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Exploratory Study of Nurse-Patient Encounters in Home Healthcare: A DissertationFalkenstrom, Mary Kate 28 April 2016 (has links)
The purpose of this study was to explore nurse-patient encounters from the perspective of the Home Healthcare Registered Nurse. A qualitative descriptive design was used to collect data from a purposive sample of 20 home healthcare registered nurses from Connecticut, Massachusetts, and Rhode Island currently or previously employed as a home healthcare nurse. Four themes and one interconnecting theme emerged from the data: Objective Language; Navigating the Unknown; Mitigating Risk; Looking for Reciprocality in the Encounter; and the interconnecting theme of Acknowledging Not All Nurse-Patient Encounters Go Well. One goal of the study was to propose an empirically informed definition of what constituted a difficult encounter. An important early finding was that the terms difficult patient and difficult encounter were not generally used by study participants. HHC RNs voiced a preference for objective and nonjudgmental language to communicate outcomes of nurse-patient encounters. Three types of HHC RN-patient interactions emerged from the data, with constructive encounters the norm and non-constructive or destructive encounters less frequent. A constructive encounter is when two or more human beings, the nurse on the one side, and the patient, caregiver, or both on the other, interact to achieve a mutually agreed upon outcome. A nonconstructive encounter is when one or more human beings obstruct efforts to achieve at least one positive outcome. A destructive encounter is when one or more human beings direct anger at or physically aggress toward another human being. Strategies to promote reciprocality are routinely employed during HHC RN-patient encounters, but HHC RNs who miss cues that a strategy is ineffective or failed may be at risk in the home. Study data lend support to key concepts, assumptions, and propositions of Travelbee’s (1971) Human-to-Human Relationship Model. Study results provide a foundation for further research to increase the understanding, recognition, and development of empirically derived responses to non-constructive or destructive encounters such that HHC RNs are safe and best able to meet patients’ healthcare needs.
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The Welfare State and Attitudes to Free Movement : How does the design of the social insurance system associate with public attitudes towards free movement in receiving EU countries?Olofsson, Johan January 2020 (has links)
The EU is faced with problems related to the unrestricted access to national welfare states of mobile EU workers. These problems are mainly framed by the growing opposition to the free movement of workers. The strongest contributing factor to these negative attitudes has commonly said to be actor-based, i.e. the media or political elites. However, more recent research has been shifting the focus to institutions as explanatory factor. I raise the question of to what extent individuals’ attitudes towards the free movement of workers are associated with their exposure to different welfare state institutional contexts. More specifically I explore the role of “earnings-relatedness” in specific social insurances for the attitudes of potential benefit claimants. Analyzing 12 EU/EFTA countries who are net receivers of mobile EU workers I find that in welfare state contexts with a high degree of earnings-relatedness the opposition to free movement is significantly lower than otherwise among the unemployed. Furthermore, I discover that these observations seem to vary depending on what part of the social insurance system one is analyzing.
