We will briefly outline a computational theory of the first stages of human vision according to which (a) the retinal image is filtered by a set of centre-surround receptive fields (of about 5 different spatial sizes) which are approximately bandpass in spatial frequency and (b) zero-crossings are detected independently in the output of each of these channels. Zero-crossings in each channel are then a set of discrete symbols which may be used for later processing such as contour extraction and stereopsis. A formulation of Logan's zero-crossing results is proved for the case of Fourier polynomials and an extension of Logan's theorem to 2-dimentsional functions is also approved. Within this framework, we shall describe an experimental and theoretical approach (developed by one of us with M. Fahle) to the problem of visual acuity and hyperacuity of human vision. The positional accuracy achieved, for instance, in reading a vernier is astonishingly high, corresponding to a fraction of the spacing between adjacent photoreceptors in the fovea. Stroboscopic presentation of a moving object can be interpolated by our visual system into the perception of continuous motion; and this "spatio-temporal" interpolation also can be very accurate. It is suggested that the known spatiotemporal properties of the channels envisaged by the theory of visual processing outlined above implement an interpolation scheme which can explain human vernier acuity for moving targets.

This dissertation reports, for the first time, on the electrical coupling of microelectromechanical (MEM) resonators for high order bandpass filter synthesis. Electrical coupling of MEM resonators has a strong potential for extension of the operating frequency of MEM bandpass filters into the ultra high frequency (UHF) range and provides higher tunability and design flexibility compared to the mechanical coupling approach. Various schemes of electrical coupling are presented in this dissertation. Electromechanical models of clamped-clamped beam resonators, and various types of electrically coupled filters are presented. Lower frequency prototypes of electrically coupled filters with operating frequencies in the hundreds of kHz are implemented using micromechanical single crystal silicon clamped-clamped beam resonators. Measurement results are in good agreement with the developed electrical equivalent models of the filters. It is demonstrated that the characteristics of electrically coupled filters can be widely tuned by changing the DC polarization voltages.

Teoricamente, conexões por sinapses elétricas entre neurônios poderiam levar ao aumento da faixa de resposta dinâmica da rede neural. A faixa de resposta dinâmica de uma rede de neurônios pode ser definida como a faixa de valores de intensidade dos estímulos de entrada para a qual o conjunto de neurônios produz resposta antes de atingir a saturação. Em um cenário biológico, propôs-se que junções gap entre células ganglionares da retina aumentariam a faixa dinâmica da retina. O teste experimental dessa proposta apresenta várias dificuldades, o que torna a modelagem computacional uma alternativa metodológica para o estudo do papel das sinapses elétricas na faixa dinâmica da camada ganglionar da retina. O objetivo deste trabalho foi a construção de um modelo biologicamente plausível da camada ganglionar da retina da salamandra, submetida a sinais de entrada realísticos conforme evidências experimentais e com a inclusão de sinapses elétricas conectando suas células, para estudar in silico os possíveis efeitos dessas sinapses elétricas sobre a faixa dinâmica da camada ganglionar. A camada ganglionar foi modelada como uma rede bidimensional cujos neurônios foram modelados pelo formalismo de Hodgkin-Huxley. Cada neurônio recebeu um de dois tipos de entrada sináptica, transiente ou sustentada. Avaliou-se o efeito da inibição pré-sináptica das células ganglionares e o efeito de diferentes padrões de conectividade mediados pelas sinapses elétricas. Os resultados sugerem que o acoplamento elétrico aumenta a sensibilidade do sistema e altera o ponto de saturação, mas não necessariamente aumenta a faixa dinâmica. / Theoretically, connections by electrical synapses between neurons could lead to an increase in their dynamic range. The dynamic range of a network of neurons can be defined as the range of input stimuli values for which the network responds before saturation. In a biological scenario, it is hypothesized that gap junctions between retinal ganglion cells may increase the dynamic range of the retina. However, the experimental testing of this hypothesis presents several difficulties, which makes computational modeling a methodological alternative to study the role of electrical synapses on the dynamic range of the ganglion cell layer of the retina. In this work we constructed a biologically plausible computational model of the ganglion cell layer of the salamander retina. A bidimensional network was built with cells modeled by the Hodgkin-Huxley formalism connected via gap junctions and subject to realistic inputs constrained by experimental evidence, to study in silico the effects of gap junctions on the dynamic range of the model. We studied the effect of different gap junction-mediated connectivity patterns, input type combinations (transient, sustained and mixed between the two) and presynaptic inhibition on the dynamic range. Our results suggest that gap junction coupling increases the network\'s sensitivity and alters the saturation point but not necessarily increases the dynamic range.

