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This thesis investigates both the physical origin and the physiological coding of pinna-based spectral features observed in experimental Head-Related Transfer Functions (HRTFs). An experimental method for measuring HRTFs using a Knowles Electronics Manikin for Acoustic Research (KEMAR) is presented. The method includes a technique for moulding individualised pinnae to be fitted on to the KEMAR's head. Experimental HRTF data obtained with this method are shown and analysed. The most remarkable pinna-based spectral features are identified and their dependency on source location is characterised, particularly for elevation-dependent spectral notches.

Auditory physiology is nearly unique in the human body because of its small-diameter neurons. When considering a single node on one neuron, the number of channels is very small, so ion fluxes exhibit randomness. Hodgkin and Huxley, in 1952, set forth a system of Ordinary Differential Equations (ODEs) to track the flow of ions in a squid motor neuron, based on a circuit analogy for electric current. This formalism for modeling is still in use today and is useful because coefficients can be directly measured. To measure auditory properties of Firing Efficiency (FE) and Post Stimulus Time (PST), we can simply measure the depolarization, or "upstroke," of a node. Hence, we reduce the four-dimensional squid neuron model to a two-dimensional system of ODEs. The stochastic variable m for sodium activation is allowed a random walk in addition to its normal evolution, and the results are drastic. The diffusion coefficient, for spreading, is inversely proportional to the number of channels; for 130 ion channels, D is closer to 1/3 than 0 and cannot be called negligible. A system of Partial Differential Equations (PDEs) is derived in these pages to model the distribution of states of the node with respect to the (nondimensionalized) voltage v and the sodium activation gate m. Initial conditions describe a distribution of (v,m) states; in most experiments, this would be a curve with mode at the resting state. Boundary conditions are Robin (Natural) boundary conditions, which gives conservation of the population. Evolution of the PDE has a drift term for the mean change of state and a diffusion term, the random change of state. The phase plane is broken into fired and resting regions, which form basins of attraction for fired and resting-state fixed points. If a stimulus causes ions to flow from the resting region into the fired region, this rate of flux is approximately the firing rate, analogous to clinically measuring when the voltage crosses a threshold. This gives a PST histogram. The FE is an integral of the population over the fired region at a measured stop time after the stimulus (since, in the reduced model, when neurons fire they do not repolarize). This dissertation also includes useful generalizations and methodology for turning other ODEs into PDEs. Within the HH modeling, parameters can be switched for other systems of the body, and may present a similar firing and non-firing separatrix (as in Chapter 3). For any system of ODEs, an advection model can show a distribution of initial conditions or the evolution of a given initial probability density over a state space (Chapter 4); a system of Stochastic Differential Equations can be modeled with an advection-diffusion equation (Chapter 5). As computers increase in speed and as the ability of software to create adaptive meshes and step sizes improves, modeling with a PDE becomes more and more efficient over its ODE counterpart.