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Revenu minimum garanti et réciprocité : une critique de l’objection de l’exploitationTrottier-Bouthillette, Arnaud 08 1900 (has links)
L’objection de l’exploitation est couramment soulevée lorsqu’il est question de l’implémentation d’un revenu minimum garanti. Celle-ci stipule que le versement d’un revenu minimum sans attente de contrepartie est fondamentalement injuste pour les travailleurs. Ce mémoire conteste cette objection en démontrant qu’un revenu minimum garanti ne pose pas de danger fondamental à la balance de la réciprocité. Dans le premier chapitre, la façon dont un revenu minimum garanti est financé est abordée. Il y est montré que les coûts d’un tel programme peuvent être couverts avec une taxe sur les ressources appartenant à la collectivité, sans toucher aux avoirs des travailleurs. Le second chapitre porte sur une comparaison entre le revenu minimum garanti et un revenu basé sur la participation. Elle permet de mettre en lumière les nombreux désavantages de cette deuxième option, sa mise en place risquant d’entraîner d’importants coûts financiers et humains. Le troisième chapitre s’attaque à l’objection de l’exploitation telle que formulée par Gijs van Donselaar. Les preuves qu’il fournit sont toutefois jugées insuffisantes et la thèse selon laquelle le versement d’un revenu minimum garanti encourage le parasitisme est réfutée. En ce qui concerne la perception qu’ont les travailleurs du revenu minimum garanti, il est argué qu’ils peuvent être convaincus du bien-fondé de ce programme en faisant appel à certaines valeurs centrales aux démocraties. Le dernier chapitre poursuit cette réflexion en montrant qu’un revenu minimum garanti peut être financé sans recourir à de nouvelles taxes, ce qui est expliqué en faisant appel à la théorie monétaire moderne. / The exploitation objection is commonly raised whenever the implementation of a universal basic income is mentionned. It stipulates that the payment of a minimum income without any form of obligation in return is fundamentally unfair to workers. This thesis challenges this objection and aims to show that a universal basic income does not pose any fundamental threat to the balance of reciprocity. In the first chapter, the way in which a universal basic income can be paid for is discussed. I show that such a program can be entirely funded using taxes on resources equally owned by all members of society, leaving workers’ earnings unspoiled. The second chapter focuses on a comparison between a universal basic income and a participation income. This discussion highlights the many disadvantages of the latter, the implementation of which could lead to significant financial and human costs. The third chapter tackles the exploitation objection directly, as formulated by Gijs van Donselaar. He provides insufficient evidence to demonstrate that the payment of a universal basic income encourages free-riding and his argument is therefore rejected. This leads to a closer inspection of the workers' perception of a universal basic income, which concludes that they can be convinced of the legitimacy of such program by appealing to intrinsically democratic values. The last chapter pursues this reflection by showing that a universal basic income can be implemented without resorting to new taxes, which is supported by modern monetary theory.
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The Role of Nature in John Muir's Conception of the Good LifeLarsen, Randy R. 30 September 2011 (has links)
No description available.
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[pt] MÉTODO PROBABILÍSTICO PARA CONSIDERAÇÃO DE INCERTEZAS BASEADO NO MÉTODO DAS FUNÇÕES DE GREEN E NO MÉTODO ESTATÍSTICO FIRST-ORDER SECONDMOMENT / [en] PROBABILISTIC METHOD FOR UNCERTAINTIES CONSIDERATION IN GEOMECHANICAL PROBLEMS BASED ON GREEN S FUNCTION APPROACH AND FIRST-ORDER SECOND-MOMENT METHODLEONARDO CARVALHO MESQUITA 04 May 2023 (has links)
[pt] O presente trabalho propõe um método estatístico computacionalmente
eficiente (chamado Green-FOSM) para consideração de incertezas em problemas
geomecânicos, com o objetivo de melhorar o processo de tomada de decisão ao
analisar problemas associados com o processo de injeção ou depleção de fluídos. A
novidade do método proposto está associada com a utilização do método das
funções de Green (GFA), que, com o auxílio do método estatístico first-order
second-moment (FOSM), é utilizado para propagar as inerentes incertezas
associadas às propriedades mecânicas do material para o campo de deslocamento
da formação geológica. Além disso, através dos conceitos de grid estocástico e
função de autocorrelação, o método proposto permite a consideração da
variabilidade espacial de variáveis aleatórias de entrada que representam essas
propriedades mecânicas. O GFA utiliza as soluções fundamentais da mecânica
clássica (solução fundamental de Kelvin, solução fundamental de Melan, entre
outras) e o teorema da reciprocidade para determinar o campo de deslocamento de
