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Frequency Domain Connectivity Identification: An Application of Partial Directed Coherence in fMRI

SATO, Joao R.; TAKAHASHI, Daniel Y.; ARCURI, Silvia M.; Sameshima, Koichi; MORETTIN, Pedro A.; Baccala, Luiz Antonio
Fonte: WILEY-LISS Publicador: WILEY-LISS
Tipo: Artigo de Revista Científica
ENG
Relevância na Pesquisa
36.72%
Functional magnetic resonance imaging (fMRI) has become an important tool in Neuroscience due to its noninvasive and high spatial resolution properties compared to other methods like PET or EEG. Characterization of the neural connectivity has been the aim of several cognitive researches, as the interactions among cortical areas lie at the heart of many brain dysfunctions and mental disorders. Several methods like correlation analysis, structural equation modeling, and dynamic causal models have been proposed to quantify connectivity strength. An important concept related to connectivity modeling is Granger causality, which is one of the most popular definitions for the measure of directional dependence between time series. In this article, we propose the application of the partial directed coherence (PDC) for the connectivity analysis of multisubject fMRI data using multivariate bootstrap. PDC is a frequency domain counterpart of Granger causality and has become a very prominent tool in EEG studies. The achieved frequency decomposition of connectivity is useful in separating interactions from neural modules from those originating in scanner noise, breath, and heart beating. Real fMRI dataset of six subjects executing a language processing protocol was used for the analysis of connectivity. Hum Brain Mapp 30:452-461...

White matter microstructure underlying default mode network connectivity in the human brain

TEIPEL, Stefan J.; BOKDE, Arun L. W.; MEINDL, Thomas; AMARO JR., Edson; SOLDNER, Jasmin; REISER, Maximilian F.; HERPERTZ, Sabine C.; MOLLER, Hans-Jurgen; HAMPEL, Harald
Fonte: ACADEMIC PRESS INC ELSEVIER SCIENCE Publicador: ACADEMIC PRESS INC ELSEVIER SCIENCE
Tipo: Artigo de Revista Científica
ENG
Relevância na Pesquisa
36.76%
Resting state functional magnetic resonance imaging (fMRI) reveals a distinct network of correlated brain function representing a default mode state of the human brain The underlying structural basis of this functional connectivity pattern is still widely unexplored We combined fractional anisotropy measures of fiber tract integrity derived from diffusion tensor imaging (DTI) and resting state fMRI data obtained at 3 Tesla from 20 healthy elderly subjects (56 to 83 years of age) to determine white matter microstructure e 7 underlying default mode connectivity We hypothesized that the functional connectivity between the posterior cingulate and hippocampus from resting state fMRI data Would be associated with the white matter microstructure in the cingulate bundle and fiber tracts connecting posterior cingulate gyrus With lateral temporal lobes, medial temporal lobes, and precuneus This was demonstrated at the p<0001 level using a voxel-based multivariate analysis of covariance (MANCOVA) approach In addition, we used a data-driven technique of joint independent component analysis (ICA) that uncovers spatial pattern that are linked across modalities. It revealed a pattern of white matter tracts including cingulate bundle and associated fiber tracts resembling the findings from the hypothesis-driven analysis and was linked to the pattern of default mode network (DMN) connectivity in the resting state fMRI data Out findings support the notion that the functional connectivity between the posterior cingulate and hippocampus and the functional connectivity across the entire DMN is based oil distinct pattern of anatomical connectivity within the cerebral white matter (C) 2009 Elsevier Inc All rights reserved; Medical Faculty Of the Ludwig-Maximilian University (Munich. Germany); Hirnliga e. V. (Nurmbrecht...

