Temporal resolution in normal-hearing and hearing-impaired listeners using frequency-modulated stimuli.

This study compares the temporal resolution of frequency-modulated sinusoids by normal-hearing and hearing-impaired subjects in a discrimination task. One signal increased linearly by 200 Hz in 50 msec. The other was identical except that its trajectory followed a series of discrete steps. Center frequencies were 500, 1000, 2000, and 4000 Hz. As the number of steps was increased, the duration of the individual steps decreased, and the subjects' discrimination performance monotonically decreased to chance. It was hypothesized that the listeners could not temporally resolve the trajectory of the step signals at short step durations. At equal sensation levels, and at equal sound pressure levels, temporal resolution was significantly reduced for the impaired subjects. The difference between groups was smaller in the equal sound pressure level condition. Performance was much poorer at 4000 Hz than at the other test frequencies in all conditions because of poorer frequency discrimination at that frequency.     

Informational masking in normal-hearing and hearing-impaired listeners

The present study aimed to test whether central, across-channel, informational auditory processing abilities are altered by hearing loss. The informational masking effect exerted on a 1 kHz tone-pip by a simultaneous four-tone masker, whose spectral content changed within as well as across trials, was measured in the left and right ears of normal-hearing subjects and hearing-impaired subjects with either symmetrical or asymmetrical hearing loss between the two ears. In the subjects with normal-hearing or symmetrical hearing loss, the level of the masker was set to 40 dB SL in each ear, in the subjects with asymmetrical hearing loss, the masker was set to 40 dB SL in the best ear and loudness-balanced in the other ear. The results failed to reveal significant differences in informational masking between normal-hearing and hearing-impaired subjects. However, in subjects with asymmetric hearing loss, less informational masking was observed in the ear with the more elevated absolute thresholds than in the opposite ear. Since the latter finding can be explained in terms of across-ear differences in loudness recruitment, it is suggested that central, across-channel, informational processing abilities are not substantially different in hearing-impaired than in normal-hearing ears.     

A comparison of psychometric functions for detection in normal-hearing and hearing-impaired listeners

Psychometric functions (PFD) for the detection of pure tones were obtained with a two-interval forced-choice procedure from a group of listeners with normal hearing and a group of listeners with sensorineural impairments of presumed cochlear origin. Five PFDs were obtained for each group at each of the four test frequencies (500, 2000, 4000, and 8000 Hz). The slopes of PFDs were abnormally steep in some of the hearing-impaired listeners, but were statistically significant only at 2000 Hz.     

Temporal loudness integration and spectral loudness summation in normal-hearing and hearing-impaired listeners.

The aim of this study was to test for differences between normal-hearing and hearing-impaired listeners regarding two fundamental aspects of intensity perception: loudness integration and loudness summation. Loudness functions for three different stimuli were measured using categorical loudness scaling in 8 normal-hearing and 12 hearing-impaired subjects. The results indicated that temporal loudness integration, defined as the difference in SPL between 16.25-ms and 300-ms noise bursts of equal loudness, was larger in the hearing-impaired than in the normal-hearing listeners. Loudness summation, defined as the difference in SPL between a 300-ms, 1,600-Hz tone pip and a white noise burst of the same duration and loudness, did not differ between the two groups. Implications of these results for hearing aid fitting strategies based on loudness normalization are discussed.     

Binaural pitch perception in normal-hearing and hearing-impaired listeners

The effects of hearing impairment on the perception of binaural-pitch stimuli were investigated. Several experiments were performed with normal-hearing and hearing-impaired listeners, including detection and discrimination of binaural pitch, and melody recognition using different types of binaural pitches. For the normal-hearing listeners, all types of binaural pitches could be perceived immediately and were musical. The hearing-impaired listeners could be divided into three groups based on their results: (a) some perceived all types of binaural pitches, but with decreased salience or musicality compared to normal-hearing listeners; (b) some could only perceive the strongest pitch types; (c) some were unable to perceive any binaural pitch at all. The performance of the listeners was not correlated with audibility. Additional experiments investigated the correlation between performance in binaural-pitch perception and performance in measures of spectral and temporal resolution. Reduced frequency discrimination appeared to be linked to poorer melody recognition skills. Reduced frequency selectivity was also found to impede the perception of binaural-pitch stimuli. Overall, binaural-pitch stimuli might be very useful tools within clinical diagnostics for detecting specific deficiencies in the auditory system.     

