Acoustic Simulations Of Cochlear Implants Research Articles

Voice Discrimination in Quiet and in Background Noise by Simulated and Real Cochlear Implant Users.

Cochlear implant (CI) users demonstrate poor voice discrimination (VD) in quiet conditions based on the speaker's fundamental frequency (fo) and formant frequencies (i.e., vocal-tract length [VTL]). Our purpose was to examine the effect of background noise at levels that allow good speech recognition thresholds (SRTs) on VD via acoustic CI simulations and CI hearing. Forty-eight normal-hearing (NH) listeners who listened via noise-excited (n = 20) or sinewave (n = 28) vocoders and 10 prelingually deaf CI users (i.e., whose hearing loss began before language acquisition) participated in the study. First, the signal-to-noise ratio (SNR) that yields 70.7% correct SRT was assessed using an adaptive sentence-in-noise test. Next, the CI simulation listeners performed 12 adaptive VDs: six in quiet conditions, two with each cue (fo, VTL, fo + VTL), and six amid speech-shaped noise. The CI participants performed six VDs: one with each cue, in quiet and amid noise. SNR at VD testing was 5 dB higher than the individual's SRT in noise (SRTn +5 dB). Results showed the following: (a) Better VD was achieved via the noise-excited than the sinewave vocoder, with the noise-excited vocoder better mimicking CI VD; (b) background noise had a limited negative effect on VD, only for the CI simulation listeners; and (c) there was a significant association between SNR at testing and VTL VD only for the CI simulation listeners. For NH listeners who listen to CI simulations, noise that allows good SRT can nevertheless impede VD, probably because VD depends more on bottom-up sensory processing. Conversely, for prelingually deaf CI users, noise that allows good SRT hardly affects VD, suggesting that they rely strongly on bottom-up processing for both VD and speech recognition.

An Improved Noise Reduction Technique for Enhancing the Intelligibility of Sinewave Vocoded Speech: Implication in Cochlear Implants

A cochlear implant (CI) is the most suitable option for individuals with severe profound hearing loss. CI restores the audibility to near perfection and offers good speech understanding in quiet. However, the speech perception in noise with CIs is less optimal as most speech coding strategies of CIs encode only the temporal envelope. Besides the current CI signal coding strategies lacks sophisticated pre-processing. In the current study, we proposed a novel pre-processing method to improve speech Intelligibility in noise and tested using the acoustic simulations of cochlear implants. The proposed noise reduction technique aims to minimize the mean square error (MSE) between the temporal envelopes of the enhanced speech and its clean speech. Therefore, the proposed method will be suitable for CI applications. This paper provides an analysis of the theoretical derivation of the noise suppression function and also the performance evaluation using objective and subjective tests. The effectiveness of the proposed method was objectively evaluated using the SRMR-CI and ESTOI. Additionally, speech recognition through the acoustic simulations of the cochlear implant was done for the subjective evaluation. Performance of the proposed method was compared with the Weiner filter (WF) and sigmoidal functions. The sinewave vocoder was used to simulate the cochlear implant perception. Both objective and subjective scores revealed that the performance of the proposed technique is superior to the WF and sigmoidal function.

Vibrotactile Stimulation Based on the Fundamental Frequency Can Improve Melodic Contour Identification of Normal-Hearing Listeners With a 4-Channel Cochlear Implant Simulation.

Cochlear implant (CI) users’ poor speech recognition in noise and music perception may be both due to their limited access to pitch cues such as the fundamental frequency (F0). Recent studies showed that similar to residual low-frequency acoustic hearing, vibrotactile presentation of the F0 significantly improved speech recognition in noise of CI users. The present study tested whether F0-based vibrotactile stimulation can improve melodic contour identification (MCI) of normal-hearing listeners with acoustically simulated CI processing. Each melodic contour consisted of five musical notes with one of nine contour patterns (rising, falling, or flat in each half of the contour). The F0 of the middle note was 220 or 880 Hz, and the frequency intervals between adjacent notes were 1, 3, or 5 semitones. The F0 of each note was extracted in real time and transposed to a vibration frequency centered around 110 Hz at the right forearm top. MCI was tested in five experimental conditions (with a 4- or 8-channel CI simulation alone, vibrotactile stimulation alone, and 4- or 8-channel CI simulation plus vibrotactile stimulation), each after the same amount of brief training was provided. Results showed that discrimination of vibrotactile stimuli significantly improved from chance to near perfect as the vibration frequency interval increased from 0.25 to 3 semitones. The MCI performance with vibrotactile stimulation alone was similar to that with the 4-channel CI simulation alone, but was significantly worse than that with the 8-channel CI simulation alone. Significant improvement in MCI performance with the addition of vibrotactile stimulation was only found with the 4-channel CI simulation when the middle F0 was 880 Hz and when the frequency intervals were 3 or 5 semitones. The improvement in MCI performance with than without vibrotactile stimulation was significantly correlated with the baseline MCI performance with 4-channel CI simulation alone or with the MCI performance difference between vibrotactile stimulation and 8-channel CI simulation. Therefore, when the simulated or real CI performance is relatively poor, vibrotactile stimulation based on the F0 may improve MCI with acoustic CI simulations and perhaps in real CI users as well.