Altered gray matter morphometry and resting-state functional and structural connectivity in social anxiety disorder

LIAO, Wei; XU, Qiang; MANTINI, Dante; DING, Jurong; MACHADO-DE-SOUSA, Joao Paulo; HALLAK, Jaime E. C.; TRZESNIAK, Clarissa; QIU, Changjian; ZENG, Ling; ZHANG, Wei; CRIPPA, Jose Alexandre S.; GONG, Qiyong; CHEN, Huafu
Fonte: ELSEVIER SCIENCE BV Publicador: ELSEVIER SCIENCE BV
Tipo: Artigo de Revista Científica
ENG
Relevância na Pesquisa
36.75%
In social anxiety disorder (SAD), impairments in limbic/paralimbic structures are associated with emotional dysregulation and inhibition of the medial prefrontal cortex (MPFq. Little is known, however, about alterations in limbic and frontal regions associated with the integrated morphometric, functional, and structural architecture of SAD. Whether altered gray matter volume is associated with altered functional and structural connectivity in SAD. Three techniques were used with 18 SAD patients and 18 healthy controls: voxel-based morphometry; resting-state functional connectivity analysis; and diffusion tensor imaging tractography. SAD patients exhibited significantly decreased gray matter volumes in the right posterior inferior temporal gyrus (ITG) and right parahippocampal/hippocampal gyrus (PHG/HIP). Gray matter volumes in these two regions negatively correlated with the fear factor of the Liebowitz Social Anxiety Scale. In addition, we found increased functional connectivity in SAD patients between the right posterior ITG and the left inferior occipital gyrus, and between the right PHF/HIP and left middle temporal gyms. SAD patients had increased right MPFC volume, along with enhanced structural connectivity in the genu of the corpus callosum. Reduced limbic/paralimbic volume...

Relative effects of fragment size and connectivity on bird community in the Atlantic Rain Forest: Implications for conservation

MARTENSEN, Alexandre C.; PIMENTEL, Rafael G.; METZGER, Jean Paul
Fonte: ELSEVIER SCI LTD Publicador: ELSEVIER SCI LTD
Tipo: Artigo de Revista Científica
ENG
Relevância na Pesquisa
36.76%
It is known that large fragment sizes and high connectivity levels are key components for maintaining species in fragments; however, their relative effects are poorly understood, especially in tropical areas. In order to test these effects, we built models for explaining understory birds occurrence in a fragmented Atlantic Rain Forest landscape with intermediate habitat cover (3%). Data from over 9000 mist-net hours from 17 fragments differing in size (2-175 ha) and connectivity (considering corridor linkages and distance to nearby fragments) were ranked under a model selection approach. A total 1293 individuals of 62 species were recorded. Species richness, abundance and compositional variation were mainly affected by connectivity indices that consider the capacity of species to use corridors and/or to cross short distances up to 30 m through the matrix. Bird functional groups were differently affected by area and connectivity: while terrestrial insectivores, omnivores and frugivores were affected by both area and connectivity, the other groups (understory insectivores, nectarivores, and others) were affected only by connectivity. In the studied landscape, well connected fragments can sustain an elevated number of species and individuals. Connectivity gives the opportunity for individuals to use multiple fragments...

Functional and Structural Connectivity Between the Perigenual Anterior Cingulate and Amygdala in Bipolar Disorder

WANG, Fei; KALMAR, Jessica H.; HE, Yong; JACKOWSKI, Marcel; CHEPENIK, Lara G.; EDMISTON, Erin E.; TIE, Karen; GONG, Gaolang; SHAH, Maulik P.; JONES, Monique; UDERMAN, Jodi; CONSTABLE, R. Todd; BLUMBERG, Hilary P.
Fonte: ELSEVIER SCIENCE INC Publicador: ELSEVIER SCIENCE INC
Tipo: Artigo de Revista Científica
ENG
Relevância na Pesquisa
36.72%
Objective: Abnormalities in the morphology and function of two gray matter structures central to emotional processing, the perigenual anterior cingulate cortex (pACC) and amygdala, have consistently been reported in bipolar disorder (BD). Evidence implicates abnormalities in their connectivity in BD. This study investigates the potential disruptions in pACC-amygdala functional connectivity and associated abnormalities in white matter that provides structural connections between the two brain regions in BD. Methods: Thirty-three individuals with BD and 31 healthy comparison subjects (HC) participated in a scanning session during which functional magnetic resonance imaging (fMRI) during processing of face stimuli and diffusion tensor imaging (DTI) were performed. The strength of pACC-amygdala functional connections was compared between BD and HC groups, and associations between these functional connectivity measures from the fMRI scans and regional fractional anisotropy (FA) from the DTI scans were assessed. Results: Functional connectivity was decreased between the pACC and amygdala in the BD group compared with HC group, during the processing of fearful and happy faces (p < .005). Moreover, a significant positive association between pACC-amygdala functional coupling and FA in ventrofrontal white matter...