Identification of envelope-expanded sentences in normal-hearing and hearing-impaired listeners

The present study examined the effects of temporal-envelope expansion on speech perception. Sentence identification performance was measured in normal-hearing and hearing-impaired listeners for stationary and fluctuating noise. Two expansion schemes were used to increase the depth of the slow (< 16 Hz) and fast (< 256 Hz) amplitude fluctuations of the stimuli. In the first scheme, the envelope of the stimuli was raised to the power 2. In the second scheme, the high- and low-level segments of the envelope were compressed and expanded, respectively. When envelope processing was applied to speech before the addition of noise, the first form of expansion generally degraded identification, while the second form generally improved identification. When envelope processing was applied to speech after the addition of noise, both forms of expansion improved or did not affect identification scores in both groups of listeners when applied to the slowest fluctuations. When applied to the broadest range of fluctuations, both forms of expansion degraded identification. However, in hearing-impaired listeners, the second form of expansion yielded an increase in performance for fluctuating noise. This complex pattern of results will be discussed in light of previous studies on envelope expansion.     

Temporal summation in hearing-impaired listeners

A study was conducted to explore variations in auditory temporal summation in listeners with normal hearing, and impairment due to otosclerosis, sensori-neural hearing loss and acoustic neuroma. Using a two-interval forced-choice procedure the detection threshold was measured for one-third octave noise bands centered at either 1000 or 4000 Hz, in combination with eight signal durations (2.5, 5, 10, 20, 40, 80, 160, and 640 ms). The results indicated that for normal listeners: (1) the slope of the function relating the detection threshold and the signal duration varied inversely with the frequency tested, and (2) the variability in the detection threshold was greater for 4000 Hz than for 1000 Hz. A comparison of performance across groups showed that the magnitude of the slope of the temporal integration function decreased as the site of lesion moved from middle ear to eighth nerve. For listeners with normal hearing and those with otosclerosis, temporal integration appeared to be incomplete at 640 ms.     

Intelligibility of synthetic speech for normal-hearing and hearing-impaired listeners

This study examined the perceived intelligibility of synthetic speech. Participants were adults aged 49-69, one group with normal hearing and one group with acquired sensorineural hearing impairment. Word lists were presented in two speech types: DECtalk (a high-quality speech synthesizer) and a natural male speaker. Results revealed differences between groups, with normal-hearing listeners scoring higher than hearing-impaired listeners, and between speech types, with higher scores for natural speech than for synthesized speech. There was no significant interaction of hearing level and speech type.     

Temporal masking curves for hearing-impaired listeners

The decay of forward masking was investigated for three subjects with moderate sensorineural hearing loss. For such subjects, compression on the basilar membrane (BM) is thought to be largely absent, enabling one to determine the decay of masking without the influence of compression. Temporal masking curves (TMCs), plots of the masker level at threshold against delay between masker offset and signal onset, were measured for delays of 0, 15, 30, 45, 60, and 75 ms, for signal frequencies, fs, of 500, 1000, 2000, 4000, and 6000 Hz. Masker frequencies were 0.5, 0.8, 1.0, 1.15, and 1.3 times fs. Most of the TMCs were well fitted with single-segment straight lines, which, except for high masker levels, were roughly parallel for each fs, supporting the belief that BM compression was largely absent in these subjects. However, the slopes of the TMCs were greater for fs = 500 and 1000 Hz than for higher frequencies, which may indicate that the decay of forward masking is not the same for all signal frequencies. The results suggest that it may not be valid to infer BM compression at low signal frequencies by using a reference TMC for a high fs.     

