Noise Susceptibility of Cochlear Implant Users: The Role of Spectral Resolution and Smearing

The latest-generation cochlear implant devices provide many deaf patients with good speech recognition in quiet listening conditions. However, speech recognition deteriorates rapidly as the level of background noise increases. Previous studies have shown that, for cochlear implant users, the absence of fine spectro-temporal cues may contribute to poorer performance in noise, especially when the noise is dynamic (e.g., competing speaker or modulated noise). Here we report on sentence recognition by cochlear implant users and by normal-hearing subjects listening to an acoustic simulation of a cochlear implant, in the presence of steady or square-wave modulated speech-shaped noise. Implant users were tested using their everyday, clinically assigned speech processors. In the acoustic simulation, normal-hearing listeners were tested for different degrees of spectral resolution (16, eight, or four channels) and spectral smearing (carrier filter slopes of –24 or –6 dB/octave). For modulated noise, normal-hearing listeners experienced significant release from masking when the original, unprocessed speech was presented (which preserved the spectro-temporal fine structure), while cochlear implant users experienced no release from masking. As the spectral resolution was reduced, normal-hearing listeners release from masking gradually diminished. Release from masking was further reduced as the degree of spectral smearing increased. Interestingly, the mean speech recognition thresholds of implant users were very close to those of normal-hearing subjects listening to four-channel spectrally smeared noise-band speech. Also, the best cochlear implant listeners performed like normal-hearing subjects listening to eight- to 16-channel spectrally smeared noise-band speech. These findings suggest that implant users susceptibility to noise may be caused by the reduced spectral resolution and the high degree of spectral smearing associated with channel interaction. Efforts to improve the effective number of spectral channels as well as reduce channel interactions may improve implant performance in noise, especially for temporally modulated noise.     

A Psychophysical Method for Measuring Spatial Resolution in Cochlear Implants

A novel psychophysical method was developed for assessing spatial resolution in cochlear implants. Spectrally flat and spectrally peaked pulse train stimuli were generated by interleaving pulses on 11 electrodes. Spectrally flat stimuli used loudness-balanced currents and the spectrally peaked stimuli had a single spatial ripple with the current of the middle electrode raised to create a peak while the currents on two electrodes equally spaced at variable distance from the peak electrode were reduced to create valleys. The currents on peak and valley electrodes were adjusted to balance the overall loudness with the spectrally flat stimulus, while keeping the currents on flanking electrodes fixed. The psychometric functions obtained from percent correct discrimination of peaked and flat stimuli versus the distance between peak and valley electrodes were used to quantify spatial resolution for each of the eight subjects. The ability to resolve the spatial ripple correlated strongly with current level difference limens measured on the peak electrode. The results were consistent with a hypothesis that a factor other than spread of excitation (such as neural response variance) might underlie much of the variability in spatial resolution. Resolution ability was not correlated with phoneme recognition in quiet or sentence recognition in quiet and background noise, consistent with a hypothesis that implantees rely on cues other than fine spectral detail to identify speech, perhaps because this detail is poorly accessible or unreliable.     

Correlations Between Pitch and Phoneme Perception in Cochlear Implant Users and Their Normal Hearing Peers

This study examined correlations between pitch and phoneme perception for nine cochlear implant users and nine normal hearing listeners. Pure tone frequency discrimination thresholds were measured for frequencies of 500, 1000, and 2000 Hz. Complex tone fundamental frequency (F0) discrimination thresholds were measured for F0s of 110, 220, and 440 Hz. The effects of amplitude and frequency roving were measured under the rationale that individuals who are robust to such perturbations would perform better on phoneme perception measures. Phoneme identification was measured using consonant and vowel materials in quiet, in stationary speech-shaped noise (SSN), in spectrally notched SSN, and in temporally gated SSN. Cochlear implant pure tone frequency discrimination thresholds ranged between 1.5 and 9.9 %, while cochlear implant complex tone F0 discrimination thresholds ranged between 2.6 and 28.5 %. On average, cochlear implant users had 5.3 dB of masking release for consonants and 8.4 dB of masking release for vowels when measured in temporally gated SSN compared to stationary SSN. Correlations with phoneme identification measures were generally higher for complex tone discrimination measures than for pure tone discrimination measures. Correlations with phoneme identification measures were also generally higher for pitch perception measures that included amplitude and frequency roving. The strongest correlations were observed for measures of complex tone F0 discrimination with phoneme identification in temporally gated SSN. The results of this study suggest that musical training or signal processing strategies that improve F0 discrimination should improve consonant identification in fluctuating noise.     

