Dissertations / Theses on the topic 'Coherent oscillations'
To see the other types of publications on this topic, follow the link: Coherent oscillations.
Create a spot-on reference in APA, MLA, Chicago, Harvard, and other styles
Consult the top 33 dissertations / theses for your research on the topic 'Coherent oscillations.'
Next to every source in the list of references, there is an 'Add to bibliography' button. Press on it, and we will generate automatically the bibliographic reference to the chosen work in the citation style you need: APA, MLA, Harvard, Chicago, Vancouver, etc.
You can also download the full text of the academic publication as pdf and read online its abstract whenever available in the metadata.
Browse dissertations / theses on a wide variety of disciplines and organise your bibliography correctly.
1
Jerebtsov, Serguei Nikolaevich. "Femtosecond time-resolved spectroscopy of coherent oscillations in nanomaterials." [College Station, Tex. : Texas A&M University, 2007. http://hdl.handle.net/1969.1/ETD-TAMU-1358.
Full text
2
Blum, Julia Maria. "Coherent brain oscillations during processes of human sensorimotor learning /." Zürich : ETH, 2008. http://e-collection.ethbib.ethz.ch/show?type=diss&nr=17951.
Full text
3
Buehlmann, Andrés. "Information processing in the cortex: the relevance of coherent oscillations for neuronal communication." Doctoral thesis, Universitat Pompeu Fabra, 2010. http://hdl.handle.net/10803/7566.
Full textAbstract:
Les oscil·lacions d'activitat neuronal són un fenomen omnipresent a l'escorça cerebral. La funció d'aquestes oscil·lacions, però, no està clara. ¿Són només un epifenomen de les elevades taxes de descàrrega de potencials d'acció, o representen un procés fonamental? Per tal d'aclarir aquesta qüestió, en aquest treball hem aplicat models computacionals basats en xarxes neurobiològicament plausibles per tal d'investigar alguns dels resultats experimentals recents més rellevants. Primerament, estudiem la rellevància de les oscil·lacions en processos d'atenció i després en un context més general de teoria d'informació. Els resultats donen suport a la idea que les oscil·lacions representen un mecanisme independent. Demostrem que l'atenció modula les oscil·lacions gamma de manera independent de la taxa de descàrrega de potencials d'acció. També es mostra que la transmissió d'informació entre àrees corticals depèn tant de la fase com de la potència espectral de les oscil·lacions. A més, la velocitat amb què es produeix aquesta transmissió d'informació augmenta en funció de la potència espectral en bandes de freqüències específiques. Aquests resultats suggereixen que les oscil·lacions representen un mecanisme biològicament plausible per mitjançar les interaccions entre àrees cerebrals i, per tant, per establir un vincle entre activitat neuronal i comportament.Oscillatory neuronal activity is an omnipresent phenomenon in the cerebral cortex. However, the actual function of these oscillations remains unclear. Are they just an epiphenomenon of elevated firing rates or do they represent a fundamental process on their own? Based on experimental work, we apply computational modeling to address this question. We first study the role of oscillations in attentional processes and then in a more general, information theoretical context. Our results support the idea that oscillations represent an independent mechanism. In particular, we show that attention modulates gamma oscillations independently of rates and that the flow of information between brain areas depends both on the phase and on the spectral power of oscillations. Moreover, we show that the speed of information exchange increases as a function of spectral power in specific frequency bands. Taken together, these results suggest that oscillations are a mechanism employed by the brain to control actual interactions between brain areas and thus likely have a link to behavior.
4
Karpov, Ivan [Verfasser], Oliver [Akademischer Betreuer] Boine-Frankenheim, and Shaukat [Akademischer Betreuer] Khan. "Damping of Coherent Oscillations in Intense Ion Beams / Ivan Karpov ; Oliver Boine-Frankenheim, Shaukat Khan." Darmstadt : Universitäts- und Landesbibliothek Darmstadt, 2017. http://d-nb.info/1125627530/34.
Full text
5
Magnuson, Matthew Evan. "Effects of severing the corpus callosum on coherent electrical and hemodynamic interhemispheric oscillations intrinsic to functional brain networks." Diss., Georgia Institute of Technology, 2013. http://hdl.handle.net/1853/47681.
Full textAbstract:
Large scale functional brain networks, defined by synchronized spontaneous oscillations between spatially distinct anatomical regions, are essential to brain function and have been implicated in disease states, cognitive capacity, and many sensing and motor processes. In this work, we sever the corpus callosum in the rodent model to determine if structural connectivity (specifically the primary interhemispheric pathway) organizes and influences bilateral functional connectivity and brain-wide spatiotemporal dynamic activity patterns.
Prior to the callosotomy work, resting state brain networks were evaluated using blood oxygen level dependent (BOLD) and cerebral blood volume (CBV) magnetic resonance imaging contrast mechanisms, and revealed that BOLD and CBV provide highly similar spatial maps of functional connectivity; however, the amplitude of BOLD connectivity was generally stronger. The effects of extended anesthetic durations on functional connectivity were also evaluated revealing extended isoflurane anesthetic periods prior to the switch to dexmedetomidine attenuates functional activity for a longer duration as compared to a shorter isoflurane paradigm. We also observed a secondary significant evolution of functional metrics occurring during long durations of dexmedetomidine use under the currently accepted and refined dexmedetomidine sedation paradigm.
Taking these previous findings into account, we moved forward with the callosotomy study. Functional network integrity was evaluated in sham and full callosotomy groups using BOLD and electrophysiology. Functional connectivity analysis indicated a similar significant reduction in bilateral connectivity in the full callosotomy group as compared to the sham group across both recording modalities. Spatiotemporal dynamic analysis revealed bilaterally symmetric propagating waves of activity in the sham data, but none were present in the full callosotomy data; however, the emergence of unilateral spatiotemporal patterns became prominent following the callosotomy. This finding suggests that the corpus callosum could be largely responsible for maintaining bilateral network integrity, but non-bilaterally symmetric propagating waves occur in the absence of the corpus callosum, suggesting a possible subcortical driver of the dynamic cascading event. This work represents a robust finding indicating the corpus callosum's influence on maintaining integrity in bilateral functional networks.
6
Marsh, Richard. "Superconducting phase coherent electron transport in nano-engineered ferromagnetic vortices." Thesis, Royal Holloway, University of London, 2013. http://repository.royalholloway.ac.uk/items/66b15a59-0b9f-4e68-b0aa-47e93f77349a/6/.
Full textAbstract:
This thesis presents an experimental study of the superconducting proximity effect in sub-micrometer sized ferromagnetic discs. Such discs belong to a class of mesoscopic ferromagnets intermediate between microscopic magnets with dimensions below about 10nm that behave as single giant spins and macroscopic structures that are larger than approximately 1 micrometer where domains are formed to minimise stray fields. The magnetic structure of mesoscopic magnets is strongly dependent on their geometric shape, allowing for purposeful engineering of magnetic structures using modern lithographic techniques. The ground magnetic state of mesoscopic ferromagnetic discs is the magnetic vortex where unusual time-asymmetric triplet superconductivity is predicted to exist and survive up to the non-magnetic coherence length, that is orders in magnitude larger than the ferromagnetic singlet coherence length. Magnetic Force Microscopy (MFM) was used to directly study the magnetic structure of the discs. To detect the proximity effect in the vortices, Andreev interferometers were used with normal parts replaced with mesoscopic ferromagnetic discs in the magnetic vortex state. The samples were fabricated using electron-beam lithography and a modified shadow evaporation technique developed within this project, allowing the whole structure to be made with highly precise alignment, without breaking vacuum and avoiding redundant ferromagnetic elements disturbing the magnetic vortices. Observations were made of superconducting phase periodic oscillations in the conductance of the Andreev interferometers. Such oscillations provide unambiguous evidence of phase coherent electron transport through the ferromagnetic vortex. Finally, further experiments are discussed that would provide a more detailed understanding of the long range proximity effect in SFS junctions.
