on AI predictions, speech-presentation levels for each subject in this experiment were selected to obtain a wide range of recognition scores. This strategy provides a method of interpreting speech-recognition performance among listeners who vary in magnitude and configuration of hearing loss by assuming that conditions which yield equal audible spectra will result in equivalent performance. It was reasoned that an association between frequency resolution and consonant recognition may be more appropriately estimated if hearing-impaired listeners' performance was measured under conditions that assured equivalent audibility of the speech stimuli. Derived auditory filter parameters indicated that filter widths and dynamic ranges were strongly associated with threshold. Stop-consonant recognition scores for most hearing-impaired listeners were not significantly poorer than predicted by the AI model. Furthermore, differences between observed recognition scores and those predicted by the AI were not associated with auditory filter characteristics, suggesting that frequency resolution and speech recognition may appear to be associated primarily because both are degraded by threshold elevation.     

Spectro-temporal analysis in normal-hearing and cochlear-impaired listeners

Detection thresholds for a 1.0-kHz pure tone were determined in unmodulated noise and in noise modulated by a 15-Hz square wave. Comodulation masking release (CMR) was calculated as the difference in threshold between the modulated and unmodulated conditions. The noise bandwidth varied between 100 and 1000 Hz. Frequency selectivity was also examined using an abbreviated notched-noise masking method. The subjects in the main experiment consisted of 12 normal-hearing and 12 hearing-impaired subjects with hearing loss of cochlear origin. The most discriminating conditions were repeated on 16 additional hearing-impaired subjects. The CMR of the hearing-impaired group was reduced for the 1000-Hz noise bandwidth. The reduced CMR at this bandwidth correlated significantly with reduced frequency selectivity, consistent with the hypothesis that the across-frequency difference cue used in CMR is diminished by poor frequency selectivity. The results indicated that good frequency selectivity is a prerequisite, but not a guarantee, of large CMR.     

Summation bandwidths at threshold in normal and hearing-impaired listeners

The bandwidths for summation at threshold were measured for subjects with normal hearing and subjects with sensorineural hearing loss. Thresholds in quiet and in the presence of a masking noise were measured for complex stimuli consisting of 1 to 40 pure-tone components spaced 20 Hz apart. The single component condition consisted of a single pure tone at 1100 Hz; additional components were added below this frequency, in a replication of the G?ssler [Acustica 4, 408-414 (1954)] procedure. For the normal subjects, thresholds increased approximately 3 dB per doubling of bandwidth for signal bandwidths exceeding the critical bandwidth. This slope was less for the hearing-impaired subjects. Summation bandwidths, as estimated from two-line fits, were wider for the hearing-impaired than for the normal subjects. These findings provide evidence that hearing-impaired subjects integrate sound energy over a wider-than-normal frequency range for the detection of complex signals. A second experiment used stimuli similar to those of Spiegel [J. Acoust. Soc. Am. 66, 1356-1363 (1979)], and added components both above and below the frequency of the initial component. Using these stimuli, the slope of the threshold increase beyond the critical bandwidth was approximately 1.5 dB per doubling of bandwidth, thus replicating the Spiegel (1979) experiment. It is concluded that the differences between the G?ssler (1954) and Spiegel (1979) studies were due to the different frequency content of the stimuli used in each study. Based upon the present results, it would appear that the slope of threshold increase is dependent upon the direction of signal expansion, and the size of the critical bands into which the signal is expanded.     

The effect of superior auditory skills on vocal accuracy

The relationship between auditory perception and vocal production has been typically investigated by evaluating the effect of either altered or degraded auditory feedback on speech production in either normal hearing or hearing-impaired individuals. Our goal in the present study was to examine this relationship in individuals with superior auditory abilities. Thirteen professional musicians and thirteen nonmusicians, with no vocal or singing training, participated in this study. For vocal production accuracy, subjects were presented with three tones. They were asked to reproduce the pitch using the vowel /a/. This procedure was repeated three times. The fundamental frequency of each production was measured using an autocorrelation pitch detection algorithm designed for this study. The musicians' superior auditory abilities (compared to the nonmusicians) were established in a frequency discrimination task reported elsewhere. Results indicate that (a) musicians had better vocal production accuracy than nonmusicians (production errors of 1/2 a semitone compared to 1.3 semitones, respectively); (b) frequency discrimination thresholds explain 43% of the variance of the production data, and (c) all subjects with superior frequency discrimination thresholds showed accurate vocal production; the reverse relationship, however, does not hold true. In this study we provide empirical evidence to the importance of auditory feedback on vocal production in listeners with superior auditory skills.     