Teoricamente, conexões por sinapses elétricas entre neurônios poderiam levar ao aumento da faixa de resposta dinâmica da rede neural. A faixa de resposta dinâmica de uma rede de neurônios pode ser definida como a faixa de valores de intensidade dos estímulos de entrada para a qual o conjunto de neurônios produz resposta antes de atingir a saturação. Em um cenário biológico, propôs-se que junções gap entre células ganglionares da retina aumentariam a faixa dinâmica da retina. O teste experimental dessa proposta apresenta várias dificuldades, o que torna a modelagem computacional uma alternativa metodológica para o estudo do papel das sinapses elétricas na faixa dinâmica da camada ganglionar da retina. O objetivo deste trabalho foi a construção de um modelo biologicamente plausível da camada ganglionar da retina da salamandra, submetida a sinais de entrada realísticos conforme evidências experimentais e com a inclusão de sinapses elétricas conectando suas células, para estudar in silico os possíveis efeitos dessas sinapses elétricas sobre a faixa dinâmica da camada ganglionar. A camada ganglionar foi modelada como uma rede bidimensional cujos neurônios foram modelados pelo formalismo de Hodgkin-Huxley. Cada neurônio recebeu um de dois tipos de entrada sináptica, transiente ou sustentada. Avaliou-se o efeito da inibição pré-sináptica das células ganglionares e o efeito de diferentes padrões de conectividade mediados pelas sinapses elétricas. Os resultados sugerem que o acoplamento elétrico aumenta a sensibilidade do sistema e altera o ponto de saturação, mas não necessariamente aumenta a faixa dinâmica. / Theoretically, connections by electrical synapses between neurons could lead to an increase in their dynamic range. The dynamic range of a network of neurons can be defined as the range of input stimuli values for which the network responds before saturation. In a biological scenario, it is hypothesized that gap junctions between retinal ganglion cells may increase the dynamic range of the retina. However, the experimental testing of this hypothesis presents several difficulties, which makes computational modeling a methodological alternative to study the role of electrical synapses on the dynamic range of the ganglion cell layer of the retina. In this work we constructed a biologically plausible computational model of the ganglion cell layer of the salamander retina. A bidimensional network was built with cells modeled by the Hodgkin-Huxley formalism connected via gap junctions and subject to realistic inputs constrained by experimental evidence, to study in silico the effects of gap junctions on the dynamic range of the model. We studied the effect of different gap junction-mediated connectivity patterns, input type combinations (transient, sustained and mixed between the two) and presynaptic inhibition on the dynamic range. Our results suggest that gap junction coupling increases the network\'s sensitivity and alters the saturation point but not necessarily increases the dynamic range.

Dans ce travail de thèse nous nous sommes intéressés aux phénomènes de transport au sein du réseau lacuno-canaliculaire (RLC) et de l'ostéon dans le tissu osseux cortical. Pour étudier la mécanotransduction ostéocytaire amenant au remodelage osseux, nous avons développé un modèle à trois échelles où sont pris en : l'électrcompte ostatique (modélisée par l'équation de Poisson Boltzmann), l'écoulement du fluide (représenté textit{via} une équation de Stokes modifiée et la conservation de la masse fluide) et le transport ionique (régi par l'équation de Nernst-Planck). L'étude de la distribution du potentiel électrique, a mis en exergue l'importance des double-couches électriques au voisinage des parois chargées des pores. Ces double-couches électriques, ainsi que la composition chimique du fluide donnent lieu à des phénomènes d'osmose et d'électroosmose intervenant dans l'écoulement du fluide interstitiel, et influençant la diffusion efficace des ions dans les pores. L'étude a démarré à l'échelle du pore canaliculaire pour être propagée à l'échelle du canalicule puis de l'ostéon, en utilisant une procédure d'homogénéisation périodique asymptotique. Une étude paramétrique nous a permis de cibler les paramètres agissant sur les phénomènes de transport et pouvant faire réagir les ostéocytes. Il est ressorti de cette étude que les effets électro-chimiques jouent rôle important. Nous avons donc choisi de nous focaliser sur la chimie et plus particulièrement sur les effets des flux ioniques physiologiques sur les ostéocytes dans le RLC. Des expériences, mises en place pour étayer ces aspects ont souligné l'importance des échanges chimiques entre les cellules et le fluide qui les entoure. Finalement, nous avons montré que les phénomènes de transports ayant lieu dans le RLC et dans l'ostéon interagissent les uns les autres, parachevant ainsi la description à trois échelles du tissu cortical / Transport phenomena appearing within the cortical bone lacuno-canalicular network (LCN) and the osteon were the objective of this study. We developped a three-scale model to investigate the osteocyte mechanotranduction which is at the origin of the bone remodeling process. This model took into account three physical phenomena : the electrostatics (through the Poisson-Boltzman equation), the interstitial fluid flow (modeled by a modified Stokes equation) and the ionic transport (governed by a Nernst-Planck equation). Analysis of the electrical potential distribution highlighted the importance of the electrical double layers close to the pore charged surface. These electrical double layers, as well as the interstitial fluid chemical composition, induce osmotic and electroosmotic fluid flows and affect the ionic effective diffusion within the pores. Using a periodic asymptotic homogeneisation procedure, the model at the canalicular pore scale was upscaled at the canalicular scale and then at the osteonal scale. A parametric study pointed out the relevant parameters acting on the transport phenomena and possibly affecting osteocyte mecanosensitivity. Our results emphasized the importance of the electro-chemical effects. We thus focused on the chemistry and more especially on the effects of the physiological ionic fluxes on the osteocyte. In vitro experiments and numrical simulations were performed to elucidate these questions. Our results underlined the importance of the chemical exchanges between the osteocyte and the surrounding fluid. Finally, we showed that fluid flow and chemical transport occuring within the LCN and the osteon interact with each other, thus achieving a three-scale description of the transport phenomena in the osteon