开放式言语评估系统单音节材料在人工耳蜗植入者中等价性的初步分析

目的：验证开放式言语评估系统中单音节言语测听材料用于人工耳蜗植入者各表间的等价性。方法在声场安静的环境中测试50例人工耳蜗植入者（年龄25．78±10．43岁,使用人工耳蜗时间43．42±49．15月）开放式言语评估系统中10张音位平衡单音节词表的言语识别率,应用RM －ANOVA测试分析其表间等价性。结果50例人工耳蜗植入者10张词表平均言语识别率依次分别为55．68％±21．50％、52．88％±22．70％、51．72％±22．57％、53．48％±22．97％、52．30％±22．96％、51．80％±22．28％、53．20％±21．11％、54．76％±22．85％、52．46％±20．83％、52．16％±22．98％,10张词表之间平均言语识别率差异有显著统计学意义（ F＝2．536,P＝0．008＜0．05）,去掉词表1时,平均言语识别率差异无统计学意义（ F＝1．391,P＝0．199）,表明词表2～10九张表之间有良好的等价性。结论本言语评估系统中的第2～10组单音节言语测试材料之间具有良好的等价性,可应用于科研及临床对汉语人工耳蜗植入者的言语评估。     

成年人工耳蜗植入者对音乐音色感知的特点与分析

目的：评价语后聋成年人工耳蜗植入者对音乐音色的识别能力,探讨人工耳蜗植入者与听力正常者在音乐音色感知中的差异及可能的机制。方法实验组为人工耳蜗植入时间≥0．5年的成年语后聋患者30例,对照组为30例听力正常成人。使用人工耳蜗音乐评估软件（M u ．S ．I ．C ）对两组受试者进行不同音色（包括来自不同乐器家族且具有代表性的六种乐器以及男高音、女高音）识别能力的测试,记录并比较两组的识别正确率。结果①实验组音色识别正确率49．23％±18．93％,对照组90．23％±12．28％,后者高于前者（ P＜0．001）;实验组识别正确率：男高音（76．7％）最高、钢琴（73．3％）次之,吉他（23．3％）最低。②实验组对每种音色的错选率均高于对照组,且选择更为分散,其中将吉他误选为钢琴的错选率最高（66．7％）;其次为长笛错选为小号（33．3％）。结论人工耳蜗植入者对音乐音色的识别能力低于听力正常者,并且容易造成弦乐器之间、管乐器之间的混淆。     

