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.

Facilitation and refractoriness of the electrically evoked compound action potential.

In this study we aim to resolve the contributions of facilitation and refractoriness at very short pulse intervals. Measurements of the refractory properties of the electrically evoked compound action potential (ECAP) of the auditory nerve in cochlear implant (CI) users at inter pulse intervals below 300 µs are influenced by facilitation and recovery effects. ECAPs were recorded using masker pulses with a wide range of current levels relative to the probe pulse levels, for three suprathreshold probe levels and pulse intervals from 13 to 200 µs. Evoked potentials were measured for 21 CI patients by using the masked response extraction artifact cancellation procedure. During analysis of the measurements the stimulation current was not used as absolute value, but in relation to the patient's individual ECAP threshold. This enabled a more general approach to describe facilitation as a probe level independent effect. Maximum facilitation was found for all tested inter pulse intervals at masker levels near patient's individual ECAP threshold, independent from probe level. For short inter pulse intervals an increased N1P1 amplitude was measured for subthreshold masker levels down to 120 CL below patient's individual ECAP threshold in contrast to the recreated state. ECAPs recorded with inter pulse intervals up to 200 µs are influenced by facilitation and recovery. Facilitation effects are most pronounced for masker levels at or below ECAP threshold, while recovery effects increase with higher masker levels above ECAP threshold. The local maximum of the ECAP amplitude for masker levels around ECAP threshold can be explained by the mutual influence of maximum facilitation and minimal refractoriness.

A clinical assessment of cochlear implant recipient performance: implications for individualized map settings in specific environments.

Individual speech intelligibility was measured in quiet and noise for cochlear Implant recipients upgrading from the Freedom to the CP900 series sound processor. The postlingually deafened participants (n = 23) used either Nucleus CI24RE or CI512 cochlear implant, and currently wore a Freedom sound processor. A significant group mean improvement in speech intelligibility was found in quiet (Freiburg monosyllabic words at 50 dBSPL) and in noise (adaptive Oldenburger sentences in noise) for the two CP900 series SmartSound programs compared to the Freedom program. Further analysis was carried out on individual's speech intelligibility outcomes in quiet and in noise. Results showed a significant improvement or decrement for some recipients when upgrading to the new programs. To further increase speech intelligibility outcomes when upgrading, an enhanced upgrade procedure is proposed that includes additional testing with different signal-processing schemes. Implications of this research are that future automated scene analysis and switching technologies could provide additional performance improvements by introducing individualized scene-dependent settings.

The Intra-Cochlear Impedance-Matrix (IIM) test for the Nucleus® cochlear implant.

OBJECTIVE: To describe the principles and operation of a new telemetry-based function test for the Nucleus® cochlear implant, known as the CS19 Intra-Cochlear Impedance Matrix (IIM) and to present results from a multicentre clinical study to establish reproducibility (test-retest reliability) and normative ranges. METHOD: The IIM test measures bipolar impedances between all electrode pairs and employs a normalization procedure based on common ground impedances in order to identify abnormal current paths among electrodes. Six European clinics collected IIM data from a total of 192 devices. RESULTS: Reproducibility was high between initial and repeat measurements. The normative analysis demonstrated narrow ranges among devices after normalization of impedance data. The IIM is able to identify abnormal current paths that are not evident from standard impedance telemetry and may otherwise only be found utilising average electrode voltage measurements (AEV). CONCLUSIONS: The IIM test was found to be straightforward to perform clinically and demonstrated reproducible data with narrow ranges in normally-functioning devices. Because this test uses a very low stimulation level the IIM test is well suited for children or multiply handicapped CI users who cannot reliably report on their auditory percepts. The new algorithms show potential to improve implant integrity testing capabilities if implemented in future clinical software.

Accuracy of measurement in electrically evoked compound action potentials.