7
Grinberg, Patricio. "Slow light in two dimensional semi-conductor photonic crystals." Phd thesis, Université Paris Sud - Paris XI, 2012. http://tel.archives-ouvertes.fr/tel-00830962.
Full textAbstract:
We report on the combination of slow light propagation with the resonance properties of a photonic crystal (PhC) cavity and with the slow mode of a PhC waveguide. We demonstrate theoretically and experimentally that slow light induced by the Coherent Population Oscillation (CPO) effect enables to have small-size and ultrahigh quality (Q) factor cavity, regardless of the technological and design issues. The experimental proof is performed in a L3 2D PhC cavity with semiconductor quantum wells as active, medium in which the CPO effect is induced. We achieve a cavity Q-factor of 520000, which corresponds to an enhancement by a factor 138 in comparison with the original Q-factor of the cavity. We present a theoretical approach to the combination of CPO-based slow light and slow mode in PhC waveguides, showing that the total group index is a multiplication of the group indices associated respectively to the CPO slow light and to the waveguide slow mode. We also set the basis for the experimental demonstration by designing and fabricating samples in the clean room facilities of LPN and addressing the challenging issue of coupling and extracting light in and from the waveguides. A particular design of the PhC in the waveguide is issued as a grating that allows to couple light perpendicularly to the plane of the PhC from free space. The vertical coupler has also been designed and fabricated along the waveguide and has been experimentally characterized. Slow light based on CPO effect in the PhC waveguides is always under experimental investigation.
8
Delagnes, Jean-Christophe. "Contrôle de la propagation d'impulsions ultracourtes : effets de déplacements lumineux." Toulouse 3, 2005. http://www.theses.fr/2005TOU30227.
Full textAbstract:
Cette thèse présente l'étude théorique et expérimentale du contrôle des phénomènes de propagation cohérente d'impulsions ultracourtes dans un milieu résonant optiquement dense. Dans une première partie, nous décrivons les phénomènes élémentaires des effets de propagation. Dans un second temps, nous développons l'idée de contrôler par un champ fort, les propriétés transitoires d'une autre impulsion faible et résonante qui se propage simultanément dans le milieu. Le champ fort induit des modifications transitoires dans le milieu, qui modifient par rayonnement le champ de l'impulsion résonante qui se propage. Nous étudions enfin une configuration où les deux impulsions, polarisées orthogonalement, excitent de manière résonante un système à quatre niveaux dégénérés. Deux chemins interfèrent modulant ainsi l'énergie de l'impulsion transmise. La combinaison des déplacements lumineux et de ces interférences, permet de contrôler aussi bien le gain que la forme temporelle de l'impulsionThe topic of this work deals with theoretical and experimental study of the control of ultrashort pulse coherent propagation in optically dense medium. First, we describe the basics of propagation phenomena. Secondly, we study the idea of using a strong driving pulse, to control the transient properties of a weak resonant pulse simultaneously propagating in the medium. The strong field induces transient modifications in the medium, which modify the electric field of the weak resonant propagating pulse. Finally, we study a configuration where two orthogonally polarized pulses, excite resonantly a four level system degenerated two by two. With the strong field mixing the states, the emission and absorption path of the weak field have similar contribution. The two paths interfere thus modulating the transmitted pulse energy. The interplay of the light shift and the interference enables us to control the gain and the pulse temporal shape as well
9
Witham, Claire. "Oscillations and corticomuscular coherence in the sensorimotor system." Thesis, University of Newcastle Upon Tyne, 2007. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.440587.
Full text
10
Titimbo, Chaparro Kelvin Ruben. "CREATION AND DETECTION OF SQUEEZED PHONONS IN PUMP AND PROBE EXPERIMENTS: A FULLY QUANTUM TREATMENT." Doctoral thesis, Università degli studi di Trieste, 2015. http://hdl.handle.net/10077/10941.
Full textAbstract:
2013/2014Femtosecond pump and probe techniques are standard experimental methodologies used for studying ultrafast dynamics in solids, in particular phonon oscillations in target materials. So far, only semiclassical methods have been employed in order to theoretically interpret the experimental data. In contrast, a fully quantum treatment will be presented here taking into account the quantum features of the generation mechanism of excited phonons by pump laser pulses, and of the process of accessing their behaviour by probe laser pulses. A single effective Hamiltonian will be used to model the interaction between photons and phonons both for the pumping and probing processes. In addition, as they interact with their environment, mainly electrons in the target, the excited phonons cannot be considered an isolated system. Their dynamics is then that typical of open quantum systems and generated by a master equation of Lindblad form, that takes into account the dissipative and noisy effects due to the environment.In this formalism, phonon oscillations can be analysed through suitable probe photon observables. Specifically, unlike in the existing literature, we will not focus only upon the scattered probe pulse intensity, namely on the probe photon number, but also on the number variance. Through the latter some quantum features of the phonon state can be accessed; in particular, specific signals of the presence of squeezed phonons can thus be identified.
Le tecniche di "pump and probe" impulsato sono metodologie sperimentali standard usate nello studio delle dinamiche ultraveloci nei solidi, in particolare delle oscillazioni di fononi. Usualmente l'interpretazione teorica dei dati sperimentali si basa su approssimazioni semiclassiche. Una descrizione completamente quantistica e` invece sviluppata nella presente trattazione: e` basata sull'introduzione di un'unica hamiltoniana di interazione tra fotoni e fononi, capace di descrivere in modo effettivo sia il processo di eccitazione che di rivelazione dei fononi. In generale, tali fononi non possono essere considerati come isolati, ma costituiscono un sistema quantistico aperto, cioe` in interazione debole con l'ambiente esterno, formato principalmente da elettroni e dagli altri costituenti del materiale in studio. La loro dinamica deve percio` venir descritta tramite una equazione master, che tenga conto di effetti di rumore e dissipazione. In questo formalismo, le proprieta` dei fononi eccitati dagli impulsi laser di "pump" possono essere analizzate attraverso lo studio di opportune osservabili caratterizzanti i fotoni di "probe". Piu` specificatamente, si e` analizzato il comportamento non solo dell'intensita` media della luce di "probe" riflessa, cioe` del numero medio di fotoni, ma anche della relativa varianza. In questo modo, si possono evidenziare alcune caratteristiche quantistiche dei fononi: in particolare, sono stati individuati segnali specifici della presenza di fononi "squeezed"
XXVII Ciclo
1986
11
Gundry, Sarah. "Implementation of an ultra-broadband high power frequency modulator based on coherent molecular oscillation." Thesis, Imperial College London, 2005. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.413552.
Full text
12
Perrier, Pierre. "Fluctuations universelles de conductance dans des systèmes mésoscopiques magnétiques." Grenoble 1, 1991. http://www.theses.fr/1991GRE10169.
Full textAbstract:
Cette these presente l'etude experimentale de certaines proprietes de fluctuations universelles de conductance dans des systemes mesoscopiques. Nous decrivons tout d'abord le transport coherent qui est l'approche du transport electronique, permettant de rendre compte du phenomene que sont les fluctuations universelles de conductance. Nous nous attacherons ensuite a montrer les differentes methodes experimentales mettant en evidence les proprietes de ces fluctuations (universalite, dependance en temperature, en champ magnetique, effets non locaux, etc). Enfin, nous nous interesserons a l'etude de ces fluctuations dans des systemes mesoscopiques qui contiennent des impuretes dont certaines portent des moments magnetiques. Nous presenterons les mesures effectuees a la fois dans des systemes paramagnetiques et dans des systemes verres de spins. Pour interpreter les resultats obtenus, nous avons developpe un modele permettant de rendre compte de l'evolution de l'amplitude des fluctuations en fonction de la temperature, du champ magnetique et de la concentration des impuretes magnetiques. Dans le cas des verres de spins, notre modele nous a aussi permis de predire dans quelles conditions la conductance sera sensible a la dynamique lente des verres de spins (retournement de paquets de spins). Nous analyserons pour terminer les raisons probables qui n'ont pas permis l'observation dans de tels systemes des sauts de conductance attendus lors des retournements de paquets de spins
13
Berger, Perrine. "Lumière lente et rapide dans les amplificateurs optiques à semi-conducteurs pour des applications en optique micro-onde et aux RADAR." Thesis, Paris 11, 2012. http://www.theses.fr/2012PA112027/document.