Changes in Pitch with a Cochlear Implant Over Time

In the normal auditory system, the perceived pitch of a tone is closely linked to the cochlear place of vibration. It has generally been assumed that high-rate electrical stimulation by a cochlear implant electrode also evokes a pitch sensation corresponding to the electrode’s cochlear place (“place” code) and stimulation rate (“temporal” code). However, other factors may affect electric pitch sensation, such as a substantial loss of nearby nerve fibers or even higher-level perceptual changes due to experience. The goals of this study were to measure electric pitch sensations in hybrid (short-electrode) cochlear implant patients and to examine which factors might contribute to the perceived pitch. To look at effects of experience, electric pitch sensations were compared with acoustic tone references presented to the non-implanted ear at various stages of implant use, ranging from hookup to 5 years. Here, we show that electric pitch perception often shifts in frequency, sometimes by as much as two octaves, during the first few years of implant use. Additional pitch measurements in more recently implanted patients at shorter time intervals up to 1 year of implant use suggest two likely contributions to these observed pitch shifts: intersession variability (up to one octave) and slow, systematic changes over time. We also found that the early pitch sensations for a constant electrode location can vary greatly across subjects and that these variations are strongly correlated with speech reception performance. Specifically, patients with an early low-pitch sensation tend to perform poorly with the implant compared to those with an early high-pitch sensation, which may be linked to less nerve survival in the basal end of the cochlea in the low-pitch patients. In contrast, late pitch sensations show no correlation with speech perception. These results together suggest that early pitch sensations may more closely reflect peripheral innervation patterns, while later pitch sensations may reflect higher-level, experience-dependent changes. These pitch shifts over time not only raise questions for strict place-based theories of pitch perception, but also imply that experience may have a greater influence on cochlear implant perception than previously thought.     

Speech recognition with a cochlear implant using triphasic charge-balanced pulses

Objective Typically, symmetrical charge-balanced biphasic current pulses are used in cochlear implants to ensure biological safety. Theoretically, monophasic pulses are more effective, but potentially noxious, stimuli. In this study we charge-balanced such monophasic pulses during selected non-stimulated intervals, effectively leading to triphasic pulses with a 4:1 amplitude ratio between the cathodic and anodic phases. Apart from ensuring safety, this is also expected to reduce power consumption and channel interaction.

Material and Methods Seven experienced Clarion CII cochlear implant users with a multichannel (12–16 channels) monopolar continuous interleaved sampling (CIS) strategy participated in the study. Three different CIS strategies were fitted using the Clarion Research Interface (CRI-2). The reference was an implementation of each subject's own CIS program. The two strategies tested used triphasic pulses on the same channels, one with half-wave rectification (TP-HWR) and one without rectification (TP-NoR) at the input. Directly after fitting (i.e. without any training), speech perception (phoneme score on consonant–vowel–consonant words) was measured in silence (sound-only) and in speech-shaped background noise with signal:noise ratios (SNRs) of +5 and 0 dB.

Results Speech perception with the reference via the CRI-2 was equal to that of the free-field condition with the subjects’ own speech processor. With the TP-NoR strategy, speech perception improved significantly (from 89% to 93%) in silence and in the 0-dB SNR condition (from 43% to 49%). With a SNR of +5 dB, performance was stable at ≈66%. With the TP-HWR strategy, performance increased significantly in the 0- and +5-dB SNR conditions, to 55% and 74%, respectively. Power consumption was reduced in both strategies, to 30% and 36% for TP-HWR and TP-NoR, respectively.

Conclusion The new triphasic strategies are most promising, with respect to both their improved speech perception and reduced power requirements. The optimal parameters will have to be identified following long-term use.     

Optimizing Performance in Adult Cochlear Implant Users through Clinician Directed Auditory Training

Clinician-directed auditory training using the KTH Speech Tracking Procedure can be a powerful approach for maximizing outcomes with adult cochlear implant (CI) users. This article first reviews prior research findings from an 8-week clinician-directed auditory training (AT) program using speech tracking that yielded significant gains in speech tracking rate and sentence recognition scores following training. The second focus of the article is to illustrate the value of intensive face-to-face long-term AT using speech tracking with adult CI users. A detailed case study report is presented that demonstrates major ongoing and progressive gains in tracking rate, sentence recognition, and improvements in self-perceived competence and confidence over the course of intensive long-term training. Given the potential of both short- and long-term clinician-directed auditory training via KTH speech tracking to help CI users reach their optimal performance level, consideration for more widespread clinical use is proposed in the overall rehabilitation of adult CI users.     

