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Dissertations / Theses on the topic 'Drift-diffusion model'
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Luzardo, A. "The Rescorla-Wagner Drift-Diffusion model." Thesis, City, University of London, 2018. http://openaccess.city.ac.uk/19210/.
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Computational models of classical conditioning have made significant contributions to the theoretic understanding of associative learning, yet they still struggle when the temporal aspects of conditioning are taken into account. Interval timing models have contributed a rich variety of time representations and provided accurate predictions for the timing of responses, but they usually have little to say about associative learning. In this thesis we present a unified model of conditioning and timing that is based on the influential Rescorla-Wagner conditioning model and the more recently developed Timing Drift-Diffusion model. We test the model by simulating 11 experimental phenomena and show that it can provide an adequate account for 9, and a partial account for the other 2. We argue that the model can account for more phenomena in the chosen set than these other similar in scope models: CSCTD, MS-TD, Learning to Time and Modular Theory. A comparison and analysis of the mechanisms in these models is provided, with a focus on the types of time representation and associative learning rule used.
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Fard, Pouyan R., Hame Park, Andrej Warkentin, Stefan J. Kiebel, and Sebastian Bitzer. "A Bayesian Reformulation of the Extended Drift-Diffusion Model in Perceptual Decision Making." Saechsische Landesbibliothek- Staats- und Universitaetsbibliothek Dresden, 2017. http://nbn-resolving.de/urn:nbn:de:bsz:14-qucosa-230313.
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Perceptual decision making can be described as a process of accumulating evidence to a bound which has been formalized within drift-diffusion models (DDMs). Recently, an equivalent Bayesian model has been proposed. In contrast to standard DDMs, this Bayesian model directly links information in the stimulus to the decision process. Here, we extend this Bayesian model further and allow inter-trial variability of two parameters following the extended version of the DDM. We derive parameter distributions for the Bayesian model and show that they lead to predictions that are qualitatively equivalent to those made by the extended drift-diffusion model (eDDM). Further, we demonstrate the usefulness of the extended Bayesian model (eBM) for the analysis of concrete behavioral data. Specifically, using Bayesian model selection, we find evidence that including additional inter-trial parameter variability provides for a better model, when the model is constrained by trial-wise stimulus features. This result is remarkable because it was derived using just 200 trials per condition, which is typically thought to be insufficient for identifying variability parameters in DDMs. In sum, we present a Bayesian analysis, which provides for a novel and promising analysis of perceptual decision making experiments.
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Schmithüsen, Bernhard. "Grid adaptation for the stationary two-dimensional drift-diffusion model in semiconductor device simulation /." Zürich : [s.n.], 2002. http://e-collection.ethbib.ethz.ch/show?type=diss&nr=14449.
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Dykhuis, Andrew Frederic. "Capturing irradiation-enhanced corrosion of zircaloy-4 with a charge-based diffusion/drift phase field model." Thesis, Massachusetts Institute of Technology, 2018. http://hdl.handle.net/1721.1/119029.
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Thesis: Ph. D., Massachusetts Institute of Technology, Department of Nuclear Science and Engineering, 2018.<br>This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.<br>Cataloged from student-submitted PDF version of thesis.<br>Includes bibliographical references (pages 389-400).<br>Zircaloy-4 has been used in pressurized water reactors (PWRs) for decades, and enhanced corrosion rates in reactors compared to out-of-pile have long been observed. However, the exact mechanism explaining the early departure from autoclave kinetics after 3-5 microns of oxide have formed has proved elusive. This thesis considers and evaluates a number of possible explanations for this early acceleration in kinetics. The bulk of the evidence points to Fe depletion from secondary phase particles (SPPs) as the culprit in enhancing Zircaloy-4 corrosion rates in PWRs. These new findings have been incorporated in a mechanistic finite-element phase field model of Zircaloy-4 corrosion called HOGNOSE. It accounts for both diffusion-and drift-based oxygen anion transport in Zircaloy-4 by including the effects of radiation-induced evolution of SPPs in changing the contribution of a local charge transport inequality through their depletion and release of iron. By addressing the imbalance in charged particle transport, the code can be adapted to model multiple zirconium-based alloys in autoclave and irradiated conditions with minimal parameter fitting. Rather than the typical empirical approach, HOGNOSE uses a physics-based methodology to capture the early agreement between autoclave and in-reactor data and the point at which reactor kinetics are enhanced compared to autoclave kinetics. HOGNOSE results agree fairly well with those observed in experiments for oxide thicknesses less than 10 microns, above which other enhancement mechanisms can no longer be safely ignored. HOGNOSE captures increasing amorphization with decreasing temperature, and more subtle corrosion rate enhancement at higher temperatures. Comparisons between HOGNOSE results and literature data suggest that the next focus for mechanistic modeling should consider additional neutron flux effects. To support HOGNOSE development, corrosion testing of Zircaloy-4 in steam at atmospheric pressure and 415 degrees Celsius was performed. Samples were analyzed using a focused ion beam/scanning electron microscope (FIB/SEM) to obtain oxide thickness measurements with greater temporal resolution than is widely provided by autoclave testing. Oxide thickness data was used to determine the thermal dependence of oxygen diffusivity in the oxide within HOGNOSE. HOGNOSE would also benefit from measurements of the concentrations and charge states of cation dopants in post-irradiated Zircaloy oxides to help determine whether this model is truly accurate in its physical description.<br>by Andrew Frederic Dykhuis.<br>Ph. D.
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Sonehag, Christian. "Modeling of Ion Injection in Oil-Pressboard Insulation Systems." Thesis, Uppsala universitet, Fasta tillståndets elektronik, 2012. http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-177600.
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To make a High Voltage Direct Current (HVDC) transmission more energy efficient, the voltage of the system has to be increased. To allow for that the components of the system must be constructed to handle the increases AC and DC stresses that this leads to. One key component in such a transmission is the HVDC converter transformer. The insulation system of the transformer usually consists of oil and oil-impregnated pressboard. Modeling of the electric DC field in the insulation system is currently done with the ion drift diffusion model, which takes into account the transport and generation of charges in the oil and the pressboard. The model is however lacking a description of how charges are being injected from the electrodes and the oil-pressboard interfaces. The task of this thesis work was to develop and implement a model for this which improves the result of the ion drift diffusion model. A theoretical study of ion injection was first carried out and proceeding from this, a model for the ion injection was formulated. By using experimental data from 5 different test geometries, the injection model could be validated and appropriate parameter values of the model could be determined. By using COMSOL Multiphysics®, the ion drift diffusion model with the injection model could be simulated for the different test geometries. The ion injection gave a substantial improvement of the ion drift diffusion model. The positive injection from electrodes into oil was found to be in the range 0.3-0.6 while the negative injection was 0.3 lower. Determination of the parameters for the injection from oil-pressboard interfaces proved to be difficult, but setting the parameters in the range 0.01-1 allowed for a good agreement with the experimental data. Here, a fit could be obtained for multiple assumptions about the set of active injection parameters. Finally it is recommended that the investigation of the ion injection continues in order to further improve the model and more accurately determine the parameters of it. Suggestions on how this work could be carried out are given in the end.