Full textAbstract:
Les techniques permettant de maitriser la vitesse de la lumière, au-delà de l'intérêt scientifique qu'elles suscitent, peuvent être appliquées au domaine radar. Elles permettent, ainsi, de remplacer avantageusement les retards optiques, jusqu'alors réalisés par des modifications géométriques du chemin optique. L’objectif de la thèse est d’étudier la lumière lente et rapide créée par oscillations cohérentes de population dans les amplificateurs à semi-conducteurs.Nous avons évalué théoriquement et expérimentalement les performances d’une ligne à retards accordables, en termes d’amplitude des retards et déphasages accordables, et de bandes passantes. Nous avons aussi étudié l’impact des oscillations cohérentes de population sur les facteurs de mérite de la liaison opto-électronique. La compréhension des mécanismes physiques mis en jeu nous a amenés à proposer des solutions pour contourner les limites identifiées du composant. Nous avons montré qu’il était possible d’utiliser les lignes à retards accordables au delà de l’inverse du temps de vie des porteurs (500 MHz) en utilisant la montée en fréquence des oscillations cohérentes de population par modulation croisée de gain. Nous avons ainsi obtenu des retards accordables de 389 ps à 16 GHz, sur une bande passante instantanée de 360 MHz. Enfin nous avons proposé une architecture permettant d’obtenir des déphasages accordables proches de 180 degrés à haute fréquence, en substituant l’effet du couplage gain-indice, révélé par l’utilisation d’un filtre optique, par l’excitation paramétrique des oscillations cohérentes de population. Nous avons utilisé ce principe, qui permet par exemple d’atteindre un déphasage accordable de 162 degrés à 2,2 GHz, pour concevoir un oscillateur optoélectronique fonctionnant à 2,2 GHz. La fréquence de ce dernier est rapidement accordable sur 6 MHz à l’aide du courant d’injection de l’amplificateur à semi-conducteurSlow and fast light is becoming a wide research field driven by an extensive effort to implement this new technology in real applications. Coherent population oscillations in semiconductor optical amplifiers constitute one of the most promising approaches, in particular for the processing of optically carried microwave signals, which includes the control of tunable true time delays and RF phase shifts.We studied theoretically and experimentally the available tunable delays and phase shifts and the associated bandwidths for a microwave photonics link including a semiconductor optical amplifier. We analyzed the influence of the coherent population oscillations on the dynamic range of the link.The understanding of the underlying physical mechanisms led us to propose new architectures in order to overcome the identified limitations of the components. We show how up-converted coherent population oscillations enable to get rid of the intrinsic limitation of the carrier lifetime (500 MHz), leading to the generation of true time delays at any high frequencies in a single semiconductor device. We demonstrated tunable delays up to 389 ps at 16 GHz, with an instantaneous bandwidth of 360 MHz.Lastly we demonstrate how to conceive a RF phase shifter up to 180 degrees at high frequency by forced coherent population oscillations. This effect replaces the enhancement of the coherent population oscillations by gain-index coupling effect, revealed by an optical filter. We used this principle, which enables to achieve a tunable phase shift up to 162 degrees at 2,2 GHz, in order to conceive an optoelectronic oscillator at 2,2 GHz. The frequency of this oscillator is fast tunable over 6 MHz by changing the current of the semiconductor amplifier
14
Kirsten, Lars, Anke Burkhardt, Jonas Golde, Julia Walther, Thomas Stoppe, Matthias Bornitz, Max Kemper, Thomas Zahnert, and Edmund Koch. "Imaging the tympanic membrane oscillation ex vivo with Doppler optical coherence tomography during simulated Eustachian catarrh." SPIE, 2015. https://tud.qucosa.de/id/qucosa%3A35042.
Full textAbstract:
Recently, optical coherence tomography (OCT) was utilized in multiple studies for structural and functional imaging of the middle ear and the tympanic membrane. Since Doppler OCT allows both, the spatially resolved measurement of the tympanic membrane oscillation and high-resolution imaging, it is regarded as a promising tool for future in vivo applications. In this study, Doppler OCT is utilized for the visualization of the tympanic membrane oscillation in temporal bones with simulated Eustachian catarrh, which was realized by generating a depression in the tympanic cavity. The transfer function, meaning the oscillation amplitude normalized to the applied sound pressure, is measured frequency resolved in the range from 0.5 kHz to 6 kHz and with a lateral spatial resolution of 0.4 mm. Typical oscillation patterns could be observed in case of ambient pressure in the tympanic cavity. Under depression the characteristic oscillation patterns were observed with widely congruent appearance but at higher frequencies.
15
Haig, Albert Roland. "Missing Links the role of phase synchronous gamma oscillations in normal cognition and their dysfunction in schizophrenia." University of Sydney. Psychological Medicine, 2002. http://hdl.handle.net/2123/848.
Full textAbstract:
SUMMARY Introduction: There has recently been a great deal of interest in the role of synchronous high-frequency gamma oscillations in brain function. This interest has been motivated by an increasing body of evidence, that oscillations which are synchronous in phase across separated neuronal populations, may represent an important mechanism by which the brain binds or integrates spatially distributed processing activity which is related to the same object. Many models of schizophrenia suggest an impairment in the integration of brain processing, such as a loosening of associations, disconnection, defective multiple constraint organization, or cognitive dysmetria. This has led to recent speculation that abnormalities of high-frequency gamma synchronization may reflect a core dimension of the disturbance underlying this disorder. However, examination of the phase synchronization of gamma oscillations in patients with schizophrenia has never been previously undertaken. Method: In this thesis a new method of analysis of gamma synchrony was introduced, which enables the phase relationships of oscillations in a specific frequency band to be examined across multiple scalp sites as a function of time. This enabled, for the first time, the phase synchronization of gamma oscillations across widespread regions, to be studied in electrical brain activity measured at the scalp in humans. Gamma synchrony responses were studied in electroencephalographic (EEG) data acquired during a commonly employed conventional auditory oddball paradigm. The research consisted of two sets of experiments. In the first set of experiments, data from 100 normal subjects, consisting of 10 males and 10 females in each age decade from 20 to 70, was examined. These experiments were designed to characterize the gamma synchonizations that occurred in response to target and background stimuli and their functional significance in normal brain activity, and to exclude the possibility of these findings being due to electromyogram (EMG) or volume conduction artifact. The examination of functional significance involved the development of an additional new analysis technique. In the second set of experiments, data acquired from 35 patients with schizophrenia and 35 matched normal controls was analyzed. The purpose of these experiments was to determine whether patients showed disturbances of gamma synchrony compared to controls, and to establish the relationship of any such disturbances to medication levels, symptom profiles, duration of illness, and a range of psychophysiological variables. Results: In the 100 normals, responses to target stimuli were characterized by two bursts of synchronous gamma oscillations, an early (evoked) and a late (induced) synchronization, with different topographic distributions. Only the early gamma synchronization was seen in response to background stimuli. The main variable modulating the magnitude of these gamma synchronizations from epoch to epoch was pre-stimulus EEG theta (3-7 Hz) and delta (1-3 Hz) power. Early and late gamma synchrony were also associated with N1 and P3 ERP component amplitude across epochs. Across subjects, the early gamma synchronization was associated with shorter latency of the ERP components P2, N2 and P3, smaller amplitude of N1 and P2, and smaller pre-stimulus beta power. The control analyses showed that these gamma responses were specific to a narrow frequency range (37 to 41 Hz), and were not present in adjacent frequency bands. The responses were not generated by EMG contamination or volume conduction. In the 35 patients with schizophrenia, significant abnormalities of both the early and late synchronizations were observed compared to the 35 normal controls, with distinctive topographic characteristics. In general, early gamma synchrony was increased in patients compared to controls, and late gamma synchrony was decreased. These gamma synchrony disturbances were not related to medication level or the four summed symptom profile scores (positive, negative, general and total). They were, however, associated with duration of illness, becoming less severe the longer the patient had suffered from the disorder. The disordered gamma synchrony in patients was not secondary to abnormalities in other psychophysiological variables, but appeared to represent a primary disturbance. Discussion: The early synchronization may relate to the binding of object representations in early sensory processing, or, given that a constant inter-stimulus interval was employed, may be anticipatory and related to active memory. The late response is probably involved in binding in relation to activation of the internal contextual model involved in late expectancy/contextual processing (context updating or context closure) for target stimuli. The across epochs effects may relate to whether the focus of attention immediately prior to stimulus presentation is internal or is directed at the task. The across subjects effects suggest that a larger magnitude of the early gamma synchronization might indicate that the subject maintains a more stable and less ambiguous internal representation of the environment, that reduces the complexity of input and facilitates target/background discrimination and subsequent processing. The early gamma synchronization findings in patients with schizophrenia suggest that anticipatory processing involving active memory and forward-prediction of the environment is subject to over-binding or the formation of inappropriate associations. The late synchronization disturbances may reflect a fragmentation of contextual processing, and an inability to maintain contextual models of the environment intact over time. Conclusion: This research demonstrates the potential importance of integrative network activity as indexed by gamma phase synchrony in relation to normal cognition, and the possible broad relevance of such activity in psychiatric disorders. In particular, the application in this study to patients with schizophrenia showed that an impairment of brain integrative activity (missing links) might be a key feature of this illness.
