Christian Berger-Vachon

Speech auditory brainstem response (speech ABR) characteristics depending on recording conditions, and hearing status An experimental parametric study.

Speech elicited auditory brainstem responses (Speech ABR) have been shown to be an objective measurement of speech processing in the brainstem. Given the simultaneous stimulation and recording, and the similarities between the recording and the speech stimulus envelope, there is a great risk of artefactual recordings. This study sought to systematically investigate the source of artefactual contamination in Speech ABR response. In a first part, we measured the sound level thresholds over which artefactual responses were obtained, for different types of transducers and experimental setup parameters. A watermelon model was used to model the human head susceptibility to electromagnetic artefact. It was found that impedances between the electrodes had a great effect on electromagnetic susceptibility and that the most prominent artefact is due to the transducers electromagnetic leakage. The only artefact-free condition was obtained with insert-earphones shielded in a Faraday cage linked to common ground. In a second part of the study, using the previously defined artefact-free condition, we recorded speech ABR in unilateral deaf subjects and bilateral normal hearing subjects. In an additional control condition, Speech ABR was recorded with the insert-earphones used to deliver the stimulation, unplugged from the ears, so that the subjects did not perceive the stimulus. No responses were obtained from the deaf ear of unilaterally hearing impaired subjects, nor in the insert-out-of-the-ear condition in all the subjects, showing that Speech ABR reflects the functioning of the auditory pathways.

Influence of stimulus frequency on NRT recordings.

Research in neural response telemetry (NRT) with the Cochlear ‘Nucleus’ CI24M Implant began a few years ago, using the first NRT software version (2.04). It has been demonstrated that NRT offers possibilities for fitting the implant speech processor. NRT sessions, however, remain lengthy for the patients, especially for children, and NRT research is seeking to reduce session time. The new version of the NRT software (3.0) allows implant stimulation up to 400 Hz, whereas the previous version stimulated only up to 80 Hz. The time gained is very significant. The goal of the present study was to observe and assess the modifications in NRT produced by these higher stimulus frequencies. Growth functions were measured in six adult patients for electrodes 5–10–15–20 at frequencies of 20, 80, 150, 250 and 365 Hz to determine NRT thresholds (NRT-T). Subjective detection thresholds (T) were also measured for the same electrodes and frequencies. The results showed that the number of valid responses, like the quality and amplitude of the NRT response, quickly decreased at these higher rates. Consequently, the number of measurement points used for the growth function decreased with frequency. Moreover, when frequency increased, the T value decreased while NRT-T increased: the gap between the two thresholds increased with frequency in a linear manner common to all patients. The growth function slopes did not change significantly with frequency. Sumario La investigación sobre telemetría de respuesta neural (NRT) con el implante Cochlear Nucleus CI24M empezó hace pocos años, al usarse la primera versión del programa (2.04). Se ha demostrado que la NRT hace posible la adaptación del procesador del lenguaje. No obstante, las sesiones de NRT son largas, especialmente con niños, por lo que la investigación trata de reducir el tiempo de las mismas. La nueva versión del programa de NRT (3.0) permite estimular el implante hasta 400 Hz en tanto la anterior solo estimulaba hasta 80 Hz. El ahorro de tiempo es significativo. El objetivo de este estudio fue observar y evaluar las modificaciones en la NRT con estas frecuencias superiores. El crecimiento de las funciones se midió en 6 adultos, en los electrodos 5, 10, 15 y 20 y en las frecuencias de 20, 80, 150, 250 y 365 Hz para determinar los umbrales de NRT (NRT-T). Los umbrales subjetivos de detección también se midieron en los mismos electrodos y frecuencias. Los resultados muestran que el número de respuestas válidas, como la calidad y amplitud de las respuestas NRT rápidamente disminuyó con tasas mayores. Así, el número de puntos de medición usado para el crecimiento de la función disminuyó con la frecuencia. Más aún, al aumentar la frecuencia, disminuyó el valor T mientras que el NRT-T aumentó: la diferencia entre los dos umbrales aumentó con la frecuencia en forma linear, similar en todos los pacientes. La pendiente de la función de crecimiento no cambió significativamente con la frecuencia.