BACKGROUND: Electrically evoked compound action potentials (ECAP) in cochlear implant (CI) patients are characterized by the amplitude of the N1P1 complex. The measurement of evoked potentials yields a combination of the measured signal with various noise components but for ECAP procedures performed in the clinical routine, only the averaged curve is accessible. To date no detailed analysis of error dimension has been published. The aim of this study was to determine the error of the N1P1 amplitude and to determine the factors that impact the outcome. NEW METHODS: Measurements were performed on 32 CI patients with either CI24RE (CA) or CI512 implants using the Software Custom Sound EP (Cochlear). N1P1 error approximation of non-averaged raw data consisting of recorded single-sweeps was compared to methods of error approximation based on mean curves. RESULTS: The error approximation of the N1P1 amplitude using averaged data showed comparable results to single-point error estimation. The error of the N1P1 amplitude depends on the number of averaging steps and amplification; in contrast, the error of the N1P1 amplitude is not dependent on the stimulus intensity. COMPARISON WITH EXISTING METHOD(S): Single-point error showed smaller N1P1 error and better coincidence with 1/√(N) function (N is the number of measured sweeps) compared to the known maximum-minimum criterion. CONCLUSION: Evaluation of N1P1 amplitude should be accompanied by indication of its error. The retrospective approximation of this measurement error from the averaged data available in clinically used software is possible and best done utilizing the D-trace in forward masking artefact reduction mode (no stimulation applied and recording contains only the switch-on-artefact).

Investigation of a matrix sentence test in noise: reproducibility and discrimination function in cochlear implant patients.

OBJECTIVE: The aim of this study was to describe common properties (reproducibility, discrimination function, and its steepness) of matrix tests used for cochlear implant (CI) users and to obtain data for the German-language version matrix test, the Oldenburg sentence test (OLSA), presented in noise. DESIGN: The speech reception thresholds (SRT) in noise were measured by means of an adaptive test procedure, and by measurement at various signal-to-noise ratios to determine the course of the entire discrimination function per subject. STUDY SAMPLE: The measurements were performed on 38 CI users fitted with a Cochlear(™) Freedom(®) or a Cochlear(™) Nucleus(®) 5 CI system. RESULTS: The test-retest reproducibility showed a significant dependence on the SRT in noise. For the better performers, the test-retest difference was found to be smaller, while for the poorer performers the difference increased. For the better performers, the slope of the discrimination function at SRT (s50) was comparable to that for individuals with normal hearing, while for the poorer performers the s50 tended to be significantly reduced. CONCLUSIONS: As the CI users differed significantly in their SRT and their s50, a unified discrimination function for CI users must not be used. Further tailoring of the procedure may be required, especially for poorer CI performers.

Evaluation of Neural Response Telemetry (NRT™) with focus on long-term rate adaptation over a wide range of stimulation rates.

Custom Sound EP™ (CSEP) is an advanced flexible software tool dedicated to recording of electrically evoked compound action potentials (ECAPs) in Nucleus® recipients using Neural Response Telemetry™ (NRT™). European multi-centre studies of the Freedom™ cochlear implant system confirmed that CSEP offers tools to effectively record ECAP thresholds, amplitude growth functions, recovery functions, spread of excitation functions, and rate adaptation functions and an automated algorithm (AutoNRT™) to measure threshold profiles. This paper reports on rate adaptation measurements. Rate adaptation of ECAP amplitudes can successfully be measured up to rates of 495 pulses per second (pps) by repeating conventional ECAP measurements and over a wide range of rates up to 8000 pps using the masked response extraction technique. Rate adaptation did not show a predictable relationship with speech perception and coding strategy channel rate preference. The masked response extraction method offers opportunities to study long-term rate adaptation with well-defined and controlled stimulation paradigms.

Results of a multicenter clinical study evaluating a new Smart algorithm to measure neural response imaging.

