Features of stimulation affecting tonal-speech perception: implications for cochlear prostheses

Tone languages differ from English in that the pitch pattern of a single-syllable word conveys lexical meaning. In the present study, dependence of tonal-speech perception on features of the stimulation was examined using an acoustic simulation of a CIS-type speech-processing strategy for cochlear prostheses. Contributions of spectral features of the speech signals were assessed by varying the number of filter bands, while contributions of temporal envelope features were assessed by varying the low-pass cutoff frequency used for extracting the amplitude envelopes. Ten normal-hearing native Mandarin Chinese speakers were tested. When the low-pass cutoff frequency was fixed at 512 Hz, consonant, vowel, and sentence recognition improved as a function of the number of channels and reached plateau at 4 to 6 channels. Subjective judgments of sound quality continued to improve as the number of channels increased to 12, the highest number tested. Tone recognition, i.e., recognition of the four Mandarin tone patterns, depended on both the number of channels and the low-pass cutoff frequency. The trade-off between the temporal and spectral cues for tone recognition indicates that temporal cues can compensate for diminished spectral cues for tone recognition and vice versa. An additional tone recognition experiment using syllables of equal duration showed a marked decrease in performance, indicating that duration cues contribute to tone recognition. A third experiment showed that recognition of processed FM patterns that mimic Mandarin tone patterns was poor when temporal envelope and duration cues were removed.     

Enhancing Chinese tone recognition by manipulating amplitude envelope: implications for cochlear implants

Tone recognition is important for speech understanding in tonal languages such as Mandarin Chinese. Cochlear implant patients are able to perceive some tonal information by using temporal cues such as periodicity-related amplitude fluctuations and similarities between the fundamental frequency (F0) contour and the amplitude envelope. The present study investigates whether modifying the amplitude envelope to better resemble the F0 contour can further improve tone recognition in multichannel cochlear implants. Chinese tone and vowel recognition were measured for six native Chinese normal-hearing subjects listening to a simulation of a four-channel cochlear implant speech processor with and without amplitude envelope enhancement. Two algorithms were proposed to modify the amplitude envelope to more closely resemble the F0 contour. In the first algorithm, the amplitude envelope as well as the modulation depth of periodicity fluctuations was adjusted for each spectral channel. In the second algorithm, the overall amplitude envelope was adjusted before multichannel speech processing, thus reducing any local distortions to the speech spectral envelope. The results showed that both algorithms significantly improved Chinese tone recognition. By adjusting the overall amplitude envelope to match the F0 contour before multichannel processing, vowel recognition was better preserved and less speech-processing computation was required. The results suggest that modifying the amplitude envelope to more closely resemble the F0 contour may be a useful approach toward improving Chinese-speaking cochlear implant patients' tone recognition.     

Faciliation of Mandarin tone perception by visual speech in clear and degraded audio: implications for cochlear implants

Cochlear implant (CI) users in tone language environments report great difficulty in perceiving lexical tone. This study investigated the augmentation of simulated cochlear implant audio by visual (facial) speech information for tone. Native speakers of Mandarin and Australian English were asked to discriminate between minimal pairs of Mandarin tones in five conditions: Auditory-Only, Auditory-Visual, CI-simulated Auditory-Only, CI-simulated Auditory-Visual, and Visual-Only (silent video). Discrimination in CI-simulated audio conditions was poor compared with normal audio, and varied according to tone pair, with tone pairs with strong non-F0 cues discriminated the most easily. The availability of visual speech information also improved discrimination in the CI-simulated audio conditions, particularly on tone pairs with strong durational cues. In the silent Visual-Only condition, both Mandarin and Australian English speakers discriminated tones above chance levels. Interestingly, tone-nai?ve listeners outperformed native listeners in the Visual-Only condition, suggesting firstly that visual speech information for tone is available, and may in fact be under-used by normal-hearing tone language perceivers, and secondly that the perception of such information may be language-general, rather than the product of language-specific learning. This may find application in the development of methods to improve tone perception in CI users in tone language environments.     

