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Notre capacité à communiquer et à apprécier la musique repose sur une discrimination de fréquences couvrant une large gamme de fréquences sonores. Cette propriété résulte de cellules mécanosensorielles « ciliées », qui sont réglées pour répondre de façon maximale à une fréquence caractéristique qui varie monotoniquement le long de l’axe de l’organe auditif, la cochlée. Les mécanismes cellulaires et moléculaires qui définissent la fréquence d’une cellule ciliée et régulent sa valeur pour différentes cellules afin de couvrir la gamme auditive demeurent néanmoins inconnus. Notre hypothèse de travail est que cette fréquence est réglée en partie par les propriétés mécaniques passives et actives de la « touffe ciliaire », l’antenne mécanosensorielle de la cellule ciliée. A l’aide d’une préparation excisée de la cochlée du rat, nous avons combiné l’iontophorèse de chélateurs de calcium (BAPTA ou EDTA) pour casser les liens de bout-de-cil qui connectent les stéréocils voisins de la touffe ciliaire, une stimulation grâce à un micro-jet de fluide pour estimer la raideur de la touffe ciliaire et des enregistrements en « patch-clamp » de courants de transduction afin de compter le nombre de liens de bout-de-cil intacts qui contribuent à la réponse. Avec les mouvements évoqués par la rupture des liens de bout-de-cil et avec nos mesures de raideur, nous avons pu estimer la tension dans toute la touffe ciliaire, ainsi que la tension dans un seul lien de bout-de-cil en connaissant le nombre de liens qui contribuent à cette tension. Dans les cellules ciliées externes, qui sont impliquées dans l’amplification du stimulus sonore mais qui n’envoient pas d’information neuronale au cerveau, nous avons observé un gradient de tension et de raideur lorsque la fréquence caractéristique de la cellule ciliée augmente, suggérant que ces paramètres physiques peuvent être impliqués dans le réglage d’une cellule ciliée à sa fréquence caractéristique. Au contraire, pour les cellules ciliées internes, les vraies cellules sensorielles de la cochlée, nos observations ne montrent pas de gradient significatif. De plus, nous avons observé des mouvements de la touffe ciliaire induits par la variation de la concentration en calcium, correspondant à une tension accrue pour des concentrations en calcium plus faibles. Ces mouvements sont similaires à ceux évoqués dans d’autres classes de vertébrés, tels que chez la grenouille ou chez la tortue. Ainsi, nos résultats réconcilient les expériences faites chez les vertébrés inférieurs et chez le mammifère, et montrent l’implication des gradients de la mécanique de la touffe ciliaire pour l’importante sélectivité fréquentielle de la cochlée / Our ability to communicate and appreciate music relies on acute frequency discrimination over a broad range of sound frequencies. This property results from the operation of mechanosensory “hair" cells, which are each tuned to respond maximally to a characteristic frequency that varies monotonically along the axis of the auditory organ, the cochlea. The cellular and molecular mechanisms that set the characteristic frequency of a hair cell and regulate its value among different cells to cover the auditory range have remained elusive. Our working hypothesis is that tuning results in part from passive and active mechanical properties of the “hair" bundle, the mechanosensory antenna of the hair cell.Using an excised preparation from the rat cochlea, we combined iontophoresis of a calcium chelator (BAPTA or EDTA) to break the tip links that interconnect neighbouring stereocilia of the hair-cell bundle, fluid-jet stimulation to estimate hair-bundle stiffness and patch-clamp recordings of transduction currents to count the number of intact transduction channels contributing to the response. From the movements evoked by tip-link breakage and our stiffness measurements, we were able to estimate tension in the whole hair bundle as well as, knowing the number of tip links contributing to this tension, in a single tip link.In outer hair cells, which are involved in sound amplification but do not send neural information to the brain, we observed a gradient of tension and stiffness from the low-frequency to the high-frequency end of the cochlea, suggesting that these physical parameters may help tune the hair cell to its characteristic frequency. Interestingly, with inner hair cells - the true sensors of the cochlea, our observations do not show any significant gradient. Furthermore, we observed calcium-evoked hair-bundle movements corresponding to an increased tension in the tip links at decreased concentrations of calcium. These movements were similar to those evoked in other classes of vertebrates, such as the frog or the turtle. Together, our results reconcile experiments performed in lower vertebrates with those performed in mammals and show the implication of hair-bundle mechanical gradients in the sharp frequency tuning of the cochlea