Effect of signal duration on categorical loudness scaling in normal and in hearing-impaired listeners.

The present study sought to determine whether the duration of white-noise bursts affects their loudness category rating in the same way for hearing-impaired as for normally-hearing subjects. Twelve normally-hearing and 12 hearing-impaired subjects took part. Categorical loudness growth functions were obtained for 16.25 ms, 32.5 ms, 75 ms, 150 ms and 300 ms white noise bursts. Temporal integration of loudness was defined as the intensity difference needed for stimuli of different durations to result in identical category ratings. In normally-hearing subjects, temporal integration of loudness occurred mainly with the short-duration (16.25 ms and 32.5 ms) stimuli, whereas it was found with almost every stimulus duration in hearing-impaired subjects. In other words, temporal integration of loudness between 16.25 ms and 300 ms stimulus duration was greater in hearing-impaired listeners and there was a difference between normal and hearing-impaired subjects regarding change in loudness perception with stimulus duration. Consequently, the use of fixed-duration stimuli hinders loudness normalization.

Temporal loudness integration and spectral loudness summation in normal-hearing and hearing-impaired listeners.

The aim of this study was to test for differences between normal-hearing and hearing-impaired listeners regarding two fundamental aspects of intensity perception: loudness integration and loudness summation. Loudness functions for three different stimuli were measured using categorical loudness scaling in 8 normal-hearing and 12 hearing-impaired subjects. The results indicated that temporal loudness integration, defined as the difference in SPL between 16.25-ms and 300-ms noise bursts of equal loudness, was larger in the hearing-impaired than in the normal-hearing listeners. Loudness summation, defined as the difference in SPL between a 300-ms, 1,600-Hz tone pip and a white noise burst of the same duration and loudness, did not differ between the two groups. Implications of these results for hearing aid fitting strategies based on loudness normalization are discussed.

Assessing auditory nerve recovery function with a modified subtraction method: results and mathematical modeling

OBJECTIVE One of the main difficulties in electrical compound action potential (ECAP) recordings is to reduce the stimulus artifact due to electrical stimulation. The neural response telemetry (NRT) system of the Nucleus 24 cochlear implant extracts the ECAP response using a forward-masking (standard) subtraction technique. However, it has been shown that this subtraction technique may distort the ECAP responses in certain situations. In order to improve ECAP recordings, a modified forward-masking subtraction technique was recently proposed (Ear Hear. 21 (2000) 280). This modified subtraction technique can be applied to recovery function measurement. The objective of this study is to compare results obtained with the modified method to those obtained using the standard method. METHODS ECAP responses were recorded in 4 adult patients using a Nucleus 24 cochlear implant. Data were collected for the 20 active electrodes. For each electrode, measurements consisted of the recovery function recording using 16 different Masker-Probe intervals. The modified method was then applied and the results compared with the standard method. RESULTS Comparison between the two methods revealed that results were different when using the standard or modified method. Using the modified method, more ECAP responses were obtained (61.8 vs. 44.2%), but the P1 peak was sometimes attenuated; when using the standard method, N1 peak was missing in many cases. A mathematical model has been established and the mathematical simulation confirms the results obtained in patients. CONCLUSIONS The results suggest that both methods have limitations and advantages. The modified subtraction method seems to be better for analyzing ECAP recordings in recovery function measurement because of the higher number of responses obtained compared to the standard method.

Simulation of an EAS Implant with a Hybrid Vocoder

Although EAS electrode arrays are shorter, they have the same number of electrodes as conventional implant arrays. The possibility of channel interaction already exists with a standard length electrode array. As electrodes on an EAS array are closer together, the likelihood of channel interaction is even greater. To compensate for this, we proposed to deactivate several channels on the electrode array, on the premise that the reduction in speech intelligibility due to fewer channels would be offset by the low frequency residual hearing. Recent studies have demonstrated that low frequency residual hearing provides significant benefit for speech understanding (Dorman et al., 2005; Turner et al., 2004). This study aims to determine the number of channels necessary to provide good speech understanding according to the degree of low frequency residual hearing, particularly the boundary or cutoff frequency (Fc) between acoustic and electrical stimulation. This study utilized an acoustic simulation of an EAS device in normal-hearing subjects.

