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  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
51

Validation of the Magneto-articulography for the Assessment of Speech Kinematics (MASK) System and Testing for Use in a Clinical Research Setting

Lau, Calvin 03 December 2013 (has links)
A novel technology, the Magneto-articulography for the Assessment of Speech Kinematics (MASK) system, which measures brain activity and oromotor movement simultaneously, was validated for its speech tracking capabilities. MASK has not been systematically tested, so its movement tracking accuracy and practicality for research was still unknown. An error testing and mapping protocol is developed to validate MASK accuracy against established electromagnetic articulography (EMA) speech tracking systems. Data from human speech experiments are also compared. MASK exhibited higher positional error and fluctuation than EMA, and more inconsistent distribution of errors. Error mapping and potential error correction protocols were also developed. MASK spatial and temporal resolutions were found insufficient for precise tracking of small and quick articulatory movements. MASK requires much improvement to reach the capabilities of EMA. Further investigation into numerical instabilities of the position calculation algorithms is encouraged. This project provides the first assessment of MASK, which may advance speech research for future applications.
52

Characterizing the Spatiotemporal Neural Representation of Concrete Nouns Across Paradigms

Sudre, Gustavo 01 December 2012 (has links)
Most of the work investigating the representation of concrete nouns in the brain has focused on the locations that code the information. We present a model to study the contributions of perceptual and semantic features to the neural code representing concepts over time and space. The model is evaluated using magnetoencephalography data from different paradigms and not only corroborates previous findings regarding a distributed code, but provides further details about how the encoding of different subcomponents varies in the space-time spectrum. The model also successfully generalizes to novel concepts that it has never seen during training, which argues for the combination of specific properties in forming the meaning of concrete nouns in the brain. The results across paradigms are in agreement when the main differences among the experiments (namely, the number of repetitions of the stimulus, the task the subjects performed, and the type of stimulus provided) were taken into consideration. More specifically, these results suggest that features specific to the physical properties of the stimuli, such as word length and right-diagonalness, are encoded in posterior regions of the brain in the first hundreds of milliseconds after stimulus onset. Then, properties inherent to the nouns, such as is it alive? and can you pick it up?, are represented in the signal starting at about 250 ms, focusing on more anterior parts of the cortex. The code for these different features was found to be distributed over time and space, and it was common for several regions to simultaneously code for a particular property. Moreover, most anterior regions were found to code for multiple features, and a complex temporal profile could be observed for the majority of properties. For example, some features inherent to the nouns were encoded earlier than others, and the extent of time in which these properties could be decoded varied greatly among them. These findings complement much of the work previously described in the literature, and offer new insights about the temporal aspects of the neural encoding of concrete nouns. This model provides a spatiotemporal signature of the representation of objects in the brain. Paired with data from carefully-designed paradigms, the model is an important tool with which to analyze the commonalities of the neural code across stimulus modalities and tasks performed by the subjects.
53

Feminism and the representations of teenaged girls in 20th century children's literature

Chou, Mei-ching, Tammy. January 2005 (has links)
Thesis (M. A.)--University of Hong Kong, 2005. / Title proper from title frame. Also available in printed format.
54

Μαθηματική ανάλυση ηλεκτροεγκεφαλογραφίας μέσω ελλειψοειδών αρμονικών εβδόμου βαθμού