Orthogonal Frequency Division Multiplexing (OFDM) is a multi-carrier modulation scheme that provides efficient bandwidth utilization and robustness against time dispersive channels. This thesis deals with self-interference, or the corruption of desired signal by itself, in OFDM systems. Inter-symbol Interference (ISI) and Inter-carrier Interference (ICI) are two types of self-interference in OFDM systems. Cyclic prefix is one method to prevent the ISI which is the interference of the echoes of a transmitted signal with the original transmitted signal. The length of cyclic prefix required to remove ISI depends on the channel conditions, and usually it is chosen according to the worst case channel scenario. Methods to find the required parameters to adapt the length of the cyclic prefix to the instantaneous channel conditions are investigated. Frequency selectivity of the channel is extracted from the instantaneous channel frequency estimates and methods to estimate related parameters, e.g. coherence bandwidth and Root-mean-squared (RMS) delay spread, are given. These parameters can also be used to better utilize the available resources in wireless systems through transmitter and receiver adaptation. Another common self-interference in OFDM systems is the ICI which is the power leakage among different sub-carriers that degrades the performance of both symbol detection and channel estimation. Two new methods are proposed to reduce the effect of ICI in symbol detection and in channel estimation. The first method uses the colored nature of ICI to cancel it in order to decrease the error rate in the detection of transmitted symbols, and the second method reduces the effect of ICI in channel estimation by jointly estimating the channel and frequency offset, a major source of ICI.

Title from PDF of title page. / Document formatted into pages; contains 93 pages. / Thesis (M.S.E.E.)--University of South Florida, 2003. / Includes bibliographical references. / Text (Electronic thesis) in PDF format. / ABSTRACT: Orthogonal Frequency Division Multiplexing (OFDM) is a multi-carrier modulation scheme that provides efficient bandwidth utilization and robustness against time dispersive channels. This thesis deals with self-interference, or the corruption of desired signal by itself, in OFDM systems. Inter-symbol Interference (ISI) and Inter-carrier Interference (ICI) are two types of self-interference in OFDM systems. Cyclic prefix is one method to prevent the ISI which is the interference of the echoes of a transmitted signal with the original transmitted signal. The length of cyclic prefix required to remove ISI depends on the channel conditions, and usually it is chosen according to the worst case channel scenario. Methods to find the required parameters to adapt the length of the cyclic prefix to the instantaneous channel conditions are investigated. / ABSTRACT: Frequency selectivity of the channel is extracted from the instantaneous channel frequency estimates and methods to estimate related parameters, e.g. coherence bandwidth and Root-mean-squared (RMS) delay spread, are given. These parameters can also be used to better utilize the available resources in wireless systems through transmitter and receiver adaptation. Another common self-interference in OFDM systems is the ICI which is the power leakage among different sub-carriers that degrades the performance of both symbol detection and channel estimation. Two new methods are proposed to reduce the effect of ICI in symbol detection and in channel estimation. The first method uses the colored nature of ICI to cancel it in order to decrease the error rate in the detection of transmitted symbols, and the second method reduces the effect of ICI in channel estimation by jointly estimating the channel and frequency offset, a major source of ICI. / System requirements: World Wide Web browser and PDF reader. / Mode of access: World Wide Web.

