Die Alkoholsucht ist ein global verbreitetes Phänomen und kennzeichnet sich durch eine Transition von kontrolliertem zu zwanghaftem Alkoholkonsum.Die Tendenz, einem neutralen Reiz eine Anreizwirkung zuzuschreiben, ist individuell unterschiedlich und stellt einen Risikofaktor für die Entwicklung einer Abhängigkeit dar.Zur Entwicklung spezifischer Präventionsstrategien ist ein besseres Verständnis der Zusammenhänge zwischen der Anreizsalienz und Alkoholabhängigkeit erforderlich.Der Übergang von mäßigem zu zwanghaftem Alkoholkonsum wurde durch das Modell des Alkoholentzugseffekts simuliert, das den menschlichen Alkoholrückfall nachahmt.Die Ratten erhielten freiwilligen Zugang zu verschieden Alkohollösungen mit wiederholten Deprivations- und Alkoholphasen.Die Ratten durchliefen zusätzlich den Pavlovian Conditioned Approach getestet, welcher die individuellen Tendenzen auf einen bedingten Reiz/Belohnung quantifiziert.Während des letzten ADE-Zyklus wurde mit Geschmacksverfälschung zwanghaftes Trinken ermittelt.Nach der Identifizierung zuverlässiger Prädiktoren für Suchtverhalten wurde präventive Neurostimulation durchgeführt, um die Tendenz der Tiere alkoholbezogenen Reizen eine motivationale Bedeutung beizumessen, zu beeinflussen, und die Manifestation eines Abhängigkeitsverhaltens zu verhindern. Weibchen tranken mehr Alkohol als Männchen und zeigten ST Verhalten im PavCA, während Männchen GT aufwiesen.Die Anwendung von transkranieller Gleichstromstimulation während PavCA führte zu mehr GT-Verhalten bei stimulierten Ratten.Frühe tDCS während des Trinkens hatte keinen Einfluss auf das akute und das Langzeit-Trinken. Die Ergebnisse zeigen eine komplexere Beziehung zwischen Anreizsalienz und Alkoholsucht und unterstreichen,individuelle Unterschiede und beide Geschlechter in der präklinischen Forschung zu berücksichtigen. / The consequences of alcohol dependence cause the global deaths of million people yearly.The ability of the environment can trigger dependent behavior and promote drinking.The tendency to attribute incentive salience to cues differs between subjects.By forming a cue-alcohol association, neutral cues receive motivational value,thereby predicting the likelihood of alcohol reward occurrence,known as Pavlovian learning.Understanding the relationship between incentive salience and alcohol addiction help inform treatment strategies.We study the relationship between incentive salience and alcohol addiction.The transition from moderate to compulsive alcohol intake can be captured by the alcohol deprivation effect rat model (mimics alcohol relapse in humans).Rats were given voluntary access to alcohol solutions with repeated abstinence/reintroduction phases.Further,rats were tested in the PavCA,which quantifies individual tendency toward a conditional cue and the reward, thus allowing to trace the process of attributing incentive salience to rewardcues.During the final ADE cycle,rats underwent a bitter taste adulteration test to assess for compulsive-like behavior.After identifying reliable predictors of addictive behavior,preventive tDCS was performed to influence the tendency of animals to attach motivational importance to alcohol-related stimuli,and to prevent the manifestation of alcohol addictive behavior.Females drank more alcohol than males and exhibited more ST behavior in the PavCA, whereas males showed GT behavior.PavCA phenotypes emerged early and remained stable.The application of tDCS during PavCA results in high GT numbers in stimulated rats.Early tDCS on drinking does not affect acute or long-term drinking.Our findings indicate a complex relationship between incentive salience and alcohol addiction and emphasize the importance of considering individual differences and both sexes in preclinical research.