uma formação geológica com geometria irregular e diferentes tipos de materiais. A
grande vantagem deste método em relação ao clássico método dos elementos finitos
(MEF) é que ele não requer a imposição de condições de contorno e a análise do
problema pode ser realizada considerando apenas o domínio do reservatório ou
outras regiões de interesse. Esta estratégia de modelagem diminui os graus de
liberdade do modelo e o tempo de processamento da análise. Desta forma, como o
GFA requer menos esforço computacional, este método torna-se ideal para ser
utilizado na propagação de incertezas em problemas geomecânicos. Inicialmente,
baseado no método das funções de Green original proposto por Peres et al. (2021),
foi proposto uma versão iterativa do método Green-FOSM, que apresenta
resultados estatísticos semelhantes aos encontrados através da clássica simulação
de Monte Carlo (SMC). Nesta versão original, o campo de deslocamento é
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calculado usando um esquema numérico iterativo que diminui o desempenho
computacional do método e pode gerar problemas de convergência. Tais limitações
tem dificultado a aplicação do GFA original e do método Green-FOSM iterativo
em problemas reais. Assim, o presente trabalho desenvolveu uma nova versão do
GFA que utiliza um esquema numérico não-iterativo. Para os problemas de
validação analisados, o método não-iterativo demonstra ser até 17.5 vezes mais
rápido do que a versão original. Além disso, esta versão demonstra ser capaz de
expandir a aplicabilidade do GFA, pois os problemas de convergência foram
eliminados e os resultados obtidos por este método, ao analisar um perfil geológico
representativo do pré-sal brasileiro, são semelhantes aos encontrados via MEF. Por
fim, a partir do GFA não-iterativo foi proposta uma versão não-iterativa do método
Green-FOSM. Esta versão não-iterativa é capaz de analisar probabilisticamente
formações geológicas complexas, como é o caso das formações geológicas do présal brasileiro. Utilizando os mesmos recursos computacionais, o método GreenFOSM não-iterativo é no mínimo 200 vezes mais rápido que o método iterativo. De
forma geral, os resultados encontrados nas análises realizadas (determinísticas e
probabilísticas) são próximos dos resultados obtidos pelo método de referência
(MEF e SMC, respectivamente). / [en] The present work proposes a computationally efficient stochastic statistical
method (called Green-FOSM) that considers uncertainties in geomechanical
problems, with the objective of improving the decision-making process related to
problems associated with the process of fluid injection or depletion. The novelty of the method lies in the use of the Green s function approach (GFA), which, together, with the first-order second-moment statistical method (FOSM), is used to propagate
uncertainties associated with the mechanical properties of material to the
displacement field of the geological formation. Furthermore, using the concepts of
stochastic grid and autocorrelation function, the proposed method allows the
consideration of the spatial variability of random variables that represent these
mechanical properties. The GFA uses the fundamental solutions of classical
mechanics (Kelvin fundamental solution, Melan fundamental solution, among
others) and the reciprocity theorem to calculate the displacement field of a
geological formation with irregular geometry, and different types of materials. The
great advantage of this method compared to the classical finite element method
(FEM) is that it does not require the imposition of boundary conditions and the
analysis of the problem can be performed considering only the reservoir or other
regions of interest. This modeling strategy decreases the degrees of freedom of the
model and the CPU time of the deterministic analysis. In this way, as the GFA
requires less computational effort, this approach becomes ideal for propagating the
uncertainties in geomechanical problems. Initially, an iterative version of the
Green-FOSM method was proposed, which presents statistical results similar to
those found through the classic Monte Carlo simulation (MCS). In this initial
version, the displacement field is calculated using an iterative numerical scheme,
which decreases the computational performance of the method and can generate
convergence problems. Such limitations would restrict the application of the
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original GFA and the iterative Green-FOSM method in real problems. Thus, the
present work also developed a new version of the GFA, which uses a non-iterative
numerical scheme. For the proposed validation problems, the non-iterative method
proved to be up to 17.5 times faster than the original version. This version is able
to expand the applicability of the GFA, since the convergence problems were
eliminated and the results obtained by this method, when analyzing a representative
geological profile of the Brazilian pre-salt, are similar to those found via FEM.