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Sales, Michael F. "Context Dependent Numerosity Representations in Children." The Ohio State University, 2019. http://rave.ohiolink.edu/etdc/view?acc_num=osu1557146188226533.
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Kaphle, VIkash. "Organic Electrochemical Transistors." Kent State University / OhioLINK, 2019. http://rave.ohiolink.edu/etdc/view?acc_num=kent1576594504410991.
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Engelbrecht, Nicholas Eugéne. "On the development and applications of a three-dimensional ab initio cosmic-ray modulation model / Nicholas Eugéne Engelbrecht." Thesis, North-West University, 2012. http://hdl.handle.net/10394/8735.
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A proper understanding of the effects of turbulence on the diffusion and drift of cosmic-rays in
the heliosphere is imperative for a better understanding of cosmic-ray modulation. This study
presents an ab initio model for cosmic-ray modulation, incorporating for the first time the results
yielded by a two-component turbulence transport model. The latter model is solved for
solar minimum heliospheric conditions, utilizing boundary values chosen in such a way that
the results of this model are in fair to good agreement with spacecraft observations of turbulence
quantities, not only in the ecliptic plane, but also along the out-of-ecliptic trajectory of the
Ulysses spacecraft. These results are employed as inputs for modelled slab and 2D turbulence
energy spectra, which in turn are used as inputs for parallel mean free paths based on those
derived from quasi-linear theory, and perpendicularmean free paths from extended nonlinear
guiding center theory. The modelled 2D spectrum is chosen based on physical considerations,
with a drop-off at the very lowest wavenumbers commencing at the 2D outerscale. There currently
exist no models or observations for this quantity, and it is the only free parameter in this
study. The use of such a spectrum yields a non-divergent 2D ultrascale, which is used as an
input for the reduction terms proposed to model the effects of turbulence on cosmic-ray drifts.
The resulting diffusion and drift coefficients are applied to the study of galactic cosmic-ray
protons, electrons, antiprotons, and positrons using a three-dimensional, steady-state numerical
cosmic-ray modulation code. The magnitude and spatial dependence of the 2D outerscale
is demonstrated to have a significant effect on computed cosmic-ray intensities. A form for the
2D outerscale was found that resulted in computed cosmic-ray intensities, for all species considered,
in reasonable agreement with multiple spacecraft observations. Computed galactic
electron intensities are shown to be particularly sensitive to choices of parameters pertaining
to the dissipation range of the slab turbulence spectrum, and certain models for the onset
wavenumber of the dissipation range could be eliminated in this study.<br>Thesis (PhD (Physics))--North-West University, Potchefstroom Campus, 2013
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Lin, Charlette. "Out of Sight Out of Mind? The Effects of Prior Study and Visual Attention on Word Identification." The Ohio State University, 2015. http://rave.ohiolink.edu/etdc/view?acc_num=osu1430322757.
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Nikitin, Vyacheslav Y. "Parameter Dependencies in an Accumulation-to-Threshold Model of Simple Perceptual Decisions." The Ohio State University, 2015. http://rave.ohiolink.edu/etdc/view?acc_num=osu1442166546.
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Chen, Wei. "Essays on Learning, Decision-making and Attention." The Ohio State University, 2017. http://rave.ohiolink.edu/etdc/view?acc_num=osu1491925104416652.
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Demir, Huseyin. "A Process-Based Model for Beach Profile Evolution." Diss., Georgia Institute of Technology, 2007. http://hdl.handle.net/1853/19811.
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Beach profile models predict the changes in bathymetry along a line perpendicular to the shoreline. These models are used to forecast bathymetric changes in response to storms, sea level rise or human activities such as dredging and beach nourishment. Process-based models achieve this by simulating the physical processes that drive the sediment transport as opposed to behavior models which simulate observed profile changes without resolving the underlying processes. Some of these processes are wave shoaling and breaking, boundary layer streaming, and offshore-directed undertow currents. These hydrodynamic processes control the sediment processes such as sediment pick-up from the bottom, diffusion of the sediment across the water column and its advection with waves and currents.
For this study, newly developed sediment transport and boundary layer models were coupled with existing models of wave transformation, nearshore circulation and bathymetry update, to predict beach profile changes. The models covered the region from the dry land to a depth of 6-8 meters, spanning up to 500 meters in the cross-shore direction. The modeling system was applied at storm time scales, extending from a couple of hours to several days. Two field experiments were conducted at Myrtle Beach, SC, involving the collection of wave, current and bathymetric data as a part of this study. The results were used to calibrate and test the numerical models along with data from various laboratory studies from the literature.
The sediment transport model computes the variation of sediment concentrations over a wave period and over the water column, solving the advection-diffusion equation using the Crank-Nicholson finite-difference numerical scheme. Using a new approach, erosion depth thickness and sediment concentrations within the bed were also predicted. The model could predict sediment transport rates for a range of conditions, within a factor of two. It successfully computed the sediment concentration profile over the water column and within the bed and its variation throughout a wave period. Erosion depth and sheet flow layer thickness were also predicted reasonably well.
Wave heights across the profile were predicted within ten percent when the empirical wave breaking parameter was tuned appropriately. Mean cross-shore velocities contain more uncertainty, even after tuning. The importance of capturing the location of the maximum, near-bottom, cross-shore velocity when predicting bar behavior was shown. Bar formation, erosion, accretion, onshore and offshore bar movement were all computed with the model successfully
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Servant, Mathieu. "Mécanismes de prise de décision dans des environnements conflictuels : approches comportementales, computationnelles et électrophysiologiques." Thesis, Aix-Marseille, 2015. http://www.theses.fr/2015AIXM4764.