The relative phonetic contributions of a cochlear implant and residual acoustic hearing to bimodal speech perception

The addition of low-passed (LP) speech or even a tone following the fundamental frequency (F0) of speech has been shown to benefit speech recognition for cochlear implant (CI) users with residual acoustic hearing. The mechanisms underlying this benefit are still unclear. In this study, eight bimodal subjects (CI users with acoustic hearing in the non-implanted ear) and eight simulated bimodal subjects (using vocoded and LP speech) were tested on vowel and consonant recognition to determine the relative contributions of acoustic and phonetic cues, including F0, to the bimodal benefit. Several listening conditions were tested (CI/Vocoder, LP, T(F0-env), CI/Vocoder + LP, CI/Vocoder + T(F0-env)). Compared with CI/Vocoder performance, LP significantly enhanced both consonant and vowel perception, whereas a tone following the F0 contour of target speech and modulated with an amplitude envelope of the maximum frequency of the F0 contour (T(F0-env)) enhanced only consonant perception. Information transfer analysis revealed a dual mechanism in the bimodal benefit: The tone representing F0 provided voicing and manner information, whereas LP provided additional manner, place, and vowel formant information. The data in actual bimodal subjects also showed that the degree of the bimodal benefit depended on the cutoff and slope of residual acoustic hearing.     

Relative contributions of spectral and temporal cues for phoneme recognition

Cochlear implants provide users with limited spectral and temporal information. In this study, the amount of spectral and temporal information was systematically varied through simulations of cochlear implant processors using a noise-excited vocoder. Spectral information was controlled by varying the number of channels between 1 and 16, and temporal information was controlled by varying the lowpass cutoff frequencies of the envelope extractors from 1 to 512 Hz. Consonants and vowels processed using those conditions were presented to seven normal-hearing native-English-speaking listeners for identification. The results demonstrated that both spectral and temporal cues were important for consonant and vowel recognition with the spectral cues having a greater effect than the temporal cues for the ranges of numbers of channels and lowpass cutoff frequencies tested. The lowpass cutoff for asymptotic performance in consonant and vowel recognition was 16 and 4 Hz, respectively. The number of channels at which performance plateaued for consonants and vowels was 8 and 12, respectively. Within the above-mentioned ranges of lowpass cutoff frequency and number of channels, the temporal and spectral cues showed a tradeoff for phoneme recognition. Information transfer analyses showed different relative contributions of spectral and temporal cues in the perception of various phonetic/acoustic features.     

Effects of cochlear implant processing and fundamental frequency on the intelligibility of competing sentences

Speech perception in the presence of another competing voice is one of the most challenging tasks for cochlear implant users. Several studies have shown that (1) the fundamental frequency (F0) is a useful cue for segregating competing speech sounds and (2) the F0 is better represented by the temporal fine structure than by the temporal envelope. However, current cochlear implant speech processing algorithms emphasize temporal envelope information and discard the temporal fine structure. In this study, speech recognition was measured as a function of the F0 separation of the target and competing sentence in normal-hearing and cochlear implant listeners. For the normal-hearing listeners, the combined sentences were processed through either a standard implant simulation or a new algorithm which additionally extracts a slowed-down version of the temporal fine structure (called Frequency-Amplitude-Modulation-Encoding). The results showed no benefit of increasing F0 separation for the cochlear implant or simulation groups. In contrast, the new algorithm resulted in gradual improvements with increasing F0 separation, similar to that found with unprocessed sentences. These results emphasize the importance of temporal fine structure for speech perception and demonstrate a potential remedy for difficulty in the perceptual segregation of competing speech sounds.     

Beneficial acoustic speech cues for cochlear implant users with residual acoustic hearing

This study investigated which acoustic cues within the speech signal are responsible for bimodal speech perception benefit. Seven cochlear implant (CI) users with usable residual hearing at low frequencies in the non-implanted ear participated. Sentence tests were performed in near-quiet (some noise on the CI side to reduce scores from ceiling) and in a modulated noise background, with the implant alone and with the addition, in the hearing ear, of one of four types of acoustic signals derived from the same sentences: (1) a complex tone modulated by the fundamental frequency (F0) and amplitude envelope contours; (2) a pure tone modulated by the F0 and amplitude contours; (3) a noise-vocoded signal; (4) unprocessed speech. The modulated tones provided F0 information without spectral shape information, whilst the vocoded signal presented spectral shape information without F0 information. For the group as a whole, only the unprocessed speech condition provided significant benefit over implant-alone scores, in both near-quiet and noise. This suggests that, on average, F0 or spectral cues in isolation provided limited benefit for these subjects in the tested listening conditions, and that the significant benefit observed in the full-signal condition was derived from implantees' use of a combination of these cues.     