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.     

The effects of age and cochlear hearing loss on temporal fine structure sensitivity, frequency selectivity, and speech reception in noise

Temporal fine structure (TFS) sensitivity, frequency selectivity, and speech reception in noise were measured for young normal-hearing (NHY), old normal-hearing (NHO), and hearing-impaired (HI) subjects. Two measures of TFS sensitivity were used: the "TFS-LF test" (interaural phase difference discrimination) and the "TFS2 test" (discrimination of harmonic and frequency-shifted tones). These measures were not significantly correlated with frequency selectivity (after partialing out the effect of audiometric threshold), suggesting that insensitivity to TFS cannot be wholly explained by a broadening of auditory filters. The results of the two tests of TFS sensitivity were significantly but modestly correlated, suggesting that performance of the tests may be partly influenced by different factors. The NHO group performed significantly more poorly than the NHY group for both measures of TFS sensitivity, but not frequency selectivity, suggesting that TFS sensitivity declines with age in the absence of elevated audiometric thresholds or broadened auditory filters. When the effect of mean audiometric threshold was partialed out, speech reception thresholds in modulated noise were correlated with TFS2 scores, but not measures of frequency selectivity or TFS-LF test scores, suggesting that a reduction in sensitivity to TFS can partly account for the speech perception difficulties experienced by hearing-impaired subjects.     

Audibility and recognition of stop consonants in normal and hearing-impaired subjects

The purpose of this study was to examine the effect of spectral-cue audibility on the recognition of stop consonants in normal-hearing and hearing-impaired adults. Subjects identified six synthetic CV speech tokens in a closed-set response task. Each syllable differed only in the initial 40-ms consonant portion of the stimulus. In order to relate performance to spectral-cue audibility, the initial 40 ms of each CV were analyzed via FFT and the resulting spectral array was passed through a sliding-filter model of the human auditory system to account for logarithmic representation of frequency and the summation of stimulus energy within critical bands. This allowed the spectral data to be displayed in comparison to a subject's sensitivity thresholds. For normal-hearing subjects, an orderly function relating the percentage of audible stimulus to recognition performance was found, with perfect discrimination performance occurring when the bulk of the stimulus spectrum was presented at suprathreshold levels. For the hearing-impaired subjects, however, it was found in many instances that suprathreshold presentation of stop-consonant spectral cues did not yield recognition equivalent to that found for the normal-hearing subjects. These results demonstrate that while the audibility of individual stop consonants is an important factor influencing recognition performance in hearing-impaired subjects, it is not always sufficient to explain the effects of sensorineural hearing loss.     

Auditory filter shapes at low center frequencies in young and elderly hearing-impaired subjects.

Auditory filter shapes were measured for two groups of hearing-impaired subjects, young and elderly, matched for audiometric loss, for center frequencies (fc) of 100, 200, 400, and 800 Hz using a modified notched-noise method [B. R. Glasberg and B. C. J. Moore, Hear. Res. 47, 103-138 (1990)]. Two noise bands, each 0.4fc wide, were used; they were placed both symmetrically and asymmetrically about the signal frequency to allow the measurement of filter asymmetry. The overall noise level was either 77 or 87 dB SPL. Stimuli were delivered monaurally using Sennheiser HD424 earphones. Although auditory filters for the hearing-impaired subjects were generally broader than for normally hearing subjects [Moore et al., J. Acoust. Soc. Am. 87, 132-140 (1990)], some hearing-impaired subjects with mild losses had normal filters. The filters tended to broaden with increasing hearing loss. There were not any clear differences in filter characteristics between young and elderly hearing-impaired subjects. The signal-to-noise ratios at the outputs of the auditory filters required for threshold (K) tended to be lower than normal for the young hearing-impaired subjects, but were not significantly different from normal for the elderly hearing-impaired subjects. The lower K values for the young hearing-impaired subjects may occur because broadened auditory filters reduce the deleterious effects on signal detection of fluctuations in the noise.     

