Comparisons of electrophysiological and psychophysical fitting methods for cochlear implants.

OBJECTIVE: This study compared two different versions of an electrophysiology-based software-guided cochlear implant fitting method with a procedure employing standard clinical software. The two versions used electrically evoked compound action potential (ECAP) thresholds for either five or all twenty-two electrodes to determine sound processor stimulation level profiles. Objective and subjective performance results were compared between software-guided and clinical fittings. DESIGN: Prospective, double-blind, single-subject repeated-measures with permuted ABCA sequences. STUDY SAMPLE: 48 post linguistically deafened adults with ≤15 years of severe-to-profound deafness who were newly unilaterally implanted with a Nucleus device. RESULTS: Speech recognition in noise and quiet was not significantly different between software- guided and standard methods, but there was a visit/learning-effect. However, the 5-electrode method gave scores on the SSQ speech subscale 0.5 points lower than the standard method. Clinicians judged usability for all methods as acceptable, as did subjects for comfort. Analysis of stimulation levels and ECAP thresholds suggested that the 5-electrode method could be refined. CONCLUSIONS: Speech recognition was not inferior using either version of the electrophysiology-based software-guided fitting method compared with the standard method. Subject-reported speech perception was slightly inferior with the five-electrode method. Software-guided methods saved about 10 min of clinician's time versus standard fittings.

A Retrospective Multicenter Study Comparing Speech Perception Outcomes for Bilateral Implantation and Bimodal Rehabilitation.

OBJECTIVES: To compare speech perception outcomes between bilateral implantation (cochlear implants [CIs]) and bimodal rehabilitation (one CI on one side plus one hearing aid [HA] on the other side) and to explore the clinical factors that may cause asymmetric performances in speech intelligibility between the two ears in case of bilateral implantation. DESIGN: Retrospective data from 2247 patients implanted since 2003 in 15 international centers were collected. Intelligibility scores, measured in quiet and in noise, were converted into percentile ranks to remove differences between centers. The influence of the listening mode among three independent groups, one CI alone (n = 1572), bimodal listening (CI/HA, n = 589), and bilateral CIs (CI/CI, n = 86), was compared in an analysis taking into account the influence of other factors such as duration of profound hearing loss, age, etiology, and duration of CI experience. No within-subject comparison (i.e., monitoring outcome modifications in CI/HA subjects becoming CI/CI) was possible from this dataset. Further analyses were conducted on the CI/CI subgroup to investigate a number of factors, such as implantation side, duration of hearing loss, amount of residual hearing, and use of HAs that may explain asymmetric performances of this subgroup. RESULTS: Intelligibility ranked scores in quiet and in noise were significantly greater with both CI/CI and CI/HA than with a CI-alone group, and improvement with CI/CI (+11% and +16% in quiet and in noise, respectively) was significantly better than with CI/HA (+6% and +9% in quiet and in noise, respectively). From the CI/HA group, only subjects with ranked preoperative aided speech scores >60% performed as well as CI/CI participants. Furthermore, CI/CI subjects displayed significantly lower preoperative aided speech scores on average compared with that displayed by CI/HA subjects. Routine clinical data available from the present database did not explain the asymmetrical results of bilateral implantation. CONCLUSIONS: This retrospective study, based on basic speech audiometry (no lateralization cues), indicates that, on average, a second CI is likely to provide slightly better postoperative speech outcome than an additional HA for people with very low preoperative performance. These results may be taken into consideration to refine surgical indications for CIs.

International Collegium of Rehabilitative Audiology (ICRA) recommendations for the construction of multilingual speech tests. ICRA Working Group on Multilingual Speech Tests.

OBJECTIVE: To provide guidelines for the development of two types of closed-set speech-perception tests that can be applied and interpreted in the same way across languages. The guidelines cover the digit triplet and the matrix sentence tests that are most commonly used to test speech recognition in noise. They were developed by a working group on Multilingual Speech Tests of the International Collegium of Rehabilitative Audiology (ICRA). DESIGN: The recommendations are based on reviews of existing evaluations of the digit triplet and matrix tests as well as on the research experience of members of the ICRA Working Group. They represent the results of a consensus process. RESULTS: The resulting recommendations deal with: Test design and word selection; Talker characteristics; Audio recording and stimulus preparation; Masking noise; Test administration; and Test validation. CONCLUSIONS: By following these guidelines for the development of any new test of this kind, clinicians and researchers working in any language will be able to perform tests whose results can be compared and combined in cross-language studies.

