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 effect of hearing impairment on the identification of speech that is modulated synchronously or asynchronously across frequency

This study investigated the effect of mild-to-moderate sensorineural hearing loss on the ability to identify speech in noise for vowel-consonant-vowel tokens that were either unprocessed, amplitude modulated synchronously across frequency, or amplitude modulated asynchronously across frequency. One goal of the study was to determine whether hearing-impaired listeners have a particular deficit in the ability to integrate asynchronous spectral information in the perception of speech. Speech tokens were presented at a high, fixed sound level and the level of a speech-shaped noise was changed adaptively to estimate the masked speech identification threshold. The performance of the hearing-impaired listeners was generally worse than that of the normal-hearing listeners, but the impaired listeners showed particularly poor performance in the synchronous modulation condition. This finding suggests that integration of asynchronous spectral information does not pose a particular difficulty for hearing-impaired listeners with mild/moderate hearing losses. Results are discussed in terms of common mechanisms that might account for poor speech identification performance of hearing-impaired listeners when either the masking noise or the speech is synchronously modulated.     

A comparison of monotic and dichotic complex-tone pitch perception in listeners with hearing loss.

The perception of fundamental pitch for two-harmonic complex tones was examined in musically experienced listeners with cochlear-based high-frequency hearing loss. Performance in a musical interval identification task was measured as a function of the average rank of the lowest harmonic for both monotic and dichotic presentation of the harmonics at 14 dB Sensation Level. Listeners with hearing loss demonstrated excellent musical interval identification at low fundamental frequencies and low harmonic numbers, but abnormally poor identification at higher fundamental frequencies and higher average ranks. The upper frequency limit of performance in the listeners with hearing loss was similar in both monotic and dichotic conditions. These results suggest that something other than frequency resolution per se limits complex-tone pitch perception in listeners with hearing loss.     

Pitch strength and pitch dominance of iterated rippled noises in hearing-impaired listeners.

Reports using a variety of psychophysical tasks indicate that pitch perception by hearing-impaired listeners may be abnormal, contributing to difficulties in understanding speech and enjoying music. Pitches of complex sounds may be weaker and more indistinct in the presence of cochlear damage, especially when frequency regions are affected that form the strongest basis for pitch perception in normal-hearing listeners. In this study, the strength of the complex pitch generated by iterated rippled noise was assessed in normal-hearing and hearing-impaired listeners. Pitch strength was measured for broadband noises with spectral ripples generated by iteratively delaying a copy of a given noise and adding it back into the original. Octave-band-pass versions of these noises also were evaluated to assess frequency dominance regions for rippled-noise pitch. Hearing-impaired listeners demonstrated consistently weaker pitches in response to the rippled noises relative to pitch strength in normal-hearing listeners. However, in most cases, the frequency regions of pitch dominance, i.e., strongest pitch, were similar to those observed in normal-hearing listeners. Except where there exists a substantial sensitivity loss, contributions from normal pitch dominance regions associated with the strongest pitches may not be directly related to impaired spectral processing. It is suggested that the reduced strength of rippled-noise pitch in listeners with hearing loss results from impaired frequency resolution and possibly an associated deficit in temporal processing.     

Age-related changes in talker recognition with reduced spectral cues

Temporal information provided by cochlear implants enables successful speech perception in quiet, but limited spectral information precludes comparable success in voice perception. Talker identification and speech decoding by young hearing children (5-7 yr), older hearing children (10-12 yr), and hearing adults were examined by means of vocoder simulations of cochlear implant processing. In Experiment 1, listeners heard vocoder simulations of sentences from a man, woman, and girl and were required to identify the talker from a closed set. Younger children identified talkers more poorly than older listeners, but all age groups showed similar benefit from increased spectral information. In Experiment 2, children and adults provided verbatim repetition of vocoded sentences from the same talkers. The youngest children had more difficulty than older listeners, but all age groups showed comparable benefit from increasing spectral resolution. At comparable levels of spectral degradation, performance on the open-set task of speech decoding was considerably more accurate than on the closed-set task of talker identification. Hearing children's ability to identify talkers and decode speech from spectrally degraded material sheds light on the difficulty of these domains for child implant users.     