Finally, based on the non-iterative GFA, a non-iterative version of the Green-FOSM
method was proposed. This non-iterative version is capable of probabilistically
analyzing complex geological formations, such as the Brazilian pre-salt geological
formations. Using the same computational resources, the non-iterative GreenFOSM method is at least 200 times faster than the iterative Green-FOSM method.
In general, the results found in the investigated analyzes (deterministic and
probabilistic) are close to the results obtained by the reference method (FEM and
MCS, respectively).
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RÄTTVISA BORTOM GRÄNSERSJÄLVRESPEKT SOM KOSMOPOLITISK PLIKT : Om global distributiv rättvisa: ett normativt rättfärdigandeAlnaji, Zezo January 2024 (has links)
This essay focus on the normative debate between cosmopolitanism and statism in the context of global distributive justice. The notion of basic structure and negative rights examines separately in two questions to understand distributive justice as a global subject rather than only national. Statists as Rawls holds the position that global distributive justice prerequisite a basic structure with coercive instrument. Pogge as cosmopolitan arguments for the existence of global basic structure, by addressing inequalities in real-world politics, in the form of negative rights violation. The aim of this study is to justify global distributive justice on cosmopolitan duties, based on normative political theory, reflective equilibrium, and conceptual analysis. The main issue is formulated into two questions in the following: • Does reciprocity constitute a global basic structure that presupposes resource distribution? • Can self-respect as foundation of rights justify global distributive justice? I do this first by analyzing the concept of basic structure, based on the notion reciprocity. This is to identify the basic structure of the global system that prerequisite global distributive justice. Second, I analyze Pogge’s formulation of negative rights as cosmopolitan rights, to modify them to a positive concept of rights. This is in purpose to avoid the libertarian counterargument presented by Narveson, that negative rights fail as a ground of cosmopolitan duties. I show first that coercion is not a necessary condition, but only sufficient for the basic structure. Thus, the global basic structure exists and prerequisite distributive justice, based on reciprocity. Unlike the national basic structure of coercive instrument, the global basic structure grounds on several global threats and challenges that tie all nations as alternative concept of coercion. Second, I show that cosmopolitan duties can be grounded on positive rights. I do this through the notion of self-respect and deontological ethics, which success to avoid the libertarian critique of cosmopolitan duties.
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Factors That Contribute to Dyadic Synchrony Among Young Latina Mothers and Their Toddlers: The Role of Maternal Behavior and Child CharacteristicsHammer, Aimee Tovah 28 February 2018 (has links)
No description available.
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論代間正義:一個羅爾斯式的觀點 / On Intergenerational Justice: A Rawlsian Perspective楊士奇, Yang, Shi-Chi Unknown Date (has links)