Auditory time constants for off-frequency forward masking in normal-hearing and hearing-impaired listeners

Previous research has shown that frequency-specific estimates of auditory time constants for recovery from short-term adaptation can be made using a fixed-probe forward-masking procedure (Nelson & Freyman, 1987) if the masker and the probe stimuli are at the same frequency. This study examines the validity of time-constant estimates for off-frequency forward-masking conditions in which the masker frequency is below (900 Hz) or above (1100 Hz) the probe frequency (1000 Hz). Fixed-probe-level temporal masking functions were obtained from four normal-hearing and four hearing-impaired listeners. Auditory time constants were estimated with iterative least-squares procedures to derive parameter values for an exponential model of recovery from forward masking. After appropriate corrections were made for attenuation to the maskers provided by the auditory filter centered at the probe frequency, recovery from forward masking produced by either off-frequency or on-frequency maskers could be described by a single time constant. That time constant was around 50 ms in normal-hearing listeners and was larger in those hearing-impaired listeners who demonstrated moderate hearing loss at the probe frequency.     

Detection and recognition of stop consonants by normal-hearing and hearing-impaired listeners.

This study examined the possibility that hearing-impaired listeners, in addition to displaying poorer-than-normal recognition of speech presented in background noise, require a larger signal-to-noise ratio for the detection of the speech sounds. Psychometric functions for the detection and recognition of stop consonants were obtained from both normal-hearing and hearing-impaired listeners. Expressing the speech levels in terms of their short-term spectra, the detection of consonants for both subject groups occurred at the same signal-to-noise ratio. In contrast, the hearing-impaired listeners displayed poorer recognition performance than the normal-hearing listeners. These results imply that the higher signal-to-noise ratios required for a given level of recognition by some subjects with hearing loss are not due in part to a deficit in detection of the signals in the masking noise, but rather are due exclusively to a deficit in recognition.     

High-level psychophysical tuning curves: forward masking in normal-hearing and hearing-impaired listeners.

Forward-masked psychophysical tuning curves (PTCs) were obtained for 1000-Hz probe tones at multiple probe levels from one ear of 26 normal-hearing listeners and from 24 ears of 21 hearing-impaired listeners with cochlear hearing loss. Comparisons between normal-hearing and hearing-impaired PTCs were made at equivalent masker levels near the tips of PTCs. Comparisons were also made of PTC characteristics obtained by fitting each PTC with three straight-line segments using least-squares fitting procedures. Abnormal frequency resolution was revealed only as abnormal downward spread of masking. The low-frequency slopes of PTCs from hearing-impaired listeners were not different from those of normal-hearing listeners. That is, hearing-impaired listeners did not demonstrate abnormal upward spread of masking when equivalent masker levels were compared. Ten hearing-impaired ears demonstrated abnormally broad PTCs, due exclusively to reduced high-frequency slopes in their PTCs. This abnormal downward spread of masking was observed only in listeners with hearing losses greater than 40 dB HL. From these results, it would appear that some, but not all, cochlear hearing losses greater than 40 dB HL influence the sharp tuning capabilities usually associated with outer hair cell function.     

Effects of lexical factors on word recognition among normal-hearing and hearing-impaired listeners

An investigation was conducted to examine the effects of lexical difficulty on spoken word recognition among young normal-hearing and middle-aged and older listeners with hearing loss. Two word lists, based on the lexical characteristics of word frequency and neighborhood density and frequency (Neighborhood Activation Model [NAM]), were developed: (1) lexically "easy" words with high word frequency and a low number and frequency of words phonemically similar to the target word and (2) lexically "hard" words with low word frequency and a high number and frequency of words phonemically similar to the target word. Simple and transformed up-down adaptive strategies were used to estimate performance levels at several locations on the performance-intensity functions of the words. The results verified predictions of the NAM and showed that easy words produced more favorable performance levels than hard words at an equal intelligibility. Although the slopes of the performance-intensity function for the hearing-impaired listeners were less steep than those of normal-hearing listeners, the effects of lexical difficulty on performance were similar for both groups.     

Recognition of digits in different types of noise by normal-hearing and hearing-impaired listeners

The objective of the study was to examine the ability to understand digits in different types of noise. Adaptive speech-in-noise tests were developed that measure the speech-reception-threshold (SRTn) i.e. signal-to-noise ratio that corresponds to 50% intelligibility. Digits were presented in continuous noise, 16-Hz interrupted noise, and 32-Hz interrupted noise. Also the standard Dutch triplet SRTn test in continuous noise was included. Results for forty-two ears of normal-hearing and hearing-impaired adult participants are presented. The ratio between the standard deviation in SRTn values between subjects and the measurement error determines the efficiency of the tests. A high efficiency could be achieved by using triplets instead of digits, or by using 16-Hz interrupted noise instead of continuous noise, because this resulted in a large spread in SRTn values. The simple calculation method of averaging presentation levels was highly efficient. The digit SRTn test in 16-Hz interrupted noise was very efficient in discriminating between normal-hearing listeners and hearing-impaired listeners, and might be used to screen for hearing loss as measured by pure-tone audiometry.     