Σατραζέμη, Κωνσταντία 25 January 2012 (has links)
Η βέλτιστη γεωμετρική προσομοίωση του εγκεφάλου επιτυγχάνεται με ένα ελλειψοειδές. Η ηλεκτροεγκεφαλογραφία (ΗΕΓ) αφορά τις μετρήσεις του ηλεκτρικού δυναμικού στην επιφάνεια του κρανίου που αναπτύσσεται από νευρωνικά ρεύματα στο εσωτερικό του εγκεφάλου. Στην παρούσα εργασία έχουν παραχθεί οι απαιτούμενες ελλειψοειδείς αρμονικές πέμπτου, έκτου και εβδόμου βαθμού. Στη συνέχεια, χρησιμοποιούμε αυτές τις νέες ελλειψοειδείς αρμονικές συναρτήσεις για να εκφράσουμε το πλήρες αναλυτικό δυναμικό της ΗΕΓ. H δομή της εργασίας είναι: Στο κεφάλαιο 1 περιγράφεται περιληπτικά η φυσιολογία ανάπτυξης δυναμικών στην περιοχή του εγκεφάλου και πως επιτυγχάνεται η μετάδοση αυτών των σημάτων μέσω των νευρώνων. Στο κεφάλαιο 2 περιγράφεται η ελλειψοειδής γεωμετρία καθώς και η μορφή του τελεστή του Laplace στο ελλειψοειδές σύστημα συντεταγμένων. Αναφερόμαστε στην επίλυση της εξίσωσης Lame, στον τρόπο κατασκευής των συναρτήσεων Lame και περιγράφουμε τις αντίστοιχες ελλειψοειδείς αρμονικές. Στο κεφάλαιο 3 καταγράφουμε τις ελλειψοειδείς αρμονικές βαθμού 3. Στα κεφάλαια 4,5,6 και 7 παράγουμε τις ελλειψοειδείς αρμονικές βαθμού 4,5,6 και 7, αντίστοιχα, όπου η εφαρμογή των οποίων στην ΗΕΓ αποτελεί και το θέμα της παρούσας εργασίας. Τέλος στο κεφάλαιο 8 χρησιμοποιούμε τις ελλειψοειδείς αρμονικές που κατασκευάσαμε για να επιτύχουμε την ακριβή συνιστώσα του ηλεκτρικού δυναμικού που ανήκει στον ελλειψοειδή αρμονικό υπόχωρο που γεννούν οι 64 πρώτες αρμονικές συναρτήσεις. / The best geometric simulation of the human brain is achieved by an ellipsoidal system. The Electroencephalography (EEG) concerns the measurements of the electric potential on the surface of the head, which is generated from neuronal current inside the head. In this thesis we have produced the necessary ellipsoidal harmonics of the fifth, sixth and seventh degree. Then, we use these new ellipsoidal harmonic functions to express the full analytical potential of EEG, up to the seventh degree. The present thesis is structured in the following way: In chapter 1 we describe, in short, the physiology which develops the potentials in and outside the brain and how they are transmitted through the neuronal current. In chapter 2 we describe the ellipsoidal geometry as well as the form of the Laplace operator in ellipsoidal coordinates. We consider the solutions of Lame equation, the way Lame constructed them and we describe the corresponding ellipsoidal harmonics. In chapter 3 we report the ellipsoidal harmonics of degree zero through three. In chapters 4, 5, 6 and 7 we develop the ellipsoidal harmonics of degree 4, 5, 6 and 7 respectively. The applications of these functions form the main part of the present master thesis. Finally, in chapter 8 we use all the known ellipsoidal harmonics to express the relative component of the electrical potential. This component belongs to the subspace generated by the first 64 ellipsoidal harmonics.
55

Dynamics of temporal anticipation in perception and action

Heideman, Simone January 2017 (has links)
The selective deployment of attention over time optimises our perception and action at the moments when relevant events are expected to happen. Such "temporal orienting" to moments when something is going to happen is especially useful when this information can be combined with predictions about where and what events are likely to occur. A large body of research has already established how temporal predictions dynamically influence our perception and action, but questions remain regarding the neural bases of these attentional mechanisms. In this thesis I present three magnetoencephalography (MEG) studies that I conducted to investigate anticipatory neural dynamics associated with spatial-temporal orienting of attention for perception and action. I also investigate and discuss how such anticipatory dynamics change with ageing and neurodegeneration in Parkinson's disease (PD), and how these anticipatory neural dynamics behave in situations where a complex, hidden spatial-temporal structure is present. In Chapter 1, I introduce the topic of this thesis by reviewing the literature on temporal orienting of attention and by introducing my specific research questions. In Chapter 2, I present an MEG study on anticipatory neural dynamics of joint spatial-temporal orienting of attention in the visual domain, in younger and older adults. This study shows that neural dynamics with spatial, temporal and spatial-temporal orienting are all differentially affected by ageing. In Chapter 3, I describe an MEG experiment that investigates anticipatory neural dynamics during spatial-temporal motor preparation and compares PD participants to healthy control participants. This study reveals that both behavioural and neural dynamics with temporal orienting are affected in PD. In Chapter 4, I describe an experiment that explores how an implicit spatial-temporal structure is utilised to predict and prepare for upcoming actions. This study shows that motor cortical excitability is dynamically modulated in anticipation of the location and timing of events, even when such expectations are hidden in complex visual-motor sequences that remain largely implicit. In Chapter 5, the General discussion, I place these results in their wider context and discuss limitations and future directions.
56