Estudo da conectividade efetiva neural através da técnica da modelagem causal dinâmica; Study of neural effective connectivity through the technique of dynamic causal modeling

Elvis Lira da Silva
Fonte: Biblioteca Digital da Unicamp Publicador: Biblioteca Digital da Unicamp
Tipo: Tese de Doutorado Formato: application/pdf
Publicado em 16/07/2010 PT
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Nas últimas décadas, vêm crescendo muito o ramo da Neurociência que estuda a integração neuronal entre áreas cerebrais, onde tal integração é mediada pela chamada conectividade efetiva. A conectividade efetiva pode ser definida como a influência que um sistema neural exerce sobre o outro, tanto ao nível sináptico quanto ao nível cortical. Neste contexto, é cada vez maior a participação de físicos e matemáticos na elaboração de técnicas matemáticas que permitam investigar o comportamento desses sistemas neurais através de experimentos baseados na Ressonância Magnètica funcional (fMRI) e na Eletroencefalografia (EEG). Uma das técnicas que vem sendo amplamente utilizada para estimar a conectividade efetiva entre áreas cerebrais é a denominada Modelagem Causal Dinâmica (DCM), que é uma técnica que incorpora à sua teoria a não-linearidade e a dinâmica de sistemas biológicos. Este trabalho teve por objetivo estudar a conectividade entre áreas cerebrais através da DCM em experimentos de fMRI. Foram estudados dois sistemas neurais. O primeiro deles, o sistema motor, nos possibilitou verificar a plausibilidade da DCM, al´em de averiguarmos as diferenças na conectividade entre as áreas do sistema motor quando indivíduos destros movimentaram os dedos da mão direita e da mão esquerda. Encontramos que a conectividade efetiva é maior quando tais sujeitos movimentaram a mão esquerda...

Relative Effects of Road Risk, Habitat Suitability, and Connectivity on Wildlife Roadkills: The Case of Tawny Owls (Strix aluco)

Santos, Sara; Lourenço, Rui; Mira, António; Beja, Pedro
Fonte: PLoS One Publicador: PLoS One
Tipo: Artigo de Revista Científica
ENG
Relevância na Pesquisa
36.72%
Background: Despite its importance for reducing wildlife-vehicle collisions, there is still incomplete understanding of factors responsible for high road mortality. In particular, few empirical studies examined the idea that spatial variation in roadkills is influenced by a complex interplay between road-related factors, and species-specific habitat quality and landscape connectivity. Methodology/Principal Findings: In this study we addressed this issue, using a 7-year dataset of tawny owl (Strix aluco) roadkills recorded along 37 km of road in southern Portugal. We used a multi-species roadkill index as a surrogate of intrinsic road risk, and we used a Maxent distribution model to estimate habitat suitability. Landscape connectivity was estimated from least-cost paths between tawny owl territories, using habitat suitability as a resistance surface. We defined 10 alternative scenarios to compute connectivity, based on variation in potential movement patterns according to territory quality and dispersal distance thresholds. Hierarchical partitioning of a regression model indicated that independent variation in tawny owl roadkills was explained primarily by the roadkill index (70.5%) and, to a much lesser extent, by landscape connectivity (26.2%)...