OBJECTIVE: To evaluate the smart algorithm in speed and reliability of threshold estimation compared with the algorithm available in the standard fitting software and to evaluate the possibility of using programs based on the smart algorithm instead of programs derived from behavioral measures. PATIENTS: Twenty subjects unilaterally implanted with a CII Bionic Ear or HiRes90K device. INTERVENTIONS: Neural response imaging thresholds (tNRI) were measured using both the smart approach within the Research Studies Platform for Objective Measures and the SoundWave fitting software. Measurements were performed intraoperatively, at first fitting, and after 3 months of implant use. Each subject received a standard behavioral program and a SmartNRI program. Speech perception tests were conducted at 3 months, and subjective preferences were documented. MAIN OUTCOME MEASURES: Smart tNRI and SoundWave tNRI at each session; speech test results and subject preferences at the 3-month session. RESULTS: High correlations were found between smart tNRI and SoundWave tNRI. The time required to obtain NRI thresholds with the smart algorithm was a quarter of the time needed with SoundWave. Although most tested subjects preferred their behavioral programs, there were no significant differences in performance between SmartNRI and behavioral programs. CONCLUSION: Neural response imaging thresholds were obtained more rapidly with the smart algorithm than with SoundWave. Because no differences were observed between SmartNRI and behavioral programs, SmartNRI programs may be a useful alternative to behavioral programs in difficult to fit cases, where user feedback is sometimes difficult to obtain.

Benefits of the HiRes 120 coding strategy combined with the Harmony processor in an adult European multicentre study.

CONCLUSION: The Harmony processor was found to be reliable, comfortable and offered a substantially increased battery life compared with the previous generation processor. No significant improvement in speech understanding with HiRes was demonstrated from objective measures, but the majority of subjects showed a clear subjective preference for the combination HiRes 120/Harmony processor. OBJECTIVES: To evaluate experience with the Harmony™ sound processor, together with the HiRes 120 strategy. METHODS: Postlingually deafened adults implanted with a CII or HiRes 90K were included and divided into three groups: (1) experienced users using the Platinum body-worn processor; (2) experienced users who had been using other processors; (3) new users with the Harmony processor from first fitting. The latter group entered a randomized crossover protocol where half were initially fitted with HiRes and half with HiRes 120. The initial strategy was used for 3 months and the alternative for a further 3 months. Speech perception tests and questionnaires were performed. RESULTS: The study included 65 subjects. Implementing HiRes 120 was straightforward. The speech test group results did not show significant differences between HiRes and HiRes 120. However, the questionnaires showed significantly higher ratings for HiRes 120 in some instances. Subjects were highly satisfied with the Harmony processor.

Clinical evaluation of cochlear implant sound coding taking into account conjectural masking functions, MP3000™.

Efficacy of the SPEAK and ACE coding strategies was compared with that of a new strategy, MP3000™, by 37 European implant centers including 221 subjects. The SPEAK and ACE strategies are based on selection of 8-10 spectral components with the highest levels, while MP3000 is based on the selection of only 4-6 components, with the highest levels relative to an estimate of the spread of masking. The pulse rate per component was fixed. No significant difference was found for the speech scores and for coding preference between the SPEAK/ACE and MP3000 strategies. Battery life was 24% longer for the MP3000 strategy. With MP3000 the best results were found for a selection of six components. In addition, the best results were found for a masking function with a low-frequency slope of 50 dB/Bark and a high-frequency slope of 37 dB/Bark (50/37) as compared to the other combinations examined of 40/30 and 20/15 dB/Bark. The best results found for the steepest slopes do not seem to agree with current estimates of the spread of masking in electrical stimulation. Future research might reveal if performance with respect to SPEAK/ACE can be enhanced by increasing the number of channels in MP3000 beyond 4-6 and it should shed more light on the optimum steepness of the slopes of the masking functions applied in MP3000.

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.

Auditory feedback does not influence random number generation: Evidence from profoundly deaf adults with cochlear implant.

Oral random number generation is a widely used neuropsychological task engaging a number of overlapping neural systems of attention, number representation, response generation, and working memory. Although phonological processing is known to be essential for random number generation no information exists on the significance of the auditory feedback of hearing one's own voice on task performance. We therefore examined the influence of auditory feedback in 15 profoundly deaf adults with cochlear implants in a device-on/off experiment. No significant effects of occluding auditory feedback on random number generation were noted, thus supporting an internal response-monitoring model independent of auditory condition.

Clinical results of AutoNRT, a completely automatic ECAP recording system for cochlear implants.