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<p>Sensorineural hearing loss imparts two serious hearing deficits in hearing-impaired people: reduced dynamic range of hearing and reduced frequency selectivity. Psychophysically, these deficits render loss of speech audibility and speech intelligibility to a hearing-impaired person. Studies of an impaired cochlea in cats have shown that the hearing loss originates from damage to or complete loss of inner and outer hair cells. Neurophysiology of an impaired cochlea in cats shows that the tuning curves of the auditory nerve fibers become elevated and broadened. Amplification in hearing aids has been used to restore audibility in hearing-impaired people. Multiband compression has been commercially available in conventional hearing aids to compensate for the reduced dynamic range of hearing. However, little has been achieved to improve the intelligibility of speech in the hearing-impaired people. The aim of this thesis is to restore not only the speech audibility in a hearing-impaired person, but also to improve their speech intelligibility through some hearing-aid signal processing. The compensation technique used in this thesis for speech intelligibility is based on the hypothesis that a narrowband response of the auditory nerve fibers to speech signals ensure phonemic discriminability in the central nervous system.</p><p>Miller et al. [1999] have proposed contrast-enhancing frequency shaping ( CEFS) to compensate for the broadband responses of the fibers to first and second formants (Fl and F2) of a speech stimulus. Bruce [2004] has shown that the multiband compression can be combined with CEFS without counteracting each other. In Bruce's algorithm, a multiband compressor is serially combined with a time-domain CEFS filter. The MICEFS algorithm, herein presented, is a combination of multiband compression and an improved version of CEFS implemented in the frequency domain. The frequency domain implementation of MICEFS has improved the time delay response of the algorithm by 10 ms as compared to series implementation proposed by Bruce. The total time delay of the MICEFS algorithm is 16 ms, which is still longer than the standard time delay of 10 ms in hearing aids. The MICEFS algorithm was tested on a computational model of auditory periphery [Bruce et al., 2003] using a synthetic vowel and a synthetic sentence. The testing paradigm consisted of five conditions: 1) unmodified speech presented to a normal cochlea; 2) speech modified with halfgain rule presented to an impaired cochlea; 3) CEFS modified speech presented to the impaired cochlea; 4) speech modified with MICEFS presented to the impaired cochlea, and; 5) MICEFS-modified speech with some added noise in the formant estimation presented to an impaired cochlea. The spectral enhancement filter used in MICEFS has improved the synchrony capture of the fibers to the first three formants of a speech stimulus. MICEFS has also restored the correct tonotopic representation in the average discharge rate of the fibers at the first three formants of the speech.</p> / Thesis / Master of Applied Science (MASc)

Die Eigenschaften einzelner Nervenzellen sind von grundlegender Bedeutung für die Verarbeitung von Informationen im Nervensystem. Neuronen antworten auf Eingangsreize durch Veränderung der elektrischen Spannung über die Zellmembran. Die Spannungsantwort wird dabei durch die Dynamik der Ionenkanäle in der Zellmembran bestimmt. In dieser Arbeit untersuche ich anhand von leitfähigkeits-basierten Modellneuronen den Einfluss von Ionenkanälen auf zwei Aspekte der Signalverarbeitung: die Frequenz-Selektivität sowie die Zuverlässigkeit und zeitliche Präzision von Aktionspotentialen. Zunächst werden die zell-intrinsischen Mechanismen identifiziert, welche the Frequenz-Selektivität und die Zuverlässigkeit bestimmen. Weiterhin wird untersucht, wie Ionenkanäle diese Mechanismen modulieren können, um die Integration von Signalen zu optimieren. Im ersten Teil der Arbeit wird demonstriert, dass der Mechanismus der unterschwelligen Resonanz, so wie er bisher für periodische Signale beobachtet wurde, auch auf nicht-periodische Signale anwendbar ist und sich ebenfalls in den Feuerraten niederschlägt. Im zweiten Teil wird gezeigt, dass zeitliche Präzision und Zuverlässigkeit von Aktionspotentialen mit der Stimulusfrequenz variieren und dass, in Abhängigkeit davon, ob das Stimulusmittel über- oder unterhalb der Feuerschwelle liegt, zwei Stimulusregime unterschieden werden müssen. In beiden Regimen existiert eine bevorzugte Stimulusfrequenz, welche durch die Gesamtleitfähigkeit und die Dynamik spezifischer Ionenkanäle moduliert werden kann. Im dritten