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Une décision perceptive est un processus délibératif consistant à choisir une proposition catégorielle ou un plan d'action parmi plusieurs alternatives sur la base d'information sensorielle. Les modèles de prise décision font l'hypothèse que l'information sensorielle est accumulée au cours du temps jusqu'à un seuil décisionnel. Ces modèles ont récemment reçu un support empirique important grâce à la découverte de neurones accumulateurs dans le cerveau de singes. Toutefois, l'étude neurophysiologique de ces système d'accumulation chez l'homme est rare. Ce travail de thèse vise à mieux comprendre les mécanismes neuronaux de prise de décision chez l'homme dans des contextes de la vie réelle, beaucoup plus complexes que ceux utilisés chez le singe<br>A perceptual decision is a deliberative process that aims to choose a categorical proposition or course of action from a set of alternatives on the basis of available sensory information. Models of perceptual decision-making assume that sensory information is accumulated to some threshold level, whence the decision terminates in a choice. The recent discovery of neural correlates of these theoretical predictions in the non-human primate brain has reinforced their validity. However, neurophysiological studies of perceptual decision-making mechanisms in humans are relatively scarce. This work aims at enhancing our understanding of the computations and neurophysiology underpinning such mechanisms in humans, through the study of decision-making contexts more complex than those used in monkey research
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Domenech, Philippe. "Une approche neuro-computationnelle de la prise de décision et de sa régulation contextuelle." Phd thesis, Université Claude Bernard - Lyon I, 2011. http://tel.archives-ouvertes.fr/tel-00847494.
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Décider, c'est sélectionner une alternative parmi l'ensemble des options possibles en accord avec nos buts. Les décisions perceptuelles, correspondant à la sélection d'une action sur la base d'une perception, résultent de l'accumulation progressive d'information sensorielle jusqu'à un seuil de décision. Aux niveaux comportemental et cérébral, ce processus est bien capturé par les modèles de décision par échantillonnage séquentiel. L'étude neurobiologique des processus de décision, guidée par l'usage de modèles computationnels, a permis d'établir un lien clair entre cette accumulation d'information sensorielle et un réseau cortical incluant le sillon intra-pariétal et le cortex dorso-latéral préfrontal. L'architecture des réseaux biologiques impliqués dans la prise de décision, la nature des algorithmes qu'ils implémentent et surtout, l'étude des relations entre structure biologique et computation est au cœur des questionnements actuels en neurosciences cognitives et constitue le fil conducteur de cette thèse. Dans un premier temps, nous nous sommes intéressés aux mécanismes neuraux et computationnels permettant l'ajustement du processus de décision perceptuelle à son contexte. Nous avons montré que l'information a priori disponible pour prédire nos choix diminue la distance au seuil de décision, régulant ainsi dynamiquement la quantité d'information sensorielle nécessaire pour sélectionner une action. Pendant la prise de décision perceptuelle, le cortex cingulaire antérieur ajuste le seuil de décision proportionnellement à la quantité d'information prédictive disponible et le cortex dorso-latéral préfrontal implémente l'accumulation progressive d'information sensorielle. Dans un deuxième temps, nous avons abordé la question de l'unicité, au travers des domaines cognitifs, des mécanismes neuro-computationnels implémentant la prise de décision. Nous avons montré qu'un modèle de décision par échantillonnage séquentiel utilisant la valeur subjective espérée de chaque option prédisait avec précision le comportement de sujets lors de choix économiques risqués. Pendant la décision, la portion médiale du cortex orbito-frontal code la différence entre les valeurs subjectives des options considérées, exprimées sur une échelle de valeur commune. Ce signal orbito-frontal médian sert d'entrée à un processus de décision par échantillonnage séquentiel implémenté dans le cortex dorso-latéral préfrontal. Pris ensemble, nos travaux précisent les contours d'une architecture fonctionnelle de la prise de décision dans le cortex préfrontal humain en établissant une cartographie des modules computationnels qu'il implémente, mais aussi en caractérisant la façon dont l'intégration fonctionnelle de ces régions cérébrales permet l'émergence de la capacité à prendre des décisions
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Champmartin, Aude. "Modélisation et étude numérique d'écoulements diphasiques : Modélisation d’un écoulement homogène équilibré : Modélisation des collisions entre gouttelettes à l’aide d’un modèle simplifié de type BGK." Thesis, Cachan, Ecole normale supérieure, 2011. http://www.theses.fr/2011DENS0007/document.
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Cette thèse décrit la modélisation et la simulation de systèmes à deux phases composées de particules évoluant dans un gaz. Les deux phases interagissent entre elles et le type de modèle à considérer dépend directement du type de simulations envisagées. Dans une première partie, les deux phases sont considérées comme des fluides, elles sont décrites à l’aide d’un modèle de mélange avec une relation de dérive (permettant de suivre une vitesse relative entre les deux phases et de prendre en compte deux vitesses) et sont supposées à l’équilibre en température et pression. Cette partie du manuscrit est composée de la dérivation des équations, de l’écriture d’un schéma numérique associé à ce jeu d’équations, d’une étude d’ordre de ce schéma ainsi que de simulations. Une étude mathématique de ce modèle (hyperbolicité dans un cadre simplifié, stabilité du système linéaire autour d’un état constant) a été réalisée dans un cadre o`u le gaz est supposé barotrope. La seconde partie de ce manuscrit est consacrée à la modélisation de l’effet de collisions inélastiques sur les gouttelettes lorsque l’on se place à un temps de simulation beaucoup plus court, pour lequel les gouttelettes ne peuvent plus être vues comme un fluide. Pour modéliser ces collisions, on construit un modèle simplifié (moins coûteux en temps) de type BGK permettant de reproduire le comportement en temps de certains moments sur les gouttelettes. Ces moments sont choisis pour être représentatifs de l’effet des collisions sur ces gouttelettes, à savoir une thermalisation en vitesse et énergie. Ce modèle est discrétisé avec une méthode particulaire et des résultats numériques sont donnés en comparaison avec ceux obtenus avec un modèle résolvant directement l’équation de Boltzmann homogène<br>This thesis describes the modelisation and the simulation of two-phase systems composed of droplets moving in a gas. The two phases interact with each other and the type of model to consider directly depends on the type of simulations targeted. In the first part, the two phases are considered as fluid and are described using a mixture model with a drift relation (to be able to follow the relative velocity between the two phases and take into account two velocities), the two-phase flows are assumed at the equilibrium in temperature and pressure. This part of the manuscript consists of the derivation of the equations, writing a numerical scheme associated with this set of equations, a study of this scheme and simulations. A mathematical study of this model (hyperbolicity in a simplified framework, linear stability analysis of the system around a steady state) was conducted in a frame where the gas is assumed barotropic. The second part is devoted to the modelisation of the effect of inelastic collisions on the particles when the time of the simulation is shorter and the droplets can no longer be seen as a fluid. We introduce a model of inelastic collisions for droplets in a spray, leading to a specific Boltzmann kernel. Then, we build caricatures of this kernel of BGK type, in which the behavior of the first moments of the solution of the Boltzmann equation (that is mass, momentum, directional temperatures, variance of the internal energy) are mimicked. The quality of these caricatures is tested numerically at the end
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Alles, Benjamin. "Coupled drift diffusion problems with implicit source functions and their applications." Aachen Shaker, 2008. http://d-nb.info/989623653/04.