Resting-state functional connectivity of ventral parietal regions associated with attention reorienting and episodic recollection

Daselaar, S. M.; Huijbers, W.; Eklund, K.; Moscovitch, M.; Cabeza, R.
Fonte: Frontiers Media S.A. Publicador: Frontiers Media S.A.
Tipo: Artigo de Revista Científica
EN_US
Relevância na Pesquisa
36.76%
In functional neuroimaging studies, ventral parietal cortex (VPC) is recruited by very different cognitive tasks. Explaining the contributions of VPC to these tasks has become a topic of intense study and lively debate. Perception studies frequently find VPC activations during tasks involving attention-reorienting, and memory studies frequently find them during tasks involving episodic recollection. According to the Attention to Memory (AtoM) model, both phenomena can be explained by the same VPC function: bottom-up attention. Yet, a recent functional MRI (fMRI) meta-analysis suggested that attention-reorienting activations are more frequent in anterior VPC, whereas recollection activations are more frequent in posterior VPC. Also, there is evidence that anterior and posterior VPC regions have different functional connectivity patterns. To investigate these issues, we conducted a resting-state functional connectivity analysis using as seeds the center-of-mass of attention-reorienting and recollection activations in the meta-analysis, which were located in the supramarginal gyrus (SMG, around the temporo-parietal junction—TPJ) and in the angular gyrus (AG), respectively. The SMG seed showed stronger connectivity with ventrolateral prefrontal cortex (VLPFC) and occipito-temporal cortex...

Functional connectivity in the first year of life in infants at-risk for autism: a preliminary near-infrared spectroscopy study

Keehn, Brandon; Wagner, Jennifer B.; Tager-Flusberg, Helen; Nelson, Charles A.
Fonte: Frontiers Media S.A. Publicador: Frontiers Media S.A.
Tipo: Artigo de Revista Científica
EN_US
Relevância na Pesquisa
36.78%
Background: Autism spectrum disorder (ASD) has been called a “developmental disconnection syndrome,” however the majority of the research examining connectivity in ASD has been conducted exclusively with older children and adults. Yet, prior ASD research suggests that perturbations in neurodevelopmental trajectories begin as early as the first year of life. Prospective longitudinal studies of infants at risk for ASD may provide a window into the emergence of these aberrant patterns of connectivity. The current study employed functional connectivity near-infrared spectroscopy (NIRS) in order to examine the development of intra- and inter-hemispheric functional connectivity in high- and low-risk infants across the first year of life. Methods: NIRS data were collected from 27 infants at high risk for autism (HRA) and 37 low-risk comparison (LRC) infants who contributed a total of 116 data sets at 3-, 6-, 9-, and 12-months. At each time point, HRA and LRC groups were matched on age, sex, head circumference, and Mullen Scales of Early Learning scores. Regions of interest (ROI) were selected from anterior and posterior locations of each hemisphere. The average time course for each ROI was calculated and correlations for each ROI pair were computed. Differences in functional connectivity were examined in a cross-sectional manner. Results: At 3-months...

Connectivity in MEG resting-state networks increases after resective surgery for low-grade glioma and correlates with improved cognitive performance☆

van Dellen, E.; de Witt Hamer, P.C.; Douw, L.; Klein, M.; Heimans, J.J.; Stam, C.J.; Reijneveld, J.C.; Hillebrand, A.
Fonte: Elsevier Publicador: Elsevier
Tipo: Artigo de Revista Científica
EN_US
Relevância na Pesquisa
36.72%
Purpose Low-grade glioma (LGG) patients often have cognitive deficits. Several disease- and treatment related factors affect cognitive processing. Cognitive outcome of resective surgery is unpredictable, both for improvement and deterioration, especially for complex domains such as attention and executive functioning. MEG analysis of resting-state networks (RSNs) is a good candidate for presurgical prediction of cognitive outcome. In this study, we explore the relation between alterations in connectivity of RSNs and changes in cognitive processing after resective surgery, as a stepping stone to ultimately predict postsurgical cognitive outcome. Methods: Ten patients with LGG were included, who had no adjuvant therapy. MEG recording and neuropsychological assessment were obtained before and after resective surgery. MEG data were recorded during a no-task eyes-closed condition, and projected to the anatomical space of the AAL atlas. Alterations in functional connectivity, as characterized by the phase lag index (PLI), within the default mode network (DMN), executive control network (ECN), and left- and right-sided frontoparietal networks (FPN) were compared to cognitive changes. Results: Lower alpha band DMN connectivity was increased after surgery...

