Matthias Hey

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°N45° and at S−45°N+45°. 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.

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.

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

Abstract 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.

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

Abstract 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.

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.

Abstract 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 μV. However, 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. Sumario El dispositivo Nucleus CI24RE “Freedom” ofrece tasas de estimulación mayores, y menores niveles de ruido en las mediciones del potencial de acción (ECAPs) que los dispositivos anteriores. Un estudio que incluyó diez grupos europeos de implantes, mostró que el efecto de los cambios en la tasa de 250 a 3500 pulsos por segundo en la inclinación y la curvatura de los perfiles T y C es insignificante. Cuando se cambian las tasas, uno puede cambiar los niveles de todos los electrodos en la misma medida. Utilizando un procedimiento automatizado, los ECAPs pueden medirse en forma rápida y confiable a un nivel de ruido de solo 1mV. Sin embargo, esto no conlleva (a) una mejoría en las correlaciones entre los parámetros de inclinación y de curvatura de los perfiles ECAP y aquellos de los perfiles de T y C. Los niveles C promedio parecen diferir marcadamente entre los centros de implante; se requiere un mejor protocolo de evaluación. Cuando se incrementa la tasa de estimulación uno debe tomar en cuenta que eso requiere una mayor carga de pulsos por segundo y mayor poder de consumo.

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).

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

Abstract 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.

Facilitation and refractoriness of the electrically evoked compound action potential

Abstract 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 &mgr;s 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 &mgr;s. 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 patients 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 patients 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 patients individual ECAP threshold in contrast to the recreated state. ECAPs recorded with inter pulse intervals up to 200 &mgr;s 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. HighlightsFacilitation for short pulse intervals is maximal for masker level at ECAP threshold.Facilitation is still present for masker level far below ECAP threshold.The amount of facilitation is independent of probe level.Refractoriness is dependent of probe level.Facilitation decreases logarithmically depending on pulse interval.