Pam W. Dawson

Clinical Evaluation of Signal-to-Noise Ratio―Based Noise Reduction in Nucleus® Cochlear Implant Recipients

Objective: The aim of this study was to investigate whether a real-time noise reduction algorithm provided speech perception benefit for Cochlear™ Nucleus® cochlear implant recipients in the laboratory. Design: The noise reduction algorithm attenuated masker-dominated channels. It estimated the signal-to-noise ratio of each channel on a short-term basis from a single microphone input, using a recursive minimum statistics method. In this clinical evaluation, the algorithm was implemented in two programs (noise reduction programs 1 [NR1] and 2 [NR2]), which differed in their level of noise reduction. These programs used advanced combination encoder (ACE™) channel selection and were compared with ACE without noise reduction in 13 experienced cochlear implant subjects. An adaptive speech reception threshold (SRT) test provided the signal-to-noise ratio for 50% sentence intelligibility in three different types of noises: speech-weighted, cocktail party, and street-side city noise. Results: In all three noise types, mean SRTs for both NR programs were significantly better than those for ACE. The greatest improvement occurred for speech-weighted noise; the SRT benefit over ACE was 1.77 dB for NR1 and 2.14 dB for NR2. There were no significant differences in speech perception scores between the two NR programs. Subjects reported no degradation in sound quality with the experimental programs. Conclusions: The noise reduction algorithm was successful in improving sentence perception in speech-weighted noise, as well as in more dynamic types of background noise. The algorithm is currently being trialed in a behind-the-ear processor for take-home use.

Clinical evaluation of expanded input dynamic range in nucleus cochlear implants

Objective: The aim of this study was to investigate whether expanded instantaneous input dynamic ranges (IIDRs) in the Nucleus cochlear implant system benefit speech perception in the laboratory and listening in the real world. Design: Until recently, Nucleus cochlear implants have used an IIDR of approximately 30 dB. In this study, an IIDR of 31 dB was compared with 46 dB and 56 dB in the SPEAR3 research processor with nine adult implant recipients. Subjects were given two, 2-wk blocks of take-home experience with each of the three IIDRs. A single IIDR setting was used in each trial period. During the take-home experience with the expanded IIDRs, subjects used two programs: a standard program (with clinically measured electrode dynamic ranges) and a program with adjusted thresholds (decreased T levels). After each block of take-home experience, speech perception testing was conducted for CNC words in quiet (at 45 dB and 55 dB SPL) and for CUNY sentences in the presence of multi-taker babble. Results: On average, CNC word recognition at low presentation levels was significantly better with the 46 dB and 56 dB IIDRs, compared with the 31 dB IIDR; however, there was no significant difference between the 46 dB and 56 dB IIDR conditions. These benefits were greater for standard programs than for reduced T level programs. For CUNY sentences in babble, group results indicated no significant difference in performance across IIDR. The three IIDRs were rated similarly in real-life listening situations, and two of the subjects expressed tolerance problems with the expanded standard IIDRs. Conclusions: IIDRs of 46 and 56 dB provided benefit in accessing low-level speech without a decrement in sentence perception in babble. Most subjects accepted the standard, wider IIDR programs in everyday life. No significant differences were found between the 46 dB and 56 dB IIDR programs.

A Different Approach to Using Neural Response Telemetry for Automated Cochlear Implant Processor Programming

Objective: This study explores the theoretical relation between the psychophysically measured current levels required for sound processor fitting in cochlear implants and the objectively measured compound action potential threshold (as measured by Neural Response Telemetry, NRT). The objective was to gain understanding of the variability across implantees in this relation and determine possible ways (using objective measures) of improving the predictability of NRT thresholds for behavioral levels needed for mapping. Design: A model of how rate of stimulation affects loudness is presented. The model can be used to understand differences among implantees in the way that rate affects loudness and hence explain the disappointing correlation between NRT and psychophysical measures. Suggestions are made, based on the model, for additional information that may improve the usefulness of NRT measurements. One such option (measuring the effect of interphase gap on NRT amplitude) was experimentally explored in eight subjects (26 electrodes). It was hypothesized that the current change required to maintain equal NRT amplitude when interphase gap was changed from 8 to 45 &mgr;sec would be correlated with the offset between behavioral and NRT thresholds. Results: The above hypothesis was not supported by the data, and several possible reasons for this outcome are discussed. Conclusions: The loudness model provides useful insights into why NRT thresholds are not good predictors of the behavioral levels needed for mapping and how NRT might be made more useful by additional objective information. These insights should be investigated in further experimental studies.