本論文題旨為:「論代間正義:一個羅爾斯式的觀點」。代間正義是晚近三十年來新興且益愈受到重視的倫理學議題之一,其主要關切的核心問題,乃在於追問「當代之於後代所應擔負的責任」。本文透過當代政治哲學與倫理學家羅爾斯(John Rawls)有關社會正義理論的設計與主張,分兩部分處理此問題。 / 第一部份所處理的問題為由帕菲特(Derek Parfit)所深化之「後代人格不同一問題」(The Non-Identity Problem),旨在探究「代間正義是否可能」。帕菲特指出,前代不同的行為選擇,將造成不同後代的存在,而這使得現存既有之各種權利與責任相對應的理論,無法合理地適用於代間關係。帕菲特主張,可以採取「忽略特定人格的比較(品質)原則」以解決此後代人格不同一問題。然而,帕菲特此舉卻陷入「不特定人格的後代無法追究前代之行為責任」的理論困境。本文主張,透過羅爾斯原初位置(original position)的理論設計啟發,即便在代間存在著「前代不同的行為選擇,將造成不同後代的存在」的後代人格不同一疑慮,當代仍可採納原初位置的理論啟示,區分人的屬性(properties)與獨特性(particular)的差異,在後代存有人格不同一問題(獨特性)的情形下,針對「屬性」而確立追問當代之於後代所應擔負責任之正當性。 / 第二部分主要處理羅爾斯有關代間正義觀點的內部論證問題,並進一步藉此說明「代間正義如何可能」。羅爾斯以「正義的儲蓄原則」(just savings principle)說明代間的分配正義問題,並《正義論》(A Theory of Justice)中將它納入正義二原則之中,成為建構社會基本體制的基本原則之一。然而,羅爾斯早期解釋與證成儲蓄原則的相關理據如動機假定(motivation assumption)與家族模式等,卻可能與其他理論假定如締約者的理性等相衝突、衍生解釋融貫上的困難,而遭到眾多學者們的質疑。羅爾斯在一九九三年的《政治自由主義》(Political Liberalism)中對此做出回應,並將關切下一代的動機假定,修正為「要求前代也承諾遵守他們所遵守的儲蓄原則,無論向前或向後追溯多遠」。除此之外,羅爾斯於《正義論》以外的其他著作,在提及正義二原則時,皆不再表述「正義的儲蓄原則」。本文認為羅爾斯後期所提出的證立主張,不僅整合了代內分配正義(差異原則)與代間分配正義(儲蓄原則)的論證理據(小中取大規則的應用),更與其主張「社會作為一世代相繼之公平的合作體系」時所標舉之「相互性理念」(the idea of reciprocity)的核心概念相符應。本文認為,在論證理據得到順利整合的前提下,羅爾斯仍可在後期表述正義二原則時,將「正義的儲蓄原則」放回其中,並可據此呼應當代永續發展理念「既滿足當代人的需求,又不對後代人滿足其需求的能力造成危害」的核心主張。 / The topic “Intergenerational Justice” is one of the newest but getting more important ethics problems to contemporaries. One of the key points of this issue is how to make sense of our obligations to the posterity (include future people) if possible. In this dissertation, I intend to clarify this problem by Rawls’s theory of justice into two parts. / The first part is “The Non-Identity Problem” held by Derek Parfit. This problem shows that “in the different outcomes, different people would be born”, and it seems inactive the traditional theories of rights. Parfit suggests that we can through it by the priinciple Q: “if in either of two outcomes the same number of people would ever live, it would be bad if those who live are worse off, or have a lower quality of life, than those who would have lived.” But this principle makes new difficulties about this problem. According Reiman, I argue that we can adjust this non-identity problem by the theory hypothesis “Original Position” of Rawls’s theory of justice, and that there are obligations from contemporaries to the future people. / The second part is about the arguments of Rawls’s theory of justice between generations. According to early Rawls in 1971, the theory of justice between generations represented by the “just savings principle” and was one part of the Two Principles of Justice in A Theory of Justice. But there are some argument troubles about the assumptions that makes the theory of justice between generations difficultly, especially on the “motivation assumption” and the family mode and so on. In 1993, Rawls changed his arguments about the theory of justice between generations, but he also take off the just savings principle from the Two Principles of Justice in other books or articles besides A Theory of Justice. I argue that latter arguments seem more reasonable to the theory of justice between generations, and they also makes the whole theory of social justice comprehensive. Then I argue that Rawls can still presents the just savings principle when he says about the Two Principles of Justice.