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Flegg, Mark Bruce. "Theoretical investigation of mechanisms of formation and interaction of nanoparticles." Thesis, Queensland University of Technology, 2010. https://eprints.qut.edu.au/31843/1/Mark_Flegg_Thesis.pdf.
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In this thesis an investigation into theoretical models for formation and interaction of nanoparticles is presented. The work presented includes a literature review of current models followed by a series of five chapters of original research. This thesis has been submitted in partial fulfilment of the requirements for the degree of doctor of philosophy by publication and therefore each of the five chapters consist of a peer-reviewed journal article. The thesis is then concluded with a discussion of what has been achieved during the PhD candidature, the potential applications for this research and ways in which the research could be extended in the future. In this thesis we explore stochastic models pertaining to the interaction and evolution mechanisms of nanoparticles. In particular, we explore in depth the stochastic evaporation of molecules due to thermal activation and its ultimate effect on nanoparticles sizes and concentrations. Secondly, we analyse the thermal vibrations of nanoparticles suspended in a fluid and subject to standing oscillating drag forces (as would occur in a standing sound wave) and finally on lattice surfaces in the presence of high heat gradients. We have described in this thesis a number of new models for the description of multicompartment networks joined by a multiple, stochastically evaporating, links. The primary motivation for this work is in the description of thermal fragmentation in which multiple molecules holding parts of a carbonaceous nanoparticle may evaporate. Ultimately, these models predict the rate at which the network or aggregate fragments into smaller networks/aggregates and with what aggregate size distribution. The models are highly analytic and describe the fragmentation of a link holding multiple bonds using Markov processes that best describe different physical situations and these processes have been analysed using a number of mathematical methods. The fragmentation of the network/aggregate is then predicted using combinatorial arguments. Whilst there is some scepticism in the scientific community pertaining to the proposed mechanism of thermal fragmentation,we have presented compelling evidence in this thesis supporting the currently proposed mechanism and shown that our models can accurately match experimental results. This was achieved using a realistic simulation of the fragmentation of the fractal carbonaceous aggregate structure using our models. Furthermore, in this thesis a method of manipulation using acoustic standing waves is investigated. In our investigation we analysed the effect of frequency and particle size on the ability for the particle to be manipulated by means of a standing acoustic wave. In our results, we report the existence of a critical frequency for a particular particle size. This frequency is inversely proportional to the Stokes time of the particle in the fluid. We also find that for large frequencies the subtle Brownian motion of even larger particles plays a significant role in the efficacy of the manipulation. This is due to the decreasing size of the boundary layer between acoustic nodes. Our model utilises a multiple time scale approach to calculating the long term effects of the standing acoustic field on the particles that are interacting with the sound. These effects are then combined with the effects of Brownian motion in order to obtain a complete mathematical description of the particle dynamics in such acoustic fields. Finally, in this thesis, we develop a numerical routine for the description of "thermal tweezers". Currently, the technique of thermal tweezers is predominantly theoretical however there has been a handful of successful experiments which demonstrate the effect it practise. Thermal tweezers is the name given to the way in which particles can be easily manipulated on a lattice surface by careful selection of a heat distribution over the surface. Typically, the theoretical simulations of the effect can be rather time consuming with supercomputer facilities processing data over days or even weeks. Our alternative numerical method for the simulation of particle distributions pertaining to the thermal tweezers effect use the Fokker-Planck equation to derive a quick numerical method for the calculation of the effective diffusion constant as a result of the lattice and the temperature. We then use this diffusion constant and solve the diffusion equation numerically using the finite volume method. This saves the algorithm from calculating many individual particle trajectories since it is describes the flow of the probability distribution of particles in a continuous manner. The alternative method that is outlined in this thesis can produce a larger quantity of accurate results on a household PC in a matter of hours which is much better than was previously achieveable.
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Lamboll, Robin Davies. "Two-dimensional modelling of novel back-contact solar cells." Thesis, University of Cambridge, 2017. https://www.repository.cam.ac.uk/handle/1810/268518.
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This dissertation computationally and analytically investigates ways to model solar cells when the lateral motion of charge carriers and light are relevant. We focus on back-contact perovskite solar cells, and assessing the experimental technique of scanning photocurrent microscopy as a means to investigate them. Solar cells are three-dimensional objects frequently modelled as being one-dimensional. However, for more complex designs of solar cell or if the cell is only point-illuminated, one-dimensional modelling is insufficient. In the first study, some conditions for reducing the complexity of two-dimensional drift-diffusion simulations are investigated for a back-contact perovskite cell. Analytic expressions for the relationship in both the low extraction velocity and high extraction velocity regimes are demonstrated, and the conditions where these approximations break down are investigated. These findings are then applied a point-excited film with an extended electrode, a problem encountered during scanning photocurrent microscopy. We show the current recorded in this case should decay exponentially with the distance between excitation and electrode, with a decay constant that can be related to device parameters. The characteristic equilibration time for the system to reach this current is demonstrated to increase linearly with distance. Between this gradient and the exponent, information about the diffusion and recombination mechanics can be extracted from a variety of systems. Photon recycling is the process in whereby photogenerated carriers recombine to generate light that is absorbed again within the solar cell. In the second section, we apply the findings of the first section to show that experimental results published elsewhere are best explained by photon recycling in methylammonium lead iodide perovskite back-contact solar cells. However we do not have an established theoretical model for long-ranged lateral optical transport in these solar cells. Three models are developed: a bimolecular model for unscattered, coherent transport; a photon diffusion model for frequently scattered, noncoherent light; and a monomolecular, assisted-diffusion model. The modal nature of coherent optical transport is considered and modifications to previous one-dimensional theories are made. The nature of the photon diffusion model is discussed, as are theoretical shortcomings. All three models are then solved numerically and compared to experimental results. The low-scattering photon diffusion models correspond well to the experiment. The third investigation involves the performance of different architectures of back-contact perovskite cells. These cells potentially offer increased current due to less shadowing by front electrodes. We compare them to each other and to traditional vertical structures. It is found that, in terms of internal transport, the back-contact solar cells give less efficient performance than the vertical design. The best of the back-contact cells investigated is a flat interdigitated design. The increase in efficiency from optical factors would have to exceed 10% for the overall efficiency of back-contact cells to be higher than vertical devices. We also develop a model of photon recycling appropriate for short-ranged, bulk 2D transport and demonstrate that in perovskites, it produces little change in power conversion efficiency (and small changes in short-circuit voltage) when compared with the standard drift-diffusion equations with the second-order recombination constant is adjusted.
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Cavalier, Jordan. "Modèles cinétiques et caractérisation expérimentale des fluctuations électrostatiques dans un propulseur à effet Hall." Thesis, Université de Lorraine, 2013. http://www.theses.fr/2013LORR0130/document.
