10 research outputs found

    Theoretical and experimental approaches for the initiation and propagation of activity in spatially embedded neuronal cultures

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    [eng] Spatial embedding and inherited metric constraints are a fundamental trait of biological neuronal circuits. However their role in shaping connectivity and dynamics has been often disregarded, with models of neuronal networks paying much more attention to the distribution of connections in the quest for understanding network's behavior. In this thesis we aim at filling this gap by studying the importance of metric features in the complex connectivity- dynamics-noise interplay that shapes spontaneous neuronal activity. This thesis combines experiments in rat dissociated neuronal cultures with theoretical analyses to better comprehend the relevance of spatial embedding. We developed a new theoretical model grounded on Ising Models to assess metric effects in neuronal cultures' behavior, and in the context of percolation approaches. Once metric effects were settled, we illustrated their relevance in shaping spontaneous activity by perturbing the structural connectivity blueprint of neuronal cultures. This was achieved by patterning the substrate where neurons grow, and by using topographical molds that dictated the connectivity of the network. Next, and since the initiation of bursting activity is governed in great manner by a complex amplification mechanism that involves metric correlations and noise, we focused on the metric-driven amplification of spontaneous single-neuron noise to derive an analytical model that predicts the frequency of bursting events in neuronal cultures. We then further investigated in an experimental context the contribution of noise to the observed activity patterns, and by implementing a moderate electrical stimulation protocol that increases the level of activity noise in cultures. Finally, the latter study was completed with experiments regarding the specific role of inhibition in neuronal networks, to provide a wider understanding of the mechanisms that govern the initiation and propagation of activity fronts in cortical cultures.[cat] L'objectiu d'aquesta tesis és investigar els mecanismes que generen l'activitat espontània i estimulada en xarxes neuronals, més concretament en cultius corticals dissociats, i fent un especial èmfasi en l’efecte de les correlacions mètriques. En aquest marc, l’activitat col·lectiva consisteix en episodis esporàdics de dispars quasi sincronitzats entre totes les neurones del cultiu, anomenats “esclats de xarxa”. Tres elements principals en determinen les característiques: connectivitat entre neurones, dinàmica intrínseca neuronal, i soroll (activacions neuronals aleatòries). La investigació s’ha centrat en cinc línies de recerca: l’estudi de correlacions mètriques en cultius neuronals; el desenvolupament d’un model teòric per descriure i predir l’esclat de xarxa; l’anàlisi de la propagació dels fronts d’activitat experimentals sota pertorbacions estructurals de la connectivitat del cultiu; l’estudi de l’efecte de la inhibició en la iniciació i propagació dels esclats ‘in vitro’; i l’estudi de la resposta experimental dels cultius sota una estimulació elèctrica moderada de baixa freqüència. En la primera línia de recerca hem comprovat que les correlacions mètriques dominen el comportament dinàmic del cultiu, fins al punt d’emmascarar la contribució de la distribució del nombre de connexions. En la segona línia hem desenvolupat un model analític que prediu semi- quantitativament la freqüència dels esclats observada experimentalment. La tercera línia s’ha centrat en l’efecte de pertorbacions estructurals en la connectivitat; la dinàmica resultant ha mostrat una gran riquesa en patrons d’activitat, esclats de xarxa a diferents escales, i propagació altament específica de cada cultiu. La quarta línia de recerca ha demostrat que les xarxes sense inhibició disminueixen la seva freqüència d’esclat respecte a les xarxes control, que la velocitat de propagació de l’activitat incrementa lleugerament quan s’ha bloquejat la inhibició, i que els punts on s’inicien ens esclats varien respecte als controls. I, finalment, la cinquena línia de recerca ha constatat que l’aplicació d’un camp elèctric feble augmenta el soroll d’activitat de la xarxa, generant un increment en la freqüència dels esclats de xarxa

    The Role of Europe in World-Wide Science and Technology: Monitoring and Evaluation in a Context of Global Competition

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    Noyons ECM, Buter RK, van Raan AFJ, Schwechheimer H, Winterhager M, Weingart P. The Role of Europe in World-Wide Science and Technology: Monitoring and Evaluation in a Context of Global Competition. Leiden: Universiteit Leiden; 2000