Audio-Visual Speech Intelligibility Benefits with Bilateral Cochlear Implants when Talker Location Varies

One of the key benefits of using cochlear implants (CIs) in both ears rather than just one is improved localization. It is likely that in complex listening scenes, improved localization allows bilateral CI users to orient toward talkers to improve signal-to-noise ratios and gain access to visual cues, but to date, that conjecture has not been tested. To obtain an objective measure of that benefit, seven bilateral CI users were assessed for both auditory-only and audio-visual speech intelligibility in noise using a novel dynamic spatial audio-visual test paradigm. For each trial conducted in spatially distributed noise, first, an auditory-only cueing phrase that was spoken by one of four talkers was selected and presented from one of four locations. Shortly afterward, a target sentence was presented that was either audio-visual or, in another test configuration, audio-only and was spoken by the same talker and from the same location as the cueing phrase. During the target presentation, visual distractors were added at other spatial locations. Results showed that in terms of speech reception thresholds (SRTs), the average improvement for bilateral listening over the better performing ear alone was 9 dB for the audio-visual mode, and 3 dB for audition-alone. Comparison of bilateral performance for audio-visual and audition-alone showed that inclusion of visual cues led to an average SRT improvement of 5 dB. For unilateral device use, no such benefit arose, presumably due to the greatly reduced ability to localize the target talker to acquire visual information. The bilateral CI speech intelligibility advantage over the better ear in the present study is much larger than that previously reported for static talker locations and indicates greater everyday speech benefits and improved cost-benefit than estimated to date.     

The Effect of Gaussian Noise on the Threshold,Dynamic Range,and Loudness of Analogue Cochlear Implant Stimuli

The deliberate addition of Gaussian noise to cochlear implant signals has previously been proposed to enhance the time coding of signals by the cochlear nerve. Potentially, the addition of an inaudible level of noise could also have secondary benefits: it could lower the threshold to the information-bearing signal, and by desynchronization of nerve discharges, it could increase the level at which the information-bearing signal becomes uncomfortable. Both these effects would lead to an increased dynamic range, which might be expected to enhance speech comprehension and make the choice of cochlear implant compression parameters less critical (as with a wider dynamic range, small changes in the parameters would have less effect on loudness). The hypothesized secondary effects were investigated with eight users of the Clarion cochlear implant; the stimulation was analogue and monopolar. For presentations in noise, noise at 95% of the threshold level was applied simultaneously and independently to all the electrodes. The noise was found in two-alternative forced-choice (2AFC) experiments to decrease the threshold to sinusoidal stimuli (100 Hz, 1 kHz, 5 kHz) by about 2.0 dB and increase the dynamic range by 0.7 dB. Furthermore, in 2AFC loudness balance experiments, noise was found to decrease the loudness of moderate to intense stimuli. This suggests that loudness is partially coded by the degree of phase-locking of cochlear nerve fibers. The overall gain in dynamic range was modest, and more complex noise strategies, for example, using inhibition between the noise sources, may be required to get a clinically useful benefit.     

人工耳蜗使用者噪声环境下言语识别能力研究

目的:评估人工耳蜗(CI)植入者在噪声环境下的言语识别能力。方法:利用普通话版噪声下言语测试(MHINT)为言语测试工具,选取22例CI受试者,按照MHINT适应性得分规则,首先确定受试者在噪声下识别50%～74%的句子内容(评分规则3),达到该水平则继续进行75%以上难度内容(评分规则2或规则1),否则停止。获得受试个体在噪声下言语识别能力的语句接受阈、言语识别率和PI function曲线。结果:22例受试者中,5例在噪声下识别能力达80%～90%,3例可达60%～70%,7例达到50%,7例未能达到MHINT评分规则3测试。获得15例受试者的PI function并与听力正常人进行比较。结论:MHINT适应性得分规则测试法,可有效地获得CI植入者在噪声下的言语识别能力。与听力正常者相比,CI植入者的PI function显示右移10dB以上,在噪声环境下仍需要更好的信噪比。     

Speech perception in Mandarin-speaking children with cochlear implants: A systematic review