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L'étude des phénomènes turbulents se développant en sortie du propulseur de Hall est nécessaire pour pouvoir modéliser le transport anormal (par opposition au transport diffusif) des électrons à travers les lignes de champ magnétique. Les relations de dispersion de deux instabilités pouvant être responsables de ce transport ont été mesurées à des échelles millimétriques à l'aide du diagnostic de diffusion collective de la lumière. Ce travail de thèse s'attache à en donner une description aussi bien théorique qu'expérimentale, pierre à l'édifice de la compréhension du transport dans le propulseur. Une instabilité se propageant majoritairement dans la direction azimutale du propulseur y est caractérisée comme étant l'instabilité de dérive électronique ExB et un modèle analytique décrivant la fréquence expérimentale y est dérivé et validé. De plus, le manuscrit présente une méthode de déconvolution du signal de la diffusion collective de la fonction d'appareil pour ce mode. Une fois déconvoluées, les relations de dispersion expérimentales peuvent être ajustées par la fréquence du modèle analytique, ce qui permet de mesurer expérimentalement et de manière originale la température et la densité électronique dans le jet d'ions énergétiques du plasma du propulseur. Enfin, la seconde instabilité, se développant autour de la direction axiale du propulseur, est caractérisée comme l'instabilité double faisceau entre les ions simplement et doublement chargés du plasma<br>The study of turbulent phenomena that grow at the exit plane of the Hall thruster is required to modelize the anomalous transport (in contrast to the diffusion transport) of electrons across the magnetic field lines. The dispersion relations of two instabilities that can be responsible for this transport have been mesured at millimetric scales by mean of the collective light scattering diagnostic. The aim of the thesis is to describe them theoretically as well as experimentally, improving the understanding of the Hall thruster transport. In the thesis, an instability that propagates principally azimuthally is caracterized as the ExB electron drift instability and an analytical model that describes the experimental frequency is derived and validated. In addition, the manuscript presents an original method to unfold the signal of the collective scattering diagnostic from the instrumental function of this mode. Once corrected, the experimental dispersion relations can be adjusted by the frequency given by the analytical model, allowing to measure experimentally and in an original way the electron temperature and density in the energetic ion jet of the Hall thruster plasma. The second instability that is mainly propagating in the axial direction is caracterized as the two-stream instability between the simply and doubly charged ions of the plasma
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Bottemanne, Laure. "Influence de la motivation liée à autrui sur la décision : corrélats computationnels et magnétoencéphalographiques chez l’Homme." Thesis, Lyon, 2019. http://www.theses.fr/2019LYSE1257/document.
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L’homme est un animal social. La majorité des décisions que nous prenons se font dans un contexte social et dépendent d’autrui, ce qui implique des calculs cérébraux complexes qui incluent tous les facteurs contextuels et environnementaux. La majorité des études ultérieures de la prise en compte d’autrui dans la décision ont utilisé des tâches de partage de récompenses entre soi et autrui. Les choix possibles amènent le décideur à considérer autrui, mais dans le but de gagner soi-même une récompense ; donc dans un contexte où les récompenses liées à soi et les récompenses liées à autrui sont confondues. Le travail présenté dans cette thèse avait pour but une meilleure compréhension des mécanismes cérébraux soutenant l’intégration d’autrui dans la prise de décision, sans que la récompense pour autrui n’interfère directement avec soi. Nous nous sommes appuyés sur le cadre théorique de la décision perceptuelle et des modèles de diffusion pour l'étude i) des modifications du processus décisionnel induites par une récompense monétaire allant à autrui et ii) de l’impact de l’effet d’audience (le fait de se sentir observé) sur la décision. Nos résultats computationnels montrent qu'une récompense pour autrui, par rapport à une récompense pour soi, et une audience, par rapport au secret, modifient le taux de dérive de la variable de décision. En magnétoencéphalographie, nos résultats indiquent que les décisions pour soi et pour autrui diffèrent pendant, mais aussi après, la prise de décision dans des zones cérébrales associées avec la transformation sensori-motrice, l'ajustement du compromis entre rapidité et justesse et avec la cognition sociale. Ainsi, le cortex temporal montre des différences de -1170 millisecondes (ms) à -1023 ms, de -993 ms à -915 ms et de -343 ms à -188 ms en amont de la réponse. Ce qui suppose une influence sur l’intégration des preuves sensorielles. Après la décision, les régions frontales ont également montré des différences entre soi et autrui, de 153 ms à 303 ms post-réponse, suggérant une différence entre soi et autrui dans l’ajustement du compris entre justesse et rapidité. Le bénéficiaire de la récompense associée à la décision modifie les paramètres décisionnels et les corrélats cérébraux de la décision perceptuelle, démontrant l’importance du contexte social dans l’implémentation de la prise de décision chez l’Homme. Ce travail appuie également l’utilité des modèles mathématiques tels que les modèles de diffusion dans la compréhension des processus décisionnels, même de ceux découlant de la cognition sociale<br>Humans are inherently social: most of human’s decisions are within a social context and depend on others. For more than a century, researchers explore aspects of social cognition. Aiming to understand human behavior in social contexts, neuro-economic researches showed that taking others into account involve complex brain computations that include all environmental and contextual factors. However, most of the work was made using money allocation tasks; mixing self-affecting and other-affecting rewards into the decision making process. The present work intended the understanding of the brain mechanisms underpinning the integration of others into the decision making process for decisions that include others and do not interfere with self-rewards.Taking advantage of mathematical models from the drift diffusion models framework, we conducted experiments investigating how others influence the mechanistic of perceptual decisions and their correlates in the human brain. We showed that taking rewards for others into account and being observed by others influence the drift rate of the decision variable. The drift rate is higher in audience than in secret and higher for self-rewards than for other-rewards. These results indicate that others are integrated into the accumulation process together with the evidence available for making a decision. At the brain level, we found difference between self and other decisions over the anterior temporal and centro-frontal cortices during decision making. This suggests that the beneficiary of a decision modifies sensory-motor transformation processes. In addition, self- and other-affecting difference showed difference over the medial frontal sensors after the decision making process, indicating a variation in the speed-accuracy tradeoff adjustment process
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Hemzalová, Zuzana. "Evoluční algoritmy pro ultrazvukovou perfúzní analýzu." Master's thesis, Vysoké učení technické v Brně. Fakulta elektrotechniky a komunikačních technologií, 2021. http://www.nusl.cz/ntk/nusl-442504.
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This thesis deals with the principles of ultrasonic perfusion analysis and methods for determining perfusion parameters. It examines Evolutionary algorithms and their ability to optimize the approximation of dilution curves from ultrasond tissue scannig. It compares the optimization performance of three evolutionary algorithms. Continuous genetic algorithm GA, algorithm SOMA and PSO. Methods are evaluated on simulated and clinical data.