Contexte : La compréhension de la parole dans le silence est dépendante des mécanismes de codage de l’enveloppe temporelle du signal sonore. Une anomalie du codage (d’origine infectieuse, immunitaire, génétique, tumorale, ou environnementale) entraine irrémédiablement une diminution des performances audiométriques vocales. Les méthodes d’exploration fonctionnelle de la cochlée (potentiels d’action composite du nerf auditif, potentiel évoqués auditifs précoces) utilisent des stimuli sonores simples (clics, bouffées tonales) pour détecter une anomalie de codage des indices temporels. Le but de cette étude était de développer une méthode électrophysiologique capable de mesurer les réponses du nerf auditif à des stimuli modulés en amplitude.Matériel et méthodes : La réponse électrophysiologique du nerf auditif a été mesurée à l’aide d’une électrode placée sur la niche de la fenêtre ronde de la cochlée de gerbilles et de rats vieillissants. Les stimuli acoustiques consistaient en des bandes de bruit de 20 secondes modulées sinusoïdalement en amplitude et centrées sur 4, 8 et 16 kHz. Nous avons étudié le niveau, la profondeur de modulation, la fréquence de modulation et la fréquence porteuse.Résultats : Notre étude sur le modèle de perte sélective neuronale (ouabain) montre que l’analyse des potentiels globaux cochléaires permet de détecter une perte de fibres à basse activité spontanée dans le nerf auditif, résultat important car indétectable (« surdités cachées ») actuellement avec les tests utilisés en routine en clinique (EcoG et PEA) (Batrel, Huet, Hasselmann et al, Plos One 2017). Ensuite, en combinant le stimulus de cette étude avec une fonction sinusoïdale, nous avons développé et validé une méthode pour évaluer la qualité de codage de l’enveloppe par le nerf auditif. Nous avons appliqué cette méthode sur un modèle de vieillissement (rat Sprague-Dawley). Nos résultats suggèrent que le viellissement entraine une modifcation du phénotype des fibres du nerf auditif sans pertes de fibres associées (article Occelli, Hasselmann et al, soumis à eNeuro). Conclusion : Notre travail démontre qu’il est indispensable d’élargir le nombre de techniques d’exploration fonctionnelle de la cochlée car les tests utilisés en routine en clinique ne permettent pas de déceler des déficits subtils d’encodage dans le nerf auditif. La mesure de l’activité soutenue des fibres permet de détecter la perte sélective des neurones à basse activité spontanée, indétectable avec les méthodes classiques. Le changement de phénotype des fibres observé au cours du vieillissement du rat Sprague-Dawley est détectable avec notre méthode alors qu’il ne l’est pas avec le potentiel d’action composite du nerf auditif. / Background: Speech intelligibility in quiet is critically dependent on the temporal envelope of a sound signal. An abnormal coding of this temporal cue (due to infectious, immune, genetic, tumoral or environmental of origin) implies a decrease of speech recognition scores. The current proxy to probe deafness in clinical framework (Compound Action Potential of the auditory nerve, auditory brainstem responses) uses simplistic stimuli (clicks, tone bursts) to detect a such abnormal coding of the temporal cues. The aim of this study was to develop a new electrophysiology method in murins able to measure the auditory nerve responses to amplitude-modulation stimuli.Material and methods: The electrophysiology response of the auditory nerve was recorded using an electrode implanted onto the round window niche on normal-hearing gerbil cochlea and aging rat cochlea. The acoustical stimuli consisted of 20 seconds sinusoidally amplitude-modulated noise-band centered on 4, 8 and 16 kHz. We have studied varying sound level, the modulation depth, the modulation frequency and the carrier frequency.Results: Our study on the selective fiber loss ouabain model show the mass potentials recorded at the round window enable the detection of low spontaneous rate fibers in gerbil auditory nerve. This result is important because the current clinical used tests aren’t enough sensitive to detect a such coding impairment (CAP, ABR) (Batrel, Huet, Hasselmann et al., 2017). Then we combined the stimulus of this previous study with a sinusoidal function to develop a new method to assess the envelope coding by the auditory nerve. We validated this new method. Last, we used our method on an aging model (Sprague-Dawley rat). Our results suggest aging leads to a phenotype change of auditory nerve fibers without associated fiber loss (article Occelli, Hasselmann et al, submitted to eNeuro).Conclusion: Our study shows it’s indispensable to expand the number of tools to probe the cochlea because the current clinical used tests aren’t enough sensitive to detect subtle deficits of encoding in the auditory nerve. The recording of the fiber sustained activity enable to detect the selective loss of low-spontaneous rate neurons. A such loss is undetectable with classical clinical tools. The phenotype change of fibers we observed in aging Sprague-Dawley rats is detectable with our method whereas it’s not using the compound action potential of the auditory nerve.

Objective: This study aimed to systematically review and analyse the available peer-reviewed literature reporting on the results of distortion product otoacoustic emissions (DPOAEs), transient evoked otoacoustic emissions (TEOAEs) and click auditory brainstem responses (c-ABRs) in adults with type 1 and type 2 diabetes mellitus (T1DM and T2DM). Method: A comprehensive literature search was conducted across three electronic databases to identify English; peer-reviewed articles that included results of OAEs (DPOAEs and TEOAEs) and c-ABR tests in adult subjects with DM. Articles were selected according to predetermined selection criteria and critically reviewed independently by two researchers. Results: 15 studies met the inclusion criteria for the systematic review while nine articles qualified for inclusion in the meta-analysis. DPOAE studies reported significantly reduced amplitudes with only one study reporting larger amplitudes. Abnormal TEOAEs were reported in all TEOAE studies, although these abnormalities were not always significant. Significantly delayed c-ABRs were reported in all ABR studies. Analysis of c-ABR mean wave latencies identified longer latencies for DM subjects, particularly for wave III and V, as well as for IPL I-III and I-V. Conclusions: Subjects with T1DM and T2DM may present with clinical or subclinical impairment of the cochlear outer hair cells and both the peripheral and central auditory pathway. / Dissertation (MA)--University of Pretoria, 2016. / Speech-Language Pathology and Audiology / MA / Unrestricted