A influência do modelo de gestão de qualidade total (GQT) na administração atual de empresas baianas que trilham o caminho rumo a excelência

Bernardino, Lis Lisboa 27 March 2014 (has links)
Submitted by Núcleo de Pós-Graduação Administração (npgadm@ufba.br) on 2017-08-07T20:04:40Z No. of bitstreams: 1 Lis Lisboa Bernardino.pdf: 3770069 bytes, checksum: 102989ede29d4e185e2a92b0c5bd9d3b (MD5) / Approved for entry into archive by Maria Angela Dortas (dortas@ufba.br) on 2017-08-18T19:43:55Z (GMT) No. of bitstreams: 1 Lis Lisboa Bernardino.pdf: 3770069 bytes, checksum: 102989ede29d4e185e2a92b0c5bd9d3b (MD5) / Made available in DSpace on 2017-08-18T19:43:55Z (GMT). No. of bitstreams: 1 Lis Lisboa Bernardino.pdf: 3770069 bytes, checksum: 102989ede29d4e185e2a92b0c5bd9d3b (MD5) / A Gestão pela Qualidade Total (GQT) alcançou grande visibilidade e popularidade nas décadas de 1980 e 1990 no mundo ocidental. No Brasil, especialmente a partir de 1990 com a abertura da economia, uma quantidade significativa de organizações passou a adotar GQT. Porém, ao final da segunda metade da década de 1990, este modelo entrou em processo de desuso, perdendo destaque no meio acadêmico e empresarial. Considerando a relevância dos estudos de casos de desuso no campo da administração, esta pesquisa, do tipo exploratória, busca compreender quais aspectos (categorizados em princípios, práticas, ferramentas e metodologias) do modelo de GQT foram capazes de transcender ao período denominado “boom da qualidade” no Brasil e quais deles foram descartados ou sofreram modificações, após adoção do Modelo de Excelência da Gestão (MEG) instituído pela Fundação Nacional da Qualidade. Para tanto, utilizou-se da estratégia de pesquisa Estudo de Casos Múltiplos realizado com quatro organizações constituídas no estado da Bahia que nos anos 1990 implantaram programas de GQT e posteriormente adotaram o MEG. Em relação aos aspectos empíricos, foram utilizadas as seguintes técnicas para coleta de dados: entrevistas exploratórias, análise documental, entrevistas semiestruturadas, questionários semiabertos e observação. Os resultados obtidos revelaram que atualmente algumas das práticas, ferramentas e metodologias oriundas do modelo de GQT, utilizadas durante a década de 1990 pelas empresas pesquisadas, não estão mais sendo amplamente utilizadas. Porém, a grande maioria dos princípios, ou seja, da filosofia de gestão ainda fazem parte do ambiente organizacional das empresas pesquisadas. Assim, a hipótese mais plausível, formulada a partir da execução desta pesquisa, é que não há propriamente uma “crise” do modelo de GQT nas empresas pesquisadas, pois muitos dos seus aspectos se encontram amplamente sedimentados, sendo implementados por meio de práticas ou programas gerenciais com nomenclaturas diferenciadas, mas que ainda guardam muitas semelhanças com os antigos programas de GQT ao estilo japonês. / Total Quality Management (TQM) achieved great visibility and popularity in the 80s and 90s in the western world. In Brazil, since 1990 with the opening of the economy, many organizations started adopting TQM. However, at the end of the second half of the 90s, this model went into disuse, losing importance in the academic and business fields. Considering the relevance of this approach to the business administration field, this research aims to understand which aspects (categorized by principles, practices, tools and methodologies) of TQM model were able to transcend brazilian “quality boom” and which ones are no longer used or have changed after the adoption of the Management Excellence Model (MEG), created by the Brazilian Foundation of Quality (FNQ). Thus, multiple case studies were conducted with four organizations located in Bahia, that in the 90s adopted TQM and subsequently adopted MEG. About the empirical aspects, the following techniques were used for data collection: exploratory interviews, document analysis, interviews, questionnaires and observation. Results obtained revealed that currently some of the practices, tools and methodologies, from the TQM model, used during the 90s for the four mentioned cases, are not being widely used anymore. However, most of principles from TQM are still part of the organizational environment of the surveyed cases, as a management philosophy. The hypothesis arising from this research is that there is not a real “TQM crisis”, since many of its aspects have been incorporated into business management of the companies surveyed, being implemented through practices or programs with different names, but still maintaining similarities with the classic TQM on Japanese way.
57

