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The history of classical conditioning is summarized. The contributions and weaknesses of several earlier models of classical conditioning are studied. Two new neuronal models are proposed. The first, called the delay-producing connections (or DPC) model, is an extension of the Klopf (1988) and Sutton & Barto (1981) models. The DPC model makes two contributions: (1) it represents the trace of each conditioned stimuli (CS) by differential equations; and (2) it replaces each CS in the activation rule with a trace of the relevant CS. A method is suggested to measure the trace of a CS. The second model, called the adaptive delays (or AD) model, is proposed as an extension of the DPC model to account for the phenomenon of inhibition of delay. Both models reproduce the shape of a CR, the curve of efficacy of conditioning as a function of the interstimulus interval (ISI), the dependence of the optimal ISI on CS duration, the extinction of a CR (even for long lasting CSs as opposed to Klopf's (1988) model), and several other properties of classical conditioning.

Cyclical thrombocytopenia (CT) is a rare hematological disease characterized by periodic oscillations in the platelet count. Although first reported in 1936, the pathogenesis and an effective therapy remain to be identified. Since besides fluctuations in platelet levels the patients hematological profile have been consistently normal, a destabilization of a peripheral control mechanism might play an important role in the genesis of this disorder. In this thesis, we investigate through computer simulations the mechanisms underlying the platelet oscillations observed in CT. First, we collected the data published in the last 40 years and quantified the significance of the platelet fluctuations using Lomb-Scargle periodograms. Our analysis reveals that the incidence of the statistically significant periodic data is equally distributed in men and women. The mathematical model proposed in this thesis captures the essential features of hematopoiesis and successfully duplicates the characteristics of CT. With the same parameter changes, the model is able to fit the platelet counts and to qualitatively reproduce the TPO oscillations (when data is available). Our results indicate that a variation in the megakaryocyte maturity, a slower relative growth rate of megakaryocytes, as well as an increased random destruction of platelets are the critical elements generating the platelet oscillations in CT. / La thrombocytopénie cyclique (TC) est une rare maladie hématologique caracteriséepar des oscillations périodiques dans les plaquettes sanguines. Bien qu'elle fût évoquéepour la première fois en 1936, la maladie et une thérapie efficace restent à trouver.Puisque malgré les fluctuations au niveau des plaquettes, les profiles hématologiquesdes patients restent toujour normaux, une destabilisation du méchanisme de contrôlepériphérique peut jouer un rôle important dans la formation de ce maladie. Dans cettethèse, nous recherchons à travers des simulations informatiques les mechanismes sousjacentaux oscillations des plaquettes observées dans TC. En premier lieu, nous avonscollecté les données publiées ces 40 dernière années et quantifié l'importance des fluctuationsdes plaquettes en utilisant les périodograms Lomb-Scargle. Notre analysestatistique révèle que les données périodiques sont équitablement distribuée chez leshommes et les femmes. Le modèle mathématique proposé dans cette thèse prenden compte les caractéristiques essentielles de la production des cellules sanguineset reproduit avec succès les charactéristiques de TC. Avec les même changementde parametèrs, le modèle reproduit bien le comportement des plaquettes sanguineset donne qualitativement les même oscillations que TPO (quand les données sontdisponibles). Nos résultats indiquent que les éléments critiques générant les oscillationsdes plaquettes dans TC sont une variation dans la maturité du mégakaryocytes,un taux de croissance relativement lent des mégakaryo cytes , ainsi que une augmentationaléatoire de destruction des plaquettes.

This thesis uses a theoretical model to consider whether plaintiffs paying lawyers on a contingent fee basis receive smaller payoffs than those retaining lawyers on an hourly basis. This is the view in England, where contingent fees are illegal. It is also a view recognised in America, where contingent fees are legal and commonly used in some areas of law. The issue revolves around whether contingent fee lawyers will settle cases too soon for their clients to receive a substantial settlement offer. In an incomplete information, multiperiod bargaining model of personal injury litigation, we show that this need not be true: even if lawyers are self-interested, plaintiffs can receive higher payoffs under contingent fees than under hourly ones. Considerable ambiguity surrounds plaintiffs' preferred fee arrangement and the speed at which any settlement occurs under the different fee contracts. The most crucial role in this comparison is played by the distribution of legal expenses between plaintiffs and their lawyers. Our model therefore confirms the payoff ambiguity found in previous literature while being the first to address the settlement timing issue. It also suggests that the issue of how different fee arrangements affect the plaintiff in litigation is somewhat more complicated than policy debates on both sides of the Atlantic have implied.