16
Supe, Kristin Edwards. "Role of Rat Neuronal Oscillations in Acquisition and Disruption of Working Memory with Acute Ethanol." The Ohio State University, 2014. http://rave.ohiolink.edu/etdc/view?acc_num=osu1404732893.
Full text
17
Wang, Yunli. "Etude expérimentale et numérique des oscillations hydrodynamiques en milieux poreux partiellement saturés." Thesis, Toulouse, INPT, 2010. http://www.theses.fr/2010INPT0127/document.
Full textAbstract:
Cette thèse vise à étudier expérimentalement, analytiquement et numériquement, les conséquences de variations et d'oscillations hydrodynamiques à forte variabilité temporelle en milieux poreux partiellement saturés. Les problèmes que nous étudions comportent des surfaces libres tant à l'extérieur qu'à l'intérieur des milieux poreux, celles-ci étant définies comme des isosurfaces de pression d'eau égale à la pression atmosphérique (Pwater = Patm). Les différentes études expérimentales réalisées en laboratoire sont, respectivement : une expérience d'imbibition dans une boite à sable avec effets capillaires importants; la transmission d'oscillations de la surface libre à travers un massif sableux intercalaire dans un petit canal à houle (IMFT, Toulouse); l'étude de la dynamique et de la propagation des oscillations des niveaux d'eau dans un grand canal à houle (HYDRALAB, Barcelone), partiellement recouvert d'un fond sableux incliné, avec mesures de niveaux d'eau en pleine eau et sous le sable, et mesures du fond sableux (érosion/dépôts). Pour les études théoriques, nous avons développés des solutions analytiques linéarisées. Un exemple de problème traité analytiquement est: l'équation linéarisée de Dupuit-Boussinesq (D-B) transitoire à surface libre, en hypothèse d'écoulements plans et vidange/remplissage instantané : oscillations forcées, transmission et dissipation d'ondes à travers une boite à sable rectangulaire. Nous avons aussi développé une solution de l'équation faiblement non linéaire de Dupuit- Boussinesq (D-B) pour étudier le problème d'imbibition avec variation abrupte du niveau d'eau amont (suivi temporel du front de saturation). Nous avons pu étudier les différents types de problèmes transitoires liés aux expériences citées plus haut par simulation numérique. En particulier, nous avons simulé des écoulements partiellement saturés et insaturés, en coupe verticale, à l'aide d'un code de calcul (BIGFLOW 3D) qui résoud l'équation de Richards généralisée en régime transitoire. Nous avons ainsi étudié numériquement en régime non saturé, l'expérience d'imbibition dans un sable initialement sec à frontières verticales (IMFT sandbox), puis l'expérience de propagation d'ondes dans le grand canal à houle de Barcelone (laboratoire HYDRALAB) comportant une plage de sable inclinée, avec un couplage complètement intégré entre les zones micro-poreuse (sable) et “macro-poreuse” (pleine eau). Pour analyser les résultats de cette dernière expérience et les comparer aux simulations, nous avons utilisé plusieurs méthodes de traitement et d'analyse des signaux : analyse de Fourier (spectres de fréquences) ; ondelettes discrètes multi-résolution (Daubechies) ; analyses corrélatoires simple et croisée. Ces méthodes sont combinées avec des méthodes de préfiltrage pour estimer dérives et résidus (moyennes mobiles ; ondelettes multi-résolution). Cette analyse des signaux a permis de comprendre et quantifier la propagation à travers une plage de sable. Au total, les différentes approches de modélisation mis en oeuvre, associé à des procédures de calage en situation de couplage transitoire non linéaire ont permis de reproduire globalement les phénomènes de propagation de teneur en eau et de niveau d'eau dans les différentes configurations étudiéesThis thesis aims at investigating experimentally, analytically and numerically, the consequences of hydrodynamic variations and oscillations with high temporal variability in partially saturated porous media. The problems investigated in this work involve “free surfaces” both outside and inside the porous media, the free surface being defined as the “atmospheric” water pressure isosurface (Pwater = Patm). The laboratory experiments studied in this work are, respectively: Lateral imbibition in a dry sand box with significant capillary effects; Transmission of oscillations of the free surface through a vertical sand box placed in a small wave canal (IMFT, Toulouse); Dynamics of free surface oscillations and wave propagation in a large wave canal (HYDRALAB, Barcelona), partially covered with sand, with measurements of both open water and groundwater levels, and of sand topography (erosion / deposition). For theoretical studies, we have developed linearized analytical solutions. Here is a sample problem that was treated analytically in this work: The linearized equation of Dupuit-Boussinesq (DB) for transient free surface flow, assuming horizontal flow and instantaneous wetting/drainage of the unsaturated zone: forced oscillations, wave transmission and dissipation through a rectangular sandbox. We also developed a weakly nonlinear solution of the Dupuit-Boussinesq equation to study the sudden imbibition (temporal monitoring of the wetting front). We have studied the different types of transient flow problems related to the experiments cited above by numerical simulation. In particular, we have simulated unsaturated or partially saturated transient flows in vertical cross-section, using a computer code (BIGFLOW 3D) which solves a generalized version of Richards’ equation. Thus, using the Richards / BIGFLOW 3D model, we have studied numerically the experiment of unsaturated imbibition in a dry sand (IMFT sandbox), and then, with the same model, we have also studied the partially saturated wave propagation experiment in the large Barcelona wave canal (HYDRALAB laboratory), focusing on the sloping sandy beach, with coupling between the micro-porous zone (sand) and the “macro-porous” zone (open water). To interpret the results of the latter experiment and compare them to simulations, we use several methods of signal analyzis and signal processing, such as: Fourier analysis, discrete multi-resolution wavelets (Daubechies), auto and cross-correlation functions. These methods are combined with pre-filtering methods to estimate trends and residuals (moving averages; discrete wavelet analyses). This signal analyzis has allowed us to interpret and quantify water propagation phenomena through a sandy beach. To sum up, different modeling approaches, combined with model calibration procedures, were applied to transient nonlinear coupled flow problems. These approaches have allowed us to reproduce globally the water content distributions and water level propagation in the different configurations studied in this work
18
Blomberg, Rina. "CORTICAL PHASE SYNCHRONISATION MEDIATES NATURAL FACE-SPEECH PERCEPTION." Thesis, Linköpings universitet, Institutionen för datavetenskap, 2015. http://urn.kb.se/resolve?urn=urn:nbn:se:liu:diva-122825.
