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Recent documents in Articlesen-usFri, 29 Apr 2016 01:59:13 PDT3600Simultaneous Modelling of Initial Conditions and Time Heterogeneity in Dynamic Networks: An Application to Foreign Direct Investments
http://arrow.dit.ie/scschmatart/216
http://arrow.dit.ie/scschmatart/216Wed, 27 Apr 2016 02:54:37 PDT
In dynamic networks, the presence of ties are subject both to endogenous network dependencies and spatial dependencies. Current statistical models for change over time are typically defined relative to some initial condition, thus skirting the issue of where the first network came from. Additionally, while these longitudinal network models may explain the dynamics of change in the network over time, they do not explain the change in those dynamics. We propose an extension to the longitudinal exponential random graph model that allows for simultaneous inference of the changes over time and the initial conditions, as well as relaxing assumptions of time-homogeneity. Estimation draws on recent Bayesian approaches for cross-sectional exponential random graph models and Bayesian hierarchical models. This is developed in the context of foreign direct investment relations in the global electricity industry in 1995–2003. International investment relations are known to be affected by factors related to: (i) the initial conditions determined by the geographical locations; (ii) time-dependent fluctuations in the global intensity of investment flows; and (iii) endogenous network dependencies. We rely on the well-known gravity model used in research on international trade to represent how spatial embedding and endogenous network dependencies jointly shape the dynamics of investment relations.
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Johan Koskinen et al.Knowledge Sharing in Organisations: a Bayesian Analysis of the Role of Reciprocity and Formal Structure
http://arrow.dit.ie/scschmatart/215
http://arrow.dit.ie/scschmatart/215Wed, 27 Apr 2016 02:54:28 PDT
We examine the conditions under which knowledge embedded in advice relations is likely to reach across intraorganizational boundaries and be shared between distant organizational members. We emphasize boundary-crossing relations because activities of knowledge transfer and sharing across subunit boundaries are systematically related to desirable organizational outcomes. Our main objective is to understand how organizational and social processes interact to sustain the transfer of knowledge carried by advice relations. Using original fieldwork and data that we have collected on members of the top management team in a multiunit industrial group, we show that knowledge embedded in task advice relations is unlikely to crosscut intra- organizational boundaries, unless advice relations are reciprocated, and supported by the pres- ence of hierarchical relations linking managers in different subunits. The results we report are based on a novel Bayesian Exponential Random Graph Models (BERGMs) framework that allows us to test and assess the empirical value of our hypotheses while at the same time accounting for structural characteristics of the intraorganizational network of advice relations. We rely on computational and simulation methods to establish the consistency of the network implied by the model we propose with the structure of the intraorganizational network that we actually observed.
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Alberto Caimo et al.Bayesian Exponential Random Graph Models with Nodal Random Effects
http://arrow.dit.ie/scschmatart/214
http://arrow.dit.ie/scschmatart/214Mon, 25 Apr 2016 05:17:55 PDT
We extend the well-known and widely used Exponential Random Graph Model (ERGM) by including nodal random effects to compensate for heterogeneity in the nodes of a network. The Bayesian framework for ERGMs proposed by Caimo and Friel (2011) yields the basis of our modelling algorithm. A central question in network models is the question of model selection and following the Bayesian paradigm we focus on estimating Bayes factors. To do so we develop an approximate but feasible calculation of the Bayes factor which allows one to pursue model selection. Two data examples and a small simulation study illustrate our mixed model approach and the corresponding model selection.