Entropy-based nonlinear analysis for electrophysiological recordings of brain activity in Alzheimer's disease

Azami, Hamed January 2018 (has links)
Alzheimer’s disease (AD) is a neurodegenerative disorder in which the death of brain cells causes memory loss and cognitive decline. As AD progresses, changes in the electrophysiological brain activity take place. Such changes can be recorded by the electroencephalography (EEG) and magnetoencephalography (MEG) techniques. These are the only two neurophysiologic approaches able to directly measure the activity of the brain cortex. Since EEGs and MEGs are considered as the outputs of a nonlinear system (i.e., brain), there has been an interest in nonlinear methods for the analysis of EEGs and MEGs. One of the most powerful nonlinear metrics used to assess the dynamical characteristics of signals is that of entropy. The aim of this thesis is to develop entropy-based approaches for characterization of EEGs and MEGs paying close attention to AD. Recent developments in the field of entropy for the characterization of physiological signals have tried: 1) to improve the stability and reliability of entropy-based results for short and long signals; and 2) to extend the univariate entropy methods to their multivariate cases to be able to reveal the patterns across channels. To enhance the stability of entropy-based values for short univariate signals, refined composite multiscale fuzzy entropy (MFE - RCMFE) is developed. To decrease the running time and increase the stability of the existing multivariate MFE (mvMFE) while keeping its benefits, the refined composite mvMFE (RCmvMFE) with a new fuzzy membership function is developed here as well. In spite of the interesting results obtained by these improvements, fuzzy entropy (FuzEn), RCMFE, and RCmvMFE may still lead to unreliable results for short signals and are not fast enough for real-time applications. To address these shortcomings, dispersion entropy (DispEn) and frequency-based DispEn (FDispEn), which are based on our introduced dispersion patterns and the Shannon’s definition of entropy, are developed. The computational cost of DispEn and FDispEn is O(N) – where N is the signal length –, compared with the O(N2) for popular sample entropy (SampEn) and FuzEn. DispEn and FDispEn also overcome the problem of equal values for embedded vectors and discarding some information with regard to the signal amplitudes encountered in permutation entropy (PerEn). Moreover, unlike PerEn, DispEn and FDispEn are relatively insensitive to noise. As extensions of our developed DispEn, multiscale DispEn (MDE) and multivariate MDE (mvMDE) are introduced to quantify the complexity of univariate and multivariate signals, respectively. MDE and mvMDE have the following advantages over the existing univariate and multivariate multiscale methods: 1) they are noticeably faster; 2) MDE and mvMDE result in smaller coefficient of variations for synthetic and real signals showing more stable profiles; 3) they better distinguish various states of biomedical signals; 4) MDE and mvMDE do not result in undefined values for short time series; and 5) mvMDE, compared with multivariate multiscale SampEn (mvMSE) and mvMFE, needs to store a considerably smaller number of elements. In this Thesis, two restating-state electrophysiological datasets related to AD are analyzed: 1) 148-channel MEGs recorded from 62 subjects (36 AD patients vs. 26 age-matched controls); and 2) 16-channel EEGs recorded from 22 subjects (11 AD patients vs. 11 age-matched controls). The results obtained by MDE and mvMDE suggest that the controls’ signals are more and less complex at respectively short (scales between 1 to 4) and longer (scales between 5 to 12) scale factors than AD patients’ recordings for both the EEG and MEG datasets. The p-values based on Mann-Whitney U-test for AD patients vs. controls show that the MDE and mvMDE, compared with the existing complexity techniques, significantly discriminate the controls from subjects with AD at a larger number of scale factors for both the EEG and MEG datasets. Moreover, the smallest p-values are achieved by MDE (e.g., 0.0010 and 0.0181 for respectively MDE and MFE using EEG dataset) and mvMDE (e.g., 0.0086 and 0.2372 for respectively mvMDE and mvMFE using EEG dataset) for both the EEG and MEG datasets, illustrating the superiority of these developed entropy-based techniques over the state-of-the-art univariate and multivariate entropy approaches. Overall, the introduced FDispEn, DispEn, MDE, and mvMDE methods are expected to be useful for the analysis of physiological signals due to their ability to distinguish different types of time series with a low computation time.
58

