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Diffusion tensor spectroscopy and imaging of the arcuate fasciculus
No full textThe arcuate fasciculus (AF) is a fiber pathway in the human brain relevant for language processes and has recently been characterized by means of diffusion tensor tractography. The observations made concerning the left and right hemisphere AF include a characterization of the trajectories and quantification of physical properties such as fractional anisotropy, DTI-based fiber density and volume. However, these observations were based on the diffusion of water, which is not particular to either the intra- or extra-axonal compartments, and thus its usefulness for tissue characterization is limited. If the diffusion properties and in turn the geometric properties of only one tissue compartment can be isolated and characterized, a better microstructural characterization of AF is possible. In this study, water-based diffusion tensor probabilistic mapping was first implemented to segment the AF. Subsequently, diffusion tensor spectroscopic measurements of N-acetyl aspartate (NAA) were performed to measure the intra-axonal specific diffusion in left and right AF. Diffusion properties of NAA, which solely reflect the intra-axonal space, indicated possible leftward asymmetry in axonal diameter, where those of water, which are not compartment-specific, showed laterality to a lesser extent. (C) 2007 Elsevier Inc. All rights reserved
Effective and structural connectivity in the human auditory cortex
No full textLanguage processing involves multiple neuronal structures in the human auditory cortex. Although a variety of neuroimaging and mapping techniques have been implemented to better understand language processing at the level of the auditory cortex, much is unknown regarding how and by what pathways these structures interact during essential tasks such as sentence comprehension. In this study, the effective and structural connectivity at the level of the auditory cortex were investigated. First, blood oxygenation level-dependent ( BOLD) responses were measured with time-resolved functional magnetic resonance imaging ( fMRI) during audition of short sentences. Once BOLD activation maps were obtained, the effective connectivity between primary auditory cortex and the surrounding auditory regions on the supratemporal plane and superior temporal gyrus (STG) were investigated using Granger causality mapping (GCM). Effective connectivity was observed between the primary auditory cortex and ( 1) the lateral planum polare and anterior STG, and ( 2) the lateral planum temporale and posterior STG. By using diffusion tensor probabilistic mapping ( DTPM), rostral and caudal fiber pathways were detected between regions depicting effective connectivity. The effective and structural connectivity results of the present study provide further insight as to how auditory stimuli (i.e., human language) is processed at the level of the auditory cortex. Furthermore, combining BOLD fMRI-based GCM and DTPM analysis could provide a novel means to study effective and structural connectivity not only in the auditory cortex, but also in other cortical regions
Function and connectivity in human primary auditory cortex: A combined fMRI and DTI study at 3 tesla
No full textHuman primary auditory cortex (PAC) is functionally organized in a tonotopic manner. Past studies have used neuroimaging to characterize tonotopic organization in PAC and found similar organization as that described in mammals. In contrast to what is known about PAC in primates and nonprimates, in humans, the structural connectivity within PAC has not been defined. In this study, strobo-scopic event-related functional magnetic resonance imaging (fMRI) was utilized to reveal mirror symmetric tonotopic organization consisting of a high-low-high frequency gradient in PAC. Furthermore, diffusion tensor tractography and probabilistic mapping was used to study projection patterns within tonotopic areas. Based on earlier physiological and histological work in nonhuman PAC, we hypothesized the existence of cross-field isofrequency (homotopic) and within-field nonisofrequency (heterotopic)-specific axonal projections in human PAC. The presence of both projections types was found in all subjects. Specifically, the number of diffusion tensor imaging (DTI) reconstructed fibers projecting between high and low-frequency regions was greater than those fibers projecting between 2 high-frequency areas, the latter of which are located in distinct auditory fields. The fMRI and DTI results indicate that functional and structural properties within early stages of the auditory processing stream are preserved across multiple mammalian species at distinct evolutionary levels
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Adaptive optics integration in lattice light-sheet microscope for depth super-resolution imaging