Abstract

Objective: This paper reviewed the literature on the trajectories and the factors significantly affecting post-implantation speech perception development in Mandarin-speaking children with cochlear implants (CIs). Design: A systematic literature search of textbooks and peer-reviewed published journal articles in online bibliographic databases was conducted. Study sample: PubMed, Scopus and Wiley online library were searched for eligible studies based on predefined inclusion and exclusion criteria. Results: A total of 14 journal articles were selected for this review. A number of consistent results were found. That is, children with CIs, as a group, exhibited steep improvement in early speech perception, from exhibiting few prelingual auditory behaviours before implantation to identifying sentences in noise after one year of CI use. After one to three years of CI use, children are expected to identify tones above chance and recognition of words in noise. In addition, early age at implantation, longer duration of CI use and higher maternal education level contributed to greater improvements in speech perception. Conclusions: Findings from this review will contribute to the establishment of appropriate short-term developmental goals for Mandarin-speaking children with CIs in mainland China and clinicians could use them to determine whether children have made appropriate progress with CIs.     

Examining the Electro-Neural Interface of Cochlear Implant Users Using Psychophysics,CT Scans,and Speech Understanding

This study examines the relationship between focused-stimulation thresholds, electrode positions, and speech understanding in deaf subjects treated with a cochlear implant (CI). Focused stimulation is more selective than monopolar stimulation, which excites broad regions of the cochlea, so may be more sensitive as a probe of neural survival patterns. Focused thresholds are on average higher and more variable across electrodes than monopolar thresholds. We presume that relatively high focused thresholds are the result of larger distances between the electrodes and the neurons. Two factors are likely to contribute to this distance: (1) the physical position of electrodes relative to the modiolus, where the excitable auditory neurons are normally located, and (2) the pattern of neural survival along the length of the cochlea, since local holes in the neural population will increase the distance between an electrode and the nearest neurons. Electrode-to-modiolus distance was measured from high-resolution CT scans of the cochleae of CI users whose focused-stimulation thresholds were also measured. A hierarchical set of linear models of electrode-to-modiolus distance versus threshold showed a significant increase in threshold with electrode-to-modiolus distance (average slope = 11 dB/mm). The residual of these models was hypothesized to reflect neural survival in each subject. Consonant–Nucleus–Consonant (CNC) word scores were significantly correlated with the within-subject variance of threshold (r² = 0.82), but not with within-subject variance of electrode distance (r² = 0.03). Speech understanding also significantly correlated with how well distance explained each subject’s threshold data (r² = 0.63). That is, subjects with focused thresholds that were well described by electrode position had better speech scores. Our results suggest that speech understanding is highly impacted by individual patterns of neural survival and that these patterns manifest themselves in how well (or poorly) electrode position predicts focused thresholds.     

Barriers to Cochlear Implant Access: Acknowledging the Challenges,Changing the Future: Determining Cochlear Implant Candidacy in Adults: Limitations,Expansions, and Opportunities for Improvement

The safety, efficacy, and success of cochlear implants (CIs) are well established and have led to changes in criteria used by clinicians to determine who should receive a CI. Such changes in clinical decision-making have out-paced the slower-occurring changes that have taken place with regulatory bodies'' and insurers'' indications. We review the historical development of indications for CIs, including those of the U.S. Food and Drug Administration (FDA), Medicare, Medicaid, and private insurers. We report on expansion to include patients with greater residual hearing, such as those who receive Hybrid and EAS devices, and report on recent FDA approvals that place less emphasis on the patient''s best-aided condition and greater emphasis on the ear to be treated. This includes expansion of CIs to patients with single-side deafness and asymmetric hearing loss. We review changes in the test materials used to determine candidacy, including transition from sentences in quiet to sentences in noise to the recent use of monosyllabic words and cognitive screening measures. Importantly, we discuss the recent trend to recommend CIs despite a patient not meeting FDA or insurers'' indications (a practice known as “off-label”), which serves as attestation that current indications need to be updated.     