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Wu, Hsin-hung, and 吳信宏. "A Quantum-Corrected Drift Diffusion Model for Ionic Channel." Thesis, 2010. http://ndltd.ncl.edu.tw/handle/83630263378464145805.
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碩士<br>國立高雄大學<br>應用數學系碩士班<br>98<br>A drift diffusion model coupled with the density gradient method as quantum mechanical corrections is proposed and numerically investigated for ionic channel. The model is completely self-adjoint for all state variables and hence provides many appealing mathematical features such as global convergence with simple initial guesses and highly parallelizable. Numerical simulations on K channel with the channel length 3.5 nm using this model have been performed and compared with that using the classical drift diffusion model. It is shown that the I-V characteristics of this ionic channel is corrected by the density-gradient equations with current drive reduced by about 6.6% comparing with that of the classical model along.
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(10725198), Yi Yang. "Electromechanical Characterization of Organic Field-Effect Transistors with Generalized Solid-State and Fractional Drift-Diffusion Models." Thesis, 2021.
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<p>The miniaturization and thinning of wearable, soft robotics and medical devices are soon to require higher performance modeling as the physical flexibility causes direct impacts on the electrical characteristics of the circuit – changing its behavior. As a representative flexible electronic component, the organic field effect transistor (OFET) has attracted much attention in its manufacturing as well as applications. However, as the strain and stress effects are integrated into multiphysics modelers with deeper interactions, the computational complexity and accuracy of OFET modeling is resurfacing as a limiting bottleneck.</p><p>The dissertation was organized into three interrelated studies. In the first study, the Mass-Spring-Damper (MSD) model for an inverted staggered thin film transistor (TFT) was proposed to investigate the TFT’s internal stress/strain fields, and the strain effects on the overall characteristics of the TFT. A comparison study with the finite element analysis (FEA) model shows that the MSD model can reduce memory usage and raises the computational convergence speed for rendering the same results as the FEA. The second study developed the generalized solid-state model by incorporating the density of trap states in the band structure of organic semiconductors (OSCs). The introduction of trap states allows the generalized solid-state model to describe the electrical characteristics of both inorganic TFTs and organic field-effect transistors (OFETs). It is revealed through experimental verification that the generalized solid-state model can accurately characterize the bending induced electrical properties of an OFET in the linear and saturation regimes. The third study aims to model the transient and steady-state dynamics of an arbitrary organic semiconductor device under mechanical strain. In this study, the fractional drift-diffusion (Fr-DD) model and its computational scheme with high accuracy and high convergence rate were proposed. Based on simulation and experimental validation, the transconductance and output characteristics of a bendable OFET were found to be well determined by the Fr-DD model not only in the linear and saturation regimes, but also in the subthreshold regime.</p>
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Lu, Li-Shuo, and 呂理碩. "The Simulation of Carrier Transport under 3D Poisson and Drift-Diffusion, Schrödinger and Landscape Model in InGaN Light Emitting Diode." Thesis, 2016. http://ndltd.ncl.edu.tw/handle/ua6e2e.
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碩士<br>國立臺灣大學<br>光電工程學研究所<br>105<br>In the classical 3-D Poisson drift-diffusion self-consistent solver developed by our lab is versatile that we can combine it with other solvers and functions to simulate the carrier transport behavior and electric characteristic. However, it is hard to couple well with Schrodinger equation and solve them self-consistently under current injection conditions. Therefore, we apply the Poisson drift-diffusion with landscape theory. The landscape theory model is able to consider the quantum effective potential. It solvesHu(r) = (-Δ+Ec;v)u(r) = 1, which is a Schrodinger-like equation with uniform right-hand side and modifies the electron and hole density according to the obtained effective potential (1/u). Not only localized landscape theory avoids solving Schrodinger equation, which is a eigenvalues and eigenvectors problem and it costs much computation time, but also provides the effective quantum potential in the classical Poisson drift-diffusion model. In this thesis, we apply the random alloy generator and strain solver to construct the atom distribution and calculate the strain distribution before solving the Poisson drift-diffusion equations. Simulation results show that quantum well potential solved with landscape model is smoother and higher, which leads to the extended carrier distribution. It also lowering the quantum barrier''s potential due to the quantum tunnelling effects. The forward voltage is smaller as a result. When the random atom distribution is obtained by random number generator, the composition map is decided by a Gaussian weighting function with broadening factor sigma. When sigma increases, the potential and carrier density becomes smoother and forward the voltage declines because of lower potential. Different average indium compositions from 11%, 14%, 17% to 20% were studied. It appears that lower piezoelectric potential would be obtained with landscape model which leads to the decrease of forward voltage. But in the In0.11Ga0.89N case, the forward voltages solved with and without landscape are closed because peizo-polarization is smaller and the bandgap is higher. In chapter 4, we simulate the carrier transport behavior in the fluctuate quantum well(QW) thickness. With fluctuated thickness in a larger scale compared to local indium fluctuation, the polarization declines and provides a percolation path at the barrier. The forward voltages solved without landscape decline with increasing fluctuate thickness. However, fluctuated thickness may leads to the stronger confinement, larger effective bandgap and reduction of forward voltage.
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Chiu, Chia-Ning, and 邱佳寧. "The Effects of Challenge, Hindrance Pressure, and Sports Participation on Flanker Task and Executive Task Performance Investigated Using the Drift Diffusion Model." Thesis, 2018. http://ndltd.ncl.edu.tw/handle/35d5d2.
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碩士<br>國立中央大學<br>認知與神經科學研究所<br>106<br>Sports can be classified as open-skill sport (strategic and interceptive sports) or closed-skill sports (static sports). Several review papers have provided an overview of the beneficial effects of fitness training on cognitive function and athletes train hard to fight to achieve good performance. However, pressure could be an important issue that would change sports performance. This thesis tried to gain a better understanding of the influence of pressure on cognitive performance and how this can be altered by different sports engagement. Nonsporting controls were compared to an exercise groups, including swimmers and runners (closed-skill, predictable action sports) and a volleyball group (an open-skill, unpredictable action sport) on tasks with three different pressure manipulations: challenge stress; hindrance stress; and task difficulty. These were used to investigate performance on the flanker tasks and an executive task.
The drift diffusion model (DDM) which negates problems such as speed-accuracy tradeoffs in cognitive tasks, and provides more specific measurement of aspects of performance, such as the rate at which decisions are made and the decision threshold was specially used to quantify the effects of stress on performance.