InterlACE Sound Coding for Unilateral and Bilateral Cochlear Implants.

OBJECTIVE: Cochlear implant signal processing strategies define the rules of how acoustic signals are converted into electrical stimulation patterns. Technological and anatomical limitations, however, impose constraints on the signal transmission and the accurate excitation of the auditory nerve. Acoustic signals are degraded throughout cochlear implant processing, and electrical signal interactions at the electrode-neuron interface constrain spectral and temporal precision. In this work, we propose a novel InterlACE signal processing strategy to counteract the occurring limitations. METHODS: By replacing the maxima selection of the Advanced Combination Encoder strategy with a method that defines spatially and temporally alternating channels, InterlACE can compensate for discarded signal content of the conventional processing. The strategy can be extended bilaterally by introducing synchronized timing and channel selection. InterlACE was explored unilaterally and bilaterally by assessing speech intelligibility and spectral resolution. Five experienced bilaterally implanted cochlear implant recipients participated in the Oldenburg Sentence Recognition Test in background noise and the spectral ripple discrimination task. RESULTS: The introduced alternating channel selection methodology shows promising outcomes for speech intelligibility but could not indicate better spectral ripple discrimination. CONCLUSION: InterlACE processing positively affects speech intelligibility, increases available unilateral and bilateral signal content, and may potentially counteract signal interactions at the electrode-neuron interface. SIGNIFICANCE: This work shows how cochlear implant channel selection can be modified and extended bilaterally. The clinical impact of the modifications needs to be explored with a larger sample size.

Visual activation of auditory cortex reflects maladaptive plasticity in cochlear implant users.

Cross-modal reorganization in the auditory cortex has been reported in deaf individuals. However, it is not well understood whether this compensatory reorganization induced by auditory deprivation recedes once the sensation of hearing is partially restored through a cochlear implant. The current study used electroencephalography source localization to examine cross-modal reorganization in the auditory cortex of post-lingually deafened cochlear implant users. We analysed visual-evoked potentials to parametrically modulated reversing chequerboard images between cochlear implant users (n = 11) and normal-hearing listeners (n = 11). The results revealed smaller P100 amplitudes and reduced visual cortex activation in cochlear implant users compared with normal-hearing listeners. At the P100 latency, cochlear implant users also showed activation in the right auditory cortex, which was inversely related to speech recognition ability with the cochlear implant. These results confirm a visual take-over in the auditory cortex of cochlear implant users. Incomplete reversal of this deafness-induced cortical reorganization might limit clinical benefit from a cochlear implant and help explain the high inter-subject variability in auditory speech comprehension.

Factors affecting auditory performance of postlinguistically deaf adults using cochlear implants: an update with 2251 patients.

OBJECTIVE: To update a 15-year-old study of 800 postlinguistically deaf adult patients showing how duration of severe to profound hearing loss, age at cochlear implantation (CI), age at onset of severe to profound hearing loss, etiology and CI experience affected CI outcome. STUDY DESIGN: Retrospective multicenter study. METHODS: Data from 2251 adult patients implanted since 2003 in 15 international centers were collected and speech scores in quiet were converted to percentile ranks to remove differences between centers. RESULTS: The negative effect of long duration of severe to profound hearing loss was less important in the new data than in 1996; the effects of age at CI and age at onset of severe to profound hearing loss were delayed until older ages; etiology had a smaller effect, and the effect of CI experience was greater with a steeper learning curve. Patients with longer durations of severe to profound hearing loss were less likely to improve with CI experience than patients with shorter duration of severe to profound hearing loss. CONCLUSIONS: The factors that were relevant in 1996 were still relevant in 2011, although their relative importance had changed. Relaxed patient selection criteria, improved clinical management of hearing loss, modifications of surgical practice, and improved devices may explain the differences.