    Accident Tolerant Barriers for Fuel Rod Cladding in Nuclear Reactors

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    Zirconium-based alloys are currently used as fuel rod cladding in nuclear power reactors because of their low neutron absorption cross-section and adequate oxidation resistance. However, zirconium (Zr) alloys undergo waterside corrosion by the primary coolant water under normal operating conditions (300-330oC) and at high temperature, especially during loss of coolant accident conditions. During the oxidation reaction, hydrogen generated which can lead to the formation of zirconium hydride precipitates in the Zr alloy cladding - and is detrimental to the mechanical integrity of the fuel rods. The principal objective of this work was to develop a surface-treated layer/coating for pure Zr in order to improve its oxidation resistance (and to determine the feasibility of such surface-treated layers/coatings being used on Zr alloys) - and thereby increase its operational longevity under both normal and abnormal operating conditions. Triode Plasma Nitriding (TPN) diffusion treatments were used to develop diffusion-treated nitride layers intended to enhance load-bearing capacity of bulk Zr while allowing for good adhesion properties to the substrate. Physical Vapour Deposition (PVD) was also used to fabricate magnesium-containing zirconium coatings with a range of magnesium concentrations to study the solubility magnesium and zirconium in a Zr-Mg coating intended to enhance hydrothermal oxidation resistance. Various characterisation techniques were used to investigate the effects of diffusion treatments and sputter deposition on surface morphology, topology and bulk substrate microstructure. The hardness, adhesion and oxidation resistance of the nitrided layers and sputter-deposited coatings were assessed. The hardness of the diffusion-treated surface of pure Zr metal was found to have been significantly improved and a higher load-bearing capacity was achieved after TPN-treatment compared to untreated pure zirconium. This was due to a ‘deep’ hardened case which was achieved at moderately low temperatures at short processing times, allowing significantly higher loads to be applied before failure occurred. TPN treatment of pure zirconium metal was found to have improved the oxidation resistance compared to untreated pure zirconium in aqueous conditions. The Zr-Mg coatings produced by PVD showed that magnesium can remain in substitutional solid solution with zirconium at high magnesium concentration and showed no evidence of phase separation. Hydrothermal oxidation tests of the Zr-Mg coatings revealed the formation of a (metastable) tetragonal zirconia phase throughout the oxide scale which plays an important role in stabilising of the oxide. The evidence presented suggests that both the diffusion treatments and sputter co-deposition process need to be optimised in order to improve oxidation resistance under aqueous conditions and abnormal reactor operating conditions

    Future as medium

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    FUTURE AS MEDIUM through critical examination develops a working premise for advancing designed visualizations of the future. In an effort to build new models of practice that consciously work towards the future, I undertake a set of delicate negotiations rooted in critical inquiry as well as creative production, all expressed across three temporal contingencies: yesterday, today and tomorrow. I work the terrain of the past as an assembled archive of speculative worlds created by architects, designers, and artists, and problematize the mediation of futures through a close reading of their world-building tools. I use this archive to make sense of my overall visions and methodology, always looking to the future. In the present, I measure these accounts against the ideological and systemic biases of designers and the complex world we inhabit, allowing opportunities for nuance, true plurality, and the incorporation of diversity for effective social engagement. Tomorrow explores adaptive models for an alternative pedagogical framework and the invention of new visualization strategies. Behaving like an “ologist”, taking a somewhat scientific and ethnographic approach to understanding my subject matter and intended audience, I finally ask (and attempt to answer) these questions: Are there ways for us to build multiple yet collective futures? Can we use the future as medium

    Freire, Wittgenstein and Criticality Scholarship

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    We are concerned with the educational concept of criticality. What do we mean by criticality? How do we use it? Criticality links education with social, political, cultural and economic existence. Yet for the connection to be significant, meaningful, we must be able to say what we mean by a critical citizenry. We explore English educational policy underscoring the notion of criticality and offer a snapshot of some international models. We examine relevant considerations arising in the philosophy of education research literature. Criticality is an emerging and fluid concept and is informed by critical theory, critical pedagogy, critical thinking scholarship and the informal logic movement. We coin a new public space ‘criticality scholarship’ in which we develop our ideas. We address a number of important questions: What is critical thinking? What is the function, the relevance, of criticality in education and in the broader society? What is the connection between skills, propensities and character traits that pertain to criticality? Who is and who is not a critical being? How should we deal with field dependency and the problem of transfer? What pedagogical strategies support the teaching of criticality? How do human beings think? Why does the rationalistic thematic and the Cartesian method assume such a privileged position? What other forms of knowledge and canons of rigour and validity are relevant to a critical education? We delve into the works of Paulo Freire and Ludwig Wittgenstein and add to our evolving conception of criticality. We examine points of commonality and of difference in respect of their lived experiences as pedagogues. We consider Freire’s idea of conscientização and Wittgenstein’s stance on encouraging his students and readers to think for themselves and of the ways in which each of these relate to the critical being developing his or her own criticality. Within the new philosophical framework of criticality scholarship, we connect criticality with the promotion of democracy and social justice. We support this with Freire’s notion of the critical being naming the world and the word and juxtapose Wittgenstein’s aphorism that philosophy ‘leaves everything as it is’. We make a case for aligning Wittgenstein’s later philosophy with Karl Marx’s eleventh thesis on Feuerbach. Also we draw on Freirean aesthetic curiosity and Wittgenstein’s deep respect for the mystical and, with it, questions touching upon aesthetics, questions of value, God and the meaning of life to envision new horizons, complimentary vistas, that criticality scholarship offers. Reflection on theory and practice as it informs educational policy leads to some key findings and recommendations for policymakers to consider in relation to criticality. We sketch out how our conception of criticality can continue to gain purchase in the new domain of criticality scholarship. We erect signposts indicating possible paths that might be taken towards imagining and bringing about a more humane and just world