In Experiment 1, the flanker task in conjunction with a time pressure manipulation was used to investigate how such pressure may influence attention. In Experiment 2, video observation combined with time pressure was used to investigate whether this different stressor influenced decision-making performance and whether this interacted with sporting expertise and fitness. Drift diffusion model analysis of Experiments 1 and 2 showed that time pressure induced reduced threshold separation, non-decision time and decreased drift rate. Higher fitness levels were associate with faster speed to make a decision under time pressure. The volleyball group seem to have a better ability to process the flanker information which may be a consequence of the characteristics of the sport. No effect of observation was found (Experiment 2
In Experiment 3, task switching with stress manipulation caused by different switch levels, was investigated to evaluate a cognitive equivalent of motor switching and flexibility. The results showed that higher fitness levels led to performing more accurately in the task switch condition. The manipulation of task difficulty resulted in a lower threshold separation and non-decision time but a faster drift rate in a relatively simple task, but higher criteria, non-decision time and lower drift rate in the difficult task. Overall, in addition to performing better on cognitive tasks, the open skill sport (volleyball) group had better performance than nonsporting and exercise group when under different pressure manipulations.
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Sakowski, Konrad. "Determination of the properties of the nitride laser diodes and light-emitting diodes by simulation based on the drift-diffusion model with the Discontinuous Galerkin Method." Doctoral thesis, 2017.
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In this dissertation we discuss numerical simulations of semiconductor devices with the drift-diffusion model. This model consists of three nonlinear elliptic differential equations known as van Roosbroeck equations. We consider two variants of discretization of this system with the Composite Discontinuous Galerkin Method, based on Symmetric Interior Penalty Galerkin method and on Weakly Over-Penalized Symmetric Interior Penalty method. We discuss these methods in context of simulations of luminescent devices based on gallium nitride and its mixed compounds. The proposed methods account for lower regularity of solutions on the interfaces between layers of different materials. It is shown that for in the equilibrium state regime the discrete problems are well posed and error estimates for the piecewise-linear finite element spaces in one- and two-dimensional domains are derived. These error bounds are then verified against results of numerical simulations, which covered both abstract settings and real semiconductor structures, in equilibrium and non-equilibrium state.It is also demonstrated that these methods may be used in determination of physical properties of the luminescent devices. For this purpose, a linearization of the discretized van Roosbroeck equations based on the Newton method and Picard method was developed. Then the numerical code was developed, which makes use of these discretizations and the linearization. Since the physical phenomena related to operation of the gallium nitride laser diodes and light-emitting diodes are not fully understood, our application allows for modification of the underlying differential model to some extent. Examples of such modifications and their effects are presented.<br>W niniejszej pracy omówione zostały numeryczne symulacje urządzeń półprzewodnikowych wykonywane za pomocą modelu dryfowo-dyfuzyjnego. Model ten składa się z trzech nieliniowych eliptycznych równań różniczkowych znanych jako równania van Roosbroecka. Rozważane są dwa warianty dyskretyzacji tego układu, wykorzystujące Złożoną Nieciągłą Metodę Galerkina, które bazują na symetrycznej wersji Nieciągłej Metody Galerkina (ang. Symmetric Interior Penalty Galerkin method) oraz wersji symetrycznej o zwiększonych wyrazach kary (ang. Weakly Over-Penalized Symmetric Interior Penalty method). Te metody są omawiane w kontekście symulacji świecących urządzeń półprzewodnikowych opartych o azotek galu oraz jego związki i uwzględniają problem niższej stabilności rozwiązań na granicach materiałów. W pracy wykazano, że w przypadku stanu równowagowego problemy dyskretne są dobrze postawione. Wyprowadzone zostały także oszacowania błędu rozwiązania problemów dyskretnych dla kawałkami liniowych funkcji dyskretnych w jednym i w dwóch wymiarach. Oszacowania te zostały skonfrontowane z rezultatami symulacji numerycznych, które zostały przeprowadzone dla realnych struktur półprzewodnikowych, jak i w przypadkach abstrakcyjnych, w przypadku równowagowym i nierównowagowym.Zostało pokazane także zastosowanie omawianych metod do znajdowania własności fizycznych świecących struktur półprzewodnikowych. W tym celu opracowana została linearyzacja zdyskretyzowanych równań van Roosbroecka, oparta o metodę Newtona oraz metodę Picarda. Opracowany został także program, który stosuje omawiane dyskretyzacje i metodę linearyzacji. Ponieważ zjawiska fizyczne dotyczące działania diod laserowych oraz diod elektroluminescencyjnych nie są do końca zrozumiane, opracowany program pozwala na wprowadzanie zmian do zaimplementowanego modelu różniczkowego. Przykłady takich modyfikacji oraz ich wpływ są również zademonstrowane.
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Michalski, Julien. "An investigation of reach decisions during ongoing action control." Thesis, 2020. http://hdl.handle.net/1866/25402.
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Les études neurophysiologiques de la prise de décision, traditionnellement ancrées dans des principes neuro-économiques, ont évoluées pour inclure une variété d’aires du cerveau. Partant d’abord du lobe frontal associé aux jugements de valeur, le champ s’est élargi pour inclure d’autres types de décisions incluant les décisions perceptuelles et les décisions incarnées qui impliquent notamment les aires sensorimotrices du cerveau. La théorie moderne de la prise de décision modèle l’activité neurale dans ces régions comme une compétition entre les différents stimuli et actions considérés par un individu. Cette compétition est résolue lorsque l’activité neurale associée à un stimulus ou une action choisie atteint un seuil critique. Toutefois, il reste à éclaircir comment ce modèle s’applique aux décisions effectuées alors que l’individu est déjà engagé dans une activité. Dans ce mémoire nous examinons ce type de décision chez des sujets humains dans une tâche de suivi continu. Des cibles « choix » apparaissaient sur un écran pendant que le sujet suivait de la main une cible qui se déplaçait doucement en continu. Le sujet pouvait ignorer ces cibles choix, ou abandonner la cible suivie pour toucher une cible choix, dans quel cas la cible sélectionnée devenait la nouvelle cible à suivre du doigt. Tel qu’attendu, nous avons observé que les sujets favorisaient les cibles plus proches, plus grandes, et les cibles alignées avec l’axe du mouvement. Toutefois nous avons été surpris de constater que les sujets ignoraient les coûts énergétiques du mouvement, tel que modélisés. Un biais pour minimiser les coûts du mouvement fut réintroduis lorsque la tâche fut divisée en séries de mouvements point-à-point, plutôt qu’un mouvement continu. Même si nous ne pouvons expliquer ce résultat surprenant, nous espérons qu’il inspire de futures études utilisant le paradigme expérimental de décision durant l’action.<br>Neurophysiological studies of decision-making have expanded over decades to involve many brain areas. The field broadened from neuroeconomics, mainly concerned with frontal regions, to perceptual or embodied decision-making involving several sensorimotor areas where neural activity is linked to the stimuli and actions necessary for the decision process. Current models of decision-making envision this neural activity as a competition between different actions that is resolved when enough activity favors one over the other. However, it is unclear how such models can explain decisions often present in natural behavior, where deliberation takes place while already engaged in an action. In this thesis, we examined the choices human subjects made as they were engaged in a continuous tracking task. While they were manually tracking a target on a flat screen, subjects were occasionally presented with a new target to which they could freely choose to switch, whereupon it became the new tracked target. As expected, we found that subjects were more likely to move to closer targets, bigger targets, or targets that were aligned to the direction of movement. However, we were surprised that subjects did not choose targets that minimized energetic cost, as calculated by a biomechanical model of the arm. A biomechanical bias was restored when the continuous movement was broken up into a series of point to point movements. While we cannot yet explain these findings with certainty, we hope they will inspire further studies using decide-while-acting paradigms.