Tinnitus before and 6 months after cochlear implantation.

In this prospective multicenter study, tinnitus loudness and tinnitus-related distress were investigated in 174 cochlear implant (CI) candidates who underwent CI surgery at a Swiss cochlear implant center. All subjects participated in two session, one preoperatively and one 6 months after device activation. In both sessions, tinnitus loudness was assessed using a visual analogue scale and tinnitus distress using a standardized tinnitus questionnaire. The data were compared with unaided pre- and postoperative pure tone thresholds, and postoperative speech reception scores. 71.8% of the subjects reported tinnitus preoperatively. Six months after CI surgery 20.0% of these reported abolition of their tinnitus, 51.2% a subjective improvement, 21.6% no change and 7.2% a deterioration. Of the 49 (28.2%) subjects with no tinnitus preoperatively, 5 developed tinnitus 6 months after CI. These 5 had poorer speech understanding after CI surgery with their device than the group who remained tinnitus free. We found no correlation between tinnitus improvement, age, duration of tinnitus, or change in unaided hearing thresholds between the two sessions.

Localization of virtual sound sources with bilateral hearing aids in realistic acoustical scenes.

Sound localization with hearing aids has traditionally been investigated in artificial laboratory settings. These settings are not representative of environments in which hearing aids are used. With individual Head-Related Transfer Functions (HRTFs) and room simulations, realistic environments can be reproduced and the performance of hearing aid algorithms can be evaluated. In this study, four different environments with background noise have been implemented in which listeners had to localize different sound sources. The HRTFs were measured inside the ear canals of the test subjects and by the microphones of Behind-The-Ear (BTEs) hearing aids. In the first experiment the system for virtual acoustics was evaluated by comparing perceptual sound localization results for the four scenes in a real room with a simulated one. In the second experiment, sound localization with three BTE algorithms, an omnidirectional microphone, a monaural cardioid-shaped beamformer and a monaural noise canceler, was examined. The results showed that the system for generating virtual environments is a reliable tool to evaluate sound localization with hearing aids. With BTE hearing aids localization performance decreased and the number of front-back confusions was at chance level. The beamformer, due to its directivity characteristics, allowed the listener to resolve the front-back ambiguity.

Predicting Cochlear Implant Electrode Placement Using Monopolar, Three-Point and Four-Point Impedance Measurements.

OBJECTIVE: This study aimed to investigate the relationship between cochlear implant (CI) electrode distances to the cochlea's inner wall (the modiolus) and electrical impedance measurements made at the CI's electrode contacts. We introduced a protocol for "three-point impedances" in which we recorded bipolar impedances in response to monopolar stimulation at a neighboring electrode. We aimed to assess the usability of three-point impedances and two existing CI impedance measurement methods (monopolar and four-point impedances) for predicting electrode positioning during CI insertion. METHODS: Impedances were recorded during stepwise CI electrode array insertions in cadaveric human temporal bones. The positioning of the electrodes with respect to the modiolus was assessed at each step using cone beam computed tomography. Linear mixed regression analysis was performed to assess the relationship between the impedances and electrode-modiolar distances. The experimental results were compared to clinical impedance data and to an existing lumped-element model of an implanted CI. RESULTS: Three-point and four-point impedances strongly correlated with electrode-modiolar distance. In contrast, monopolar impedances were only minimally affected by changes in electrode positioning with respect to the modiolus. An overall model specificity of 62% was achieved when incorporating all impedance parameters. This specificity could be increased beyond 73% when prior expectations of electrode positioning were incorporated in the model. CONCLUSION: Three-point and four-point impedances are promising measures to predict electrode-modiolar distance in real-time during CI insertion. SIGNIFICANCE: This work shows how electrical impedance measurements can be used to predict the CI's electrode positioning in a biologically realistic model.

Phenomenological model of auditory nerve population responses to cochlear implant stimulation.

