Wai Kong Lai

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 CI24M 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.

Normative Findings of Electrically Evoked Compound Action Potential Measurements Using the Neural Response Telemetry of the Nucleus CI24M Cochlear Implant System

One hundred and forty-seven adult recipients of the Nucleus® 24 cochlear implant system, from 13 different European countries, were tested using neural response telemetry to measure the electrically evoked compound action potential (ECAP), according to a standardised postoperative measurement procedure. Recordings were obtained in 96% of these subjects with this standardised procedure. The group results are presented in terms of peak amplitude and latency, slope of the amplitude growth function and ECAP threshold. The effects of aetiological factors and the duration of deafness on the ECAP were also studied. While large intersubject variability and intrasubject variability (across electrodes) were found, results fell within a consistent pattern and a normative range of peak amplitudes and latencies was established. The aetiological factors had little effect on the ECAP characteristics. However, age affected ECAP amplitude and slope of the amplitude growth function significantly; i.e., the amplitude is higher in the lowest age category (15–30 years). Principal component analysis of the ECAP thresholds shows that the thresholds across 5 electrodes can be described by two factors accounting for 92% of the total variance. The two factors represent the overall level of the threshold profiles (‘shift’) and their slopes across the electrode array (‘tilt’). Correlation between these two factors and the same factors describing the T- and C-levels appeared to be moderate, in the range of 0.5–0.6.

A Simple Two-Component Model of the Electrically Evoked Compound Action Potential in the Human Cochlea

Neural response telemetry (NRT) permits in situ intracochlear recordings of the electrically evoked compound action potential from the auditory nerves using scala tympani electrodes. The recorded NRT waveforms can generally be categorized under either single positive peak or double positive peak waveforms. This is similar to the observations from Stypulkowski and van den Honert, who suggested that the double peak complex arises from two components that could be axonal and dendritic in origin, respectively. Using a simple mathematical model which linearly combines two separate waveforms similar in shape but differing in amplitude and latencies, it was possible to simulate the various NRT waveform categories. The simulation results support the view that the two waveform components originate from dendritic or axonal processes and implies that the shape of the response waveform may provide information about the degree of neural survival in the stimulated cochlea. This information could be useful for determining optimal speech coding parameters for cochlear implant users on an individual basis.

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.

Longitudinal behaviour of neural response telemetry (NRT) data and clinical implications

Neural response telemetry (NRT) data from 63 subjects equipped with the Nucleus CI24M Cochlear Implant System generally exhibited little change over up to 4years. Larger changes, when they occurred, were seen only within the first 15 months postoperatively, and these changes diminished over time. Intraoperative NRT data were generally stable enough to be used for assisting in the initial speech processor fitting sessions. It was not possible to predict changes in the subjective map threshold and comfortable loudness levels (T and C levels, respectively) based on observed changes in the NRT data. The long-term stability of the neural response amplitude and the neural response threshold, however, implies that NRT may be useful as a routine diagnostic tool to detect changes to the neural periphery over time. Sumario Los datos sobre la telemetría de respuesta neural (NRT) en 63 sujetos que utilizan un Sistema de Implante Coclear Nucleus CI24M exhibieron pocos cambios durante un período de hasta 4 años. Los cambios mayores, cuando existieron, fueron vistos solamente dentro de los primeros 15 meses posteriores a la cirugía, y estos cambios disminuyeron con el tiempo. Los datos de la NRT intra-operatoria fueron tan estables como para ser utilizados como apoyo en la sesión inicial de adaptación del procesador de lenguaje. No fue posible predecir los cambios en el mapa subjetivo de umbrales y de niveles confortables de sonoridad (niveles T y C, respectivamente), con base en los cambios observados en los datos de la NRT. La estabilidad a largo plazo del umbral y la amplitud de la respuesta neural, sin embargo, implica que la NRT puede ser útil como una herramienta diagnóstica rutinaria para detectar cambios en el tiempo en la periferia neural.

A software tool for analyzing multichannel cochlear implant signals.

A useful and convenient means to analyze the radio frequency (RF) signals being sent by a speech processor to a cochlear implant would be to actually capture and display them with appropriate software. This is particularly useful for development or diagnostic purposes. sCILab (Swiss Cochlear Implant Laboratory) is such a PC-based software tool intended for the Nucleus family of Multichannel Cochlear Implants. Its graphical user interface provides a convenient and intuitive means for visualizing and analyzing the signals encoding speech information. Both numerical and graphic displays are available for detailed examination of the captured CI signals, as well as an acoustic simulation of these CI signals. sCILab has been used in the design and verification of new speech coding strategies, and has also been applied as an analytical tool in studies of how different parameter settings of existing speech coding strategies affect speech perception. As a diagnostic tool, it is also useful for troubleshooting problems with the external equipment of the cochlear implant systems.

