478 research outputs found
On the Origin of Asymmetries in Bilateral Supernova Remnants
AIMS: We investigate whether the morphology of bilateral supernova remnants
(BSNRs) observed in the radio band is determined mainly either by a non-uniform
interstellar medium (ISM) or by a non-uniform ambient magnetic field.
METHODS: We perform 3-D MHD simulations of a spherical SNR shock propagating
through a magnetized ISM. Two cases of shock propagation are considered: 1)
through a gradient of ambient density with a uniform ambient magnetic field; 2)
through a homogeneous medium with a gradient of ambient magnetic field
strength. From the simulations, we synthesize the synchrotron radio emission,
making different assumptions about the details of acceleration and injection of
relativistic electrons.
RESULTS: We find that asymmetric BSNRs are produced if the line-of-sight is
not aligned with the gradient of ambient plasma density or with the gradient of
ambient magnetic field strength. We derive useful parameters to quantify the
degree of asymmetry of the remnants that may provide a powerful diagnostic of
the microphysics of strong shock waves through the comparison between models
and observations.
CONCLUSIONS: BSNRs with two radio limbs of different brightness can be
explained if a gradient of ambient density or, most likely, of ambient magnetic
field strength is perpendicular to the radio limbs. BSNRs with converging
similar radio arcs can be explained if the gradient runs between the two arcs.Comment: 14 pages, 8 Figures; paper accepted for publication in A&A; the paper
with high-resolution figures can be downloaded at
http://www.astropa.unipa.it/~orlando/PAPERS/sorlando_6045.pd
Symmetry in Human Evolution, from Biology to Behaviours
Our knowledge of human evolution has made particular progress recently, due to the discovery of new fossils, the use of new methods and multidisciplinary approaches. Moreover, studies on the departure from symmetry, including variations in fluctuating or directional asymmetries, have contributed to the expansion of this knowledge. This Special Issue brings together articles that deal with symmetry and human evolution. The notion of symmetry is addressed, including whether to reconstruct deformed fossil specimens, study biological variations within hominins or compare them with extant primates, address the shape of the brain or seek possible relationships between biological and behavioural data
Three Dimensional Nonlinear Statistical Modeling Framework for Morphological Analysis
This dissertation describes a novel three-dimensional (3D) morphometric analysis framework for building statistical shape models and identifying shape differences between populations. This research generalizes the use of anatomical atlases on more complex anatomy as in case of irregular, flat bones, and bones with deformity and irregular bone growth. The foundations for this framework are: 1) Anatomical atlases which allow the creation of homologues anatomical models across populations; 2) Statistical representation for output models in a compact form to capture both local and global shape variation across populations; 3) Shape Analysis using automated 3D landmarking and surface matching. The proposed framework has various applications in clinical, forensic and physical anthropology fields. Extensive research has been published in peer-reviewed image processing, forensic anthropology, physical anthropology, biomedical engineering, and clinical orthopedics conferences and journals.
The forthcoming discussion of existing methods for morphometric analysis, including manual and semi-automatic methods, addresses the need for automation of morphometric analysis and statistical atlases. Explanations of these existing methods for the construction of statistical shape models, including benefits and limitations of each method, provide evidence of the necessity for such a novel algorithm. A novel approach was taken to achieve accurate point correspondence in case of irregular and deformed anatomy. This was achieved using a scale space approach to detect prominent scale invariant features. These features were then matched and registered using a novel multi-scale method, utilizing both coordinate data as well as shape descriptors, followed by an overall surface deformation using a new constrained free-form deformation.
Applications of output statistical atlases are discussed, including forensic applications for the skull sexing, as well as physical anthropology applications, such as asymmetry in clavicles. Clinical applications in pelvis reconstruction and studying of lumbar kinematics and studying thickness of bone and soft tissue are also discussed
Barrels, jets and smoke-rings: Understanding the bizarre shapes of radio supernova remnants
This thesis considers the various morphologies of radio supernova remnants (SNRs), and attempts to determine whether their appearance results from the properties of the progenitor star and its supernova explosion, or from the structure of the interstellar medium (ISM) and ambient magnetic field into which a SNR consequently expands. High-resolution observations of Supernova 1987A show a young remnant whose appearance and evolution are completely dominated by the structure of its progenitor wind. A statistical study of the Galactic population of bilateral SNRs demonstrates that the symmetry axes of these remnants run parallel to the Galactic Plane. This result can be explained by the interaction of main sequence stellar wind-bubbles with the ambient magnetic field; expansion of SNRs into the resulting elongated cavities results in a bilateral appearance with the observed alignment. Radio observations of SNR G296.8-00.3 show a double-ringed morphology which is best explained by expansion either into an anisotropic main-sequence progenitor wind or into multiple cavities in the ISM. Data on SNRs G309.2-00.6 and G320.4-01.2 (MSH 15-52) make a strong case that the appearance of both remnants is significantly affected by collimated outflows from a central source; for G309.2-00.6 the source itself is not detected, but for G320.4-01.2 there is now compelling evidence that the remnant is associated with and is interacting with the young pulsar PSR B1509-58. I conclude that, while the youngest SNRs are shaped by their progenitor's circumstellar material, the appearance of most SNRs reflects the properties of the local ISM and magnetic field. Remnants which interact with an associated pulsar or binary system appear to be rare, and are easily distinguished by their unusual and distorted morphologies
CLARCS, a C++ Library for Automated Registration and Comparison of Surfaces: Medical Applications
International audienceIn this paper we present the methods implemented in the CLARCS (C++ Library for Automated Registration and Comparison of Surfaces) library. This library allows some basic and high level processing on free-form surfaces, represented as point sets or meshes. Three methods are the "building bricks" of CLARCS; they allow (i) the rigid/affine/non-linear registration of two point sets, (ii) the computation of the mid-sagittal plane of one point set, (iii) the computation of a mean point set from several point sets, and the variability around this mean. These methods are all based on a common methodological framework, in which the point sets/meshes are represented either as a Gaussian mixture model or as a draw of such a model. We propose some applications of the methods implemented in CLARCS on different sets of medical data
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The natverse, a versatile toolbox for combining and analysing neuroanatomical data.