Teil wird belegt, dass Ionenkanäle die Zuverlässigkeit auch direkt über eine Veränderung der Sensitivität einer Zelle gegenüber neuronalem Rauschen bestimmen können. Die Ergebnisse der Arbeit lassen auf eine wichtige Rolle der dynamischen Regulierung der Ionenkanäle für die Frequenz-Selektivität und die zeitliche Präzision und Zuverlässigkeit der Spannungsantworten schließen. / The properties of individual neurons are of fundamental importance for the processing of information in the nervous system. The generation of voltage responses to input signals, in particular, depends on the properties of ion channels in the cell membrane. Within this thesis, I employ conductance-based model neurons to investigate the effect of ionic conductances and their dynamics on two aspects of signal processing: frequency-selectivity and temporal precision and reliability of spikes. First, the cell-intrinsic mechanisms that determine frequency selectivity and spike timing reliability are identified on the basis of conductance-based model neurons. Second, it is analyzed how ionic conductances can serve to modulate these mechanisms in order to optimize signal integration. In the first part, the frequency selectivity of subthreshold response amplitudes previously observed for periodic stimuli is proven to extend to nonperiodic stimuli and to translate into firing rates. In the second part, it is demonstrated that spike timing reliability is frequency-selective and that two different stimulus regimes have to be distinguished, depending on whether the stimulus mean is below or above threshold. In both cases, resonance effects determine the most reliable stimulus frequency. It is shown that this frequency preference can be modulated by the peak conductance and dynamics of specific ion channels. In the third part, evidence is provided that ionic conductances determine spike timing reliability beyond changes in the preferred frequency. It is demonstrated that ionic conductances also exert a direct influence on the sensitivity of the timing of spikes to neuronal noise. The findings suggest an important role for dynamic neuromodulation of ion channels with regard to frequency selectivity and spike timing reliability.

This thesis aims to verify the function of the cochlea as a spectrum analyzer based on computational modeling of macro-mechanics of the cochlea using FEM. I aim to identify the spectrum of the basilar membrane, which is dependent on the variability of geometry, material characteristics and the presence of liquid environments. Interactions between liquid and solid phases is described by fluid-structure interaction in the system ANSYS. The model is linear and does not pursue an active policy of metabolic processes. Further the work focuses on the decomposition of the sound and check of two best known hypotheses about the transmission frequency of the sound to the brain.

La surdité professionnelle constitue un problème de santé publique important, avec une prévalence estimée de 10 à 16%. Les études animales ont mis en évidence une perte des synapses entre les cellules ciliées internes et le nerf auditif et des fibres auditives présentant une activité spontanée faible. Cette synaptopathie cochléaire se manifesterait avant la dégradation des seuils auditifs et la perte des cellules ciliées externes, étant ainsi un précurseur à la surdité professionnelle. L’audiogramme utilisé en milieu clinique ne permet pas de mesurer la synaptopathie cochléaire, se contentant seulement de quantifier la perte d’audibilité causée par l’exposition au bruit. Des études post-mortem humaines ont permis d’identifier une synaptopathie cochléaire, similaire à celle observée dans le modèle animal. Étant donné que la quantification des synapses chez l'humain vivant n'est pas possible, les chercheurs se sont concentrés sur le développement d’outils pouvant servir de marqueur indirect de la synaptopathie cochléaire. À cet égard, les résultats sont divergents d’une étude à l’autre. Il est possible que ces outils ne soient pas sensibles ou que l’exposition au bruit investiguée dans la littérature ne soit pas suffisante pour entraîner une synaptopathie cochléaire chez l’humain. L’objectif de cette thèse est donc d’évaluer les effets d’une exposition au bruit industriel continu et au bruit impulsionnel, qui pourraient s’avérer plus nocives et entraîner un processus précoce de synaptopathie cochléaire. Des individus présentant des seuils auditifs et des émissions otoacoustiques dans la normale ont été investigués à l’aide de mesures électrophysiologiques et psychoacoustiques. Dans la première étude, 40 participants exposés au bruit industriel continu et 40 participants sans exposition au