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Damage to the nervous system may cause sensorimotor impairment, often resulting in chronic neuropathic pain. Phantom limb pain affects multiple amputees and the treatment options are limited. A promising treatment option to reduce chronic pain is by training sensory discrimination. At the Center for Bionics and Pain Research, a sensory training device has been developed. Transcranial direct current stimulation (tDCS) is a technique to stimulate different regions of the cerebral cortex. In studies, anodal tDCS of the sensorimotor cortex has shown to improve tactile acuity. Until now, the effect of the sensory discrimination training, performed with the sensory training device, in combination with tDCS has not been tested. The purpose of this master’s thesis was to determine the effect of tDCS applied on the sensorimotor cortex on the outcomes of sensory discrimination training. The purpose was also to compare the effect of two different methods of stimulation, namely conventional and High Definition tDCS. 16 able-bodied participants underwent three single sessions with 40 minutes of sensory discrimination training: one session combined with conventional tDCS, one session combined with High Definition tDCS and one session without tDCS. The tactile acuity was determined by the two-point discrimination test and the Semmes-Weinstein monofilament test, prior to and one hour after each session. This study showed that 40 minutes of sensory discrimination training was sufficient to improve the two-point discrimination in the sensory trained areas, compared to the sensory untrained areas (p=0.02). However, the improvement in two-point discrimination was not statistically significant between the interventions, i.e. the improvement in two-point discrimination for the sessions with brain modulation was not statistically significant compared to the session without brain modulation. The monofilament assessments showed an improvement in monofilament score for the sensory untrained skin patches (p=0.053). This study concluded that single sessions of training was enough to improve two-point discrimination but not monofilament score at the site of stimulation. This study lays a foundation for what parameters to include in future studies.

Transformers emit a characteristic humming noise due to magnetostriction which is the continuous change in dimensions during magnetization. The noise is amplified if the induced frequencies match the core’s natural frequencies, consequently avoiding geometries that create resonance is critical in order to fulfill customer sound level requirements. In this thesis, a high voltage direct current transformer core with two main limbs and two return limbs is studied. Using finite element analysis (FEA), the core can be modeled and analyzed in a computer environment. The main contributors of noise are the first bending and longitudinal resonance modes. Data for how these modes change with geometric alterations is collected and stored through parametric studies. An analytical expression is then constructed through Rayleigh’s energy method with added coefficients that can be correlated to FEA datasets achieving a verified model via data-fitting. A satisfactory model is created for both resonance modes. / Transformatorer avger ett karaktäristiskt surrande ljud. Ljudet uppkommer på grund av magnetostriktion vilket är förändringar i geometri som uppkommer då kärnan kontinuerligt magnetiseras. Ljudet förstärks om induktionsfrekvenserna matchar kärnans naturliga frekvenser, så att undvika kärngeometrier som skapar resonans är viktigt för att klara ljudnivåkrav som kunden har. I denna studie betraktas resonansfenomenet i en högspänningstransformator för likström (HVDC) med två lindade ben och två sidoben. Med avstamp i en finita elementanalys (FEA) kan kärnan modelleras och analyseras i en datormiljö. Data för hur resonansmoderna som bidrar mest till ljud förändras med förändringar av geometriska parametrar samlas genom parametriska studier. I detta fall analyseras de första böj- och longitudinella resonansmoderna. Ett analytiskt uttryck skapas sedan med hjälp av Rayleigh’s energimetod där coefficienter anpassas efter FEA-datan. Detta leder slutligen till en verifierad modell som fungerar väl för uppskattning av de båda relevanta resonansmoderna.