Early Sentence Recognition in Adult Cochlear Implant Users.

Normal-hearing subjects listening to acoustic simulations of cochlear implants (CI) can obtain sentence recognition scores near 100% in quiet and in 10 dB signal-to-noise ratio (SNR) noise with acute exposure. However, average sentence recognition scores for real CI listeners are generally lower, even after months of experience, and there is a high degree of heterogeneity. Our aim was to identify the relative importance and strength of factors that prevent CI listeners from achieving early, 1-mo scores as high as those for normal-hearing-listener acoustic simulations. Sentence recognition scores (100 words/list, 65 dB SPL) using CI alone were collected for all adult unilateral CI listeners implanted in our center over a 5-yr period. Sentence recognition scores in quiet and in 10 dB SNR 8-talker babble, collected from 1 to 12 mo, were reduced to a single dependent variable, the "initial" score, via logarithmic regression. "Initial" scores equated to an improved estimate of 1-mo scores, and integrated the time to rise above zero score for poorer performing subjects. Demographic, device, and medical data were collected for 118 subjects who met standard CI candidacy criteria. Computed tomography of the electrode array allowing determination of the insertion depth as an angle, and the presence or absence of scala dislocation was available for 96 subjects. Predictive factors for initial scores were selected using stepwise multiple linear regression. The relative importance of predictive factors was estimated as partial r with a low bias method, and statistical significance tested with type II analysis of variance. The etiologies chronic otitis and autoimmune disease were associated with lower, widely variable sentence recognition scores in the long-term. More than 60% of CI listeners scored >50/100 in quiet at 1 mo. Congenital hearing loss was associated with significantly lower initial scores in quiet (r 0.23, p < 0.001), as was longer duration of hearing loss (r 0.12, p < 0.001, -0.76 pts per year). Initial scores were negatively correlated with insertion depth (r 0.09, p < 0.001, -0.1 pts per degree), with the highest initial scores being obtained for insertion depths of 300° to 400°. A much greater proportion of scala dislocations was found for perimodiolar arrays compared with straight arrays. Scores were negatively correlated with the proportion of the active electrode array found in scala vestibuli for Nucleus perimodiolar devices (r 0.14, p < 0.01, coefficient -25). Similar overall results were obtained for sentence recognition scores in noise (+10 dB SNR). The intercept value for the obtained regression functions indicated that CI listeners with the least limiting factors generally scored ~95/100 in quiet and ~90/100 in noise. In addition, CI listeners with insertion angles as low as 315° to 360° could obtain sentence recognition scores >80/100 even at 1 day after activation. Insertion depths of 360° were estimated to produce frequency-place mismatches of about one octave upward shift. Patient-related factors etiology and duration of deafness together explained ~40% of the variance in early sentence recognition scores, and electrode position factors ~20%. CI listeners with insertion depths of about one turn obtained the highest early sentence recognition scores in quiet and in noise, and these were comparable with those reported in the literature for normal-hearing subjects listening to 8 to 12 channel vocoder simulations. Differences between device brands were largely explained by differences in insertion depths. This indicates that physiological frequency-place mismatches of about one octave are rapidly accommodated by CI users for understanding sentences, between 1 day to 1 mo postactivation, and that channel efficiency may be significantly poorer for more deeply positioned electrode contacts.

On the color of speech: The effect of speaking styles on speech recognition by cochlear-implant users

Speaking styles are a common source of real-life variability that may affect cochlear implant (CI) speech communication. Speech transmitted via a CI is heavily degraded in acoustic-phonetic cues. Therefore, the extent to which (and for what conditions) individual CI users benefit from carefully articulated speech (or are hindered by casually articulated/reduced speech) is unclear. First, we examined the impact of speaking styles on CI speech recognition in favorable listening conditions. CI users and normal-hearing (NH) listeners tested with 8- and 4-channel acoustic CI simulations completed a sentence recognition task with casual and careful sentences (with similar lexical and semantic content) in the quiet. Second, we explored individual performance on this task and its relation with standard speech recognition scores and self-reported real-life hearing abilities. Listeners were overall more accurate on careful sentences than the casual sentences, but casual sentences were disproportionately more diffic...