BACKGROUND: Models of auditory nerve fiber (ANF) responses to electrical stimulation are helpful to develop advanced coding for cochlear implants (CIs). A phenomenological model of ANF population responses to CI electrical stimulation with a lower computational complexity compared to a biophysical model would be beneficial to evaluate new CI coding strategies. NEW METHOD: This study presents a phenomenological model which combines four temporal characteristics of ANFs (refractoriness, facilitation, accommodation and spike rate adaptation) in addition to a spatial spread of the electric field. RESULTS: The model predicts the performances of CI subjects in the melodic contour identification (MCI) experiment. The simulations for the MCI experiment were consistent with CI recipients' experimental outcomes that were not predictable from the electrical stimulation patterns themselves. COMPARISON WITH EXISTING METHODS: Previously, no phenomenological population model of ANFs has combined all four aforementioned temporal phenomena. CONCLUSIONS: The proposed model would help the further investigations of ANFs responses to different electrical stimulation patterns and comparison of different sound coding strategies in CIs.

Simultaneous Intra- and Extracochlear Electrocochleography During Cochlear Implantation to Enhance Response Interpretation.

The use of electrocochleography (ECochG) for providing real-time feedback of cochlear function during cochlear implantation is receiving increased attention for preventing cochlear trauma and preserving residual hearing. Although various studies investigated the relationship between intra-operative ECochG measurements and surgical outcomes in recent years, the limited interpretability of ECochG response changes leads to conflicting study results and prevents the adoption of this method for clinical use. Specifically, the movement of the recording electrode with respect to the different signal generators in intracochlear recordings makes the interpretation of signal changes with respect to cochlear trauma difficult. Here, we demonstrate that comparison of ECochG signals recorded simultaneously from intracochlear locations and from a fixed extracochlear location can potentially allow a differentiation between traumatic and atraumatic signal changes in intracochlear recordings. We measured ECochG responses to 500 Hz tone bursts with alternating starting phases during cochlear implant insertions in six human cochlear implant recipients. Our results show that an amplitude decrease with associated near 180° phase shift and harmonic distortions in the intracochlear difference curve during the first half of insertion was not accompanied by a decrease in the extracochlear difference curve's amplitude (n = 1), while late amplitude decreases in intracochlear difference curves (near full insertion, n = 2) did correspond to extracochlear amplitude decreases. These findings suggest a role for phase shifts, harmonic distortions, and recording location in interpreting intracochlear ECochG responses.

Evaluation of evoked potentials to dyadic tones after cochlear implantation.

Auditory evoked potentials are tools widely used to assess auditory cortex functions in clinical context. However, in cochlear implant users, electrophysiological measures are challenging due to implant-created artefacts in the EEG. Here, we used independent component analysis to reduce cochlear implant-related artefacts in event-related EEGs of cochlear implant users (n = 12), which allowed detailed spatio-temporal evaluation of auditory evoked potentials by means of dipole source analysis. The present study examined hemispheric asymmetries of auditory evoked potentials to musical sounds in cochlear implant users to evaluate the effect of this type of implantation on neuronal activity. In particular, implant users were presented with two dyadic tonal intervals in an active oddball design and in a passive listening condition. Principally, the results show that independent component analysis is an efficient approach that enables the study of neurophysiological mechanisms of restored auditory function in cochlear implant users. Moreover, our data indicate altered hemispheric asymmetries for dyadic tone processing in implant users compared with listeners with normal hearing (n = 12). We conclude that the evaluation of auditory evoked potentials are of major relevance to understanding auditory cortex function after cochlear implantation and could be of substantial clinical value by indicating the maturation/reorganization of the auditory system after implantation.

Multicenter evaluation of signal enhancement algorithms for hearing aids.

In the framework of the European HearCom project, promising signal enhancement algorithms were developed and evaluated for future use in hearing instruments. To assess the algorithms' performance, five of the algorithms were selected and implemented on a common real-time hardware/software platform. Four test centers in Belgium, The Netherlands, Germany, and Switzerland perceptually evaluated the algorithms. Listening tests were performed with large numbers of normal-hearing and hearing-impaired subjects. Three perceptual measures were used: speech reception threshold (SRT), listening effort scaling, and preference rating. Tests were carried out in two types of rooms. Speech was presented in multitalker babble arriving from one or three loudspeakers. In a pseudo-diffuse noise scenario, only one algorithm, the spatially preprocessed speech-distortion-weighted multi-channel Wiener filtering, provided a SRT improvement relative to the unprocessed condition. Despite the general lack of improvement in SRT, some algorithms were preferred over the unprocessed condition at all tested signal-to-noise ratios (SNRs). These effects were found across different subject groups and test sites. The listening effort scores were less consistent over test sites. For the algorithms that did not affect speech intelligibility, a reduction in listening effort was observed at 0 dB SNR.

