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  • 1.
    Berthet, Pierre
    Stockholm University, Faculty of Science, Numerical Analysis and Computer Science (NADA).
    Computational Modeling of the Basal Ganglia: Functional Pathways and Reinforcement Learning2015Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    We perceive the environment via sensor arrays and interact with it through motor outputs. The work of this thesis concerns how the brain selects actions given the information about the perceived state of the world and how it learns and adapts these selections to changes in this environment. Reinforcement learning theories suggest that an action will be more or less likely to be selected if the outcome has been better or worse than expected. A group of subcortical structures, the basal ganglia (BG), is critically involved in both the selection and the reward prediction.

    We developed and investigated a computational model of the BG. We implemented a Bayesian-Hebbian learning rule, which computes the weights between two units based on the probability of their activations. We were able test how various configurations of the represented pathways impacted the performance in several reinforcement learning and conditioning tasks. Then, following the development of a more biologically plausible version with spiking neurons, we simulated lesions in the different pathways and assessed how they affected learning and selection.

    We observed that the evolution of the weights and the performance of the models resembled qualitatively experimental data. The absence of an unique best way to configure the model over all the learning paradigms tested indicates that an agent could dynamically configure its action selection mode, mainly by including or not the reward prediction values in the selection process. We present hypotheses on possible biological substrates for the reward prediction pathway. We base these on the functional requirements for successful learning and on an analysis of the experimental data. We further simulate a loss of dopaminergic neurons similar to that reported in Parkinson’s disease. We suggest that the associated motor symptoms are mostly causedby an impairment of the pathway promoting actions, while the pathway suppressing them seems to remain functional.

  • 2.
    Elenius, Måns
    Stockholm University, Faculty of Science, Numerical Analysis and Computer Science (NADA) (together with KTH).
    Computer Simulations of Simple Liquids with Tetrahedral Local Order: the Supercooled Liquid, Solids and Phase Transitions2009Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    The understanding of complex condensed matter systems is an area of intense study. In this thesis, some properties of simple liquids with strong preference for tetrahedral local ordering are explored. These liquids are amenable to supercooling, and give complex crystalline structures on eventual crystallisation. All liquids studied are simple, monatomic and are similar to real metallic liquids.

    The vibrational density of states of a glass created in simulation is calculated. We show a correspondence between the vibrational properties of the crystal and the glass, indicating that the vibrational spectra of crystals can be used to understand the more complex vibrational spectra of the glass of the same substance.

    The dynamics of supercooled liquids is investigated using a previously not implemented comprehensive measure of structural relaxation. This new measure decays more slowly in the deeply supercooled domain than the commonly used measure.

    A new atomic model for octagonal quasicrystals is presented. The model is based on findings from a molecular dynamics simulation that resulted in 45˚ twinned β-Mn. A decoration is derived from the β-Mn unit cell and the unit cell of the intermediate structure found at the twinning interface.

    Extensive simulations are used to explore the phase diagram of a liquid at low densities. The resulting phase diagram shows a spinodal line and a phase coexistence region between a liquid and a crystalline phase ending in a critical point. This contradicts the old conclusion of the Landau theory -- that continuous transitions between liquids and crystals cannot exist

    The same liquid is explored at higher densities. Upon cooling the liquid performs a first order liquid-liquid phase transition. The low temperature liquid is shown to be strong and to have very good glass forming abilities. This result offers new insights into fragile to strong transitions and suggests the possibility of a good metallic glass former.

  • 3.
    Glimming, Johan
    Stockholm University, Faculty of Science, Numerical Analysis and Computer Science(NADA) (together with KTH).
    Primitive Direcursion and Difunctorial Semantics of Typed Object Calculus2007Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    In the first part of this thesis, we contribute to the semantics of typed object calculus by giving (a) a category-theoretic denotational semantics using partial maps making use of an algebraic compactness assumption, (b) a notion of "wrappers'' by which algebraic datatypes can be represented as object types, and (c) proofs of computational soundness and adequacy of typed object calculus via Plotkin's FPC (with lazy operational semantics), thus making models of FPC suitable also for first-order typed object calculus (with recursive objects supporting method update, but not subtyping). It follows that a valid equation in the model induces operationally congruent terms in the language, so that program algebras can be studied. For (c), we also develop an extended first-order typed object calculus, and prove subject reduction. The second part of the thesis concerns recursion principles on datatypes including the untyped lambda calculus as a special case. Freyd showed that in certain domain theoretic categories, locally continuous functors have minimal invariants, which possess a structure that he termed dialgebra. This gives rise to a category of dialgebras and homomorphisms, where the minimal invariants are initial, inducing a powerful recursion scheme (direcursion) on a complete partial order. We identify a problem that appears when we translate (co)iterative functions to direcursion, and as a solution to this problem we develop a recursion scheme (primitive direcursion). This immediately gives a number of examples of direcursive functions, improving on the situation in the literature where only a few examples have appeared. By means of a case study, this line of work is connected to object calculus models.

  • 4.
    Holst, Anders
    Stockholm University.
    The Use of a Bayesian Neural Network Model for Classification Tasks.1997Doctoral thesis, monograph (Other academic)
  • 5.
    Larsson, Mariah
    Stockholm University, Faculty of Humanities, Department of Media Studies.
    A national/transnational genre: pornography in transition2015In: Nordic genre film: small nation film cultures in the global marketplace / [ed] Tommy Gustafsson, Pietari Kääpä, Edinburgh: Edinburgh University Press, 2015, p. 217-229Chapter in book (Refereed)
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