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Current cochlear implant systems use fast pulsatile stimulation to deliver the temporal modulations of speech and to, potentially, improve the neural representation of such modulations by restoring the independence of neural firing. The realization of these benefits may vary with other pulse rate-dependent temporal interactions that occur at the neural membrane, e.g., per(i)stimulatory adaptation and its post-stimulatory or forward masking effects. This study attempted to characterize adaptation and recovery of the electrically evoked compound action potential (ECAP) using probe pulses delivered within and following brief (100 ms) high-rate masker (1800 pps) pulse trains at various current levels in adults and children. With this stimulus paradigm, the ECAP amplitude typically achieved a steady state during the course of pulse train stimulation. The ECAP amplitude at steady state was, on average, a similar proportion (50-70%) of the amplitude at onset for various stimulus levels and in both age groups. However, long-term adaptation effects, evidenced by the decrease in onset ECAP amplitude, were greater in adults particularly at lower levels in the ECAP dynamic range. Instances of alternation in ECAP amplitude were seen at stimulus levels that were higher in the ECAP dynamic range. The forward masking effects of pulse train stimulation were quantified by the ECAP amplitude in response to a subsequent probe pulse normalized by the response to the same pulse presented alone. Pulse train forward masking increased with the level of the masker pulse train and decreased with the level of the probe stimulus. The recovery of the ECAP for probes that were lower in level than the masker pulse train was incomplete at 600 ms after masker offset, consistent with long-term cumulative effects observed in the response to the probe alone. Masker pulse trains that are lower in level than the probe pulse produced proportionally small decrements in the ECAP amplitude with complete recovery within 250 ms of pulse train offset particularly in adults. ECAP recovery of a probe preceded by a masker pulse train of equal level followed a monotonic or non-monotonic pattern consistent with a hypothesis of both adaptation and facilitation occurring with pulse train stimulation. The various patterns of recovery may attest to the occurrence of more than a single process in the same subset of nerve fibers or in different fibers. We hypothesize that the variations in the recovery patterns may be attributable to individual differences in the status of the auditory nerve and possibly, the variations in temporal interactions across the spatial domain at different stimulus levels. Finally, the probe-evoked ECAP amplitude at steady state in children and briefly, e.g., 20 ms, after pulse train offset in both age groups could be predicted by the ECAP amplitude in response to the same probe pulse when preceded at a brief interval (1.2 or 2 ms) by a single masker pulse of the same level as the masker pulse train. Further investigation may reveal if the observed differences in neural responsiveness to pulsatile stimulation, among individuals account for differences in psychophysical measures, including speech perception and whether there may be an "optimal" neural output that could be evoked by an individually "optimized" signal.

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Effective cochlear implant fitting (or programming) is essential for providing good hearing outcomes, yet it is a subjective and error-prone task. The initial objective of this research was to automate the procedure using the auditory nerve electrically evoked compound action potential (the ECAP) and machine intelligence. The Nucleus?? cochlear implant measures the ECAP via its Neural Response Telemetry (NRT™) system. AutoNRT™, a commercial intelligent system that measures ECAP thresholds with the Nucleus Freedom™ implant, was firstly developed in this research. AutoNRT uses decision tree expert systems that automatically recognise ECAPs. The algorithm approaches threshold from lower stimulus levels, ensuring recipient safety during postoperative measurements. Clinical studies have demonstrated success on approximately 95% of electrodes, measured with the same efficacy as a human expert. NRT features other than ECAP threshold, such as the ECAP recovery function, could not be measured with similar success rates, precluding further automation and loudness prediction from data mining results. Despite this outcome, a better application of the ECAP threshold profile towards fitting was established. Since C-level profiles (the contour of maximum acceptable stimulus levels across the implant array) were observed to be flatter than T-level profiles (the contour of minimum audibility), a flattening of the ECAP threshold profile was adopted when applied as a fitting profile at higher stimulus levels. Clinical benefits of this profile scaling technique were demonstrated in a 42 subject study. Data mining results also provided an insight into the ECAP recovery function and refractoriness. It is argued that the ECAP recovery function is heavily influenced by the size of the recruited neural population, with evidence gathered from a computational model of the cat auditory nerve and NRT measurements with 21 human subjects. Slower ECAP recovery, at equal loudness, is a consequence of greater neural recruitment leading to lower mean spike probabilities. This view can explain the counterintuitive association between slower ECAP recovery and greater temporal responsiveness to increasing stimulation rate. This thesis presents the first attempt at achieving completely automated cochlear implant fitting via machine intelligence; a future generation implant, capable of high fidelity auditory system measurements, may realise the ultimate objective.