Understanding speech in modulated interference: cochlear implant users and normal-hearing listeners

Many competing noises in real environments are modulated or fluctuating in level. Listeners with normal hearing are able to take advantage of temporal gaps in fluctuating maskers. Listeners with sensorineural hearing loss show less benefit from modulated maskers. Cochlear implant users may be more adversely affected by modulated maskers because of their limited spectral resolution and by their reliance on envelope-based signal-processing strategies of implant processors. The current study evaluated cochlear implant users' ability to understand sentences in the presence of modulated speech-shaped noise. Normal-hearing listeners served as a comparison group. Listeners repeated IEEE sentences in quiet, steady noise, and modulated noise maskers. Maskers were presented at varying signal-to-noise ratios (SNRs) at six modulation rates varying from 1 to 32 Hz. Results suggested that normal-hearing listeners obtain significant release from masking from modulated maskers, especially at 8-Hz masker modulation frequency. In contrast, cochlear implant users experience very little release from masking from modulated maskers. The data suggest, in fact, that they may show negative effects of modulated maskers at syllabic modulation rates (2-4 Hz). Similar patterns of results were obtained from implant listeners using three different devices with different speech-processor strategies. The lack of release from masking occurs in implant listeners independent of their device characteristics, and may be attributable to the nature of implant processing strategies and/or the lack of spectral detail in processed stimuli.     

Temporal modulation transfer functions obtained using sinusoidal carriers with normally hearing and hearing-impaired listeners

Temporal modulation transfer functions were obtained using sinusoidal carriers for four normally hearing subjects and three subjects with mild to moderate cochlear hearing loss. Carrier frequencies were 1000, 2000 and 5000 Hz, and modulation frequencies ranged from 10 to 640 Hz in one-octave steps. The normally hearing subjects were tested using levels of 30 and 80 dB SPL. For the higher level, modulation detection thresholds varied only slightly with modulation frequency for frequencies up to 80 Hz, but decreased for high modulation frequencies. The decrease can be attributed to the detection of spectral sidebands. For the lower level, thresholds varied little with modulation frequency for all three carrier frequencies. The absence of a decrease in the threshold for large modulation frequencies can be explained by the low sensation level of the spectral sidebands. The hearing-impaired subjects were tested at 80 dB SPL, except for two cases where the absolute threshold at the carrier frequency was greater than 70 dB SPL; in these cases a level of 90 dB was used. The results were consistent with the idea that spectral sidebands were less detectable for the hearing-impaired than for the normally hearing subjects. For the two lower carrier frequencies, there were no large decreases in threshold with increasing modulation frequency, and where decreases did occur, this happened only between 320 and 640 Hz. For the 5000-Hz carrier, thresholds were roughly constant for modulation frequencies from 10 to 80 or 160 Hz, and then increased monotonically, becoming unmeasurable at 640 Hz. The results for this carrier may reflect "pure" effects of temporal resolution, without any influence from the detection of spectral sidebands. The results suggest that temporal resolution for deterministic stimuli is similar for normally hearing and hearing-impaired 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.     

The relationship between frequency selectivity and pitch discrimination: sensorineural hearing loss

This study tested the relationship between frequency selectivity and the minimum spacing between harmonics necessary for accurate fo discrimination. Fundamental frequency difference limens (fo DLs) were measured for ten listeners with moderate sensorineural hearing loss (SNHL) and three normal-hearing listeners for sine- and random-phase harmonic complexes, bandpass filtered between 1500 and 3500 Hz, with fo's ranging from 75 to 500 Hz (or higher). All listeners showed a transition between small (good) fo DLs at high fo's and large (poor) fo DLs at low fo's, although the fo at which this transition occurred (fo,tr) varied across listeners. Three measures thought to reflect frequency selectivity were significantly correlated to both the fo,tr and the minimum fo DL achieved at high fo's: (1) the maximum fo for which fo DLs were phase dependent, (2) the maximum modulation frequency for which amplitude modulation and quasi-frequency modulation were discriminable, and (3) the equivalent rectangular bandwidth of the auditory filter, estimated using the notched-noise method. These results provide evidence of a relationship between fo discrimination performance and frequency selectivity in listeners with SNHL, supporting "spectral" and "spectro-temporal" theories of pitch perception that rely on sharp tuning in the auditory periphery to accurately extract fo information.     

Spectral peak resolution and speech recognition in quiet: normal hearing, hearing impaired, and cochlear implant listeners

Spectral peak resolution was investigated in normal hearing (NH), hearing impaired (HI), and cochlear implant (CI) listeners. The task involved discriminating between two rippled noise stimuli in which the frequency positions of the log-spaced peaks and valleys were interchanged. The ripple spacing was varied adaptively from 0.13 to 11.31 ripples/octave, and the minimum ripple spacing at which a reversal in peak and trough positions could be detected was determined as the spectral peak resolution threshold for each listener. Spectral peak resolution was best, on average, in NH listeners, poorest in CI listeners, and intermediate for HI listeners. There was a significant relationship between spectral peak resolution and both vowel and consonant recognition in quiet across the three listener groups. The results indicate that the degree of spectral peak resolution required for accurate vowel and consonant recognition in quiet backgrounds is around 4 ripples/octave, and that spectral peak resolution poorer than around 1-2 ripples/octave may result in highly degraded speech recognition. These results suggest that efforts to improve spectral peak resolution for HI and CI users may lead to improved speech recognition.     