To analyse neuron data at scale, neuroscientists expend substantial effort reading documentation, installing dependencies and moving between analysis and visualisation environments. To facilitate this, we have developed a suite of interoperable open-source R packages called the natverse. The natverse allows users to read local and remote data, perform popular analyses including visualisation and clustering and graph-theoretic analysis of neuronal branching. Unlike most tools, the natverse enables comparison across many neurons of morphology and connectivity after imaging or co-registration within a common template space. The natverse also enables transformations between different template spaces and imaging modalities. We demonstrate tools that integrate the vast majority of Drosophila neuroanatomical light microscopy and electron microscopy connectomic datasets. The natverse is an easy-to-use environment for neuroscientists to solve complex, large-scale analysis challenges as well as an open platform to create new code and packages to share with the community
Recommended from our members
The natverse, a versatile toolbox for combining and analysing neuroanatomical data.
To analyse neuron data at scale, neuroscientists expend substantial effort reading documentation, installing dependencies and moving between analysis and visualisation environments. To facilitate this, we have developed a suite of interoperable open-source R packages called the natverse. The natverse allows users to read local and remote data, perform popular analyses including visualisation and clustering and graph-theoretic analysis of neuronal branching. Unlike most tools, the natverse enables comparison across many neurons of morphology and connectivity after imaging or co-registration within a common template space. The natverse also enables transformations between different template spaces and imaging modalities. We demonstrate tools that integrate the vast majority of Drosophila neuroanatomical light microscopy and electron microscopy connectomic datasets. The natverse is an easy-to-use environment for neuroscientists to solve complex, large-scale analysis challenges as well as an open platform to create new code and packages to share with the community
Hemispheric differences in semantic cognition and their contribution to behaviour
This thesis investigated hemispheric differences in semantic cognition and their contribution to behaviour, using resting-state and task-based fMRI in conjunction with automated meta-analyses and cognitive decoding. The controlled semantic cognition framework proposes that distinct brain regions support the long-term representation of heteromodal conceptual knowledge and semantic control processes that retrieve currently-relevant aspects of knowledge. However, previous studies have not investigated whether these components have distinct patterns of lateralisation. Chapter 2 assessed intrinsic connectivity of four regions implicated in semantic cognition: anterior temporal lobe, angular gyrus, inferior frontal gyrus, and posterior middle temporal gyrus. Semantic sites in the left hemisphere showed connectivity with both control regions and default mode network, whilst their right hemisphere homotopes showed connectivity with control regions and visual and attentional systems. Semantic control regions showed the strongest lateralisation. Chapter 3 examined hemispheric specialisation of the anterior temporal lobes, strongly implicated in semantic representation. It assessed the relationship between differential intrinsic connectivity and behaviour outside the scanner on a semantic categorisation task previously shown to be sensitive to lateralisation. Graded differences in connectivity between left and right anterior temporal lobes, and from right anterior temporal cortex to the visual system related to semantic efficiency. Finally, Chapter 4 tested the specificity of the semantic control system and its relationship to domain-general control. Using a task known to engage domain-general inhibition, but introducing semantic content, this chapter yields evidence that regions implicated in semantic control are not sensitive to challenging tasks that require exercising controlled processing, and instead are specific to semantic processing. Together, these results constitute evidence for a component-process architecture in the semantic cognition system, with different patterns of lateralisation for the semantic representation and control systems. Within these systems, the results confirm the specific nature of semantic control, and fit with the graded-hub architecture of semantic representation
Neural representations underlying mental imagery as unveiled by representation similarity analysis
It is commonly acknowledged that visual imagery and perception rely on the same content-dependent brain areas in the high-level visual cortex (HVC). However, the way in which our brain processes and organizes previous acquired knowledge to allow the generation of mental images is still a matter of debate. Here, we performed a representation similarity analysis of three previous fMRI experiments conducted in our laboratory to characterize the neural representation underlying imagery and perception of objects, buildings and faces and to disclose possible dissimilarities in the neural structure of such representations. To this aim, we built representational dissimilarity matrices (RDMs) by computing multivariate distances between the activity patterns associated with each pair of stimuli in the content-dependent areas of the HVC and HC. We found that spatial information is widely coded in the HVC during perception (i.e. RSC, PPA and OPA) and imagery (OPA and PPA). Also, visual information seems to be coded in both preferred and non-preferred regions of the HVC, supporting a distributed view of encoding. Overall, the present results shed light upon the spatial coding of imagined and perceived exemplars in the HVC
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