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La sécurité collective dans l’unipolarité : la crise nucléaire iranienneLounnas, Djallil 09 1900 (has links)
L’argument central de notre thèse est qu’une structure internationale unipolaire non hégémonique favorise la sécurité collective. Après avoir montré que telle est la structure actuelle et avoir justifié notre positionnement théorique néolibéral, nous avons eu recours au modèle d’interaction du « leader-suiveur de Stackelberg », pour démontrer qu’une coopération conflictuelle entre États rationnels est possible, ce qui leur permet de surmonter leur dilemme de l’action collective. Une coopération possible en raison de l’existence d’un intérêt mutuel et d’un leader favorisant la coopération entre ces États, ainsi que de conditions leur permettant de mettre en place une ‘stratégie de la réciprocité’. Ils forment alors ce qu’on appelle le « groupe de Stackelberg ». Le suiveur de la périphérie, ou ‘défecteur’ doit, pour sa part, non seulement s’ajuster à l’intérêt mutuel ainsi défini, mais aussi coopérer et négocier avec le groupe, et ce, sous la pression de sanctions, voire d’un usage ultime de la force si besoin est. Après l’éventuel succès de ces négociations, un équilibre de Stackelberg favorisé par le leader, soit la puissance unipolaire et non hégémonique, est alors atteint et la coopération permet, alors, à chacun de retirer un bénéfice de cet intérêt mutuel.
Dans notre cas, le groupe de Stackelberg est constitué des membres du G5 + 1, soit les cinq membres permanents du Conseil de Sécurité de l’ONU et de l’Allemagne, agissant sous le leadership américain; l’Iran est un suiveur de la périphérie, soupçonné de défection, et l’intérêt mutuel est celui de la lutte contre la prolifération des ADM. Notre évaluation empirique montre que les conditions de la réciprocité des membres de ce groupe sont réunies et que celles de l’Iran sont en cours de négociation. / The main argument of our thesis is that a unipolar and non hegemonic structure of the international system furthers collective security. After having shown that the present international structure is as such and having justified our neoliberal theoretical approach, we have adopted the interaction model of the Stackelberg leadership model to demonstrate that a conflicting cooperation between rational States, that are respectful of the international order, is possible, which allows them to overcome their collective action dilemma. This cooperation depends on the existence of shared interests, on the ability of the leader to foster cooperation among them, and on structural conditions as well, which enable the setting up of “a reciprocity strategy”. They, thus, form what we call as a “Stackelberg’s group”. With regard to the periphery follower, or defector, he should adjust to the mutual interest of the group, as defined by this latter, as well as cooperate and negotiate with it, under the pressure of sanctions or the use of force if necessary. In the event of a successful negotiation, a Stackelberg equilibrium, furthered by the leader of the group, that is the unipolar and non hegemonic power, is then reached where cooperation between the members of the group, i.e. the States, allows them to get their share, that is a benefit from their mutual interest.
In our case study, the Stackelberg’s group includes the five permanent members of the U.N Security Council plus Germany, all acting under the US leadership. Iran is a periphery follower suspected of defection, and the mutual interest is the international security against WMD proliferation. Our empirical assessment shows that the respective conditions, which enable a reciprocity strategy within the Stackelberg’s group, are then met by its members, whereas those of Iran are still under negotiations.
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Conventional and Reciprocal Approaches to the Forward and Inverse Problems of ElectroencephalographyFinke, Stefan 03 1900 (has links)
Le problème inverse en électroencéphalographie (EEG) est la localisation de sources de courant dans le cerveau utilisant les potentiels de surface sur le cuir chevelu générés par ces sources. Une solution inverse implique typiquement de multiples calculs de potentiels de surface sur le cuir chevelu, soit le problème direct en EEG. Pour résoudre le problème direct, des modèles sont requis à la fois pour la configuration de source sous-jacente, soit le modèle de source, et pour les tissues environnants, soit le modèle de la tête. Cette thèse traite deux approches bien distinctes pour la résolution du
problème direct et inverse en EEG en utilisant la méthode des éléments de frontières (BEM): l’approche conventionnelle et l’approche réciproque.