Use of Amplitude Modulation Cues Recovered from Frequency Modulation for Cochlear Implant Users When Original Speech Cues Are Severely Degraded

Won et al. (J Acoust Soc Am 132:1113–1119, 2012) reported that cochlear implant (CI) speech processors generate amplitude-modulation (AM) cues recovered from broadband speech frequency modulation (FM) and that CI users can use these cues for speech identification in quiet. The present study was designed to extend this finding for a wide range of listening conditions, where the original speech cues were severely degraded by manipulating either the acoustic signals or the speech processor. The manipulation of the acoustic signals included the presentation of background noise, simulation of reverberation, and amplitude compression. The manipulation of the speech processor included changing the input dynamic range and the number of channels. For each of these conditions, multiple levels of speech degradation were tested. Speech identification was measured for CI users and compared for stimuli having both AM and FM information (intact condition) or FM information only (FM condition). Each manipulation degraded speech identification performance for both intact and FM conditions. Performance for the intact and FM conditions became similar for stimuli having the most severe degradations. Identification performance generally overlapped for the intact and FM conditions. Moreover, identification performance for the FM condition was better than chance performance even at the maximum level of distortion. Finally, significant correlations were found between speech identification scores for the intact and FM conditions. Altogether, these results suggest that despite poor frequency selectivity, CI users can make efficient use of AM cues recovered from speech FM in difficult listening situations.     

research advances in post-operative rehabilitation following cochlear implant

Cochlear implantation is a unique method to re-construct audition for patients with severe to profound hearing loss. With increasing clinical application of this technique, its rehabilitative effects in patients has become an important topic of hearing and speech rehabilitation research. This article is an overview of hearing and speech rehabilitative training, rehabilitative efficacy evaluation and factors affecting rehabilitation efficacy following cochlear implantation.     

Delayed Auditory Brainstem Responses in Prelingually Deaf and Late-Implanted Cochlear Implant Users

Neurophysiological studies in animals and humans suggest that severe hearing loss during early development impairs the maturation of the auditory brainstem. To date, studies in humans have mainly focused on the neural activation of the auditory brainstem in children treated with a cochlear implant (CI), but little is known about the pattern of activation in adult CI users with early onset of deafness (prelingual, before the age of 2 years). In this study, we compare auditory brainstem activation in prelingually deaf and late-implanted adult CI users to that in postlingually deaf CI users. Electrically evoked auditory brainstem responses (eABRs) were recorded by monopolar stimulation, separately using a middle and an apical electrode of the CI. Comparison of the eABR latencies revealed that wave V was significantly delayed in the prelingually deaf CI users on both electrode locations. Accordingly, when the apical electrode was stimulated, the III–V interwave interval was significantly longer in the prelingually deaf group. These findings suggest a slower neural conduction in the auditory brainstem, probably caused by impairment of maturation during the long duration of severe hearing loss in infancy. Shorter wave V latencies, reflecting a more mature brainstem, appeared to be a predictor for better speech perception.     

Evaluation of a non-linear spectral subtraction noise suppression scheme in cochlear implant users

Abstract

The aim of the study was to determine benefit to speech recognition in noise by adult cochlear implant users with the non-linear spectral subtraction (NSS) noise suppression strategy. Users of the Nucleus 22 or Nucleus 24 cochlear implant systems were tested with sentence materials combined with stationary noise at +5 and +10 dB signal to noise ratio (SNR), with and without NSS processing applied offline. Sentence scores were significantly higher with NSS processing, for both SNRs. The effect was greater at +5 dB SNR (12% improvement with NSS) than at +10 dB SNR (5% improvement with NSS). These results are promising and suggest that online implementation of NSS as part of cochlear implant processors has the potential to yield benefits for speech recognition in noise. Copyright © 2006 John Wiley & Sons, Ltd.