Rhythmic neural activity is thought to underlie many high-level functions of the nervous system. Our goals are to understand rhythmic activity starting with small networks, using theoretical and experimental tools. Phase resetting theory describes essential properties that cause and destroy rhythms. We validate and extend one branch of this theory, testing it in bursting neurons coupled by excitation and then extending the theory to account for temporal variability found in our experimental data. We show that the theory makes good predictions of rhythmic activity in heterogeneous networks. We also note differences in mathematical structure between inhibition- and excitation-coupling that cause them to behave differently in noisy contexts and may explain why all central pattern generators (CPGs) found in nature are dominated by inhibition. Our extension of the theory gives a method that is useful to compare experimental and model data and shows that noise may either create or destroy a rhythm. Finally, we described the cellular mechanisms in Aplysia that switch the feeding CPG from arrhythmic to rhythmic behavior in response to reward stimuli. Previous studies showed that a Dopamine reward signal is correlated to changes in electrical coupling and excitability in several important neurons in the CPG. Using the dynamic clamp and an in vitro analog of the full behavioral system, we were able to determine that electrical coupling alone controls rhythmicity, while excitability independently controls the rate of activity. These results beg for further study, including new theory to explain them fully.

Afin de rendre les machines synchrones à aimants permanents plus attractives pour l'aéronautique (actionneurs de commande de vol, systèmes de freinage, train d'atterrissage ...), il peut être intéressant de réduire le nombre de modules d'électronique de puissance utilisés en les mutualisant. De nombreuses études ont été réalisées pour des systèmes composés de plusieurs machines asynchrones et principalement en traction ferroviaire, mais peu concernent les machines synchrones. Après avoir étudié différentes structures envisageables, les travaux développés lors de cette thèse présentent une étude originale d'un système composé de deux machines synchrones à aimants permanents connectées en parallèle sur un onduleur unique mutualisé. Ces machines ont des caractéristiques identiques ou proches et doivent être pilotées à la même vitesse. La structure de commande retenue lors de cette thèse est de type maître-esclave: seule une des deux machines est autopilotée (la machine maître), l'autre (la machine esclave) fonctionnant en boucle ouverte. Afin d'assurer la stabilité d'un tel système, le synchronisme des deux moteurs doit toujours être respecté. Une stratégie de commande, qui choisit quel est le moteur maître, en prenant en compte la variation des paramètres internes et externes du système, est alors instaurée. Dans ces conditions, les évolutions théoriques des différentes variables sont déterminées. Pour valider ces résultats, un processus expérimental est mis en place. Les nombreux résultats obtenus en simulation et expérimentalement permettent alors de confirmer les résultats théoriques: que ce soient les paramètres mécaniques ou électriques qui varient, la stabilité du système est toujours garantie. Le cas supplémentaire où les deux machines déplacent une charge mécanique commune avec une liaison mécanique rigide entre les deux machines, est finalement développé. Une autre stratégie de commande, dont la structure et les résultats sont également présentés dans cette thèse, est alors nécessaire. / To make permanent magnet synchronous machines more attractive for aerospace (flight control actuators, braking systems, landing gear ...), it may be advantageous to reduce the number of power electronic modules used in the pooling. Many studies have been performed for systems composed of several machines and asynchronous traction primarily, but little concern synchronous machines. After considering various possible structures, this Ph.D. thesis presents an original study of a system consisting of two permanent magnet synchronous machines connected in parallel on a single shared inverter. These machines have characteristics identical or similar and must be driven at the same speed. The control structure chosen in this Ph.D. thesis is a master-slave : only one machine, called master machine is self-piloted, the other (the slave machine) operating in open loop. To ensure the stability of such a system, the timing of the two engines should always be respected. A control strategy, which selects which is the master motor, taking into account the variation of internal and external parameters of the system is then introduced. Under these conditions, the theoretical developments of the different variables are determined. To validate these results, an experimental process is established. The numerous results obtained in simulation and experiments are then used to confirm the theoretical results : whatever the mechanical or electrical parameters variation, system stability is always guaranteed. The additional case, when both machines move a mechanical load with a common rigid mechanical connection between two machines, is finally developed. Another required control strategy, the structure and the results are also presented in this Ph.D. thesis.