Temporal integration, frequency resolution, and off-frequency listening in normal-hearing and cochlear-impaired listeners

Temporal integration for a 1000-Hz signal was determined for normal-hearing and cochlear hearing-impaired listeners in quiet and in masking noise of variable bandwidth. Critical ratio and 3-dB critical band measures of frequency resolution were derived from the masking data. Temporal integration for the normal-hearing listeners was markedly reduced in narrow-band noise, when contrasted with temporal integration in quiet or in wideband noise. The effect of noise bandwidth on temporal integration was smaller for the hearing-impaired group. Hearing-impaired subjects showed both reduced temporal integration and reduced frequency resolution for the 200-ms signal. However, a direct relation between temporal integration and frequency resolution was not indicated. Frequency resolution for the normal-hearing listeners did not differ from that of the hearing-impaired listeners for the 20-ms signal. It was suggested that some of the frequency resolution and temporal integration differences between normal-hearing and hearing-impaired listeners could be accounted for by off-frequency listening.     

Effect of stimulus bandwidth on the perception of /s/ in normal- and hearing-impaired children and adults.

Recent studies with adults have suggested that amplification at 4 kHz and above fails to improve speech recognition and may even degrade performance when high-frequency thresholds exceed 50-60 dB HL. This study examined the extent to which high frequencies can provide useful information for fricative perception for normal-hearing and hearing-impaired children and adults. Eighty subjects (20 per group) participated. Nonsense syllables containing the phonemes /s/, /f/, and /O/, produced by a male, female, and child talker, were low-pass filtered at 2, 3, 4, 5, 6, and 9 kHz. Frequency shaping was provided for the hearing-impaired subjects only. Results revealed significant differences in recognition between the four groups of subjects. Specifically, both groups of children performed more poorly than their adult counterparts at similar bandwidths. Likewise, both hearing-impaired groups performed more poorly than their normal-hearing counterparts. In addition, significant talker effects for /s/ were observed. For the male talker, optimum performance was reached at a bandwidth of approximately 4-5 kHz, whereas optimum performance for the female and child talkers did not occur until a bandwidth of 9 kHz.     

Pitch strength decreases as F0 and harmonic resolution increase in complex tones composed exclusively of high harmonics

A melodic pitch experiment was performed to demonstrate the importance of time-interval resolution for pitch strength. The experiments show that notes with a low fundamental (75 Hz) and relatively few resolved harmonics support better performance than comparable notes with a higher fundamental (300 Hz) and more resolved harmonics. Two four note melodies were presented to listeners and one note in the second melody was changed by one or two semitones. Listeners were required to identify the note that changed. There were three orthogonal stimulus dimensions: F0 (75 and 300 Hz); lowest frequency component (3, 7, 11, or 15); and number of harmonics (4 or 8). Performance decreased as the frequency of the lowest component increased for both F0's, but performance was better for the lower F0. The spectral and temporal information in the stimuli were compared using a time-domain model of auditory perception. It is argued that the distribution of time intervals in the auditory nerve can explain the decrease in performance as F0, and spectral resolution increase. Excitation patterns based on the same time-interval information do not contain sufficient resolution to explain listener's performance on the melody task.     

Some effects of signal bandwidth and spectral density on the acoustic-reflex threshold in the elderly

The acoustic-reflex thresholds (ART) for multicomponent tonal complexes of varying bandwidth and spectral density were obtained from 20 normal-hearing (air-conduction thresholds less than or equal to 20 dB HL at 250-8000 Hz) young adults ranging in age from 20-30 years and 20 normal-hearing, old subjects ranging in age from 60-71 years. The results revealed that the ART decreased with spectral density, plateauing after seven components in the young group and after five components in the old group; the decrease in the acoustic-reflex threshold as a result of the increase in spectral density was less in the old than in the young group. The bandwidth effect (when bandwidth was plotted in hertz or octaves) on the acoustic-reflex threshold was present in the young adults, but substantially reduced in the elderly, as evidenced by the statistically significant interaction between subject group and signal bandwidth. The spectral density results are discussed in terms of their theoretic implications for the energy summation capacity and frequency resolution of the auditory system. The bandwidth results are discussed in terms of their theoretic implications for the frequency-resolving power of the auditory system.     

Sensorineural hearing loss and the discrimination of vowel-like stimuli

It has been hypothesized that the wider-than-normal auditory bandwidths attributed to sensorineural hearing loss lead to a reduced ability to discriminate spectral characteristics in speech signals. In order to investigate this possibility, the minimum detectable depth of a spectral "notch" between the second (F2) and third (F3) formants of a synthetic vowel-like stimulus was determined for normal and hearing-impaired subjects. The minimum detectable notch for all subjects was surprisingly small; values obtained were much smaller than those found in actual vowels. An analysis of the stimuli based upon intensity discrimination within a single critical band predicted only small differences in performance on this task for rather large differences in the size of the auditory bandwidth. These results suggest that impairments of auditory frequency resolution in sensorineural hearing loss may not be critical in the perception of steady-state vowels.     