L’approche conventionnelle pour le problème direct comporte le calcul des potentiels de surface en partant de sources de courant dipolaires. D’un autre côté, l’approche réciproque détermine d’abord le champ électrique aux sites des sources dipolaires quand les électrodes de surfaces sont utilisées pour injecter et retirer un courant unitaire. Le produit scalaire de ce champ électrique avec les sources dipolaires donne ensuite les potentiels de surface. L’approche réciproque promet un nombre d’avantages par rapport à l’approche conventionnelle dont la possibilité d’augmenter la précision des potentiels de surface et de réduire les exigences informatiques pour les solutions inverses.
Dans cette thèse, les équations BEM pour les approches conventionnelle et réciproque sont développées en utilisant une formulation courante, la méthode des résidus pondérés. La réalisation numérique des deux approches pour le problème direct est décrite pour un seul modèle de source dipolaire. Un modèle de tête de trois sphères concentriques pour lequel des solutions analytiques sont disponibles est utilisé. Les potentiels de surfaces sont calculés aux centroïdes ou aux sommets des éléments de discrétisation BEM utilisés. La performance des approches conventionnelle et réciproque pour le problème direct est évaluée pour des dipôles radiaux et tangentiels d’excentricité variable et deux valeurs très différentes pour la conductivité du crâne.
On détermine ensuite si les avantages potentiels de l’approche réciproquesuggérés par les simulations du problème direct peuvent êtres exploités pour donner des solutions inverses plus précises. Des solutions inverses à un seul dipôle sont obtenues en utilisant la minimisation par méthode du simplexe pour à la fois l’approche conventionnelle et réciproque, chacun avec des versions aux centroïdes et aux sommets. Encore une fois, les simulations numériques sont effectuées sur un modèle à trois sphères concentriques pour des dipôles radiaux et tangentiels d’excentricité variable. La précision des solutions inverses des deux approches est comparée pour les deux conductivités différentes du crâne, et leurs sensibilités relatives aux erreurs de conductivité du crâne et au bruit sont évaluées.
Tandis que l’approche conventionnelle aux sommets donne les solutions directes les plus précises pour une conductivité du crâne supposément plus réaliste, les deux approches, conventionnelle et réciproque, produisent de grandes erreurs dans les potentiels du cuir chevelu pour des dipôles très excentriques. Les approches réciproques produisent le moins de variations en précision des solutions directes pour différentes valeurs de conductivité du crâne. En termes de solutions inverses pour un seul dipôle, les approches conventionnelle et réciproque sont de précision semblable. Les erreurs de localisation sont petites, même pour des dipôles très excentriques qui produisent des grandes erreurs dans les potentiels du cuir chevelu, à cause de la nature non linéaire des solutions inverses pour un dipôle. Les deux approches se sont démontrées également robustes aux erreurs de conductivité du crâne quand du bruit est présent.
Finalement, un modèle plus réaliste de la tête est obtenu en utilisant des images par resonace magnétique (IRM) à partir desquelles les surfaces du cuir chevelu, du crâne et du cerveau/liquide céphalorachidien (LCR) sont extraites. Les deux approches sont validées sur ce type de modèle en utilisant des véritables potentiels évoqués somatosensoriels enregistrés à la suite de stimulation du nerf médian chez des sujets sains. La précision des solutions inverses pour les approches conventionnelle et réciproque et leurs variantes, en les comparant à des sites anatomiques connus sur IRM, est encore une fois évaluée pour les deux conductivités différentes du crâne. Leurs avantages et inconvénients incluant leurs exigences informatiques sont également évalués. Encore une fois, les approches conventionnelle et réciproque produisent des petites erreurs de position dipolaire. En effet, les erreurs de position pour des solutions inverses à un seul dipôle sont robustes de manière inhérente au manque de précision dans les solutions directes, mais dépendent de l’activité superposée d’autres sources neurales. Contrairement aux attentes, les approches réciproques n’améliorent pas la précision des positions dipolaires comparativement aux approches conventionnelles. Cependant, des exigences informatiques réduites en temps et en espace sont les avantages principaux des approches réciproques. Ce type de localisation est potentiellement utile dans la planification d’interventions neurochirurgicales, par exemple, chez des patients souffrant d’épilepsie focale réfractaire qui ont souvent déjà fait un EEG et IRM. / The inverse problem of electroencephalography (EEG) is the localization of current sources within the brain using surface potentials on the scalp generated by these sources. An inverse solution typically involves multiple calculations of scalp surface potentials, i.e., the EEG forward problem. To solve the forward problem, models are needed for both the underlying source configuration, the source model, and the surrounding tissues, the head model. This thesis treats two distinct approaches for the resolution of the EEG forward and inverse problems using the boundary-element method (BEM): the conventional approach and the reciprocal approach.