Solubilidade de cloreto de c?lcio em misturas de ?gua e monoetileno glicol pelo m?todo anal?tico via densidade / Calcium chloride solubility in aqueous mixtures of ethylene glycol by the analytical method via density

Silva, I?da Let?cia Mendes 09 November 2016 (has links)
Submitted by Automa??o e Estat?stica (sst@bczm.ufrn.br) on 2017-03-20T20:20:19Z No. of bitstreams: 1 IedaLeticiaMendesSilva_DISSERT.pdf: 33772951 bytes, checksum: 825956e46d9efc41cc4e776db6779930 (MD5) / Approved for entry into archive by Arlan Eloi Leite Silva (eloihistoriador@yahoo.com.br) on 2017-03-22T21:57:01Z (GMT) No. of bitstreams: 1 IedaLeticiaMendesSilva_DISSERT.pdf: 33772951 bytes, checksum: 825956e46d9efc41cc4e776db6779930 (MD5) / Made available in DSpace on 2017-03-22T21:57:01Z (GMT). No. of bitstreams: 1 IedaLeticiaMendesSilva_DISSERT.pdf: 33772951 bytes, checksum: 825956e46d9efc41cc4e776db6779930 (MD5) Previous issue date: 2016-11-09 / Dentro do universo da ind?stria petrol?fera, a forma??o de hidratos se apresenta como um dos grandes empecilhos atrelados a po?os produtores de g?s. Uma das t?cnicas utilizadas para evitar a forma??o de hidratos, ? a utiliza??o de compostos inibidores, destacando-se entre esses o monoetilenoglicol (MEG), sendo este injetado na cabe?a dos po?os produtores. No entanto, ao retornar ? superf?cie a corrente de MEG carreia tanto sedimentos como ?gua, advindos das forma??es dos po?os. Dessa forma, a regenera??o do MEG se faz necess?ria para a rentabilidade do processo. A regenera??o do MEG envolve etapas de pr?-tratamento, remo??o de sais e remo??o de ?gua. A fim de otimizar as condi??es operacionais, este trabalho se prop?s a determinar a solubilidade especificamente do sal cloreto de c?lcio em misturas de ?gua e MEG, ?s temperaturas de 25 e 50?C. A metodologia proposta foi a anal?tica e teve por intuito obter dados de solubilidade por meio de aferi??o de grandezas indiretas, sendo elas, densidade e condutividade. Sendo assim, as an?lises desenvolvidas nesse trabalho foram, principalmente, densimetria, condutimetria, e, al?m delas, termogravimetria e titula??o volum?trica de Karl Fischer, utilizadas para a determina??o do teor de ?gua apresentado pelos reagentes. Atrav?s das curvas elaboradas de densidade e condutividade versus concentra??o de cloreto de c?lcio, percebeu-se um comportamento monot?nico da grandeza densidade em rela??o ? concentra??o de sal, enquanto a condutividade apresentou comportamento n?o-monot?nico. Nos experimentos de solubilidade foram coletadas amostras que permitiram determinar a concentra??o a partir dos dados de densidade, grandeza mais adequada para este fim. Foram obtidas isotermas a 25 e 50?C de solubilidade versus concentra??o de MEG na mistura de solventes. A an?lise das isotermas permite observar que o efeito salting-out, provocado pela presen?a do MEG no solvente, ? pouco significativo at? a concentra??o m?dia de 33% m?ssico de MEG livre de sal. No entanto, ap?s esta zona de concentra??o ? percept?vel uma significativa altera??o na inclina??o de ambas as isotermas, e a diminui??o da solubilidade passa a ser mais acentuada. Em rela??o ao aumento de temperatura, a solubilidade do cloreto de c?lcio apresenta efeito positivo. A equa??o de Setchenov e o modelo UNIQUAC+DH, juntamente com os dados termoqu?micos, foram aplicados para descrever os dados experimentais. Na abordagem com UNIQUAC+DH n?o foi considerada a altera??o na fase s?lida formada com a concentra??o de MEG atrav?s dos dados termoqu?micos. Por outro lado, semelhantemente a Setchenov, foram estimados dois conjuntos de par?metros para cada isoterma, considerado cada fase s?lida distinta. A descri??o dos dados pela modelo UNIQUAC+DH resultou pr?xima da incerteza experimental, i.e., em m?dia para a solubilidade do cloreto de c?lcio 2,00 %. / In the oil and gas industry, hydrates formation is one of the issues related to the natural gas production. One of the most employed techniques to avoid hydrates formation is the injection of inhibitors compounds, like the monoethylene glycol (MEG) into the well. However, MEG stream returns to the surface impregnated with water and many salts coming from the rock sediments. Then, MEG must be regenerated to assure process environmental and economic viability. In order to optimize the operationals conditions of the reclamation process, this work aimed to determine the solubility of the calcium chloride in aqueous mixtures of MEG, in the temperatures of 25 and 50?C. The proposed methodology is analytical and applies indirect properties, density and conductivity, to determine solubility data. Therefore, the main data were obtained from densimetry, conductimetry, and beyond them, thermogravimetry and Karl Fischer titration to verify purity and moisture content. Observing the developed curves of density and conductivity versus calcium chloride concentration, it was detected the monotone and linear behavior for the density as function of the salt concentration. However, for the conductivity, it was observed a no-monotone and non-linear behavior. Thus, in order to determine solubility data, samples were evaluated with the aid of concentration versus density calibration data. The developed isotherms illustrate the calcium chloride solubility versus MEG concentration behavior. The isotherms have demonstrated the slight relevance of the salting-out effect, caused by the MEG presence in the solvent, until an average concentration of 33% MEG (salt-free). From that point, there are changes in the inclination of both isotherms and the solubility decreases more pronouncedly. In terms of the temperature increase, the calcium chloride solubility showed a positive effect. Setchenov equation and UNIQUAC+DH model were applied to describe the experimental data. In the UNIQUAC approach, it was not considered the change in the solid phase expressed by the corresponding thermochemical data due to MEG concentration. In the other hand, similarly to Setchenov correlation, two set of parameters were estimated for each isotherm, considering each solid phase formed. The description provided by the UNIQUAC+DH resulted closed to the experimental uncertainty, i.e., in average 2,00 % for the calcium chloride solubility.
59