The first chapter of this dissertation uses a three-sector intertemporal general equilibrium model to examine the effects of various taxes and favorable depreciation rules on capital accumulation and the depletion of natural resources. We find that in a world of infinitely lived firms and households, under certain conditions, taxation causes the steady-state level of capital to be lower than the no-tax level. This divergence, however is reduced by lower capital gains tax rates and favorable depreciation rules. We also find that taxation causes the extraction rate of natural resources to be faster than the social optimum. Although this difference is lessened by lower capital gains tax rates, it is exacerbated by a generous depreciation system. These results challenge the "conventional wisdom" that depreciation rules should be the same for all sectors. / Although the first chapter highlights the importance of a general equilibrium analysis of a multi-sector economy with intertemporally optimizing firms and consumers, there are a number of issues that it neglects. Among these are the adjustment costs associated with investments, and the fact that consumers might plan only for a finite horizon. The second and third chapters of this dissertation take up these issues. / The second chapter, which also uses an intertemporal model, determines theoretically how various taxes and adjustment costs affect the growth paths of domestic corporations and foreign subsidiaries of American multinationals. We find that the effects of capital gains taxation and adjustment costs on the growth paths of domestic corporations and foreign subsidiaries of American multinationals are likely to be unfavorable. / Finally, the third chapter uses a three period overlapping generations model to examine the effects of the capital income tax, the labour income tax, and favorable depreciation rules on human and physical capital accumulation. We find that, under certain conditions, the capital income tax has a negative impact on steady-state levels of physical and human capital, and the steady-state physical capital to human capital ratio. Moreover favorable depreciation rules are shown to reduce the impact of the capital income tax.

This thesis investigates the fundamental assumptions made in recent continuous-time equilibrium models of the term structure of interest rates. It finds that the number and the stochastic processes of state variables are strikingly different from those assumed in the literature. It develops a three-factor empirical term structure model, based on 22 years of cross-maturity time series data. The results show that the price differences, between the well-known Vasicek, and Cox, Ingersoll and Ross models and the three-factor empirical model, for interest-rate sensitive securities are of substantial economic significance.

An analysis of 5 minute resolution Quadra data on day 261 of GATE (0953 - 1451 GMT) is made to yield statistics of maximum area, echo top, lifetime and maximum reflectivity factor in medium-sized convective cells. The procedure generates a unique data set which is used to compare with the results of a three-dimensional cloud model and to study cloud interactions and merging processes between convective echoes. The results, obtained by tracking 140 echoes throughout their lifetime, indicate that the maximum area is log-normally distributed, 90% of the echoes being smaller than 40 km('2) and living shorter than 60 min. The modes of the maximum echo top and maximum reflectivity factor distributions are around 2.5 km and 30 dBz, respectively. / A fully three-dimensional cloud model including precipitation processes is used to simulate convective clouds on this day. A number of single cloud experiments are carried out to verify the numerical results against the radar observations. In addition, several two-cloud developments are simulated to study cloud interactions and merging processes. / Comparison of the modelled echo parameters against the observed ones indicates a fair degree of realism in the simulations. The computed maximum reflectivity factor, however, is considerably higher than that of the observations because of the unrealistic drop-size distribution assumed in the model. / The two-cloud simulations suggest that both the alignment of the clouds in relation to the wind shear vector and the spacing between the cells are important factors in determining the type of cloud interactions. Merging takes place when the spacing between the two elements becomes small enough. The numerical simulations indicate that the perturbation pressure structure is crucial to trigger echo merging.

Reach defined analysis concentrated on the water temperature change that occurred in a stream/river segment over the course of one full diurnal cycle. Digital thermistors, data loggers and computer model development were utilized in reach analysis to link parameters of the stream system to a specific temperature change. The methodology employed was relatively simple and fast, and many successive stream segments were analyzed simultaneously. Reach analysis of stream temperature change identified the existing components of the stream system that caused increased water temperature and predicted the effectiveness of managed improvements to the stream system. Stream and river temperature regulation has focused on system and basin wide management. Often, the source of increased water temperature originates in only a part of the stream system. Reach defined analysis identified the portions of the stream system in which most water temperature change occurred, offered an explanation for the temperature response and provided specific information about the alternate strategies that may ameliorate undesired water temperatures. The development of the computer model Heat Source included physically based mathematical descriptions of stream energy and hydrologic processes. An implicit finite difference numerical method was implemented for simultaneous solution. The methodology presented in Heat Source is portable and applicable to all streams, rivers and open channels. / Graduation date: 1997

Graduation date: 1976