OBJECTIVE: AutoNRT is the completely automatic electrically evoked compound action potential (ECAP) measuring algorithm in the recently released Nucleus Freedom cochlear implant system. AutoNRT allows clinicians to automatically record T-NRT profiles that in turn can be used as a guide for initial fitting. The algorithm consists of a pattern recognition part that judges if the traces contain an ECAP and an intelligent flow that optimizes the measurement parameters and finds the ECAP threshold (T-NRT). The objective of this study was to determine how accurate, reliable, and fast the automatic measurements are. DESIGN: Data on more than 400 electrodes were collected as part of the multicenter clinical trial of the Nucleus Freedom cochlear implant system. T-NRT values determined by the algorithm were compared with T-NRT determinations on the same data by different human observers. Also, the time the measurements took was analyzed. RESULTS: In 90% of the cases, the absolute difference between the AutoNRT and the human observer determined T-NRT was less than 9 CL; the median absolute difference was 3 CL. A second experiment, in which a group of human observers were asked to analyze NRT data, showed high variability in T-NRT; in some cases, two experienced clinicians disagreed by more than 30 current levels. Compared with the group, AutoNRT performed as well as the "average" clinician, with the advantage that the AutoNRT threshold determinations are objective. Analysis of the timing data showed an average intraoperative measurement time of less than 20 sec per electrode with a standard deviation of 5 sec, suggesting that the total array of 22 electrodes can be measured intraoperatively in about 7 minutes on average. CONCLUSIONS: AutoNRT provides comparable accuracy to an average clinician but with the added benefit of significant time savings over manual recordings. This makes it a valuable tool for clinical measurement of ECAP threshold in cochlear implant recipients.

Measuring the refractoriness of the electrically stimulated auditory nerve.

Intracochlear recordings in cochlear implant recipients provide access to the electrically evoked compound action potential (ECAP). ECAP thresholds are potential predictors of speech processor map's threshold and comfortable loudness levels. The auditory nerve's refractory properties can influence these levels due to high-rate stimulation with interpulse intervals within the relative refractory period. Recovery functions were investigated at 84 stimulation sites in 14 Nucleus CI24 recipients using neural response telemetry and a modified forward masking technique. This technique introduces a reference masker-probe interval (MPI). In our study, an appropriate value between 300 and 375 micro s was determined for this reference MPI, and the use of a reference MPI of 300 micro s is suggested for recovery and amplitude growth functions. A median absolute refractory period of about 390 micro s and a median time constant of about 425 micro s were obtained by fitting an exponential model to the data. Hence, the auditory nerve is usually in relative refractory state when standard neural response telemetry forward masking is selected because of its default MPI of 500 micro s. This can bias the measurement of ECAP thresholds. Additionally, the shape of standard forward masking recovery functions was explained by the influence of latency shift of the neural response.

Impact to the head increases cochlear implant reimplantation rate in children.

OBJECTIVE: To describe the outcome of cochlear implantation in children and to discuss the cause and management of cochlear reimplantation. STUDY DESIGN: Retrospective chart review. METHODS: The medical records of 110 patients younger than 18 years of age, who underwent cochlear implantation at the Department of ORL, Head and Neck Surgery, of the University of Kiel, Germany, were reviewed for demographics, complications, and history of revision surgery. The patients had previously had implantation with either Nucleus (including the Contour) devices or MED-EL devices. RESULTS: Length of use before cochlear explanation ranged from 4 days to 3.9 years. Reimplantation was caused by traumatic device failure, wrong electrode insertion and infection of implanted area. Results indicated a reimplantation rate of 5.4% in children compared to 0.8% in adults, mostly resulting from the greater risk of children receiving an impact to the head. Postoperative performance data showed no decrease in scores taken before failure. CONCLUSIONS: Though young children who are developing their motor skills are probably at greater risk of a cochlear reimplantation resulting from device failure following head trauma, surgical revision with reimplantation can be performed safely and without decrement to performance.

Benefits of bilateral electrical stimulation with the nucleus cochlear implant in adults: 6-month postoperative results.