bruit industriel continu ont été recrutés et évalués à l’aide d’un test de perception de la parole dans le bruit (SPiN) et de différentes composantes du potentiels évoqués auditifs du tronc cérébral (PEATC). L’exposition au bruit des participants a été mesurée par dosimétrie. Les résultats ne montrent pas d’association entre l’exposition au bruit et les variables du PEATC et du SPiN. Dans la deuxième étude, 27 participants militaires exposés au bruit impulsionnel et 13 participants sans exposition au bruit impulsionnel ont été recrutés. Les PEATC, l’électrocochléographie, le SPiN et la largeur des filtres auditifs rectangulaires équivalents (ERB) ont été mesurés. L’exposition au bruit des participants a été quantifiée à l’aide du Noise Exposure Structured Interview. Les résultats montrent une réduction de l’amplitude de l’onde I, un allongement de la latence de l’onde V, des performances réduites de SPiN et un ERB plus large à 4 kHz chez les militaires exposés au bruit impulsionnel, en comparaison aux participants sans exposition au bruit impulsionnel. Cette thèse est importante d’un point de vue de santé publique puisqu’elle suggère que certains outils cliniques simples, comme la mesure des filtres auditifs, pourraient permettre de détecter les premiers signes d’un dommage auditif avant l’apparition d’une surdité professionnelle mesurée par l’audiogramme. Les résultats renforcent l’importance de la sensibilisation aux risques induits par l’exposition au bruit afin de prévenir l’apparition des troubles de communication et des situations de handicap découlant de la présence d’une surdité professionnelle. / Occupational hearing loss constitutes an important public health problem, with an estimated prevalence of 10 to 16%. Animal studies have shown a phenomenon of synapses dysfunction between the inner hair cells and the auditory nerve and a preferential loss of low spontaneous rate auditory fibers. This cochlear synaptopathy manifests itself before the degradation of hearing thresholds and the loss of outer hair cells, thus being a precursor damage to occupational hearing loss. The audiogram used in a clinical setting does not measure cochlear synaptopathy, only quantifying the loss of audibility caused by noise exposure. In humans, post-mortem studies have identified a process of cochlear synaptopathy, similar to that observed in the animal model. Since quantification of synapses in living humans is not possible, researchers focused on developing a noninvasive measurement that could serve as an indirect marker for cochlear synaptopathy. Several tools have been proposed, but the results vary from one study to another. It is possible that these tools are not sensitive or that noise exposures investigated in the literature is not sufficient to cause cochlear synaptopathy in humans. The objective of this thesis is therefore to evaluate the effects of exposure to continuous industrial noise and impulse noise, which could prove to be more harmful and lead to an accelerated process of cochlear synaptopathy. To this end, individuals with normal hearing thresholds and otoacoustic emissions were investigated using electrophysiological and psychoacoustical measurements. In the first study, 40 participants with occupational noise exposure and 40 participants without occupational noise exposure were recruited and evaluated using a speech perception in noise (SPiN) test and different components of the auditory brainstem response (ABR). Participants’ noise exposure was measured by dosimetry. The results do not show an association between noise exposure and the ABR and SPiN variables. In the second study, 27 military participants exposed to impulse noise and 13 participants without exposure to impulse noise were recruited. ABR, electrocochleography, SPiN and the equivalent rectangular bandwidth (ERB) of auditory filters were measured. Participants' noise exposure was quantified using the Noise Exposure Structured Interview. Results show a reduced wave I amplitude, a lengthened wave V latency, a reduced SPiN performance, and a broader ERB at 4 kHz in military recruits exposed to impulse noise, compared to participants without exposure to impulse noise. This thesis is important from a public health point of view since it suggests that certain simple clinical tools, such as the measurement of auditory filters, might make it possible to detect the first signs of auditory damage before the onset of hearing loss measured by the audiogram. Results reinforce the importance of raising awareness to the risks induced by noise exposure in order to prevent the appearance of communication disorders and handicaps resulting from the presence of occupational hearing loss.