The limits of sensory processing during sleep: how far can we interact with memory contents?

Farthouat, Juliane 16 December 2016 (has links)
Depuis une dizaine d'années, de nouveaux outils ont été développés visant à stimuler la consolidation mnésique opérant au cours du sommeil. Sur base des connaissances récentes sur la perception sensorielle résiduelle chez le sujet endormi, ainsi que des théories suggérant que la consolidation de souvenirs a lieu suite à leurs réactivations durant le sommeil, de nouveaux paradigmes ont vus le jour, permettant de biaiser cette réactivation spontanée à l'aide de stimulations externes. Toutefois, les limites et les conditions nécessaires de ce processus n'ont pas été clairement définies. Par ailleurs, si la question de stimuler la consolidation en mémoire durant le sommeil a été fortement étudiée, la possibilité d'au contraire interférer avec le contenu mnésique reste en suspens. Enfin, des études récentes suggèrent que la stimulation auditive permet non seulement de stimuler la mémoire, mais qu'il est également possible de créer de nouvelles associations entre un stimulus et une réponse respiratoire durant le sommeil. La possibilité d'établir des associations plus complexes entre stimuli reste à établir.Au cours de cette dissertation, les trois études principales de ma thèse seront successivement présentées, durant lesquelles nous nous sommes attachés à déterminer les limites du traitement sensoriel durant le sommeil et ses possibles interactions avec le contenu mnésique. Dans une première étude, nous nous sommes intéressés à la possibilité de perturber un apprentissage de paires de mots en représentant durant le sommeil des paires de mots interférentes ainsi que les oscillations cérébrales associées à la réactivation du contenu mnésique. Dans une seconde étude, nous avons développé un paradigme et une technique d'analyse des potentiels évoqués stationnaires en magnétoencéphalographie permettant de marquer la segmentation ayant lieu au cours de l'écoute d'une séquence statistique ainsi que la progression temporelle du processus d'apprentissage associé. Enfin, dans une dernière étude, nous avons testé à l'aide des potentiels évoqués stationnaires si la détection de régularités statistiques est possible au cours du sommeil. / Over the last decade, new tools have been developed to boost the memory consolidation processes taking place during sleep. Based on recent knowledge about residual sensory processing in sleeping adults, and on theoretical accounts that memory consolidation during sleep occurs via the reactivation of memory content, new paradigms have been proposed to bias spontaneous memory reactivation using sensory stimulation. However, the limits and necessary conditions for successful memory reactivation remain unclear. Also, if boosting consolidation processes has been widely studied, it remains to be tested whether it is possible, to the contrary, to interfere with memory acquisition using stimulations. Finally, recent studies suggest that auditory stimulation during sleep not only boost memory, but also that new associations can be created between stimuli and breathing responses. Whether it is possible to create more complex associations between stimuli while sleeping is an enduring question.In this doctoral dissertation, I will present 3 main studies in which we studied the limits of auditory processing during sleep and how it can interact with memory content. In a first study, we attempted to interfere with consolidation of learned word pairs by presenting interfering material during sleep, and we studied the neural oscillations associated with the reactivation of memory content. In a second study, we developed a new paradigm using auditory frequency-tagged responses and magnetoencephalography to highlight segmentation processes that take place when listening to auditory statistical streams, and visualizing their temporal evolution. In the third and last study, we tested using steady-states analyses whether statistical learning can be achieved during sleep. / Doctorat en Sciences psychologiques et de l'éducation / info:eu-repo/semantics/nonPublished
60