The effects of binaural spectral resolution mismatch on Mandarin speech perception in simulated electric hearing

This study assessed the effects of binaural spectral resolution mismatch on the intelligibility of Mandarin speech in noise using bilateral cochlear implant simulations. Noise-vocoded Mandarin speech, corrupted by speech-shaped noise at 0 and 5?dB signal-to-noise ratios, were presented unilaterally or bilaterally to normal-hearing listeners with mismatched spectral resolution between ears. Significant binaural benefits for Mandarin speech recognition were observed only with matched spectral resolution between ears. In addition, the performance of tone identification was more robust to noise than that of sentence recognition, suggesting factors other than tone identification might account more for the degraded sentence recognition in noise.     

Auditory-visual speech perception in normal-hearing and cochlear-implant listeners

The present study evaluated auditory-visual speech perception in cochlear-implant users as well as normal-hearing and simulated-implant controls to delineate relative contributions of sensory experience and cues. Auditory-only, visual-only, or auditory-visual speech perception was examined in the context of categorical perception, in which an animated face mouthing ba, da, or ga was paired with synthesized phonemes from an 11-token auditory continuum. A three-alternative, forced-choice method was used to yield percent identification scores. Normal-hearing listeners showed sharp phoneme boundaries and strong reliance on the auditory cue, whereas actual and simulated implant listeners showed much weaker categorical perception but stronger dependence on the visual cue. The implant users were able to integrate both congruent and incongruent acoustic and optical cues to derive relatively weak but significant auditory-visual integration. This auditory-visual integration was correlated with the duration of the implant experience but not the duration of deafness. Compared with the actual implant performance, acoustic simulations of the cochlear implant could predict the auditory-only performance but not the auditory-visual integration. These results suggest that both altered sensory experience and improvised acoustic cues contribute to the auditory-visual speech perception in cochlear-implant users.     

Effect of stimulation rate on phoneme recognition by nucleus-22 cochlear implant listeners

This study investigated the effect of pulsatile stimulation rate on medial vowel and consonant recognition in cochlear implant listeners. Experiment 1 measured phoneme recognition as a function of stimulation rate in six Nucleus-22 cochlear implant listeners using an experimental four-channel continuous interleaved sampler (CIS) speech processing strategy. Results showed that all stimulation rates from 150 to 500 pulses/s/electrode produced equally good performance, while stimulation rates lower than 150 pulses/s/electrode produced significantly poorer performance. Experiment 2 measured phoneme recognition by implant listeners and normal-hearing listeners as a function of the low-pass cutoff frequency for envelope information. Results from both acoustic and electric hearing showed no significant difference in performance for all cutoff frequencies higher than 20 Hz. Both vowel and consonant scores dropped significantly when the cutoff frequency was reduced from 20 Hz to 2 Hz. The results of these two experiments suggest that temporal envelope information can be conveyed by relatively low stimulation rates. The pattern of results for both electrical and acoustic hearing is consistent with a simple model of temporal integration with an equivalent rectangular duration (ERD) of the temporal integrator of about 7 ms.     

Enhancement of temporal periodicity cues in cochlear implants: effects on prosodic perception and vowel identification

Standard continuous interleaved sampling processing, and a modified processing strategy designed to enhance temporal cues to voice pitch, were compared on tests of intonation perception, and vowel perception, both in implant users and in acoustic simulations. In standard processing, 400 Hz low-pass envelopes modulated either pulse trains (implant users) or noise carriers (simulations). In the modified strategy, slow-rate envelope modulations, which convey dynamic spectral variation crucial for speech understanding, were extracted by low-pass filtering (32 Hz). In addition, during voiced speech, higher-rate temporal modulation in each channel was provided by 100% amplitude-modulation by a sawtooth-like wave form whose periodicity followed the fundamental frequency (F0) of the input. Channel levels were determined by the product of the lower- and higher-rate modulation components. Both in acoustic simulations and in implant users, the ability to use intonation information to identify sentences as question or statement was significantly better with modified processing. However, while there was no difference in vowel recognition in the acoustic simulation, implant users performed worse with modified processing both in vowel recognition and in formant frequency discrimination. It appears that, while enhancing pitch perception, modified processing harmed the transmission of spectral information.     