Changes of functional connectivity in the left frontoparietal network following aphasic stroke

Zhu, Dan; Chang, Jingling; Freeman, Sonya; Tan, Zhongjian; Xiao, Juan; Gao, Ying; Kong, Jian
Fonte: Frontiers Media S.A. Publicador: Frontiers Media S.A.
Tipo: Artigo de Revista Científica
EN_US
Relevância na Pesquisa
36.8%
Language is an essential higher cognitive function supported by large-scale brain networks. In this study, we investigated functional connectivity changes in the left frontoparietal network (LFPN), a language-cognition related brain network in aphasic patients. We enrolled 13 aphasic patients who had undergone a stroke in the left hemisphere and age-, gender-, educational level-matched controls and analyzed the data by integrating independent component analysis (ICA) with a network connectivity analysis method. Resting state functional magnetic resonance imaging (fMRI) and clinical evaluation of language function were assessed at two stages: 1 and 2 months after stroke onset. We found reduced functional connectivity between the LFPN and the right middle frontal cortex, medial frontal cortex, and right inferior frontal cortex in aphasic patients as compared to controls. Correlation analysis showed that stronger functional connectivity between the LFPN and the right middle frontal cortex and medial frontal cortex coincided with more preserved language comprehension ability after stroke. Network connectivity analysis showed reduced LFPN connectivity as indicated by the mean network connectivity index of key regions in the LFPN of aphasic patients. The decreased LFPN connectivity in stroke patients was significantly associated with the impairment of language function in their comprehension ability. We also found significant association between recovery of comprehension ability and the mean changes in intrinsic LFPN connectivity. Our findings suggest that brain lesions may influence language comprehension by altering functional connectivity between regions and that the patterns of abnormal functional connectivity may contribute to the recovery of language deficits.

Increased resting state functional connectivity in the fronto-parietal and default mode network in anorexia nervosa

Boehm, Ilka; Geisler, Daniel; King, Joseph A.; Ritschel, Franziska; Seidel, Maria; Deza Araujo, Yacila; Petermann, Juliane; Lohmeier, Heidi; Weiss, Jessika; Walter, Martin; Roessner, Veit; Ehrlich, Stefan
Fonte: Frontiers Media S.A. Publicador: Frontiers Media S.A.
Tipo: Artigo de Revista Científica
EN_US
Relevância na Pesquisa
36.75%
The etiology of anorexia nervosa (AN) is poorly understood. Results from functional brain imaging studies investigating the neural profile of AN using cognitive and emotional task paradigms are difficult to reconcile. Task-related imaging studies often require a high level of compliance and can only partially explore the distributed nature and complexity of brain function. In this study, resting state functional connectivity imaging was used to investigate well-characterized brain networks potentially relevant to understand the neural mechanisms underlying the symptomatology and etiology of AN. Resting state functional magnetic resonance imaging data was obtained from 35 unmedicated female acute AN patients and 35 closely matched healthy controls female participants (HC) and decomposed using spatial group independent component analyses (ICA). Using validated templates, we identified components covering the fronto-parietal “control” network, the default mode network (DMN), the salience network, the visual and the sensory-motor network. Group comparison revealed an increased functional connectivity between the angular gyrus and the other parts of the fronto-parietal network in patients with AN in comparison to HC. Connectivity of the angular gyrus was positively associated with self-reported persistence in HC. In the DMN...