Full textAbstract:
It is a challenging task for researchers to determine how the brain solves multisensory perception, and the neural mechanisms involved remain subject to theoretical conjecture. According to a hypothesised cortical model for natural audiovisual stimulation, phase synchronised communications between participating brain regions play a mechanistic role in natural audiovisual perception. The purpose of this study was to test the hypothesis by investigating oscillatory dynamics from ongoing EEG recordings whilst participants passively viewed ecologically realistic face-speech interactions in film. Lagged-phase synchronisation measures were computed for conditions of eye-closed rest (REST), speech-only (auditory-only, A), face-only (visual-only, V) and face-speech (audio-visual, AV) stimulation. Statistical contrasts examined AV > REST, AV > A, AV > V and AV-REST > sum(A,V)-REST effects. Results indicated that cross-communications between the frontal lobes, intraparietal associative areas and primary auditory and occipital cortices are specifically enhanced during natural face-speech perception and that phase synchronisation mediates the functional exchange of information associated with face-speech processing between both sensory and associative regions in both hemispheres. Furthermore, phase synchronisation between cortical regions was modulated in parallel within multiple frequency bands.
19
Vahidnia, Arash. "Wide area control through aggregation of power systems." Thesis, Queensland University of Technology, 2013. https://eprints.qut.edu.au/61186/1/Arash_Vahidnia_Thesis.pdf.
Full textAbstract:
This thesis was a step forward in improving the stability of power systems by applying new control and modelling techniques. The developed methods use the data obtained from voltage angle measurement devices which are synchronized with GPS signals to stabilize the system and avoid system-wide blackouts in the event of severe faults.
New approaches were developed in this research for identifying and estimating reduced dynamic system models using phasor measurement units. The main goal of this research is achieved by integrating the developed methods to obtain a feasible wide-area control system for stabilizing the power systems.
20
Huang, Wenyu. "Fundamental studies of the interaction between femtosecond laser and patterned monolayer plasmonic nanostructures." Diss., Atlanta, Ga. : Georgia Institute of Technology, 2007. http://hdl.handle.net/1853/24786.
Full textAbstract:
Thesis (Ph. D.)--Chemistry and Biochemistry, Georgia Institute of Technology, 2008.Committee Chair: El-Sayed, Mostafa A.; Committee Member: Perry, Joseph W.; Committee Member: Srinivasarao, Mohan; Committee Member: Whetten, Robert L.; Committee Member: Zhang, Z. John.
21
Karpov, Ivan. "Damping of Coherent Oscillations in Intense Ion Beams." Phd thesis, 2017. https://tuprints.ulb.tu-darmstadt.de/5983/1/Karpov_thesis.pdf.
Full textAbstract:
Transverse decoherence of a displaced ion bunch is an important phenomenon in synchrotrons and storage rings. An offset can be caused by an injection error after the bunch-to-bucket transfer between synchrotrons or by an externally generated kick. Decoherence results in a transverse emittance blowup, which can cause particle losses
and a beam quality degradation. To prevent the beam blowup, a transverse feedback system (TFS) can be used. The damping time should be shorter than the characteristic decoherence time, which can be strongly affected by the interplay of different intensity effects (e.g., space charge and impedances).
This thesis describes the development of the analytical models that explain decoherence and emittance growth with chromaticity, space charge, and image charges within the first synchrotron period. The pulsed response function including intensity effects was derived from the model for beam transfer functions. For a coasting beam, the two-
dimensional model shows that space charge slows down and above intensity threshold suppresses decoherence. These predictions were confirmed by particle tracking simulations with self-consistent space charge fields. Additionally, halo buildup and losses during decoherence were observed in simulations. These effects were successfully interpreted using a non self-consistent particle-core model.
The two-dimensional model was extended to the bunched beams. The simulation results reproduce the analytical predictions. The intensity threshold of decoherence suppression is higher in comparison to a coasting beam, image charges can restore decoherence.
In the present work dedicated experiments were performed in the SIS18 synchrotron at GSI Darmstadt and the results were compared with simulations and analytical predictions. The contribution of nonlinearities and image charges is negligible while chromaticity and space charge dominate decoherence.
To study the damping efficiency of TFS, a comprehensive TFS module was developed for simulations. The system bandwidth should cover the characteristic spectrum including intensity effects. Delay errors, which can cause a beam instability, and the level of noise, which results in an emittance blowup, were determined.
22
Barannyk, Oleksandr. "Shear layer instabilities and flow-acoustic coupling in valves: application to power plant components and cardiovascular devices." Thesis, 2014. http://hdl.handle.net/1828/5372.
Full textAbstract:
In the first part of this dissertation, the phenomenon of self-sustained pressure os-cillations due to the flow past a circular, axisymmetric cavity, associated with inline gate valves, was investigated. In many engineering applications, such as flows through open gate valves, there exists potential for coupling between the vortex shedding from the up-stream edge of the cavity and a diametral mode of the acoustic pressure fluctuations. The effects of the internal pipe geometry immediately upstream and downstream of the shal-low cavity on the characteristics of partially trapped diametral acoustic modes were in-vestigated numerically and experimentally on a scaled model of a gate valve mounted in a pipeline that contained convergence-divergence sections in the vicinity of the valve. The resonant response of the system corresponded to the second acoustic diametral mode of the cavity. Excitation of the dominant acoustic mode was accompanied by pressure oscillations, and, in addition to that, as the angle of the converging-diverging section of the main pipeline in the vicinity of the cavity increased, the trapped behavior of the acoustic diametral modes diminished, and additional antinodes of the acoustic pressure wave were observed in the main pipeline.
In addition to that, the effect of shallow chamfers, introduced at the upstream and/or downstream cavity edges, was investigated in the experimental system that con-tained a deep, circular, axisymmetric cavity. Through the measurements of unsteady pressure and associated acoustic mode shapes, which were calculated numerically for several representative cases of the internal cavity geometry, it was possible to identify the configuration that corresponded to the most efficient noise suppression. This arrangement also allowed calculation of the azimuthal orientation of the acoustic modes, which were classified as stationary, partially spinning or spinning. Introduction of shallow chamfers at the upstream and the downstream edges of the cavity resulted in changes of azimuthal orientation and spinning behaviour of the acoustic modes. In addition, introduction of splitter plates in the cavity led to pronounced change in the spatial orientation and the spinning behaviour of the acoustic modes. The short splitter plates changed the behaviour of the dominant acoustic modes from partially spinning to stationary, while the long split-ter plates enforced the stationary behaviour across all resonant acoustic modes.
Finally, the evolution of fully turbulent, acoustically coupled shear layers that form across deep, axisymmetric cavities and the effects of geometric modifications of the cavity edges on the separated flow structure were investigated using digital particle image velocimetry (PIV). Instantaneous, time- and phase-averaged patterns of vorticity pro-vided insight into the flow physics during flow tone generation and noise suppression by the geometric modifications. In particular, the first mode of the shear layer oscillations was significantly affected by shallow chamfers located at the upstream and, to a lesser degree, the downstream edges of the cavity.