Musician effect in cochlear implant simulated gender categorization

Musicians have been shown to better perceive pitch and timbre cues in speech and music, compared to non-musicians. It is unclear whether this "musician advantage" persists under conditions of spectro-temporal degradation, as experienced by cochlear-implant (CI) users. In this study, gender categorization was measured in normal-hearing musicians and non-musicians listening to acoustic CI simulations. Recordings of Dutch words were synthesized to systematically vary fundamental frequency, vocal-tract length, or both to create voices from the female source talker to a synthesized male talker. Results showed an overall musician effect, mainly due to musicians weighting fundamental frequency more than non-musicians in CI simulations.

ENHANCED SIGNAL PROCESSING FOR COCHLEAR IMPLANTS

The restoration of melody perception is a key remaining challenge in cochlear implants. A novel sound coding strategy is proposed that converts an input audio signal into time-varying electrically stimulating pulse trains. A sound is first split into several frequency sub-bands with a fixed filter bank or a dynamic filter bank tracking harmonics in sounds. Each sub-band signal is coherently downward shifted to a low-frequency base band. These resulting coherent envelope signals have Hermitian symmetric frequency spectrums and are thus real-valued. A peak detector or high-rate sampler of half-wave rectified coherent envelope signals in each sub-band further converts the coherent envelopes into rate-varying, interleaved pulse trains. Acoustic simulations of cochlear implants using this new technique with normal hearing listeners, showed significant improvement in melody recognition over the most common conventional stimulation approach used in cochlear implants.

Voice gender discrimination provides a measure of more than pitch-related perception in cochlear implant users

Objectives: (1) To investigate whether voice gender discrimination (VGD) could be a useful indicator of the spectral and temporal processing abilities of individual cochlear implant (CI) users; (2) To examine the relationship between VGD and speech recognition with CI when comparable acoustic cues are used for both perception processes. Design: VGD was measured using two talker sets with different inter-gender fundamental frequencies (F0), as well as different acoustic CI simulations. Vowel and consonant recognition in quiet and noise were also measured and compared with VGD performance. Study sample: Eleven postlingually deaf CI users. Results: The results showed that (1) mean VGD performance differed for different stimulus sets, (2) VGD and speech recognition performance varied among individual CI users, and (3) individual VGD performance was significantly correlated with speech recognition performance under certain conditions. Conclusions: VGD measured with selected stimulus sets might be useful for assessing not only pitch-related perception, but also spectral and temporal processing by individual CI users. In addition to improvements in spectral resolution and modulation detection, the improvement in higher modulation frequency discrimination might be particularly important for CI users in noisy environments.SumarioObjetivos: (1) Investigar si la discriminación de voz por género (VGD) puede ser un indicador útil de las habilidades de procesamiento espectral y temporal de usuarios de implantes cocleares (IC); (2) Examinar la relación entre VGD y el reconocimiento del lenguaje con IC, cuando se usan claves acústicas comparables para ambos procesos de percepción. Diseño: Se midió el VGD usando dos series de hablantes con diferentes frecuencias fundamentales (Fo) inter-género así como diferentes simulaciones acústicas de IC. El reconocimiento de vocales y consonantes en silencio y con ruido también se midió y comparó con el rendimiento VGD. Muestra: Once usuarios sordos de IC post-lingüísticos. Resultados: Los resultados mostraron que 1) el rendimiento promedio VGD difirió para diferentes grupos de estímulos; 2) el rendimiento para el reconocimiento de VGD y de lenguaje varió entre usuarios de IC y 3) el rendimiento individual de VGD correlacionó significativamente con el reconocimiento del lenguaje bajo ciertas condiciones. Conclusiones: El VGD medido con grupos seleccionados de estímulos puede ser útil para evaluar no solamente la percepción relacionada con la frecuencia sino también el procesamiento espectral y temporal en usuarios de IC. Además de la mejoría en la resolución espectral y en la detección de la modulación, la mejoría en la discriminación en frecuencias de alta modulación puede ser particularmente importante para los usuarios de IC en ambientes de ruido.