Code-Switching Between Cultures And Languages : Creative connectivity

Shafiq, Muna
Fonte: Université de Montréal Publicador: Université de Montréal
Tipo: Thèse ou Mémoire numérique / Electronic Thesis or Dissertation
EN
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36.75%
Problème: Ma thèse porte sur l’identité individuelle comme interrogation sur les enjeux personnels et sur ce qui constitue l’identification hybride à l’intérieur des notions concurrentielles en ce qui a trait à l’authenticité. Plus précisément, j’aborde le concept des identifications hybrides en tant que zones intermédiaires pour ce qui est de l’alternance de codes linguistiques et comme négociation des espaces continuels dans leur mouvement entre les cultures et les langues. Une telle négociation engendre des tensions et/ou apporte le lien créatif. Les tensions sont inhérentes à n’importe quelle construction d’identité où les lignes qui définissent des personnes ne sont pas spécifiques à une culture ou à une langue, où des notions de l’identité pure sont contestées et des codes communs de l’appartenance sont compromis. Le lien créatif se produit dans les exemples où l’alternance de code linguistique ou la négociation des espaces produit le mouvement ouvert et fluide entre les codes de concurrence des références et les différences à travers les discriminations raciales, la sexualité, la culture et la langue. Les travaux que j’ai sélectionnés représentent une section transversale de quelques auteurs migrants provenant de la minorité en Amérique du Nord qui alternent les codes linguistiques de cette manière. Les travaux détaillent le temps et l’espace dans leur traitement de l’identité et dans la façon dont ils cernent l’hybridité dans les textes suivants : The Woman Warrior de Maxine Hong Kingston (1975-76)...

A connectivity index for discrete fracture networks

Xu, C.; Dowd, P.; Mardia, K.; Fowell, R.
Fonte: Kluwer Academic/Plenum Publ Publicador: Kluwer Academic/Plenum Publ
Tipo: Artigo de Revista Científica
Publicado em //2006 EN
Relevância na Pesquisa
36.75%
Connectivity is an important measure for assessing flow transport in rock, especially through fractures. In this paper, rock fracture systems are modelled by a discrete fracture model simulated by a marked point process. A connectivity index is then introduced to quantify the connectivity between any two points in space. Monte Carlo simulation is used to evaluate the connectivity index for stationary cases and relationships between the connectivity index and the parameters of the discrete fracture model are analysed. The average number of intersections per fracture, Xf, and the fracture intensity, P12 (P32), are calculated and the relationships between these parameters and the connectivity index are investigated, concluding that Xf is the more suitable parameter for the classification of rock mass flow properties. The relationships between the connectivity index and the percolation state of the fractured medium are also discussed. An edge correction is briefly discussed and a practical example is used to demonstrate the method of computing the connectivity index.; C. Xu, P. A. Dowd, K. V. Mardia and R. J. Fowell

Application of connectivity measures in enhanced geothermal systems

Fadakar Alghalandis, Y.; Dowd, P.; Xu, C.
Fonte: Geoscience Australia; Australia Publicador: Geoscience Australia; Australia
Tipo: Conference paper
Publicado em //2012 EN
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36.72%
The three major determinant factors for the productivity of hot dry rock geothermal reservoirs are fractures, fluid and heat. Fractures create an interconnected network that provides the pathways for fluid flow, which in turn facilitates heat exchange from the rock masses. The connection between fractures is therefore a critical characteristic of a successful heat producing geothermal system. Connectivity analysis is also an important component in the design, assessment and development of fracture-based reservoirs particularly enhanced geothermal systems. In this paper, we evaluate the application of two connectivity measures: the connectivity field and the connectivity index, of a fracture network. Both measures are well suited to stochastic modelling, which provides a means of incorporating the uncertainty due to lack of data. We demonstrate the effectiveness of both measures in the determination of preferential pathways through the fracture network. We also demonstrate the use of the connectivity field in determining the optimal location of an injection or production well so as to maximise the reservoir performance. The two measures show good correlation with other established connectivity measures such as Xf and P21 (P32). They are also shown to be useful in the evaluation of percolation state of a fractured rock mass.; Younes Fadakar-Alghalandis...