Test of auditory comprehension: a comparison of hearing-impaired and normal-hearing listeners

Normal-hearing (NH) children (N:35) aged 9-11 yrs in regular classrooms were given all 10 subtests of the Test of Auditory Comprehension (TAC) (Trammel, 1976). Group means of raw scores, measures of dispersion, and of internal consistency, alpha, and of test-retest reliability, r, were computed and are presented here for the first time for NH Ss. These data were compared with TAC norms published for a group of 750 hearing-impaired (HI) children with moderate to severe sensorineural hearing loss. Group means for the NH Ss were substantially higher (p less than or equal to .05) on 9 of the 10 subtests than for the HI Ss, but the dispersion was always substantially greater. Furthermore, estimates of alpha and of r were greater on 8 of the 10 subtests for alpha and on 8 for r, the 8 tests not altogether overlapping. The implication is that different auditory processing information may be used by the two different populations, generally supporting the use in the clinic of only those tests specifically designed and demonstrated to be applicable for specific populations.     

Recognition of digits in different types of noise by normal-hearing and hearing-impaired listeners

The objective of the study was to examine the ability to understand digits in different types of noise. Adaptive speech-in-noise tests were developed that measure the speech-reception-threshold (SRT_n) i.e. signal-to-noise ratio that corresponds to 50% intelligibility. Digits were presented in continuous noise, 16-Hz interrupted noise, and 32-Hz interrupted noise. Also the standard Dutch triplet SRT_n test in continuous noise was included. Results for forty-two ears of normal-hearing and hearing-impaired adult participants are presented. The ratio between the standard deviation in SRT_n values between subjects and the measurement error determines the efficiency of the tests. A high efficiency could be achieved by using triplets instead of digits, or by using 16-Hz interrupted noise instead of continuous noise, because this resulted in a large spread in SRT_n values. The simple calculation method of averaging presentation levels was highly efficient. The digit SRT_n test in 16-Hz interrupted noise was very efficient in discriminating between normal-hearing listeners and hearing-impaired listeners, and might be used to screen for hearing loss as measured by pure-tone audiometry.     

Temporal envelope expansion of speech in noise for normal-hearing and hearing-impaired listeners: effects on identification performance and response times

The effects of expanding the temporal envelope of speech sounds on speech identification in noise were investigated in normal-hearing and hearing-impaired listeners. Expansion was performed by applying a power-law transformation to the low-frequency temporal modulations (<500 Hz) of vowel-consonant-vowel logatomes presented against a background noise. Stimuli were spectrally degraded, allowing a direct examination of the perceptual effects induced by the modification of the temporal envelope alone on speech reception. This study extended a previous study conducted by Lorenzi et al. [1999. Hear. Res. 136, 131--138] by measuring the effects of envelope expansion on both identification performance and response times in normal-hearing and hearing-impaired listeners. Overall, the results show that temporal expansion yields only small improvements in identification scores (approximately 5%) in normal-hearing listeners. No effect of expansion on identification scores was observed in hearing-impaired listeners. On the other hand, the results show that expansion led to a significant decrease in response times in both normal-hearing and hearing-impaired listeners. The average benefit from expansion was about 65 ms in both groups. These results suggest that expanding the temporal envelope of speech sounds presented in noise may improve 'ease of listening' in both normal-hearing and hearing-impaired listeners.     