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Carland, Matthew A. "A theoretical and experimental dissociation of two models of decision‐making." Thèse, 2014. http://hdl.handle.net/1866/12038.
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La prise de décision est un processus computationnel fondamental dans de nombreux aspects du comportement animal. Le modèle le plus souvent rencontré dans les études portant sur la prise de décision est appelé modèle de diffusion. Depuis longtemps, il explique une grande variété de données comportementales et neurophysiologiques dans ce domaine. Cependant, un autre modèle, le modèle d’urgence, explique tout aussi bien ces mêmes données et ce de façon parcimonieuse et davantage encrée sur la théorie. Dans ce travail, nous aborderons tout d’abord les origines et le développement du modèle de diffusion et nous verrons comment il a été établi en tant que cadre de travail pour l’interprétation de la plupart des données expérimentales liées à la prise de décision. Ce faisant, nous relèveront ses points forts afin de le comparer ensuite de manière objective et rigoureuse à des modèles alternatifs. Nous réexaminerons un nombre d’assomptions implicites et explicites faites par ce modèle et nous mettrons alors l’accent sur certains de ses défauts. Cette analyse servira de cadre à notre introduction et notre discussion du modèle d’urgence. Enfin, nous présenterons une expérience dont la méthodologie permet de dissocier les deux modèles, et dont les résultats illustrent les limites empiriques et théoriques du modèle de diffusion et démontrent en revanche clairement la validité du modèle d'urgence. Nous terminerons en discutant l'apport potentiel du modèle d'urgence pour l'étude de certaines pathologies cérébrales, en mettant l'accent sur de nouvelles perspectives de recherche.<br>Decision‐making is a computational process of fundamental importance to many aspects of animal behavior. The prevailing model in the experimental study of decision‐making is the drift‐diffusion model, which has a long history and accounts for a broad range of behavioral and neurophysiological data. However, an alternative model – called the urgency‐gating model – has been offered which can account equally well for much of the same data in a more parsimonious and theoretically‐sound manner. In what follows, we will first trace the origins and development of the DDM, as well as give a brief overview of the manner in which it has supplied an explanatory framework for a large number of behavioral and physiological studies in the domain of decision‐making. In so doing, we will attempt to build a strong and clear case for its strengths so that it can be fairly and rigorously compared to potential alternative models. We will then re‐examine a number of the implicit and explicit theoretical assumptions made by the drift‐diffusion model, as well as highlight some of its empirical shortcomings. This analysis will serve as the contextual backdrop for our introduction and discussion of the urgency‐gating model. Finally, we present a novel experiment, the methodological design of which uniquely affords a decisive empirical dissociation of the models, the results of which illustrate the empirical and theoretical shortcomings of the drift‐diffusion model and instead offer clear support for the urgency‐gating model. We finish by discussing the potential for the urgency gating model to shed light on a number of clinical disorders, highlighting a number of future directions for research.
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(6594413), Farzin Shamloo. "A STUDY OF RULE-BASED CATEGORIZATION WITH REDUNDANCY." Thesis, 2019.
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In tasks with more than one path to succeed, it is possible that participants’ strategies vary and therefore, participants should not be analyzed as a homogeneous group. This thesis investigates individual differences in a two-dimensional categorization task with redundancy (i.e., a task where any of the two dimensions by itself suffices for perfect performance). Individual differences in learned knowledge and used knowledge are considered and studied. Participants first performed a categorization task with redundancy (training phase), and afterward were asked to do categorizations in which the previously redundant knowledge becomes decisive (testing phase). Using the data from the testing phase, dimension(s) learned by each participant were determined and the response patterns of each participant in the training phase was used to determine which dimension(s) were used. The used knowledge was assessed using two separate analyses, both of which look at accuracy and response time patterns, but in different ways. Analysis 1 uses iterative decision bound modeling and RT-distance hypothesis and Analysis 2 uses the stochastic version of general recognition theory. In Analysis 1, more errors and slower response times close to a decision bound perpendicular to a dimension indicate that a participant is using that dimension. Analysis 2 goes a step further and in addition to determining which dimension(s) are used, specifies in what way they were used (i.e., identifying the strategy of each participant). Possible strategies are described heuristically (unidimensional, time efficient and conservative) and then each heuristic is translated into a drift diffusion model by the unique way that strategy is assumed to affect trial-by-trial difficulty of the task. Finally, a model selection criterion is used to pick the strategy that is used by each participant.
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Vorster, Michael Johannes. "Modelling the evolution of pulsar wind nebulae / Michael Johannes Vorster." Thesis, 2014. http://hdl.handle.net/10394/10861.
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This study focusses on modelling important aspects of the evolution of pulsar wind nebulae
using two different approaches. The first uses a hydrodynamic model to simulate the morphological
evolution of a spherically-symmetric composite supernova remnant that is expanding
into a homogeneous interstellar medium. In order to extend this model, a magnetic field is
included in a kinematic fashion, implying that the reaction of the fluid on the magnetic field
is taken into account, while neglecting any counter-reaction of the field on the fluid. This approach
is valid provided that the ratio of electromagnetic to particle energy in the nebula is
small, or equivalently, for a large plasma β environment. This model therefore allows one to
not only calculate the evolution of the convection velocity but also, for example, the evolution
of the average magnetic field.
The second part of this study focusses on calculating the evolution of the energy spectra of
the particles in the nebula using a number of particle evolution models. The first of these is
a spatially independent temporal evolution model, similar to the models that can be found
in the literature. While spatially independent models are useful, a large part of this study
is devoted to developing spatially dependent models based on the Fokker-Planck transport
equation. Two such models are developed, the first being a spherically-symmetric model that
includes the processes of convection, diffusion, adiabatic losses, as well as the non-thermal
energy loss processes of synchrotron radiation and inverse Compton scattering. As the magnetic
field geometry can lead to the additional transport process of drift, the previous model is
extended to an axisymmetric geometry, thereby allowing one to also include this process.<br>PhD (Space Physics), North-West University, Potchefstroom Campus, 2014