Spectral integration of speech bands in normal-hearing and hearing-impaired listeners

This investigation examined whether listeners with mild-moderate sensorineural hearing impairment have a deficit in the ability to integrate synchronous spectral information in the perception of speech. In stage 1, the bandwidth of filtered speech centered either on 500 or 2500 Hz was varied adaptively to determine the width required for approximately 15%-25% correct recognition. In stage 2, these criterion bandwidths were presented simultaneously and percent correct performance was determined in fixed block trials. Experiment 1 tested normal-hearing listeners in quiet and in masking noise. The main findings were (1) there was no correlation between the criterion bandwidths at 500 and 2500 Hz; (2) listeners achieved a high percent correct in stage 2 (approximately 80%); and (3) performance in quiet and noise was similar. Experiment 2 tested listeners with mild-moderate sensorineural hearing impairment. The main findings were (1) the impaired listeners showed high variability in stage 1, with some listeners requiring narrower and others requiring wider bandwidths than normal, and (2) hearing-impaired listeners achieved percent correct performance in stage 2 that was comparable to normal. The results indicate that listeners with mild-moderate sensorineural hearing loss do not have an essential deficit in the ability to integrate across-frequency speech information.     

Effects of noise and distortion on speech quality judgments in normal-hearing and hearing-impaired listeners

Noise and distortion reduce speech intelligibility and quality in audio devices such as hearing aids. This study investigates the perception and prediction of sound quality by both normal-hearing and hearing-impaired subjects for conditions of noise and distortion related to those found in hearing aids. Stimuli were sentences subjected to three kinds of distortion (additive noise, peak clipping, and center clipping), with eight levels of degradation for each distortion type. The subjects performed paired comparisons for all possible pairs of 24 conditions. A one-dimensional coherence-based metric was used to analyze the quality judgments. This metric was an extension of a speech intelligibility metric presented in Kates and Arehart (2005) [J. Acoust. Soc. Am. 117, 2224-2237] and is based on dividing the speech signal into three amplitude regions, computing the coherence for each region, and then combining the three coherence values across frequency in a calculation based on the speech intelligibility index. The one-dimensional metric accurately predicted the quality judgments of normal-hearing listeners and listeners with mild-to-moderate hearing loss, although some systematic errors were present. A multidimensional analysis indicates that several dimensions are needed to describe the factors used by subjects to judge the effects of the three distortion types.     

Relationship between perception of spectral ripple and speech recognition in cochlear implant and vocoder listeners

Spectral resolution has been reported to be closely related to vowel and consonant recognition in cochlear implant (CI) listeners. One measure of spectral resolution is spectral modulation threshold (SMT), which is defined as the smallest detectable spectral contrast in the spectral ripple stimulus. SMT may be determined by the activation pattern associated with electrical stimulation. In the present study, broad activation patterns were simulated using a multi-band vocoder to determine if similar impairments in speech understanding scores could be produced in normal-hearing listeners. Tokens were first decomposed into 15 logarithmically spaced bands and then re-synthesized by multiplying the envelope of each band by matched filtered noise. Various amounts of current spread were simulated by adjusting the drop-off of the noise spectrum away from the peak (40-5 dBoctave). The average SMT (0.25 and 0.5 cyclesoctave) increased from 6.3 to 22.5 dB, while average vowel identification scores dropped from 86% to 19% and consonant identification scores dropped from 93% to 59%. In each condition, the impairments in speech understanding were generally similar to those found in CI listeners with similar SMTs, suggesting that variability in spread of neural activation largely accounts for the variability in speech perception of CI listeners.     

Factors affecting masking release for speech in modulated noise for normal-hearing and hearing-impaired listeners

The Speech Reception Threshold for sentences in stationary noise and in several amplitude-modulated noises was measured for 8 normal-hearing listeners, 29 sensorineural hearing-impaired listeners, and 16 normal-hearing listeners with simulated hearing loss. This approach makes it possible to determine whether the reduced benefit from masker modulations, as often observed for hearing-impaired listeners, is due to a loss of signal audibility, or due to suprathreshold deficits, such as reduced spectral and temporal resolution, which were measured in four separate psychophysical tasks. Results show that the reduced masking release can only partly be accounted for by reduced audibility, and that, when considering suprathreshold deficits, the normal effects associated with a raised presentation level should be taken into account. In this perspective, reduced spectral resolution does not appear to qualify as an actual suprathreshold deficit, while reduced temporal resolution does. Temporal resolution and age are shown to be the main factors governing masking release for speech in modulated noise, accounting for more than half of the intersubject variance. Their influence appears to be related to the processing of mainly the higher stimulus frequencies. Results based on calculations of the Speech Intelligibility Index in modulated noise confirm these conclusions.     

