1,721,067 research outputs found
Multimodal imaging: an evaluation of univariate and multivariate methods for simultaneous EEG/fMRI
No full textThe combination of electroencephalography (EEG) and functional magnetic resonance imaging (fMRI) has been proposed as a tool to study brain dynamics with both high temporal and high spatial resolution. Multimodal imaging techniques rely on the assumption of a common neuronal source for the different recorded signals. In order to maximally exploit the combination of these techniques, one needs to understand the coupling (i.e., the relation) between electroencephalographic (EEG) and fMRI blood oxygen level-dependent (BOLD) signals. Recently, simultaneous EEG-fMRI measurements have been used to investigate the relation between the two signals. Previous attempts at the analysis of simultaneous EEG-fMRI data reported significant correlations between regional BOLD activations and modulation of both event-related potential (ERP) and oscillatory EEG power, mostly in the alpha but also in other frequency bands. Beyond the correlation of the two measured brain signals, the relevant issue we address here is the ability of predicting the signal in one modality using information from the other modality. Using multivariate machine learning-based regression, we show how it is possible to predict EEG power oscillations from simultaneously acquired fMRI data during an eyes-open/eyes-closed task using either the original channels or the underlying cortically distributed sources as the relevant EEG signal for the analysis of multimodal data
Diffusion weighted magnetic resonance imaging : validation, correction and applications
Get PDFDiffusion weighted Magnetic Resonance Imaging (DW-MRI) images the structure of the white substance in the brains and connections between various brain parts. To validate this technique a test object (phantom) was developed, the structure of which resembles the structure of the white substance. This phantom was brought on the market and by now over 15 universities and hospitals own one. Furthermore, DW-MRI was applied to a patient with Landau-Kleffner syndrome. This syndrome involves losing the ability of using and understanding language as a consequence of epilepsy. Intensive therapy can restore the language ability. Important connections for language processing look different in these patients and the brains use more visual information in communication. The dissertation also describes further studies with a blindsight patient. In these patients the eyes still function, but the visual information is no longer consciously processed. And yet, the deep brain parts that unconsciously process emotions and visual information appear active. The necessary brain connections were shown in this patient and they were not present in control persons. Therefore, the patient has created new connections
Methods of diffusion-weighted and functional magnetic resonance imaging investigated in the human brain at ultra-high-field
Get PDFMagnetic resonance imaging (MRI) allows to obtain various types of brain images. A particular MRI method is diffusion tensor imaging (DTI), which can visualize nerve fiber pathways. The method is indirect, involving complicated mathematical models to infer this kind of information from the raw data. Therefore, it has to be investigated whether the results match the actual fiber architecture in practice. Parts of this PhD thesis are concerned with this problem: DTI results were compared to microscopical scans of brain tissue, where nerve fibers had been stained. Doing so, it was shown that DTI indeed works as theoretically supposed to
dMRI: Diffusion Magnetic Resonance Imaging as a Window onto Structural Brain Networks and White Matter Microstructure
No full textDiffusion magnetic resonance imaging (dmri) can be used to probe the connectivity and microstructure of human brain tissue non-invasively in vivo. The diffusion-weighted mr signal has sensitivity to micrometer-scale tissue properties averaged over the imaging voxel size, for example, intra-cellular and extra-cellular volumes and the 3d orientations of axonal bundles. It derives its contrast from sensitivity to the bulk displacement of water through what is known as thermal motion, brownian motion, or passive (self-)diffusion. In the brain, cell membranes, organelles, and myelin sheaths create barriers and form biological compartments that constrain the displacement of water molecules, modifying the statistical behavior of bulk diffusion over time. This review therefore focuses on the use of dmri for estimates of the local orientations and estimates of microstructure of fiber tracts, and on an understanding of dmri signal mechanisms and appropriate signal processing and modeling for this purpose. It first discusses basic diffusion mri acquisition principles and diffusion contrast and the constraints the acquisition places on the modeling of diffusion. It then sets out the diffusion tensor model, the most used model in dmri that underlies diffusion tensor imaging, in a way which prepares a discussion of its limitations. The next sections set out advances in dmri beyond dti as focusing on tractography and connectomics, with a need to accurately model spatially complex fiber configurations and on diffusion microstructure, with a need to accurately model restricted diffusion and compartmentalization. Throughout, the emphasis is on a thorough understanding of basic principles and assumptions underlying techniques, as well as their possibilities and limitations for inference of brain connectivity, with a minimum of technical detail and mathematics. This review ends with an outlook on future developments emanating from current trends
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
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
High-Resolution Diffusion Tensor Imaging and Tractography of the Human Optic Chiasm at 9.4 T
Get PDFThe optic chiasm with its complex fiber micro-structure is a challenge for diffusion tensor models and tractography methods. Likewise, it is an ideal candidate for evaluation of diffusion tensor imaging tractography approaches in resolving inter-regional connectivity because the macroscopic connectivity of the optic chiasm is well known. Here, high-resolution (156 pm in-plane) diffusion tensor imaging of the human optic chiasm was performed ex vivo at ultra-high field (9.4 T). Estimated diffusion tensors at this high resolution were able to capture complex fiber configurations such as sharp curves, and convergence and divergence of tracts, but were unable to resolve directions at sites of crossing fibers. Despite the complex microstructure of the fiber paths through the optic chiasm, all known connections could be tracked by a line propagation algorithm. However, fibers crossing from the optic nerve to the contralateral tract were heavily underrepresented, whereas ipsilateral nerve-to-tract connections, as well as tract-to-tract connections, were overrepresented, and erroneous nerve-to-nerve connections were tracked. The effects of spatial resolution and the varying degrees of partial volume averaging of complex fiber architecture on the performance of these methods could be investigated. Errors made by the tractography algorithm at high resolution were shown to increase at lower resolutions closer to those used in vivo. This study shows that increases in resolution, made possible by higher field strengths, improve the accuracy of DTI-based tractography. More generally, post-mortem investigation of fixed tissue samples with diffusion imaging at high field strengths is important in the evaluation of MR-based diffusion models and tractography algorithms
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