La multiplication des projets HVDC de par le monde démontre l'engouement toujours croissant pour cette technologie de transport de l'électricité. La grande majorité de ces transmissions HVDC correspondent à des liaisons point-à-point et se basent sur des convertisseurs AC/DC de type LCC ou VSC à 2 ou 3 niveaux. Les travaux de cette thèse se focalisent sur l'étude, le contrôle et la commande de systèmes HVDC de type multi-terminal (MTDC), avec des convertisseurs de type VSC classique ou modulaire multi-niveaux. La première étape consiste à obtenir les modèles moyens du VSC classique et du MMC. La différence fondamentale entre ces deux convertisseurs, à savoir la possibilité pour le MMC de stocker et de contrôler l'énergie des condensateurs des sous-modules, est détaillée et expliquée. Ces modèles et leurs commandes sont ensuite linéarisés et mis sous forme de représentations d'état, puis validés en comparant leur comportement à ceux de modèles de convertisseurs plus détaillés à l'aide de logiciels de type EMT. Une fois validés, les modèles d'état peuvent être utilisés afin de générer le modèle d'état de tout système de transmissions HVDC, qu'il soit point-à-point ou MTDC. La comparaison d'une liaison HVDC à base de VSCs classiques puis de MMCs est alors réalisée. Leurs valeurs propres sont étudiées et comparées, et les modes ayant un impact sur la tension DC sont identifiés et analysés. Cette étude est ensuite étendue à un système MTDC à 5 terminaux, et son analyse modale permet à la fois d'étudier la stabilité du système, mais aussi de comprendre l'origine de ses valeurs propres ainsi que leur impact sur la dynamique du système. La méthode de décomposition en valeurs singulières permet ensuite d'obtenir un intervalle de valeurs possibles pour le paramètre de"voltage droop", permettant ainsi le contrôle du système MTDC tout en s'assurant qu'il soit conforme à des contraintes bien définies, comme l'écart maximal admissible en tension DC. Enfin, une proposition de "frequency droop" (ou "statisme"), permettant aux convertisseurs de participer au réglage de la fréquence des réseaux AC auxquels ils sont connectés, est étudiée. Le frequency droop est utilisé conjointement avec le voltage droop afn de garantir le bon fonctionnement de la partie AC et de la partie DC. Cependant, l'utilisation des deux droop génère un couplage indésirable entre les deux commandes. Ces interactions sont mathématiquement quantifiées et une correction à apporter au paramètre de frequency droop est proposée. Ces résultats sont ensuite validés par des simulations EMT et par des essais sur la plate-forme MTDC du laboratoire L2EP. / HVDC transmission systems are largely used worldwide, mostly in the form of back-to-back and point-to-point HVDC, using either thyristor-based LCC or IGBT-based VSC. With the recent deployment of the INELFE HVDC link between France and Spain, and the commissioning in China of a three-terminal HVDC transmission system using Modular Multilevel Converters (MMCs), a modular design of voltage source converters, the focus of the scientific community has shifted onto the analysis and control of MMC-based HVDC transmission systems. In this thesis, the average value models of both a standard 2-level VSC and an MMC are proposed and the most interesting difference between the two converter technologies -the control of the stored energy in the MMC- is emphasised and explained. These models are then linearised, expressed in state-space form and validated by comparing their behaviour to more detailed models under EMT programs. Afterwards, these state-space representations are used in the modelling of HVDC transmission systems, either point-to-point or Multi-Terminal HVDC (MTDC). A modal analysis is performed on an HVDC link, for both 2-level VSCs and MMCs. The modes of these two systems are specifed and compared and the independent control of the DC voltage and the DC current in the case of an MMC is illustrated. This analysis is extended to the scope of a 5-terminal HVDC system in order to perform a stability analysis, understand the origin of the system dynamics and identify the dominant DC voltage mode that dictates the DC voltage response time. Using the Singular Value Decomposition method on the MTDC system, the proper design of the voltage-droop gains of the controllers is then achieved so that the system operation is ensured within physical constraints, such as the maximum DC voltage deviation and the maximum admissible current in the power electronics. Finally, a supplementary droop "the frequency-droop control" is proposed so that MTDC systems also participate to the onshore grids frequency regulation. However, this controller interacts with the voltage-droop controller. This interaction is mathematically quantified and a corrected frequency-droop gain is proposed. This control is then illustrated with an application to the physical converters of the Twenties project mock-up.