Modulation of amygdala response and connectivity in depression by serotonin transporter polymorphism and diagnosis

Costafreda, S.; McCann, P.; Saker, P.; Cole, J.; Cohen-Woods, S.; Farmer, A.; Aitchison, K.; McGuffin, P.; Fu, C.
Fonte: Elsevier Science BV Publicador: Elsevier Science BV
Tipo: Artigo de Revista Científica
Publicado em //2013 EN
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36.75%
BACKGROUND: Polymorphisms in the serotonin transporter gene (5-HTTLPR) modulate amygdala activity in healthy individuals. Increased responses to negative stimuli in carriers of low transcription alleles have been proposed to contribute to the pathogenesis of depression. We sought to investigate the effects of genotype as well as diagnosis in patients with depression. METHODS: Subjects with recurrent depression (n=67) and matched healthy controls (n=49) participated in a fMRI task of implicit processing of sad facial stimuli. Effects of biallelic (short (S) and long (L) alleles) and triallelic (including rs25531 A/G single nucleotide variation) models of 5-HTTLPR polymorphisms on amygdala activity and connectivity were investigated. RESULTS: Significant effects were observed of both genotype and diagnosis on amygdala activity. Increased amygdala activity was associated with 5-HTTLPR genotype in low transcription allele carriers as well as with a diagnosis of depression. The connectivity analysis revealed a main effect of genotype with reduced connectivity to the subgenual region of the anterior cingulate in carriers of the low transcription alleles. There was also a main effect of diagnosis with reduced connectivity to the dorsal region of the anterior cingulate and to the dorsolateral prefrontal cortex in depression. There were no interaction effects between genotype and diagnosis in amygdala activity or connectivity. CONCLUSIONS: Significant independent effects of genotype and diagnosis on amygdala responsivity were revealed. The effects of genotype and diagnosis on amygdala connectivity showed a regional segregation...

Connectivity field: a measure for characterising fracture networks

Alghalandis, Y.F.; Dowd, P.A.; Xu, C.
Fonte: Springer Publicador: Springer
Tipo: Artigo de Revista Científica
Publicado em //2015 EN
Relevância na Pesquisa
36.75%
Analysis of the connectivity of a fracture network is an important component of the design, assessment and development of fracture-based reservoirs in geothermal, petroleum and groundwater resource applications. It is a useful means of characterising the flow pathways and the mechanical behaviours of reservoirs. An appropriate practical measure is required for connectivity characterisation because of the extreme complexity of fracture networks. In this paper, we propose the connectivity field (CF), as a useful measure to evaluate the spatial connectivity characteristics of fractures in a fracture network. The CF can be applied on both a particular realisation of a fracture network model (for deterministic evaluation) and on stochastic fracture network models using stochastic modelling and Monte Carlo simulations (for probabilistic evaluation with uncertainties). Two extensions are also proposed: the generalised connectivity field, a measure that is independent of support size, and the probabilistic connectivity field. Potential applications of the CF and its extensions are in determining the optimal location of an injection or production well so as to maximise reservoir performance and in determining potential flow pathways in fracture networks. The average CF map shows strong correlations with the Xf and P21 measures. The relationships between the CF measures...