Effect of changing pulse rate on profile parameters of perceptual thresholds and loudness comfort levels and relation to ECAP thresholds in recipients of the Nucleus CI24RE device.

The Nucleus CI24RE 'Freedom' device offers higher stimulation rates and lower noise levels in action potential measurements (ECAPs) than previous devices. A study including ten European implant teams showed that the effect of changes in rate from 250 to 3500 pulses per second on tilt and curvature of the T and C profiles is insignificant. When changing rate one may change the levels at all electrodes by the same amount. Using an automated procedure ECAPs could be measured quickly and reliably at a noise level of only 1 microV, this did not result in improved correlations between the tilt and curvature parameters of the ECAP profiles and those of the T and C profiles. Average C levels appear to differ markedly among implant centers; a better assessment protocol is required. When increasing stimulus rate one should take into account that this requires higher pulse charges per second and more power consumption.

Speech perception performance as a function of stimulus pulse rate and processing strategy preference for the Cochlear Nucleus CI24RE device: relation to perceptual threshold and loudness comfort profiles.

Current cochlear implants can operate at high pulse rates. The effect of increasing pulse rate on speech performance is not yet clear. Habituation to low rates may affect the outcome. This paper presents the results of three subsequent studies using different experimental paradigms, applying the Nucleus CI24RE device, and conducted by ten European implant teams. Pulse rate per channel varied from 500 to 3500 pulses per second with ACE and from 1200 to 3500 pps with CIS strategy. The results showed that the first rate presented had little effect on the finally preferred rate. Lower rates were preferred. The effect of pulse rate on word scores of post-linguistic implantees was small; high rates tended to give lower scores. However, there were no significant differences between the word scores across subjects if collected at the individually preferred pulse rate. High pulse rates were preferred when the post-implantation threshold was low.

A bio-inspired coding (BIC) strategy for cochlear implants.

A bio-inspired coding (BIC) strategy was implemented in this study with the goal of better representation of spectral and temporal information. The auditory nerve fibers' (ANFs) characteristics such as refractory recovery, facilitation and spatial spread were obtained from ECAP data recorded in 11 CI recipients. These characteristics, together with a non-individualized model-derived adaptation effect, were integrated into the BIC strategy for a better selection of channels. Two variations of the BIC strategy were compared to the conventional advanced combination encoder (ACE) coding strategy: the BIC-I strategy based on the individual CI recipients' ECAP parameters, and the BIC-G strategy based on the median values of ECAP parameters from all CI recipients who participated in the study. The melodic contour identification (MCI) and Oldenburg sentence recognition in noise (OLSA) tests were used to assess and compare the three coding strategies. A significantly better performance in the transformed MCI test results with the rationalized arcsine transformation, was observed for both BIC strategy variations compared to the ACE strategy. There was no significant difference between the two variations of the BIC strategy and the ACE strategy in the OLSA test. No correlation was found between recovery time constants, absolute refractory periods, left and right width of SOE functions from three test electrodes and CI recipients' performances in the two experiments. However, significant correlations were found between facilitation time constant and amplitude and the results of the MCI and OLSA tests for the two variations of the BIC strategy.

Measuring temporal response properties of auditory nerve fibers in cochlear implant recipients.

Auditory nerve fibers' (ANFs) refractoriness and facilitation can be quantified in electrically evoked compound action potentials (ECAPs) recorded via neural response telemetry (NRT). Although facilitation has been observed in animals and human cochlear implant (CI) recipients, no study has modeled this in human CI users until now. In this study, recovery and facilitation effects at different masker and probe levels for three test electrodes (E6, E12 and E18) in 11 CI subjects were recorded. The ECAP recovery and facilitation were modeled by exponential functions and the same function used for +10 CL masker offset condition can be applied to all other masker offsets measurements. Goodness of fit was evaluated for the exponential functions. A significant effect of probe level was observed on a recovery time constant which highlights the importance of recording the recovery function at the maximum acceptable stimulus level. Facilitation time constant and amplitude showed no dependency on the probe level. However, facilitation was stronger for masker level at or around the threshold of the ECAP (T-ECAP). There was a positive correlation between facilitation magnitude and amplitude growth function (AGF) slope, which indicates that CI subjects with better peripheral neural survival have stronger facilitation.