In the second part of the dissertation, the performance of aortic heart valve pros-thesis was assessed in geometries of the aortic root associated with certain types of valve diseases, such as aortic valve stenosis and aortic valve insufficiency. The control case that corresponds to the aortic root of a patient without valve disease was used as a reference. By varying the aortic root geometry, it was possible to investigate corresponding changes in the levels of Reynolds shear stress and establish the possibility of platelet activation and, as a result of that, the formation of blood clots.Graduate
0541
0546
0548
0986
alexbn024@gmail.com
23
Feist, Armin. "Next-Generation Ultrafast Transmission Electron Microscopy – Development and Applications." Thesis, 2018. http://hdl.handle.net/11858/00-1735-0000-002E-E48B-B.
Full text
24
Kim, Jisun Ph D. "Study of quantum thin films : phase relationship, surface reactivity, and coherent coupling." Thesis, 2011. http://hdl.handle.net/2152/ETD-UT-2011-08-3842.
Full textAbstract:
When an electronic system is confined in one or more dimensions to a length scale comparable to the de Broglie wavelength, quantum confinement occurs. In metallic quantum thin films grown on semiconductor substrates, such confinement occurs between the vacuum-solid and the solid-solid interfaces, which results in the formation of distinctive quantum well states (QWS). Due to this confinement, many physical phenomena occurring in the thin metal system are totally different from the bulk system, which makes the study of quantum thin films interesting and important. In this thesis, quantum thin film studies, mainly based on the Pb/Si(111) system, were performed utilizing low-temperature scanning tunneling microscopy/spectroscopy (STM/STS) with a focus on three main aspects: phase relationship, surface reactivity, and coherent coupling. The Pb/Si(111) system is chosen due to its unique phase matching between the Fermi wavelength and the lattice spacing along [111], leading to a bi-layer quantum oscillation in many physical properties, including the surface energy and the work function. Surprisingly, STM/STS measurement revealed that quantum oscillations of work function and surface energy have identical phase, in contrast to a theoretically predicted 1/4 wavelength phase shift in the phase relationship. Here, a new solution to this puzzle is provided. Furthermore, it is found out that the oxidation rate of Pb/Si(111) system is greatly enhanced in the presence of atomic scale catalyst -- Cs substitutional atoms, while the reactivity to CO is saturated after the initial enhanced nucleation. Finally, by inserting thin Ag layers in between Pb/Si(111) system, the coherent coupling of double quantum wells (a Pb quantum well and a Ag quantum well) are probed, where combined QWS features are observed by STS measurement. The growth mechanism of these heterostructures -- Pb/Ag/Si(111) -- is also investigated.text
25
Shen, Yu-Kai, and 沈昱凱. "Investigation of ultrafast carrier dynamics and coherent acoustic phonons oscillation in ZnO by the transient pump-probe technique." Thesis, 2013. http://ndltd.ncl.edu.tw/handle/w2t8sp.
Full textAbstract:
碩士國立臺北科技大學
光電工程系研究所
101
In this thesis, we investigated the ultrafast carrier dynamics and the coherent acoustic phonons (CAPs) in high quality epitaxial ZnO thin films on c-Si substrate by the degenerate transient reflection pump-probe technique, using a frequency doubling femtosecond Ti:sapphire laser as the light source. In order to success grow the ZnO on the silicon substrate, yttrium-doped hafnium oxide (YDH) is used as a buffer layer. The reflection and photoluminescence spectrum of the sample reveal two emission peaks at 378 and 385 nm (3.28 and 3.22 eV) that corresponds to the free exciton emission and LO-phonon replica. In pump-probe measurement, the central wavelength of the UV pulse is tuned from 355 to 385 nm corresponding to the conduction band to shallow band-tail (3.49-3.22eV) of this sample. As excited photon energy above conduction band (355-369 nm, 3.49-3.36 eV), the measured positive transient differential reflectance change (TDRC) is due to the band filling effect. The free carrier relaxations include two exponential decays, the fast decay is non-radiation recombination time, and the slow decay is due to radiation recombination time. In addition, a relative fast decay time below 1-ps around exciton resonance is due to exciton-exciton scattering (375-379 nm, 3.31-3.27 eV). When the photo-excited energy is located at shallow band-tail (380-385 nm, 3.26-3.22 eV), the TDRC transfer from positive to negative is due to the resonance. The subsequent coherent acoustic phonons (CAPs) oscillation is superimposed on the TDRC within 200 ps, with the order of amplitude around 10−4, and the oscillation frequency around 80 GHz. It is recognized that the CAPs is mainly due to electric stress of deformation potential coupling. From the oscillation frequency of CAPs, the refractive index of ZnO is obtained, which is similar with the estimated value from Sellmeier equation. The velocity of CAPs around 6089 m/s propagation in ZnO can be obtained, from the plot of the angular frequency versus wave vector. The obtained velocity is close to the theory calculation of sound wave propagation velocity in ZnO.
26
Riese, Michael. "Study of large-scale coherent structures in the near field and transition regions of a mechanically oscillated planar jet." 2009. http://hdl.handle.net/2440/54150.
Full textAbstract:
Enhancing the performance of mixing and fluid entrainment by excitation of quasi-steady jets has been a subject of research for more than three decades. During the 1980s a special emphasis was placed on mechanically oscillating planar jets and the possibility to augment thrust of V/STOL aircraft. However, during this time, little attention was paid to the classification of flow regimes, the development of coherent structures or the existence of different regions in the flow within the jet near field. For the present study, a large aspect ratio nozzle was oscillated in the direction transverse to the width of the nozzle in simple harmonic motion. For a constant nozzle height, the stroke length, oscillation frequency and jet velocity were systematically varied. Over 240 flow cases were examined using a novel method of phase-locked flow visualisation. Following an initial analysis of the acquired data, a small subset of flow conditions was selected for further quantitative investigation using Particle Image Velocimetry (PIV). The phase-locked flow visualisation led to the identification and classification of three separate flow regimes, the Base Flow, the Resonant Flow and the Bifurcation Flow Regimes. Each regime is linked to the other regimes by the presence of a small number of repetitive coherent structures in the form of starting and stopping vortices. The analysis revealed a relationship between the stroke-to-nozzle height ratio and the ratio of the forcing frequency to the natural vortex shedding frequency in the planar jet. This directly contradicts the relationship between the Strouhal and Reynolds numbers of the jet that was proposed by previous investigators. Comparison of phase-locked PIV and flow visualisation data confirms both, the validity of the new regime classification and the identification of relevant large-scale structures. Time-averaged vorticity data are also used to further illustrate the differences between the three flow regimes. Investigation of the time-averaged qualitative data for the Base and Resonant Flow Regimes show that three distinct flow regions exist within both regimes. Adjacent to the nozzle is the initial formation region, where all large-scale structures form. This is followed by a coherent near-field region in which the jet exhibits very little spread for both the Base and Resonant Flow Regimes. Within this region no pairing of the large-scale vortices from the opposing sides of the flow can be found. This region is followed by a transition region that is marked by the sudden breakup and dissipation of all visible large-scale coherent structures. The vortex formation distance is then investigated using the available PIV data and compared with the results of previous investigations. The data show that the formation distance depends on the jet velocity, oscillation frequency and the stroke length. The agreement with previous data is poor due to differences in the method of measurement. Quantitative data are also used to investigate the centreline velocity decay in relation to changes of the jet Reynolds number and stroke-to-nozzle height ratio. The results show that the velocity decay rate increases with increasing stroke length as is expected from findings of earlier studies. In addition the centreline velocity decay rates in the mean jet transition region appear to be constant for each stroke length in the cases examined. Finally, conclusions are drawn and recommendations for future work are presented.http://proxy.library.adelaide.edu.au/login?url= http://library.adelaide.edu.au/cgi-bin/Pwebrecon.cgi?BBID=1349701
Thesis (Ph.D.) -- University of Adelaide, School of Mechanical Engineering, 2009
27
Cannon, Jonathan. "On the interaction of gamma-rhythmic neuronal populations." Thesis, 2014. https://hdl.handle.net/2144/15290.