Relations between intelligibility of narrow-band speech and auditory functions, both in the 1-kHz frequency region

Relations between perception of suprathreshold speech and auditory functions were examined in 24 hearing-impaired listeners and 12 normal-hearing listeners. The speech intelligibility index (SII) was used to account for audibility. The auditory functions included detection efficiency, temporal and spectral resolution, temporal and spectral integration, and discrimination of intensity, frequency, rhythm, and spectro-temporal shape. All auditory functions were measured at 1 kHz. Speech intelligibility was assessed with the speech-reception threshold (SRT) in quiet and in noise, and with the speech-reception bandwidth threshold (SRBT), previously developed for investigating speech perception in a limited frequency region around 1 kHz. The results showed that the elevated SRT in quiet could be explained on the basis of audibility. Audibility could only partly account for the elevated SRT values in noise and the deviant SRBT values, suggesting that suprathreshold deficits affected intelligibility in these conditions. SII predictions for the SRBT improved significantly by including the individually measured upward spread of masking in the SII model. Reduced spectral resolution, reduced temporal resolution, and reduced frequency discrimination appeared to be related to speech perception deficits. Loss of peripheral compression appeared to have the smallest effect on the intelligibility of suprathreshold speech.     

Measurement of auditory temporal processing using modified masking period patterns

A common metric of auditory temporal processing is the difference in the threshold for a pure-tone signal masked by either unmodulated or amplitude-modulated noise. This technique may be viewed as a modification of the masking period pattern technique. Such measurements have been proposed as an efficient means of estimating auditory temporal resolution in a clinical setting, although in many cases threshold differences may reflect additional spectro-temporal processes. The primary purpose of the present experiment was to examine interactions among signal frequency and masker bandwidth and the effects of modulation frequency on modified masking period patterns. The results revealed unmodulated-modulated threshold differences that increased with increasing masker bandwidth and decreased with increasing modulation frequency. There was little effect of signal frequency for narrow-band noise maskers that were equal in absolute bandwidth across frequency. However, unmodulated-modulated threshold differences increased substantially with increasing signal frequency for bandwidths proportional to the signal frequency and for wideband maskers. Although the results are interpreted in terms of a combination of both within-channel and across-channel cues, the specific contributions of these cues in particular conditions are difficult to ascertain. Because modified masking period patterns depend strongly upon a number of specific stimulus parameters, and because it is difficult to determine with any precision the underlying perceptual processes, this technique is not recommended for use as a clinical measure of auditory temporal processing.     

Forward- and simultaneous-masked thresholds in bandlimited maskers in subjects with normal hearing and cochlear hearing loss

Forward- and simultaneous-masked thresholds were measured at 0.5 and 2.0 kHz in bandpass maskers as a function of masker bandwidth and in a broadband masker with the goal of estimating psychophysical suppression. Suppression was operationally defined in two ways: (1) as a change in forward-masked threshold as a function of masker bandwidth, and (2) as a change in effective masker level with increased masker bandwidth, taking into account the nonlinear growth of forward masking. Subjects were younger adults with normal hearing and older adults with cochlear hearing loss. Thresholds decreased as a function of masker bandwidth in forward masking, which was attributed to effects of suppression; thresholds remained constant or increased slightly with increasing masker bandwidth in simultaneous masking. For subjects with normal hearing, slightly larger estimates of suppression were obtained at 2.0 kHz rather than at 0.5 kHz. For hearing-impaired subjects, suppression was reduced in regions of hearing loss. The magnitude of suppression was strongly correlated with the absolute threshold at the signal frequency, but did not vary with thresholds at frequencies remote from the signal. The results suggest that measuring forward-masked thresholds in bandlimited and broadband maskers may be an efficient psychophysical method for estimating suppression.     