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The aim of this study is to ascertain what misconceptions N2 students have about DC resistive circuits and how screencasts could effect on the rectification of these misconceptions. This study was conducted at the Kathu Campus of the Northern Cape Rural Further Education and Training College in the town Kathu in the arid Northern Cape. The empirical part of this study was conducted during the first six months of 2013. A design-based research (DBR) method consisting of four phases was used. DBR function is to design and develop interventions such as a procedure, new teachinglearning strategies, and in the case of this study a technology-enhanced learning (TEL) tool (screencast) with the purpose of solving a versatile didactic problem and to acquire information about the interventions of the TEL tool (screencast) on the learning of a student. In the first and second phase of DBR quantitative data for this research were gathered with the Determining and Interpreting Resistive Electric circuits Concepts Test (DIRECT) in order to determine the four most common misconceptions. The DIRECT test was conducted in the first trimester to find the misconceptions; the test was conducted in the second trimester also to confirm the misconceptions. Further quantitative data were collected from a demographic questionnaire. The qualitative data were collected by individual interviews in the fourth phase of the research project. Phase three of this study was the development of screencasts in the four most prominent misconceptions in DC resistive circuits of the students. The respondents of this study were non-randomly chosen and comprised of two groups, one in the first trimester of the year and one in the second trimester of the year, which enrolled for the N2 Electrical or Millwright courses. The respondents were predominant male and representing the three main cultural groups in the Northern Cape namely: Black, Coloured and White. The four misconceptions on DC resistive circuits that were identified were: (i) understanding of concepts, (ii) understanding of short circuit, (iii) battery as a constant current source, and (iv) rule application error. Screencasts clarifying the four misconceptions were developed and distributed to the respondents. On the foundation of the results of this research, it can be concluded that the students have several misconceptions around direct current resistive direct current circuits and that the use of TEL like screencasts can be used to solve some of these misconceptions. Screencasts could supplement education when they were incorporated into the tutoring and learning for supporting student understanding. The results of this research could lead to the further development and refinement of screencasts on DC resistive circuits and also useable guidelines in creating innovative screencasts on DC resistive circuits. / MEd (Curriculum Development), North-West University, Potchefstroom Campus, 2014

Le traitement visuel répété d’un visage inconnu entraîne une suppression de l’activité neuronale dans les régions préférentielles aux visages du cortex occipito-temporal. Cette «suppression neuronale» (SN) est un mécanisme primitif hautement impliqué dans l’apprentissage de visages, pouvant être détecté par une réduction de l’amplitude de la composante N170, un potentiel relié à l’événement (PRE), au-dessus du cortex occipito-temporal. Le cortex préfrontal dorsolatéral (CPDL) influence le traitement et l’encodage visuel, mais sa contribution à la SN de la N170 demeure inconnue. Nous avons utilisé la stimulation électrique transcrânienne à courant direct (SETCD) pour moduler l’excitabilité corticale du CPDL de 14 adultes sains lors de l’apprentissage de visages inconnus. Trois conditions de stimulation étaient utilisées: inhibition à droite, excitation à droite et placebo. Pendant l’apprentissage, l’EEG était enregistré afin d’évaluer la SN de la P100, la N170 et la P300. Trois jours suivant l’apprentissage, une tâche de reconnaissance était administrée où les performances en pourcentage de bonnes réponses et temps de réaction (TR) étaient enregistrées. Les résultats indiquent que la condition d’excitation à droite a facilité la SN de la N170 et a augmentée l’amplitude de la P300, entraînant une reconnaissance des visages plus rapide à long-terme. À l’inverse, la condition d’inhibition à droite a causé une augmentation de l’amplitude de la N170 et des TR plus lents, sans affecter la P300. Ces résultats sont les premiers à démontrer que la modulation d’excitabilité du CPDL puisse influencer l’encodage visuel de visages inconnus, soulignant l’importance du CPDL dans les mécanismes d’apprentissage de base. / Repeated visual processing of an unfamiliar face suppresses neural activity in face-specific areas of the occipito-temporal cortex. This "repetition suppression" (RS) is a primitive mechanism involved in learning of unfamiliar faces, which can be detected through amplitude reduction of the N170 event-related potential (ERP). The dorsolateral prefrontal cortex (DLPFC) exerts top-down influence on early visual processing. However, its contribution to N170 RS and learning of unfamiliar faces remains unclear. Transcranial direct current stimulation (tDCS) transiently increases or decreases cortical excitability, as a function of polarity. We hypothesized that DLPFC excitability modulation by tDCS would cause polarity-dependent modulations of N170 RS during encoding of unfamiliar faces. tDCS-induced N170 RS enhancement would improve long-term recognition reaction time (RT) and/or accuracy rates, whereas N170 RS impairment would compromise recognition ability. Participants underwent three tDCS conditions in random order at ~72 hour intervals: right anodal/left cathodal, right cathodal/left anodal and sham. Immediately following tDCS conditions, an EEG was recorded during encoding of unfamiliar faces for assessment of P100 and N170 visual ERPs. P300 was analyzed to detect prefrontal function modulation. Recognition tasks were administered ~72 hours following encoding. Results indicate the right anodal/left cathodal condition facilitated N170 RS and induced larger P300 amplitudes, leading to faster recognition RT. Conversely, the right cathodal/left anodal condition caused increases in N170 amplitudes and RT, but did not affect P300. These data are the first to demonstrate that DLPFC excitability modulation can influence early visual encoding of unfamiliar faces, highlighting the importance of DLPFC in basic learning mechanisms.