Full textAbstract:
Local gamma-band (~30-100Hz) oscillations in the brain, produced by feedback inhibition on a characteristic timescale, appear in multiple areas of the brain and are associated with a wide range of cognitive functions. Some regions producing gamma also receive gamma-rhythmic input, and the interaction and coordination of these rhythms has been hypothesized to serve various functional roles. This thesis consists of three stand-alone chapters, each of which considers the response of a gamma-rhythmic neuronal circuit to input in an analytical framework. In the first, we demonstrate that several related models of a gamma-generating circuit under periodic forcing are asymptotically drawn onto an attracting invariant torus due to the convergence of inhibition trajectories at spikes and the convergence of voltage trajectories during sustained inhibition, and therefore display a restricted range of dynamics. In the second, we show that a model of a gamma-generating circuit under forcing by square pulses cannot maintain multiple stably phase-locked solutions. In the third, we show that a separation of time scales of membrane potential dynamics and synaptic decay causes the gamma model to phase align its spiking such that periodic forcing pulses arrive under minimal inhibition. When two of these models are mutually coupled, the same effect causes excitatory pulses from the faster oscillator to arrive at the slower under minimal inhibition, while pulses from the slower to the faster arrive under maximal inhibition. We also show that such a time scale separation allows the model to respond sensitively to input pulse coherence to an extent that is not possible for a simple one-dimensional oscillator. We draw on a wide range of mathematical tools and structures including return maps, saltation matrices, contraction methods, phase response formalism, and singular perturbation theory in order to show that the neuronal mechanism of gamma oscillations is uniquely suited to reliably phase lock across brain regions and facilitate the selective transmission of information.
28
Yang, Shang-Lin, and 楊尚霖. "Laser spiking and relaxation oscillation in optical vortex formed by the coherent superposition of off-axis multiple pass transverse modes in an azimuthal symmetry-breaking laser resonator." Thesis, 2018. http://ndltd.ncl.edu.tw/handle/h2jnfn.
Full textAbstract:
碩士國立中山大學
光電工程學系研究所
106
Optical vortex (OV) is a special morphological beam with orbital angular momentum. It has a ring shape with no energy at the center. In recent years, there have been many applications associated with optical vortex beam, such as STED microscopy, OAM multiplexing optical communication, optical tweezers and the like. Upon the demand of the applications, high quality optical vortex is necessary and there are many different methods to achieve this goal, such as the pump beam shaping, fiber-based amplifier with mode conversion, and degenerate cavity with intra-cavity spiral phase plate (SPP). In particular, a nearly hemi-spherical cavity with SPP produces OV with ultrahigh vortex purity of more than 99.99%. In such a laser configuration, the OV is formed as a coherent superposition of off-axis multiple pass transverse modes circulating in the resonator. They retrace a periodic orbital when travels twice roundtrips and form a V-shaped optical path. The phases of the beamlets traveling a MPT modes are then correlated by the strong coupling during the stimulated emission process in the laser gain medium and can be understood by the Kuromoto model under the minimum energy criterion. In this new laser configuration, built up dynamics has never been explored. Multi-mode relaxation oscillations in lasers can be modeled by Tang-Statz-deMars (TSD) equation. It is found that modes with unbalanced gain volume results in irregular relaxation oscillation for each mode while the intensity sum of all modes remains regular. However, the relaxation dynamics in our optical vortex produces irregular but synchronized spiking and relaxation oscillation across the spatial measurement. By introducing coupling terms between modes in to the photon equations in TSD equations, the spiking and relaxation synchronizations among modes can be explained. Experiments with a varying gain volume and coupling strength showed degradation in the correlation of signals taken from different oscillating modes as can be expected by TSD equations with photon mode couplings. Moreover it is also found that during the laser spiking phase, the optical vortex output shows a regular high and low changes, which we believe is due to the interplay between competition and couplings among the resonating modes.
29
Scalo, CARLO. "Large-eddy simulation and modelling of dissolved oxygen transport and depletion in water bodies." Thesis, 2012. http://hdl.handle.net/1974/7298.
Full textAbstract:
In the present doctoral work we have developed and tested a model for dissolved oxygen (DO) transfer from water to underlying flat and cohesive sediment beds populated with DO-absorbing bacteria. The model couples Large-Eddy Simulation (LES) of turbulent transport in the water-column, a biogeochemical model for DO transport and consumption in the sediment, and Darcy’s Law for the pore water-driven solute dispersion and advection. The model’s predictions compare well against experimental data for low friction-Reynolds numbers (Re). The disagreement for higher Re is investigated by progressively increasing the complexity of the model. A sensitivity analysis shows that the sediment-oxygen uptake (or demand, SOD) is approximately proportional to the bacterial content of the sediment layer, and varies with respect to fluid dynamics conditions, in accordance to classic high-Schmidt-number mass-transfer laws. The non- linear transport dynamics responsible for sustaining a statistically steady SOD are investigated by temporal- and-spatial correlations and with the aid of instantaneous visualizations: the near-wall coherent structures modulate the diffusive sublayer, which exhibits complex spatial and temporal filtering behaviours; its slow and quasi-periodic regeneration cycle determines the streaky structure of the DO field at the sediment-water interface (SWI), retained in the deeper layers of the porous medium. Oxygen depletion dynamics are then simulated by preventing surface re-areation with turbulent mixing driven by an oscillating low-speed current — an idealization of hypolimnetic DO depletion in the presence of a non-equilibrium periodic forcing. The oxygen distribution exhibits a self-similar pattern of decay with, during the deceleration phase, oscillations modulated by the periodic ejection of peaks of high turbulent mass flux (pumping oxygen towards the SWI), generated at the edge of the diffusive sublayer at the end of the acceleration phase. These fronts of highly turbulent mixing propagate away from the SWI, at approximately constant speed, in layers of below-average oxygen concentration. Finally, the model has been tested in a real geophysical framework, reproducing published in-situ DO measurements of a transitional flow in the bottom boundary layer of lake Alpnach. A simple model for the SOD is then derived for eventual inclusion in RANSE biogeochemical management-type models for similar applications.Thesis (Ph.D, Mechanical and Materials Engineering) -- Queen's University, 2012-07-04 11:13:24.936
30
Pabel, Stefanie Corinna. "Manipulation of the Working Memory Performance in Humans using Transcranial Alternating Current Stimulation over the Frontoparietal Network." Doctoral thesis, 2018. http://hdl.handle.net/11858/00-1735-0000-002E-E4FF-9.
Full text
31
Santos, João Pedro Mateus Gens dos. "Brain circuits involved in self-paced motion: the influence of 0.1 Hz waves." Master's thesis, 2016. http://hdl.handle.net/10362/18305.
Full textAbstract:
The neural mechanisms behind human voluntary motion are not fully characterized
yet, in spite of numerous research studies. Slow ( 0.1 Hz) brain oscillations are known to have a powerful modulatory effect on several cognitive and physiological phenomena, including free movement.
This study is based on fMRI data acquired from 25 young, healthy subjects. The tasks
were: rest, self-paced motion, motion paced by a periodic 0.1 Hz stimulus. The temporal
resolution was finer than standard fMRI protocols (TR=871 ms). After preprocessing, the signal from brain regions of interest was extracted, and functional connectivity was computed between brain regions using wavelet phase coherence. Complementarily, effective connectivity was measured using Granger causality. The final output was Phase-Locking (PL) and Granger Causality (GC) matrices reflecting inter-regional phase coherence and causal interactions, respectively, around 0.1 Hz.
Using the GraphVar toolbox, inter-task and inter-group comparisons were performed.
In inter-task comparisons PL matrices showed encouraging results unlike GC matrices.