OBJECTIVE: To evaluate the benefits of bilateral electrical stimulation for hearing-impaired adult subjects using the Nucleus 24 cochlear implant in a multicenter study, and to compare and quantify performance on speech perception measures in quiet and in noise and localization ability for unilateral and bilateral cochlear implant use. DESIGN: : Repeated single subject measures were carried out for each subject, with each subject serving as their own control. Assessment of unilateral and bilateral listening conditions for performance on tests of speech comprehension and sound localization were performed. Speech comprehension measures were performed in quiet at 0 degree azimuth and in the presence of background noise simultaneously presented from the same speaker and spatially separated by 90 degrees, at S+45 degrees N45 degrees and at S-45 degrees N+45 degrees . Test materials included Freiburger monosyllabic words, Oldenburger sentences, and the Hochmair-Schulz-Moser sentences. Tests of localization were performed in the horizontal plane with 12 speaker locations 30 degrees apart using a shortened sentence stimulus from the Hochmair-Schulz-Moser sentences at two possible presentation levels of 55 and 70 dB sound pressure level for assessment of directionality. The binaural advantage provided by bilateral stimulation was calculated with respect to each ear separately, classified as either the better or poorer performing ear for each speech material in quiet and in noise test conditions. For localization of sound, the binaural advantage was compared with left and right ears separately. Paired comparisons for performance data in all conditions were carried out by considering measurements for each subject in different conditions as paired observations and applying the Student's t test to determine the statistical difference between the data sets. SETTING: Tertiary referral centers with a cochlear implant program. PATIENTS: Thirty-seven profoundly hearing-impaired adults were enrolled in the study, 22 simultaneously and 15 sequentially bilaterally implanted. All patients received the Nucleus 24 cochlear implant and used the Nucleus SPrint or ESPrit 3G speech processor, with the vast majority using the ACE speech coding strategy. RESULTS: For spatially separated speech in noise conditions, an interaural performance advantage for the ear closest to the speech source (i.e., with a superior signal to noise ratio) compared with that for the ear closest to the noise source (i.e., with an inferior signal to noise ratio) is consistently demonstrated regardless of whether it is the better or poorer performing ear closest to the speech signal. This is referred to as a significant binaural head-shadow benefit, resulting in a mean improvement between -10 dB and -11.4 dB in the critical signal to noise ratio required for 50% speech comprehension for the Olden-burger sentences and a mean improvement in the maximum score of 42% to 55% for the ear closest to the speech signal over the ear farthest away for the Hochmair-Schulz-Moser sentences. Bilateral stimulation is always observed to provide a performance advantage over the unilateral listening condition for either ear when ipsilateral to the noise source. In addition, as demonstrated by approximately half the subjects tested in noise with the Hochmair-Schulz-Moser sentences, a performance advantage of bilateral stimulation may be observed over the better ear alone when positioned ipsilateral to the speech signal, which is referred to as a binaural squelch effect. On average, for the group, this resulted in a statistically significant improvement in speech comprehension scores of 8% in the bilateral listening condition compared with the scores for the better ear alone. Through assessment of comprehension of coincidental speech in noise and speech in quiet, a significant benefit of binaural redundancy was noted for the group for Oldenburger sentence scores in noise and in quiet compared with unilateral scores for either ear and for the Freiburger monosyllabic words in quiet in comparison with the better ear alone scores. Binaural stimulation also led to a significant improvement in localization ability over either monaural condition, with the root mean square degrees of error reduced by 38 degrees compared with that observed for unilateral stimulation. CONCLUSION: Similar to what has been observed for bilateral acoustic stimulation in the past, bilateral electrical stimulation provides the foundation for the potential advantages of the head-shadow effect, providing a binaural head-shadow benefit and binaural auditory processing such as binaural redundancy and binaural squelch effects, all of which combine to lead to improved speech comprehension over unilateral listening conditions. The combination of improved speech comprehension and improved localization ability made available through bilateral electrical stimulation provides the necessary foundation to further assist the hearing-impaired listener to better cope with communication in the everyday listening situation both in noise and in quiet.

Measurement of the electrically evoked compound action potential via a neural response telemetry system.

The main aim of this study was to validate a new technique, neural response telemetry (NRT), for measuring the electrically evoked compound action potential in adult cochlear implant users via their Nucleus C124M implant. Thirty-eight adults were evaluated with a variety of measurement procedures with the NRT software. Electrically evoked compound action potentials were obtained in 31 of the 38 adults (81.6%) and in 132 of the 160 electrodes (82.5%) tested. In addition to validating this technique, we also established a set of default clinical test parameters.