The effects of habitat connectivity and regional heterogeneity on artificial pond metacommunities

Pedruski, Michael
Fonte: Quens University Publicador: Quens University
Tipo: Tese de Doutorado Formato: 3294139 bytes; application/pdf
EN; EN
Relevância na Pesquisa
36.79%
While much evidence suggests that ecosystem functioning is closely related to biodiversity, present rates of biodiversity loss are high. With the emergence of the metacommunity concept ecologists have become increasingly aware that both local processes (e.g. competition, predation), and regional processes (e.g. dispersal and regional heterogeneity) affect ecological communities at multiple spatial scales. I experimentally investigated the effects of habitat connectivity and regional heterogeneity on biodiversity, community composition, and ecosystem functioning of artificial pond metacommunities of freshwater invertebrates at the local (α), among-community (β), and regional (γ) spatial levels. There was a significant effect of habitat connectivity on mean local richness, but mean local Simpson diversity, mean local functional diversity (FD), and all the three indices of ecosystem functioning investigated (regional abundance, invertebrate biomass, and chlorophyll a concentration) were unaffected by connectivity levels. Regional heterogeneity had no effect on local diversity, but enhanced both among-community richness and among-community Simpson diversity. Conversely, connectivity reduced among-community Simpson diversity. All indices of regional diversity were unaffected by either connectivity or heterogeneity. Despite expectations that there would be strong interactions between the effects of connectivity and heterogeneity on species richness...

Street Connectivity as a Determinant of Health in Canadian Youth

Mecredy, Graham C
Fonte: Quens University Publicador: Quens University
Tipo: Tese de Doutorado
EN; EN
Relevância na Pesquisa
36.78%
Background: Street connectivity, an important aspect of the built environment, refers to the directness of links and the density of connections in road networks. Street connectivity of youth’s neighborhoods may impact upon both physical activity patterns and the occurrence of physical activity injuries in street locations. Objectives: The objectives of the two studies comprising this thesis were to examine, among Canadian youth in grades 6 to 10: (1) the relationship between street connectivity and physical activity, and (2) the relationship between street connectivity and physical activity injuries occurring in the street. Methods: 9,717 youth in grades 6-10 from 188 schools across Canada participated in the 2006 Health Behaviour in School-aged Children (HBSC) survey. A street connectivity composite measure was gathered objectively using a geographic information system database, and was based on intersection density, average block length, and connected node ratio in a 5km radius around each school. Physical activity was measured via students’ self-reported hours of moderate-to-vigorous physical activity per week outside of school hours. Injury was measured via students’ self-reports to questions regarding whether they had suffered an injury while playing in the street in the past 12 months. Multi-level regression analyses were used to examine the relations between street connectivity and both physical activity and injury. Results: Compared to students living in the highest street connectivity quartile...

Assessing and mitigating road effects on functional landscape connectivity: a case study with common genets (Genetta Genetta) in a mediterranean context

Carvalho, Filipe Granja de
Fonte: Universidade de Évora Publicador: Universidade de Évora
Tipo: Tese de Doutorado
ENG
Relevância na Pesquisa
36.75%
A conectividade funcional da paisagem para uma espécie depende do seu comportamento e capacidade de movimento e influencia a sua persistência em paisagens humanizadas. Porém os efeitos desta componente da paisagem nos carnívoros florestais são pouco conhecidos. Esta tese usa dados de movimento, comportamento e genética para avaliar os efeitos das estradas e da heterogeneidade dos habitats na conectividade funcional da geneta numa paisagem florestal agrícola, do Sul de Portugal. Especificamente, pretende-se alcançar os seguintes objetivos: i) avaliar os fatores ambientais que influenciam o comportamento de repouso; ii) determinar a influência da intensidade da monitorização na análise do comportamento de repouso; iii) analisar trajetórias de movimentos para modelar a conectividade da paisagem e avaliar os fatores ambientais que a influenciam, a diferentes escalas; iv) combinar dados de mortalidade rodoviária, radiotelemetria, e análises genéticas para inferir atravessamentos de estradas e dispersão efetiva. As cavidades arbóreas foram o abrigo mais seguro, particularmente na época húmida, e permitiram o abrigo junto a rodovias, sempre que necessário. A monitorização diária foi a que melhor permitiu caracterizar os comportamentos de repouso. A conectividade da paisagem aumentou em grandes áreas florestais...