Cochlear implant optimized noise reduction

Noise-reduction methods have provided significant improvements in speech perception for cochlear implant recipients, where only quality improvements have been found in hearing aid recipients. Recent psychoacoustic studies have suggested changes to noise-reduction techniques specifically for cochlear implants, due to differences between hearing aid recipient and cochlear implant recipient hearing. An optimized noise-reduction method was developed with significantly increased temporal smoothing of the signal-to-noise ratio estimate and a more aggressive gain function compared to current noise-reduction methods. This optimized noise-reduction algorithm was tested with 12 cochlear implant recipients over four test sessions. Speech perception was assessed through speech in noise tests with three noise types; speech-weighted noise, 20-talker babble and 4-talker babble. A significant speech perception improvement using optimized noise reduction over standard processing was found in babble noise and speech-weighted noise and over a current noise-reduction method in speech-weighted noise. Speech perception in quiet was not degraded. Listening quality testing for noise annoyance and overall preference found significant improvements over the standard processing and over a current noise-reduction method in speech-weighted and babble noise types. This optimized method has shown significant speech perception and quality improvements compared to the standard processing and a current noise-reduction method.

Investigating cochlear implant place‐pitch perception with the Modified Melodies test

Abstract There has been speculation that cochlear implant place-of-excitation cues could be more akin to the brightness attribute of timbre (the spectral profile) than to melodic pitch. As brightness can be ordered on a low-to-high scale, it would allow high scores on pitch-ranking tests. In contrast, the Modified Melodies test measures pitch perception in a melodic context. In each trial, a familiar melody was presented twice. In one presentation, randomly selected, the pitch was deliberately modified. The subjects task was to select the un-modified melody. Six Nucleus implant recipients were tested with melodies presented as pure tones in the frequency range C5–C6 (523–1046 Hz) through the ACE strategy on the Freedom processor. All subjects were able to identify incorrect melodic contours and three subjects were able to recognize errors in musical intervals. These results are consistent with the hypothesis that cochlear implant place cues alone are sufficient to convey a melody. Copyright

Evaluation of NRT and behavioral measures for MAPping elderly cochlear implant users

We investigated the acceptability of electrophysiologically derived MAPs and the effect of these MAPs on speech perception in elderly adults using Nucleus 24 cochlear implants. Eight implant recipients aged 75 years or older trialed an electrophysiologically derived MAP and a behavioral MAP. The electrophysiologically derived MAP was based on the threshold and maximum comfort level for electrode 10 and evoked compound action potential thresholds measured on six electrodes using neural response telemetry (NRT). Word perception at 55 dB SPL and sentence perception in noise at 70 dB SPL were assessed after six weeks take-home experience and again after an additional two weeks of experience. During the final two weeks of take-home experience participants indicated their preferred MAP for different listening situations. The NRT derived MAP estimated behavioral T levels well, but underestimated behavioral C levels for apical electrodes in some subjects. Speech perception with NRT derived MAPs was comparable to speech perception with behaviorally measured MAPs. MAPs estimated from NRT data provided good speech perception outcomes for elderly implant recipients and were well tolerated.

Factors Predicting Postoperative Unilateral and Bilateral Speech Recognition in Adult Cochlear Implant Recipients with Acoustic Hearing.

Objectives: The first objective was to examine factors that could be predictive of postoperative unilateral (cochlear implant alone) speech recognition ability in a group of subjects with greater degrees of preoperative acoustic hearing than has been previously examined. Second, the study aimed to identify factors predictive of speech recognition in the best-aided, bilateral listening condition. Design: Participants were 65 postlinguistically hearing-impaired adults with preoperative phoneme in quiet scores of greater than or equal to 46% in one or both ears. Preoperative demographic and audiometric factors were assessed as predictors of 12-month postoperative unilateral and bilateral monosyllabic word scores in quiet and of bilateral speech reception threshold (SRT) in babble. Results: The predictive regression model accounted for 34.1% of the variance in unilateral word recognition scores in quiet. Factors that predicted better scores included: a shorter duration of severe to profound hearing loss in the implanted ear; and poorer pure-tone-averaged thresholds in the contralateral ear. Predictive regression models of postimplantation bilateral function accounted for 36.0% of the variance for word scores in quiet, and 30.9% of the variance for SRT in noise. A shorter duration of severe to profound hearing loss in the implanted ear, a lower age at the time of implantation, and better contralateral hearing thresholds were associated with higher bilateral word recognition in quiet and SRT in noise. Conclusions: In this group of cochlear implant recipients with preoperative acoustic hearing, a shorter duration of severe to profound hearing loss in the implanted ear was shown to be predictive of better unilateral and bilateral outcomes. However, further research is warranted to better understand the impact of that factor in a larger number of subjects with long-term hearing impairment of greater than 30 years. Better contralateral hearing was associated with poorer unilateral word scores with the implanted ear alone, but better absolute bilateral speech recognition. As a result, it is clear that different models would need to be developed to predict unilateral and bilateral postimplantation scores.