    Functional Role of Critical Dynamics in Flexible Visual Information Processing

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    Recent experimental and theoretical work has established the hypothesis that cortical neurons operate close to a critical state which signifies a phase transition from chaotic to ordered dynamics. Critical dynamics are suggested to optimize several aspects of neuronal information processing. However, although signatures of critical dynamics have been demonstrated in recordings of spontaneously active cortical neurons, little is known about how these dynamics are affected by task-dependent changes in neuronal activity when the cortex is engaged in stimulus processing. In fact, some in vivo investigations of the awake and active cortex report either an absence of signatures of criticality or relatively weak ones. In addition, the functional role of criticality in optimizing computation is often reported in abstract theoretical studies, adopting minimalistic models with homogeneous topology and slowly-driven networks. Consequently, there is a lack of concrete links between information theoretical benefits of the critical state and neuronal networks performing a behaviourally relevant task. In this thesis we explore such concrete links by focusing on the visual system, which needs to meet major computational challenges on a daily basis. Among others, the visual system is responsible for the rapid integration of relevant information from a large number of single channels, and in a flexible manner depending on the behavioral and environmental contexts. We postulate that critical neuronal dynamics in the form of cascades of activity spanning large populations of neurons may support such quick and complex computations. Specifically, we consider two notable examples of well-known phenomena in visual information processing: First the enhancement of object discriminability under selective attention, and second, a feature integration and figure-ground segregation scenario. In the first example, we model the top-down modulation of the activity of visuocortical neurons in order to selectively improve the processing of an attended region in a visual scene. In the second example, we model how neuronal activity may be modulated in a bottom-up fashion by the properties of the visual stimulus itself, which makes it possible to perceive different shapes and objects. We find in both scenarios that the task performance may be improved by employing critical networks. In addition, we suggest that the specific task- or stimulus-dependent modulations of information processing may be optimally supported by the tuning of relevant local neuronal networks towards or away from the critical point. Thus, the relevance of this dissertation is summarized by the following points: We formally extend the existing models of criticality to inhomogeneous systems subject to a strong external drive. We present concrete functional benefits for networks operating near the critical point in well-known experimental paradigms. Importantly, we find emergent critical dynamics only in the parts of the network which are processing the behaviourally relevant information. We suggest that the implied locality of critical dynamics in space and time may help explain why some studies report no signatures of criticality in the active cortex

    Ongoing and Visually-Evoked Cortical Activity Measured Across Multiple Spatial Scales

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    The visual cortex is a densely-interconnected network of neurons, which receives sensory input from the early visual pathway, and represents information about the visual world in the resulting spatiotemporal activity patterns. These patterns are mediated by cortical interactions that span multiple spatial scales (i.e., neuron-neuron, neuron-population, and population-population). Such interactions are a fundamental property of cortical function, and capture the contribution of the cortex to sensory coding. Yet these interactions are only beginning to be understood, in part because of the challenge of recording ongoing and visually-evoked activity across multiple spatial scales in individual studies. In this thesis, we present the results of such a study. Using the ex vivo turtle eye-attached whole-brain preparation, we simultaneously recorded the membrane potentials from pairs of cortical pyramidal neurons, as well as the nearby local field potential (LFP). In a parallel set of experiments, we recorded the LFP from multiple locations using a microelectrode array. To identify relevant cortical properties, we supplemented our experimental approach with network simulations. Our results quantify cortical interactions using a variety of coordination measures, and suggest that ongoing cortical activity, synaptic clustering, synaptic adaptation, and emergent network phenomena fundamentally shape the distribution and dynamics of coordination levels. Together, these results contribute to a clearer picture of cortical visual processing, and the roles of specific anatomical and emergent cortical properties in network function
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