Pairs of regions for which PL differs significantly between rest and self-paced movement were identified. These include mainly the Postcentral gyrus, Putamen, the Anterior Cingulum, the Precentral gyrus, the Calcarine, the Lingual and the Insula (all in the left hemisphere). Topological changes in the brain wiring were identified across the tasks by computing the node degree and global efficiency. Inter-group comparisons took into account the inter movement interval and the coupling between BOLD and heart rate beatto-beat interval signals and showed changes in brain activity depending on the regularity of movement intervals and specific connectivity patterns for neural BOLD oscillations, respectively.
This methodological approach allowed to make a contribution towards the characterization of the functional connectivity of brain circuits related to voluntary motor behavior.
32
Helmer, Markus. "Attention: A Complex System." Doctoral thesis, 2015. http://hdl.handle.net/11858/00-1735-0000-0028-867C-6.
Full text
33
Τσαρούχας, Νικόλαος. "Electrophysiologιcal study of brain hypoxia." Thesis, 2009. http://nemertes.lis.upatras.gr/jspui/handle/10889/4120.
Full textAbstract:
The current research work aims at the development of Biomedical Neuroengineering tools (Biotechnologies) for the in-depth functional study, rapid diagnosis, continuous monitoring and well-timed management of acute and chronic brain disorders, of individuals that are subjected to or suffer from any kind of systemic hypoxaemia or more localized brain hypoxia; as well as the functional assessment and continuous control of adaptability during the training of “altinauts” and generally of individuals that practice activities and function within environments of increased visual-cognitive-motor response demands (a type of brain “stress test”). For this purpose, we subject the entire visuocognitive system, from the elementary sensory to the most complex cognitive level, to an experimental test of categorical discrimination of complex visuocognitive stimuli, following ultra-rapid visual stimulation that leads to a motor response upon categorization of targets (images of animals elicit productive responses) and to its suppression upon categorization of nontargets (images of nonanimals elicit inhibitory responses). The oscillatory electro-physiological responses that are concurrently recorded at the occipital-temporal-parietal brain areas are analyzed in the time-domain (<20Hz) and in the joint time-frequency domain broadband (1-60Hz) with the Continuous Wavelet Transform that optimizes the multiresolution analysis of the high frequency (≥20Hz) γ-band oscillatory activity. This visuocognitive categorization test takes place in normoxaemic as well as hypoxaemic conditions (monitored reduction in the blood oxygen saturation from ≥97% to around 80% under conditions of hypobaric hypoxia within a hypobaric chamber), in order to assess electrophysiological markers that can detect and capture in the most sensitive and dynamic way even so transient, short-living and rather mild changes in brain function. The statistical parametric analysis of the time-frequency maps and the generalized, statistically safer, method of analysis of variance have established as the most sensitive and reliable the following markers: the major deflections of the evoked potentials, the phase-coherence factor of the oscillations across single-trials and the elicited energy of the evoked/phase-locked and the induced/total oscillatory activity. These electrophysiological markers in conjunction with psychometric tests allow for the investigation of the stages/levels of the decline as well as of the compensatory reserves in the visual-perceptive and cognitive-mental brain functions in order to determine the functional sensitivity thresholds of different brain functions to hypoxia. They open up the way for the functional characterization, the diagnosis and monitoring of brain insults or other acute and chronic pathological brain conditions.Η παρούσα ερευνητική εργασία στοχεύει στην ανάπτυξη εργαλείων Βιοϊατρικής Νευρομηχανικής (Βιοτεχνολογίες) για την σε βάθος λειτουργική μελέτη, ταχεία διάγνωση, συνεχή παρακολούθηση και έγκαιρη αντιμετώπιση οξέων και χρόνιων εγκεφαλικών διαταραχών, ατόμων που υπόκεινται σε ή πάσχουν από οιαδήποτε μορφή συστηματικής υποξαιμίας ή πιο εντοπισμένης εγκεφαλικής υποξίας, καθώς και για την λειτουργική αξιολόγηση και το συνεχή έλεγχο της προσαρμοστικότητας κατά την εξάσκηση των «υψιβατών», και γενικότερα ατόμων που ασκούν δραστηριότητες και λειτουργούν μέσα σε περιβάλλοντα αυξημένων οπτικο-γνωστικο-κινητικών απαιτήσεων (ένα είδος «στρες τεστ» για τον εγκέφαλο). Για το σκοπό αυτό υποβάλλουμε ολόκληρο το οπτικογνωστικό σύστημα, από το στοιχειώδες αισθητηριακό έως το πιο πολύπλοκο νοητικό επίπεδο, σε μια πειραματική δοκιμασία κατηγορικής διάκρισης σύνθετων οπτικογνωστικών ερεθισμάτων, μετά από υπερταχεία οπτική διέγερση που οδηγεί στην έκλυση κινητικής απάντησης κατά την κατηγοριοποίηση στόχων (εικόνες «ζώων» εκλύουν παραγωγικές αποκρίσεις) και στην καταστολή της κατά την κατηγοριοποίηση μη-στόχων (εικόνες «μη-ζώων» εκλύουν ανασταλτικές αποκρίσεις). Οι ταλαντωτικές ηλεκτροφυσιολογικές αποκρίσεις που συγχρόνως καταγράφονται στις ινιακές-κροταφικές-βρεγματικές περιοχές του εγκεφάλου αναλύονται στο πεδίο του χρόνου (<20Hz) και στο συζευγμένο χρονοφασματικό πεδίο ευρυζωνικά (1-60Hz) με το συνεχή μετασχηματισμό του κυματίου που βελτιστοποιεί την πολυφασματική ανάλυση της υψίσυχνης (≥20Hz) γ-ταλαντωτικής δραστηριότητας. Αυτή η δοκιμασία οπτικογνωστικής κατηγοριοποίησης λαμβάνει χώρα τόσο σε νορμοξαιμικές όσο και υποξαιμικές συνθήκες (ελεγχόμενη μείωση στον κορεσμό του αίματος σε οξυγόνο από ≥97% γύρω στο 80% για 15 λεπτά κάτω από συνθήκες υποβαρικής υποξίας μέσα σε υποβαρικό θάλαμο), προκειμένου να ελέγξουμε ηλεκτροφυσιολογικούς δείκτες που μπορούν να ανιχνεύσουν και να συλλάβουν με τον πιο ευαίσθητο και δυναμικό τρόπο ακόμη και τόσο βραχύβιες και σχετικά ήπιες μεταβολές της εγκεφαλικής λειτουργίας. Η στατιστική παραμετρική ανάλυση των χρονοφασματικών χαρτών και η γενικευμένη, στατιστικά πιο ασφαλής, μέθοδος ανάλυσης των διακυμάνσεων ανέδειξαν ως πλέον ευαίσθητους και αξιόπιστους τους ακόλουθους δείκτες: τις κύριες αιχμές των προκλητών δυναμικών, τον παράγοντα φασικής συνάφειας των ταλαντώσεων μεταξύ των μοναδιαίων καταγραφών και την εκλυόμενη ενέργεια των προκλητών/φασικά-κλειδωμένων και επαγόμενων/ολικών ταλαντώσεων. Οι ηλεκτροφυσιολογικοί αυτοί δείκτες σε συνδυασμό με ψυχομετρικές δοκιμασίες επιτρέπουν τη διερεύνηση των σταδίων/επιπέδων κάμψης καθώς και των αποθεμάτων αντιρρόπησης των οπτικο-αντιληπτικών και γνωστικών-νοητικών λειτουργιών του εγκεφάλου για τον καθορισμό των λειτουργικών ουδών ευαισθησίας διάφορων εγκεφαλικών λειτουργιών στην υποξία. Ανοίγουν μάλιστα το δρόμο. για το λειτουργικό χαρακτηρισμό, τη διάγνωση και την παρακολούθηση εγκεφαλικών προσβολών ή άλλων οξέων και χρόνιων παθολογικών καταστάσεων του εγκεφάλου.