Influence of contralateral acoustic hearing on adult bimodal outcomes after cochlear implantation.

Abstract Objective: To examine post-implantation benefit and time taken to acclimate to the cochlear implant for adult candidates with more hearing in the contralateral non-implanted ear than has been previously considered within local candidacy guidelines. Design: Prospective, within-subject experimental design. Study sample: Forty postlingual hearing-impaired adult subjects with a contralateral ear word score in quiet ranging from 27% to 100% (median 67%). Results: Post-implantation improvement of 2.4 dB and 4.0 dB was observed on a sentence in coincident babble test at presentation levels of 65 and 55 dB SPL respectively, and a 2.1 dB benefit in spatial release from masking (SRM) advantage observed when the noise location favoured the implanted side. Significant post-operative group mean change of between 2.1 and 3.0 was observed on the sub-scales of the speech, spatial, and qualities (SSQ) questionnaire. Degree of post-implantation speech reception threshold (SRT) benefit on the coincident babble test and on perception of soft speech and sounds in the environment was greater for subjects with less contralateral hearing. The degree of contralateral acoustic hearing did not affect time taken to acclimate to the device. Conclusions: The findings from this study support cochlear implantation for candidates with substantial acoustic hearing in the contralateral ear, and provide guidance regarding post-implantation expectations.

Results using the OPAL strategy in Mandarin speaking cochlear implant recipients

Abstract Objective: To evaluate the effectiveness of an experimental pitch-coding strategy for improving recognition of Mandarin lexical tone in cochlear implant (CI) recipients. Design: Adult CI recipients were tested on recognition of Mandarin tones in quiet and speech-shaped noise at a signal-to-noise ratio of +10 dB; Mandarin sentence speech-reception threshold (SRT) in speech-shaped noise; and pitch discrimination of synthetic complex-harmonic tones in quiet. Two versions of the experimental strategy were examined: (OPAL) linear (1:1) mapping of fundamental frequency (F0) to the coded modulation rate; and (OPAL+) transposed mapping of high F0s to a lower coded rate. Outcomes were compared to results using the clinical ACE™ strategy. Study sample: Five Mandarin speaking users of Nucleus® cochlear implants. Results: A small but significant benefit in recognition of lexical tones was observed using OPAL compared to ACE in noise, but not in quiet, and not for OPAL+ compared to ACE or OPAL in quiet or noise. Sentence SRTs were significantly better using OPAL+ and comparable using OPAL to those using ACE. No differences in pitch discrimination thresholds were observed across strategies. Conclusions: OPAL can provide benefits to Mandarin lexical tone recognition in moderately noisy conditions and preserve perception of Mandarin sentences in challenging noise conditions.

A Wavelet-Based Noise Reduction Algorithm and Its Clinical Evaluation in Cochlear Implants

Noise reduction is often essential for cochlear implant (CI) recipients to achieve acceptable speech perception in noisy environments. Most noise reduction algorithms applied to audio signals are based on time-frequency representations of the input, such as the Fourier transform. Algorithms based on other representations may also be able to provide comparable or improved speech perception and listening quality improvements. In this paper, a noise reduction algorithm for CI sound processing is proposed based on the wavelet transform. The algorithm uses a dual-tree complex discrete wavelet transform followed by shrinkage of the wavelet coefficients based on a statistical estimation of the variance of the noise. The proposed noise reduction algorithm was evaluated by comparing its performance to those of many existing wavelet-based algorithms. The speech transmission index (STI) of the proposed algorithm is significantly better than other tested algorithms for the speech-weighted noise of different levels of signal to noise ratio. The effectiveness of the proposed system was clinically evaluated with CI recipients. A significant improvement in speech perception of 1.9 dB was found on average in speech weighted noise.