Effects of stimulus presentation level on stop consonant identification in normal-hearing and hearing-impaired listeners

The purpose of the present study was to determine whether varying the presentation level of stop consonant stimuli resulted in similar phonetic boundary shifts for listeners with normal and impaired hearing. Sixteen normal-hearing and 16 hearing-impaired listeners categorized synthetic speech stimuli as /b/, /d/, or /g/. The onset frequency of F2 varied from 900 to 2300 Hz (100-Hz steps), and the presentation level varied from 92 to 62 dB SPL (10-dB steps) for each stimulus presentation. Hearing-impaired listeners had significantly more missing boundary values than normal-hearing listeners; however, the correlation between the number of missing boundary values and hearing sensitivity was not significant. Comparison of boundary shift with level demonstrated that hearing-impaired listeners had a smaller boundary shift with increasing level than normal-hearing listeners. The amount of boundary shift was not correlated with audibility. The results of the current study suggest that increasing the presentation level of a signal does not result in performance similar to that of listeners with normal hearing.     

Effect of training on word-recognition performance in noise for young normal-hearing and older hearing-impaired listeners

OBJECTIVE: The objective of this study was to evaluate the effectiveness of a training program for hearing-impaired listeners to improve their speech-recognition performance within a background noise when listening to amplified speech. Both noise-masked young normal-hearing listeners, used to model the performance of elderly hearing-impaired listeners, and a group of elderly hearing-impaired listeners participated in the study. Of particular interest was whether training on an isolated word list presented by a standardized talker can generalize to everyday speech communication across novel talkers. DESIGN: Word-recognition performance was measured for both young normal-hearing (n = 16) and older hearing-impaired (n = 7) adults. Listeners were trained on a set of 75 monosyllabic words spoken by a single female talker over a 9- to 14-day period. Performance for the familiar (trained) talker was measured before and after training in both open-set and closed-set response conditions. Performance on the trained words of the familiar talker were then compared with those same words spoken by three novel talkers and to performance on a second set of untrained words presented by both the familiar and unfamiliar talkers. The hearing-impaired listeners returned 6 mo after their initial training to examine retention of the trained words as well as their ability to transfer any knowledge gained from word training to sentences containing both trained and untrained words. RESULTS: Both young normal-hearing and older hearing-impaired listeners performed significantly better on the word list in which they were trained versus a second untrained list presented by the same talker. Improvements on the untrained words were small but significant, indicating some generalization to novel words. The large increase in performance on the trained words, however, was maintained across novel talkers, pointing to the listener's greater focus on lexical memorization of the words rather than a focus on talker-specific acoustic characteristics. On return in 6 mo, listeners performed significantly better on the trained words relative to their initial baseline performance. Although the listeners performed significantly better on trained versus untrained words in isolation, once the trained words were embedded in sentences, no improvement in recognition over untrained words within the same sentences was shown. CONCLUSIONS: Older hearing-impaired listeners were able to significantly improve their word-recognition abilities through training with one talker and to the same degree as young normal-hearing listeners. The improved performance was maintained across talkers and across time. This might imply that training a listener using a standardized list and talker may still provide benefit when these same words are presented by novel talkers outside the clinic. However, training on isolated words was not sufficient to transfer to fluent speech for the specific sentence materials used within this study. Further investigation is needed regarding approaches to improve a hearing aid user's speech understanding in everyday communication situations.     

Frequency discrimination as a function of signal frequency and level in normal-hearing and hearing-impaired listeners.

Frequency difference limens (DLFs) for pure tones were obtained over a wide range of frequencies and levels from 7 normal-hearing subjects and 16 ears of 12 listeners with sensorineural hearing losses. The normal data were fitted with a general prediction equation. Variability of the data around the DLFs estimated by the equation was quantified and used to evaluate the DLFs from the hearing-impaired listeners. The majority of DLFs from impaired listeners were poorer than one standard deviation above the estimates of the normal equation at all frequencies and sensation levels (SLs). The portion of the equation concerned with sensation level was fitted to each listener's data at each frequency. The slopes of these functions indicated that, on average, the rate of improvement of the DLF with sensation level was similar in the two groups of subjects. These results suggest that it would be reasonable to compare DLFs from normal-hearing and hearing-impaired listeners at equivalent sensation levels. The intercepts of the DLF-intensity functions represent asymptotic values obtained at high SLs. These asymptotic DLFs were abnormal in the majority of hearing-impaired subjects, with more than half the data in excess of two standard deviations above normal. However, among those subjects, the correlation between the DLF deficit and the amount of hearing loss at the test frequency was not strong (r = +.27).