Detection and rate discrimination of amplitude modulation in electrical hearing

Three experiments were designed to examine temporal envelope processing by cochlear implant (CI) listeners. In experiment 1, the hypothesis that listeners' modulation sensitivity would in part determine their ability to discriminate between temporal modulation rates was examined. Temporal modulation transfer functions (TMTFs) obtained in an amplitude modulation detection (AMD) task were compared to threshold functions obtained in an amplitude modulation rate discrimination (AMRD) task. Statistically significant nonlinear correlations were observed between the two measures. In experiment 2, results of loudness-balancing showed small increases in the loudness of modulated over unmodulated stimuli beyond a modulation depth of 16%. Results of experiment 3 indicated small but statistically significant effects of level-roving on the overall gain of the TMTF, but no impact of level-roving on the average shape of the TMTF across subjects. This suggested that level-roving simply increased the task difficulty for most listeners, but did not indicate increased use of intensity cues under more challenging conditions. Data obtained with one subject, however, suggested that the most sensitive listeners may derive some benefit from intensity cues in these tasks. Overall, results indicated that intensity cues did not play an important role in temporal envelope processing by the average CI listener.     

Fundamental frequency is critical to speech perception in noise in combined acoustic and electric hearing

Cochlear implant (CI) users have been shown to benefit from residual low-frequency hearing, specifically in pitch related tasks. It remains unclear whether this benefit is dependent on fundamental frequency (F0) or other acoustic cues. Three experiments were conducted to determine the role of F0, as well as its frequency modulated (FM) and amplitude modulated (AM) components, in speech recognition with a competing voice. In simulated CI listeners, the signal-to-noise ratio was varied to estimate the 50% correct response. Simulation results showed that the F0 cue contributes to a significant proportion of the benefit seen with combined acoustic and electric hearing, and additionally that this benefit is due to the FM rather than the AM component. In actual CI users, sentence recognition scores were collected with either the full F0 cue containing both the FM and AM components or the 500-Hz low-pass speech cue containing the F0 and additional harmonics. The F0 cue provided a benefit similar to the low-pass cue for speech in noise, but not in quiet. Poorer CI users benefited more from the F0 cue than better users. These findings suggest that F0 is critical to improving speech perception in noise in combined acoustic and electric hearing.     

Coding of the fundamental frequency in continuous interleaved sampling processors for cochlear implants

In this study the perception of the fundamental frequency (F0) of periodic stimuli by cochlear implant users is investigated. A widely used speech processor is the Continuous Interleaved Sampling (CIS) processor, for which the fundamental frequency appears as temporal fluctuations in the envelopes at the output. Three experiments with four users of the LAURA (Registered trade mark of Philips Hearing Implants, now Cochlear Technology Centre Europe) cochlear implant were carried out to examine the influence of the modulation depth of these envelope fluctuations on pitch discrimination. In the first experiment, the subjects were asked to discriminate between two SAM (sinusoidally amplitude modulated) pulse trains on a single electrode channel differing in modulation frequency ( deltaf = 20%). As expected, the results showed a decrease in the performance for smaller modulation depths. Optimal performance was reached for modulation depths between 20% and 99%, depending on subject, electrode channel, and modulation frequency. In the second experiment, the smallest noticeable difference in F0 of synthetic vowels was measured for three algorithms that differed in the obtained modulation depth at the output: the default CIS strategy, the CIS strategy in which the F0 fluctuations in the envelope were removed (FLAT CIS), and a third CIS strategy, which was especially designed to control and increase the depth of these fluctuations (F0 CIS). In general, performance was poorest for the FLAT CIS strategy, where changes in F0 are only apparent as changes of the average amplitude in the channel outputs. This emphasizes the importance of temporal coding of F0 in the speech envelope for pitch perception. No significantly better results were obtained for the F0 CIS strategy compared to the default CIS strategy, although the latter results in envelope modulation depths at which sub-optimal scores were obtained in some cases of the first experiment. This indicates that less modulation is needed if all channels are stimulated with synchronous F0 fluctuations. This hypothesis is confirmed in a third experiment where subjects performed significantly better in a pitch discrimination task with SAM pulse trains, if three channels were stimulated concurrently, as opposed to only one.     

Bimodal listeners are not sensitive to interaural time differences in unmodulated low-frequency stimuli (L)

Sensitivity to interaural time differences (ITDs) with unmodulated low-frequency stimuli was assessed in bimodal listeners who had previously shown to be good performers in ITD experiments. Two types of stimuli were used: (1) an acoustic sinusoid combined with an electric transposed signal and (2) an acoustic sinusoid combined with an electric clicktrain. No or very low sensitivity to ITD was found for these stimuli, even though subjects were highly trained on the task and were intensively tested in multiple test sessions. In previous studies with users of a cochlear implant (CI) and a contralateral hearing aid (HA) (bimodal listeners), sensitivity was shown to ITD with modulated stimuli with frequency content between 600 and 3600 Hz. The outcomes of the current study imply that in speech processing design for users of a CI in combination with a HA on the contralateral side, the emphasis should be more on providing salient envelope ITD cues than on preserving fine-timing ITD cues present in acoustic signals.     