No full textLa microscopie de fluorescence est devenue un outil indispensable pour les étudesbiologiques en permettant d’observer des structures d’intérêts de manière spécifique etcontrastée au sein d’échantillons fixés ou vivant. Parmi ces techniques, le sectionnementoptique offert par la microscopie à feuille de lumière jouit d’un grand avantage enminimisant le bruit de fond et en réduisant phototoxicité/photoblanchiment. Lamicroscopie à feuille de lumière « lattice » (LLSM) maximise ses caractéristiques enutilisant une feuille de lumière, d’épaisseur constante sur un plus grand champ de vue.Ainsi, elle offre la possibilité d'imager la fluorescence à l'intérieur d’échantillons épaisjusqu’à environ 30 μm, avec une très grande résolution spatiale, temporelle et une trèsfaible phototoxicité.Toutefois, comme d’autres techniques de microscopie, elle souffre de deux problématiquesliées à la nature de la lumière, qui dégradent la qualité des images acquises. (1) Lesaberrations optiques, induites par les interfaces et l’inhomogénéité des échantillons,croissantes avec la profondeur d’imagerie, perturbent le front d’onde et donc la résolutionfinale de l’image. (2) La limite de diffraction contraint la résolution à ~200 nm minimumet empêche donc l’observation de structures de taille inférieure à cette limite.Pour contrer ces phénomènes, nous avons décidé de combiner la LLSM avec deuxméthodes complémentaires : (1) l’optique adaptative pour minimiser les aberrationsoptiques en profondeur d’échantillons épais et (2) la microscopie à super-résolution àtravers une technique de microscopie par localisation de molécules uniques (SMLM),nommée DNA-PAINT, pour atteindre des résolutions nanométriques. Plusparticulièrement, notre méthode, nommée AIO, s’appuie sur les images enregistrées parla caméra, à travers : une refocalisation de la feuille de lumière (AF) et une correction parmesure indirecte du front d’onde, nommée (3N+).Cette méthode, dont le paramétrage a été étudié au cours de cette thèse, améliore larésolution et le contraste des images en résolution limitée par diffraction et en SMLM. (1)En résolution « classique », l’AIO permet de récupérer un front d’onde plan avec une erreurinférieure à 50 nm RMS et en moins de 40 secondes. La correction des aberrations optimiseégalement le processus de déconvolution en restaurant la PSF du système. Ce gain estillustré à travers l’imagerie d’épines dendritiques à 40 μm sous la surface de tranche decerveaux organotypiques et l’acquisition de structures sub-micrométrique dans ladeuxième couche cellulaire de racines d’Arabidopsis. (2) En super-résolution, nous avonsdémontré l’application d’un protocole DNA-PAINT jusqu’à ~50 μm sous la surface detranches de cerveaux et l’intérêt de l’optique adaptative pour améliorer la densité dedétection et la précision de localisation en reconstruction SMLM.Fluorescence microscopy has become an indispensable tool for biological studies, allowingobservation of structures of interest. Used on both fixed or living samples, it provides greatspecificity and high-contrast. Among these techniques, the optical sectioning offered bylight-sheet microscopy (LSFM or SPIM) is a game-changer. It maximises image contrastwhilst avoiding too rapid fluorescence loss of the samples. Lattice light-sheet microscopy(LLSM) improves these characteristics by using a thinner light-sheet over a larger field ofview. As a result, it offers the possibility of fluorescence imaging inside thick samples downto around 30 μm. In addition, it allows with very high spatial and temporal resolutionwhilst almost obliviating phototoxicity.As other microscopy techniques, it suffers from two drawbacks, both linked to the natureof the light, impairing the image’s quality. (1) Multiple interfaces and the inhomogeneityof samples induce optical aberrations that increase with imaging depth. Hence, disruptionof the wavefront highly impairs the final resolution of the image. (2) The diffraction limitconstrains the resolution to a minimum of ~200 nm and therefore prevents the observationof structures smaller than this limit.As a workaround to these phenomena, we decided to implement two methods on theLLSM: (1) adaptive optics to minimise optical aberrations in the depth of thick samplesand (2) super-resolution microscopy using a single molecule localisation microscopy(SMLM) technique, called DNA-PAINT, to achieve nanometric resolutions. Our method,called AIO, is based on the images recorded by the camera, through refocusing of the lightsheet (AF) and correction in response to indirect measurement of the wavefront, called(3N+).This thesis presents the original design of the method, and the optimization of itsparameters. AIO improves the resolution and contrast of diffraction-limited and SMLMresolution images. (1) In "classical" resolution, the AIO enables a plane wavefront to berecovered with an error of less than 50 nm RMS and in less than 40 seconds. Correctingaberrations also optimises the deconvolution process by restoring the system's PSF. Thisgain is illustrated by the imaging of dendritic spines at 40 μm below the surface oforganotypic brain slices and the acquisition of the second cell layer of Arabidopsis roots.(2) In super-resolution, we demonstrated the application of a DNA-PAINT protocol downto ~50 μm below the surface of brain slices and the value of adaptive optics for improvingdetection density and localisation accuracy in SMLM reconstruction
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Get PDFWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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