Source-Space Analyses in MEG/EEG and Applications to Explore Spatio-temporal Neural Dynamics in Human Vision

Yang, Ying 01 February 2017 (has links)
Human cognition involves dynamic neural activities in distributed brain areas. For studying such neural mechanisms, magnetoencephalography (MEG) and electroencephalography (EEG) are two important techniques, as they non-invasively detect neural activities with a high temporal resolution. Recordings by MEG/EEG sensors can be approximated as a linear transformation of the neural activities in the brain space (i.e., the source space). However, we only have a limited number sensors compared with the many possible locations in the brain space; therefore it is challenging to estimate the source neural activities from the sensor recordings, in that we need to solve the underdetermined inverse problem of the linear transformation. Moreover, estimating source activities is typically an intermediate step, whereas the ultimate goal is to understand what information is coded and how information flows in the brain. This requires further statistical analysis of source activities. For example, to study what information is coded in different brain regions and temporal stages, we often regress neural activities on some external covariates; to study dynamic interactions between brain regions, we often quantify the statistical dependence among the activities in those regions through “connectivity” analysis. Traditionally, these analyses are done in two steps: Step 1, solve the linear problem under some regularization or prior assumptions, (e.g., each source location being independent); Step 2, do the regression or connectivity analysis. However, biases induced in the regularization in Step 1 can not be adapted in Step 2 and thus may yield inaccurate regression or connectivity results. To tackle this issue, we present novel one-step methods of regression or connectivity analysis in the source space, where we explicitly modeled the dependence of source activities on the external covariates (in the regression analysis) or the cross-region dependence (in the connectivity analysis), jointly with the source-to-sensor linear transformation. In simulations, we observed better performance by our models than by commonly used two-step approaches, when our model assumptions are reasonably satisfied. Besides the methodological contribution, we also applied our methods in a real MEG/EEG experiment, studying the spatio-temporal neural dynamics in the visual cortex. The human visual cortex is hypothesized to have a hierarchical organization, where low-level regions extract low-level features such as local edges, and high-level regions extract semantic features such as object categories. However, details about the spatio-temporal dynamics are less understood. Here, using both the two-step and our one-step regression models in the source space, we correlated neural responses to naturalistic scene images with the low-level and high-level features extracted from a well-trained convolutional neural network. Additionally, we also studied the interaction between regions along the hierarchy using the two-step and our one-step connectivity models. The results from the two-step and the one-step methods were generally consistent; however, the one-step methods demonstrated some intriguing advantages in the regression analysis, and slightly different patterns in the connectivity analysis. In the consistent results, we not only observed an early-to-late shift from low-level to high-level features, which support feedforward information flow along the hierarchy, but also some novel evidence indicating non-feedforward information flow (e.g., topdown feedback). These results can help us better understand the neural computation in the visual cortex. Finally, we compared the empirical sensitivity between MEG and EEG in this experiment, in detecting dependence between neural responses and visual features. Our results show that the less costly EEG was able to achieve comparable sensitivity with that in MEG when the number of observations was about twice of that in MEG. These results can help researchers empirically choose between MEG and EEG when planning their experiments with limited budgets.

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