Comparing neural response telemetry amplitude growth functions with loudness growth functions: preliminary results

Comparisons of the subjective loudness growth function and the objective evoked compound action potential (ECAP) amplitude growth function indicate that both functions are exponential in nature. This implies that a more accurate estimate of the ECAP threshold would be obtained using exponential regression of the amplitude growth function instead of the currently used linear regression. The perceptual threshold and the ECAP threshold seem to approach each other when the stimulation rate is lowered to reduce temporal summation effects. The effect of the stimulation rate on the perceptual threshold will have to be taken into account when trying to use the ECAP threshold for predicting the perceptual threshold.

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.

Abstract 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. Sumario Los implantes cocleares actuales puede operara a tasas de pulso altas. El efecto de la tasa de pulso en aumento sobre el desempeño en el lenguaje aún no es claro. La habituación a tasas bajas puede afectar el resultado. Este trabajo presenta los resultados de tres estudios subsiguientes que usaron diferentes paradigmas experimentales, utilizando el dispositivo Nucleus CI24RE, y llevado a cabo por diez equipos europeos de implante. La tasa de pulso por canal varió entre 500 a 3500 pulsos por segundo con la estrategia ACE y de 1200 a 3500 pps con la estrategia CIS. Los resultados mostraron que la primera tasa presentada tenía poco efecto sobre la tasa preferida final. Se prefirieron tasas más bajas. El efecto de la tasa de pulso sobre los puntajes de reconocimiento de palabras en implantados post-lingüísticos fue pequeño; tasas más altas tendieron a dar puntajes más bajos. Sin embargo, no existieron diferencias significativas entre los puntajes de reconocimiento de palabras en los diferentes sujetos si se recogían a la tasa preferida individual de pulso. Se prefirieron tasas de pulso más altas cuando el umbral post-implantación era bajo.

A Series of Case Studies of Tinnitus Suppression With Mixed Background Stimuli in a Cochlear Implant

PURPOSE Background sounds provided by a wearable sound playback device were mixed with the acoustical input picked up by a cochlear implant speech processor in an attempt to suppress tinnitus. METHOD First, patients were allowed to listen to several sounds and to select up to 4 sounds that they thought might be effective. These stimuli were programmed to loop continuously in the wearable playback device. Second, subjects were instructed to use 1 background sound each day on the wearable device, and they sequenced the selected background sounds during a 28-day trial. Patients were instructed to go to a website at the end of each day and rate the loudness and annoyance of the tinnitus as well as the acceptability of the background sound. Patients completed the Tinnitus Primary Function Questionnaire (Tyler, Stocking, Secor, & Slattery, 2014) at the beginning of the trial. RESULTS Results indicated that background sounds were very effective at suppressing tinnitus. There was considerable variability in sounds preferred by the subjects. CONCLUSION The study shows that a background sound mixed with the microphone input can be effective for suppressing tinnitus during daily use of the sound processor in selected cochlear implant users.

A neural-based vocoder implementation for evaluating cochlear implant coding strategies ☆

Most simulations of cochlear implant (CI) coding strategies rely on standard vocoders that are based on purely signal processing techniques. However, these models neither account for various biophysical phenomena, such as neural stochasticity and refractoriness, nor for effects of electrical stimulation, such as spectral smearing as a function of stimulus intensity. In this paper, a neural model that accounts for stochastic firing, parasitic spread of excitation across neuron populations, and neuronal refractoriness, was developed and augmented as a preprocessing stage for a standard 22-channel noise-band vocoder. This model was used to subjectively and objectively assess consonant discrimination in commercial and experimental coding strategies. Stimuli consisting of consonant-vowel (CV) and vowel-consonant-vowel (VCV) tokens were processed by either the Advanced Combination Encoder (ACE) or the Excitability Controlled Coding (ECC) strategies, and later resynthesized to audio using the aforementioned vocoder model. Baseline performance was measured using unprocessed versions of the speech tokens. Behavioural responses were collected from seven normal hearing (NH) volunteers, while EEG data were recorded from five NH participants. Psychophysical results indicate that while there may be a difference in consonant perception between the two tested coding strategies, mismatch negativity (MMN) waveforms do not show any marked trends in CV or VCV contrast discrimination.