Effects of speaker variability and noise on Mandarin fricative identification by native and non-native listeners

Speaker variability and noise are two common sources of acoustic variability. The goal of this study was to examine whether these two sources of acoustic variability affected native and non-native perception of Mandarin fricatives to different degrees. Multispeaker Mandarin fricative stimuli were presented to 40 native and 52 non-native listeners in two presentation formats (blocked by speaker and mixed across speakers). The stimuli were also mixed with speech-shaped noise to create five levels of signal-to- noise ratios. The results showed that noise affected non-native identification disproportionately. By contrast, the effect of speaker variability was comparable between the native and non-native listeners. Confusion patterns were interpreted with reference to the results of acoustic analysis, suggesting native and non-native listeners used distinct acoustic cues for fricative identification. It was concluded that not all sources of acoustic variability are treated equally by native and non-native listeners. Whereas noise compromised non-native fricative perception disproportionately, speaker variability did not pose a special challenge to the non-native listeners.     

Predicting the intelligibility of vocoded and wideband Mandarin Chinese

Due to the limited number of cochlear implantees speaking Mandarin Chinese, it is extremely difficult to evaluate new speech coding algorithms designed for tonal languages. Access to an intelligibility index that could reliably predict the intelligibility of vocoded (and non-vocoded) Mandarin Chinese is a viable solution to address this challenge. The speech-transmission index (STI) and coherence-based intelligibility measures, among others, have been examined extensively for predicting the intelligibility of English speech but have not been evaluated for vocoded or wideband (non-vocoded) Mandarin speech despite the perceptual differences between the two languages. The results indicated that the coherence-based measures seem to be influenced by the characteristics of the spoken language. The highest correlation (r = 0.91-0.97) was obtained in Mandarin Chinese with a weighted coherence measure that included primarily information from high-intensity voiced segments (e.g., vowels) containing F0 information, known to be important for lexical tone recognition. In contrast, in English, highest correlation was obtained with a coherence measure that included information from weak consonants and vowel/consonant transitions. A band-importance function was proposed that captured information about the amplitude envelope contour. A higher modulation rate (100 Hz) was found necessary for the STI-based measures for maximum correlation (r = 0.94-0.96) with vocoded Mandarin and English recognition.     

Development and validation of the Mandarin speech perception test

Currently there are few standardized speech testing materials for Mandarin-speaking cochlear implant (CI) listeners. In this study, Mandarin speech perception (MSP) sentence test materials were developed and validated in normal-hearing subjects listening to acoustic simulations of CI processing. Percent distribution of vowels, consonants, and tones within each MSP sentence list was similar to that observed across commonly used Chinese characters. There was no significant difference in sentence recognition across sentence lists. Given the phonetic balancing within lists and the validation with spectrally degraded speech, the present MSP test materials may be useful for assessing speech performance of Mandarin-speaking CI listeners.     

The use of acoustic cues for phonetic identification: effects of spectral degradation and electric hearing

Although some cochlear implant (CI) listeners can show good word recognition accuracy, it is not clear how they perceive and use the various acoustic cues that contribute to phonetic perceptions. In this study, the use of acoustic cues was assessed for normal-hearing (NH) listeners in optimal and spectrally degraded conditions, and also for CI listeners. Two experiments tested the tense/lax vowel contrast (varying in formant structure, vowel-inherent spectral change, and vowel duration) and the word-final fricative voicing contrast (varying in F1 transition, vowel duration, consonant duration, and consonant voicing). Identification results were modeled using mixed-effects logistic regression. These experiments suggested that under spectrally-degraded conditions, NH listeners decrease their use of formant cues and increase their use of durational cues. Compared to NH listeners, CI listeners showed decreased use of spectral cues like formant structure and formant change and consonant voicing, and showed greater use of durational cues (especially for the fricative contrast). The results suggest that although NH and CI listeners may show similar accuracy on basic tests of word, phoneme or feature recognition, they may be using different perceptual strategies in the process.