Lexical tone identification and consonant recognition in acoustic simulations of cochlear implants

Conclusion: Cochlear implant (CI) recipients’ performance of lexical tone identification and consonant recognition can be enhanced by providing greater spectral details. Objective: To evaluate the effects of increasing the number of total spectral channels on the lexical tone identification and consonant recognition by normally hearing listeners who are native speakers of Mandarin Chinese. Subjects and methods: Lexical tone identification and consonant recognition were measured in 15 Mandarin-speaking, normal-hearing (NH) listeners with varied numbers of total spectral channels (i.e. 4, 6, 8, 10, 12, 16, 20, and 24), using acoustic simulations of CIs. Results: The group of NH listeners’ performance of lexical tone identification ranged from 44.53% to 66.60% with 4–24 spectral channels. The performance of tone identification between channels 4 and 16 remained similar; between channels 16 and 20 performance improved significantly. As regards consonant recognition, the NH listeners’ overall accuracy ranged from 73.17% to 95.33% with 4–24 channels. Steady improvement in consonant recognition accuracy was observed as a function of increasing the spectral channels. With about 12–16 spectral channels, the NH listeners’ overall accuracy in consonant recognition began to be comparable to their accuracy with the unprocessed stimuli.

Effects of Talker Variability on Vowel Recognition in Cochlear Implants

To investigate the effects of talker variability on vowel recognition by cochlear implant (CI) users and by normal-hearing (NH) participants listening to 4-channel acoustic CI simulations. CI users were tested with their clinically assigned speech processors. For NH participants, 3 CI processors were simulated, using different combinations of carrier type and temporal envelope cutoff frequency (noise band/160 Hz, sine wave/160 Hz, and sine wave/20 Hz). Vowel recognition was measured for 4 talkers, presented in either a single-talker context (1 talker per test block) or a multi-talker context (4 talkers per test block). CI users' vowel recognition was significantly poorer in the multi-talker context than in the single-talker context. When noise-band carriers were used in the simulations, NH performance was not significantly affected by talker variability. However, when sine-wave carriers were used in the simulations, NH performance was significantly affected by talker variability in both envelope filter conditions. Because fundamental frequency was not preserved by the 20-Hz envelope filter and only partially preserved by the 160-Hz envelope filter, both spectral and temporal cues contributed to the talker variability effects observed with sine-wave carriers. Similarly, spectral and temporal cues may have contributed to the talker variability effects observed with CI participants.

Effects on performance of partial misalignments of spectral information in acoustic simulations of cochlear implants

Objective: To assess the effects on speech understanding of frequency misalignments in channel outputs for an acoustic model of a cochlear implant. Design: Consonants, vowels, and sentences were processed through a four-channel sine-wave simulation of a cochlear implant. In the five experimental conditions, channels 1 and 3 were always output at the correct frequency while channels 2 and 4 were output at frequencies varying from the correct frequency to frequencies 25%, 50%, and 75% lower than appropriate. In a fifth condition (2-of-4 condition), channels 2 and 4 were turned off. Results: Consonant recognition was reduced significantly with a 50% shift in channels 2 and 4. Vowel and sentence recognition were reduced significantly with a 75% shift in channels 2 and 4. For all material performance in the 75% shift condition was better than in the 2-of-4 condition. Conclusions: The perceiving system that underlies speech recognition is relatively tolerant of misplaced frequency information if other information is presented to the correct frequency location. This flexibility may account for some of the success of cochlear implants. [Research supported by a grant from NIDCD (No. RO1 00654-12) to the second author.]

Hot topics in speech communication

Speech communication is a robust and flexible process. For listeners with normal hearing, speech remains intelligible when a substantial proportion of the spectrum is eliminated by filtering, when portions of the wave form are replaced with silence, and in the presence of interfering sounds, including sounds made by competing talkers. The resilience of speech depends on several interacting systems which exploit the redundancy of speech in different ways. New results are reported on the perceptual processing of talker variability, monaural and binaural mechanisms of sound separation, audio-visual integration in lip reading, and perceptual compensation for channel distortion. For listeners with hearing impairment, new research in the field of cochlear implants has focused on the development of new electrode arrays, the efficacy of bilateral implants and the development of sophisticated signal processing techniques to combat noise. The new electrode arrays are designed to limit electrode interaction and thus increase the number of perceptual channels of information. The benefit of bilateral implants is investigated in terms of sound localization and speech perception in noise. New results are reported on bilateral cochlear implants, speech-processing strategies for noisy environments and acoustic simulations of cochlear implants which aim to quantify the factors contributing to speech intelligibility.