Neuroanatomical and resting state EEG power correlates of central hearing loss in older adults.

To gain more insight into central hearing loss, we investigated the relationship between cortical thickness and surface area, speech-relevant resting state EEG power, and above-threshold auditory measures in older adults and younger controls. Twenty-three older adults and 13 younger controls were tested with an adaptive auditory test battery to measure not only traditional pure-tone thresholds, but also above individual thresholds of temporal and spectral processing. The participants' speech recognition in noise (SiN) was evaluated, and a T1-weighted MRI image obtained for each participant. We then determined the cortical thickness (CT) and mean cortical surface area (CSA) of auditory and higher speech-relevant regions of interest (ROIs) with FreeSurfer. Further, we obtained resting state EEG from all participants as well as data on the intrinsic theta and gamma power lateralization, the latter in accordance with predictions of the Asymmetric Sampling in Time hypothesis regarding speech processing (Poeppel, Speech Commun 41:245-255, 2003). Methodological steps involved the calculation of age-related differences in behavior, anatomy and EEG power lateralization, followed by multiple regressions with anatomical ROIs as predictors for auditory performance. We then determined anatomical regressors for theta and gamma lateralization, and further constructed all regressions to investigate age as a moderator variable. Behavioral results indicated that older adults performed worse in temporal and spectral auditory tasks, and in SiN, despite having normal peripheral hearing as signaled by the audiogram. These behavioral age-related distinctions were accompanied by lower CT in all ROIs, while CSA was not different between the two age groups. Age modulated the regressions specifically in right auditory areas, where a thicker cortex was associated with better auditory performance in older adults. Moreover, a thicker right supratemporal sulcus predicted more rightward theta lateralization, indicating the functional relevance of the right auditory areas in older adults. The question how age-related cortical thinning and intrinsic EEG architecture relates to central hearing loss has so far not been addressed. Here, we provide the first neuroanatomical and neurofunctional evidence that cortical thinning and lateralization of speech-relevant frequency band power relates to the extent of age-related central hearing loss in older adults. The results are discussed within the current frameworks of speech processing and aging.

Investigating the use of a Gammatone filterbank for a cochlear implant coding strategy.

BACKGROUND: Contemporary speech processing strategies in cochlear implants (CIs) such as the Advanced Combination Encoder (ACE) use a standard Fast Fourier Transform (FFT) filterbank to extract envelopes. The assignment of the FFT bins to approximate the frequency resolution of the basilar membrane is only partly based on physiology, especially since the bins are distributed linearly below 1000Hz and logarithmically above 1000Hz. NEW METHOD: A Gammatone filterbank which provides a closer approximation to the bandwidths of filters in the human auditory system could replace the standard FFT filterbank in the ACE strategy. An infinite impulse response (IIR) all-pole design of the Gammatone filterbank was compared to the FFT filterbank with 128, 256 and 512 points resolutions and the effect of the frequency boundaries of the filters was also investigated. RESULTS: Melodic contour identification (MCI) and just noticeable difference (JND) experiments, both involving synthetic clarinet notes in octaves 3 and 4, were conducted with 6 normal hearing (NH) participants using noise vocoded stimuli; and 10 CI recipients just performed the MCI experiment. The MCI results for both NH and CI subjects, showed a significant effect of the filterbank on the percentage correct responses of the participants. COMPARISON WITH EXISTING METHODS: The Gammatone filterbank can better resolve the harmonics of tested synthetic clarinet notes which led to better performances in the MCI experiment. CONCLUSIONS: The total delay of the Gammatone filterbank can be made smaller than the delay of the FFT filterbank with the same frequency resolution at low frequencies.