Comparison of speech vs speech‐analogue temporal encoding in human: An auditory brainstem response and categorical perception assessment

Temporal speech cues, such as periodicity and voice onset time (VOT), are particularly salient information for speech intelligibility. In addition, speech elicited auditory brainstem response (Speech ABR) to consonant vowels (CV) has been shown to mimic the temporal periodicity of the acoustic signal very accurately. Consequently, Speech ABR has been described as a potential objective tool to explore the temporal integration of speech excerpts in human brainstem. We aimed at putting in evidence the specialization of temporal encoding of speech vs nonspeech signals through clinical and behavioral measurements. A sum of five sinusoids, at the five formant frequencies, modulated by the corresponding CV temporal envelope, was taken as speech‐analogue. Accordingly, Speech and Analogue ABRs components were compared, although a priori due to be similar. Furthermore, categorical perception assessment was processed to both analogue and full‐speech continuums. If confirmed, these preliminary results (seven normal h...

Chapter 16 – Contribution of Noise Reduction Algorithms: Perception Versus Localization Simulation in the Case of Binaural Cochlear Implant (BCI) Coding

Communication and warning are two basic tasks devoted to the auditory system and it is worth to see them together when assistive techniques are considered for hearing rehabilitation. French phonemes recognition in noisy conditions and acoustic source localization, in the case of a cochlear implant (CI) coding simulation, are compared in this chapter. Three binaural noise reduction systems have been considered: the Beamformer algorithm, the Doerbeckers processing combined with the Ephraim & Malahs noise estimator or with the Scalarts noise reduction strategy. This study has been conducted with twenty normally hearing subjects. Results show that the Beamformer algorithm and the Doerbeckers processing improved the phoneme recognition scores. The best results in recognition were obtained using the Beamformer algorithm. On the contrary, the beamformer algorithm and the Doerbeckers processing lowered the source localization. A small reinjection of the signal (20%) was profitable to the Beamformer algorithm; this improvement was not seen with the Doerbeckers processing.

A Filtered Speech Test to Better Evaluate Electric Acoustic Stimulation (EAS) Candidacy

Electric Acoustic Stimulation (EAS) is a new treatment option for partial deafness. Most of these patients are unable to use their high frequencies acoustically. Turner (2006) showed that high-frequency amplification was efficient only if the hearing loss is less than 60–80 dB HL. He suggested use of a filtered speech test when the clinician doubts the effectiveness of amplification. Despite hearing preservation, some EAS patients may not have improvement of speech understanding with the combination of electric and acoustic stimulation compared to the implant alone. For patients with high-frequency hearing loss, this study evaluated the intelligibility provided by the residual low frequencies in order to predict if a patient is a good EAS candidate.

Simulation of an electro‐acoustic implant (EAS) with a hybrid vocoder

Electroacoustic stimulation (EAS) is indicated for hearing impaired patients with enough residual hearing in low frequencies and severe hearing loss in high frequencies. We aimed at simulating the speech intelligibility provided by EAS with a hybrid vocoder model. The French Fournier word set was used in this study. We therefore tested several parameters on 24 normal hearing adults. First, the boundary between acoustic and electric stimulation frequency areas (Fc) was taken at 500, 707, 1000 and 1414Hz. Second, we assessed the effect of electrical stimulation channel numbers (1 to 4). Third, we tested the effect of background noise with a cocktail party noise at ‐6, 0 and +6 dB SNR. It appeared that the 3 electrical channels & 707 Hz Fc condition produced normal‐hearing‐like results (at least in quiet). In noisy auditory scene, 4 electrical channels & 500 Hz Fc could produce fair speech intelligibility. [Work supported by CNRS, Lyon1 University and Medel].