The conventional approach to the forward problem entails calculating the surface potentials starting from source current dipoles. The reciprocal approach, on the other hand, first solves for the electric field at the source dipole locations when the surface electrodes are reciprocally energized with a unit current. A scalar product of this electric field with the source dipoles then yields the surface potentials. The reciprocal approach promises a number of advantages over the conventional approach, including the possibility of increased surface potential accuracy and decreased computational requirements for inverse solutions.
In this thesis, the BEM equations for the conventional and reciprocal approaches are developed using a common weighted-residual formulation. The numerical implementation of both approaches to the forward problem is described for a single-dipole source model. A three-concentric-spheres head model is used for which analytic solutions are available. Scalp potentials are calculated at either the centroids or the vertices of the BEM discretization elements used. The performance of the conventional and reciprocal approaches to the forward problem is evaluated for radial and tangential dipoles of varying eccentricities and two widely different skull conductivities.
We then determine whether the potential advantages of the reciprocal approach suggested by forward problem simulations can be exploited to yield more accurate inverse solutions. Single-dipole inverse solutions are obtained using simplex minimization for both the conventional and reciprocal approaches, each with centroid and vertex options. Again, numerical simulations are performed on a three-concentric-spheres model for radial and tangential dipoles of varying eccentricities. The inverse solution accuracy of both approaches is compared for the two different skull conductivities and their relative sensitivity to skull conductivity errors and noise is assessed.
While the conventional vertex approach yields the most accurate forward solutions for a presumably more realistic skull conductivity value, both conventional and reciprocal approaches exhibit large errors in scalp potentials for highly eccentric dipoles. The reciprocal approaches produce the least variation in forward solution accuracy for different skull conductivity values. In terms of single-dipole inverse solutions, conventional and reciprocal approaches demonstrate comparable accuracy. Localization errors are low even for highly eccentric dipoles that produce large errors in scalp potentials on account of the nonlinear nature of the single-dipole inverse solution. Both approaches are also found to be equally robust to skull conductivity errors in the presence of noise.
Finally, a more realistic head model is obtained using magnetic resonance imaging (MRI) from which the scalp, skull, and brain/cerebrospinal fluid (CSF) surfaces are extracted. The two approaches are validated on this type of model using actual somatosensory evoked potentials (SEPs) recorded following median nerve stimulation in healthy subjects. The inverse solution accuracy of the conventional and reciprocal approaches and their variants, when compared to known anatomical landmarks on MRI, is again evaluated for the two different skull conductivities. Their respective advantages and disadvantages including computational requirements are also assessed.
Once again, conventional and reciprocal approaches produce similarly small dipole position errors. Indeed, position errors for single-dipole inverse solutions are inherently robust to inaccuracies in forward solutions, but dependent on the overlapping activity of other neural sources. Against expectations, the reciprocal approaches do not improve dipole position accuracy when compared to the conventional approaches. However, significantly smaller time and storage requirements are the principal advantages of the reciprocal approaches. This type of localization is potentially useful in the planning of neurosurgical interventions, for example, in patients with refractory focal epilepsy in whom EEG and MRI are often already performed.
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