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.     

Relating binaural pitch perception to the individual listener's auditory profile

The ability of eight normal-hearing listeners and fourteen listeners with sensorineural hearing loss to detect and identify pitch contours was measured for binaural-pitch stimuli and salience-matched monaurally detectable pitches. In an effort to determine whether impaired binaural pitch perception was linked to a specific deficit, the auditory profiles of the individual listeners were characterized using measures of loudness perception, cognitive ability, binaural processing, temporal fine structure processing, and frequency selectivity, in addition to common audiometric measures. Two of the listeners were found not to perceive binaural pitch at all, despite a clear detection of monaural pitch. While both binaural and monaural pitches were detectable by all other listeners, identification scores were significantly lower for binaural than for monaural pitch. A total absence of binaural pitch sensation coexisted with a loss of a binaural signal-detection advantage in noise, without implying reduced cognitive function. Auditory filter bandwidths did not correlate with the difference in pitch identification scores between binaural and monaural pitches. However, subjects with impaired binaural pitch perception showed deficits in temporal fine structure processing. Whether the observed deficits stemmed from peripheral or central mechanisms could not be resolved here, but the present findings may be useful for hearing loss characterization.     

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.     

The resolution of complex spectral patterns by cochlear implant and normal-hearing listeners

The differences in spectral shape resolution abilities among cochlear implant (CI) listeners, and between CI and normal-hearing (NH) listeners, when listening with the same number of channels (12), was investigated. In addition, the effect of the number of channels on spectral shape resolution was examined. The stimuli were rippled noise signals with various ripple frequency-spacings. An adaptive 41FC procedure was used to determine the threshold for resolvable ripple spacing, which was the spacing at which an interchange in peak and valley positions could be discriminated. The results showed poorer spectral shape resolution in CI compared to NH listeners (average thresholds of approximately 3000 and 400 Hz, respectively), and wide variability among CI listeners (range of approximately 800 to 8000 Hz). There was a significant relationship between spectral shape resolution and vowel recognition. The spectral shape resolution thresholds of NH listeners increased as the number of channels increased from 1 to 16, while the CI listeners showed a performance plateau at 4-6 channels, which is consistent with previous results using speech recognition measures. These results indicate that this test may provide a measure of CI performance which is time efficient and non-linguistic, and therefore, if verified, may provide a useful contribution to the prediction of speech perception in adults and children who use CIs.     

Vowel perception by noise masked normal-hearing young adults

This study examined vowel perception by young normal-hearing (YNH) adults, in various listening conditions designed to simulate mild-to-moderate sloping sensorineural hearing loss. YNH listeners were individually age- and gender-matched to young hearing-impaired (YHI) listeners tested in a previous study [Richie et al., J. Acoust. Soc. Am. 114, 2923-2933 (2003)]. YNH listeners were tested in three conditions designed to create equal audibility with the YHI listeners; a low signal level with and without a simulated hearing loss, and a high signal level with a simulated hearing loss. Listeners discriminated changes in synthetic vowel tokens /I e epsilon alpha ae/ when Fl or F2 varied in frequency. Comparison of YNH with YHI results failed to reveal significant differences between groups in terms of performance on vowel discrimination, in conditions of similar audibility by using both noise masking to elevate the hearing thresholds of the YNH and applying frequency-specific gain to the YHI listeners. Further, analysis of learning curves suggests that while the YHI listeners completed an average of 46% more test blocks than YNH listeners, the YHI achieved a level of discrimination similar to that of the YNH within the same number of blocks. Apparently, when age and gender are closely matched between young hearing-impaired and normal-hearing adults, performance on vowel tasks may be explained by audibility alone.     

Spectral weighting strategies for hearing-impaired listeners measured using a correlational method

Spectral weighting strategies using a correlational method [R. A. Lutfi, J. Acoust. Soc. Am. 97, 1333-1334 (1995); V. M. Richards and S. Zhu, J. Acoust. Soc. Am. 95, 423-424 (1994)] were measured in ten listeners with sensorineural-hearing loss on a sentence recognition task. Sentences and a spectrally matched noise were filtered into five separate adjacent spectral bands and presented to listeners at various signal-to-noise ratios (SNRs). Five point-biserial correlations were computed between the listeners' response (correct or incorrect) on the task and the SNR in each band. The stronger the correlation between performance and SNR, the greater that given band was weighted by the listener. Listeners were tested with and without hearing aids on. All listeners were experienced hearing aid users. Results indicated that the highest spectral band (approximately 2800-11 000 Hz) received the greatest weight in both listening conditions. However, the weight on the highest spectral band was less when listeners performed the task with their hearing aids on in comparison to when listening without hearing aids. No direct relationship was observed between the listeners' weights and the sensation level within a given band.