NoSo and NoS pi detection as a function of masker bandwidth in normal-hearing and cochlear-impaired listeners

NoSo and NoS pi detection thresholds for a 500-Hz pure-tone signal were measured as a function of masking noise bandwidth in normal-hearing and cochlear hearing-impaired subjects. NoSo and NoS pi critical bands were derived from the bandlimited noise functions. A notched noise measure of the monaural critical band was also obtained for each ear. One hypothesis tested was that an asymmetrical monaural critical band would result in a relatively steep improvement of the NoS pi detection threshold as a function of decreasing masker bandwidth and would, therefore, be associated with a wider binaural critical band. This was hypothesized because the outputs of the left and right auditory filters would be more decorrelated the greater the interaural difference in the monaural critical band. However, as the noise bandwidth was narrowed, the decorrelation would lessen, resulting in a relatively steep improvement in NoS pi detection. Results indicated that the masking level difference (MLD) was smaller and that the monaural critical bands were generally wider in cochlear-impaired listeners. NoSo and NoS pi critical bands were somewhat larger in the cochlear hearing-impaired listeners having relatively wide monaural critical bands. There was a significant correlation between monaural critical band asymmetry and the NoS pi critical band; however, this correlation was insignificant when a control was employed for the critical band in the worse ear. Therefore, the present results did not support a strong association between monaural critical band asymmetry and the width of the NoS pi critical band.     

The binaural intelligibility level difference in hearing-impaired listeners: the role of supra-threshold deficits

Reduced binaural performance of hearing-impaired listeners may not only be caused by raised hearing thresholds (reduced audibility), but also by supra-threshold coding deficits in signal cues. This question was investigated in the present study using binaural intelligibility level difference (BILD) comparisons: the improvement of speech-reception threshold scores for N(0)S(π) relative to N(0)S(0) presentation conditions. Investigated was what types of supra-threshold deficits play a role in reducing BILDs in hearing-impaired subjects. BILDs were investigated for 25 mild to moderate sensorineural hearing-impaired listeners, under conditions where optimal audibility was assured. All stimuli were bandpass filtered (250-4000 Hz). A distortion-sensitivity approach was used to investigate the sensitivity of subjects BILDs to external stimulus perturbations in the phase, frequency, time, and intensity domains. The underlying assumption of this approach was that an auditory coding deficit occurring in a signal cue in a particular domain will result in a low sensitivity to external perturbations applied in that domain. Compared to reference data for listeners with normal BILDs, distortion-sensitivity data for a subgroup of eight listeners with reduced BILDs suggests that these reductions in BILD were caused by coding deficits in the phase and time domains.     

Effects of degradation of intensity, time, or frequency content on speech intelligibility for normal-hearing and hearing-impaired listeners

Many hearing-impaired listeners suffer from distorted auditory processing capabilities. This study examines which aspects of auditory coding (i.e., intensity, time, or frequency) are distorted and how this affects speech perception. The distortion-sensitivity model is used: The effect of distorted auditory coding of a speech signal is simulated by an artificial distortion, and the sensitivity of speech intelligibility to this artificial distortion is compared for normal-hearing and hearing-impaired listeners. Stimuli (speech plus noise) are wavelet coded using a complex sinusoidal carrier with a Gaussian envelope (1/4 octave bandwidth). Intensity information is distorted by multiplying the modulus of each wavelet coefficient by a random factor. Temporal and spectral information are distorted by randomly shifting the wavelet positions along the temporal or spectral axis, respectively. Measured were (1) detection thresholds for each type of distortion, and (2) speech-reception thresholds for various degrees of distortion. For spectral distortion, hearing-impaired listeners showed increased detection thresholds and were also less sensitive to the distortion with respect to speech perception. For intensity and temporal distortion, this was not observed. Results indicate that a distorted coding of spectral information may be an important factor underlying reduced speech intelligibility for the hearing impaired.     

Auditory filter asymmetry in the hearing impaired

Thresholds for 2-kHz sinusoidal signals were determined in the presence of a notched-noise masker, for six normal-hearing listeners and 12 listeners with cochlear hearing losses. Following Patterson and Nimmo Smith [J. Acoust. Soc. Am. 67, 229-245 (1980)], conditions were used where the notch was placed both symmetrically and asymmetrically about the signal frequency. The auditory filter shape for both the low- and high-frequency side of the filter was calculated using the rounded-exponential form of the filter. In six hearing-impaired listeners, the auditory filter shape showed a shallow low-frequency skirt indicating pronounced susceptibility to the upward spread of masking. In two hearing-impaired listeners, the filter shape showed a shallow high-frequency skirt, indicating pronounced susceptibility to the downward spread of masking. Two other listeners with mild threshold losses had steeper and more symmetric filters than normal, suggesting either a small conductive loss or an attenuation factor of sensorineural origin not associated with a degradation of frequency resolution. In the remaining two listeners, the auditory filter had too little selectivity for its shape to be reliably determined.