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Stephanie Thiemichen et al.Efficient Computational Strategies for Doubly Intractable Problems with Applications to Bayesian Social Networks
http://arrow.dit.ie/scschmatart/213
http://arrow.dit.ie/scschmatart/213Mon, 25 Apr 2016 04:58:25 PDT
Powerful ideas recently appeared in the literature are adjusted and combined to design improved samplers for doubly intractable target distributions with a focus on Bayesian exponential random graph models. Different forms of adaptive Metropolis–Hastings proposals (vertical, horizontal and rectangular) are tested and merged with the delayed rejection (DR) strategy with the aim of reducing the variance of the resulting Markov chain Monte Carlo estimators for a given computational time. The DR is modified in order to integrate it within the approximate exchange algorithm (AEA) to avoid the computation of intractable normalising constant that appears in exponential random graph models. This gives rise to the AEA + DR: a new methodology to sample doubly intractable distributions that dominates the AEA in the Peskun ordering (Peskun Biometrika 60:607– 612, 1973) leading to MCMC estimators with a smaller asymptotic variance. The Bergm package for R (Caimo and Friel J. Stat. Softw. 22:518–532, 2014) has been updated to incorporate the AEA + DR thus including the possibility of adding a higher stage proposals and different forms of adaptation.
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Alberto Caimo et al.Bergm: Bayesian Exponential Random Graphs in R
http://arrow.dit.ie/scschmatart/212
http://arrow.dit.ie/scschmatart/212Mon, 25 Apr 2016 04:32:59 PDT
In this paper we describe the main featuress of the Bergm package for the open-source R software which provides a comprehensive framework for Bayesian analysis for exponential random graph models: tools for parameter estimation, model selection and goodness-of-fit diagnostics. We illustrate the capabilities of this package describing the algorithms through a tutorial analysis of three network datasets.
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Alberto Caimo et al.Delayed Rejection Algorithm to Estimate Bayesian Social Networks
http://arrow.dit.ie/scschmatart/211
http://arrow.dit.ie/scschmatart/211Mon, 25 Apr 2016 04:28:30 PDT
Statistical social network analysis has become a very active and fertile area of research in the recent past. Recent developments in Bayesian computational methods have been successfully applied to estimate social network models. The Delayed rejection (DR) strategy is a modification of the Metropolis-Hastings (MH) algorithms that reduces the variance of the resulting Markov chain Monte Carlo estimators and allows partial adaptation of the proposal distribution. In this paper we show how the DR strategy can be exploited to estimate dyadic independence social network models leading to an average 40% variance reduction relative to the competing MH algorithm, confirming that DR dominates, in terms of Peskun ordering, the MH algorithm.
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Alberto Caimo et al.Bayesian Model Selection for Exponential Random Graph Models
http://arrow.dit.ie/scschmatart/210
http://arrow.dit.ie/scschmatart/210Mon, 25 Apr 2016 04:18:23 PDT
Exponential random graph models are a class of widely used exponential family models for social networks. The topological structure of an observed network is modelled by the relative prevalence of a set of local sub-graph configurations termed network statistics. One of the key tasks in the application of these models is which network statistics to include in the model. This can be thought of as statistical model selection problem. This is a very challenging problem---the posterior distribution for each model is often termed ``doubly intractable'' since computation of the likelihood is rarely available, but also, the evidence of the posterior is, as usual, intractable. The contribution of this paper is the development of a fully Bayesian model selection method based on a reversible jump Markov chain Monte Carlo algorithm extension of Caimo & Friel (2011) which estimates the posterior probability for each competing model.
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Alberto Caimo et al.Bayesian Inference for Exponential Random Graph Models
http://arrow.dit.ie/scschmatart/209
http://arrow.dit.ie/scschmatart/209Mon, 25 Apr 2016 04:08:12 PDT
Exponential random graph models are extremely difficult models to handle from a statistical viewpoint, since their normalising constant, which depends on model parameters, is available only in very trivial cases. We show how inference can be carried out in a Bayesian framework using a MCMC algorithm, which circumvents the need to calculate the normalising constants. We use a population MCMC approach which accelerates convergence and improves mixing of the Markov chain. This approach improves performance with respect to the Monte Carlo maximum likelihood method of Geyer and Thompson (1992).
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Alberto Caimo et al.Hawking Radiation Screening and Penrose Process Shielding in the Kerr Black Hole
http://arrow.dit.ie/scschmatart/208
http://arrow.dit.ie/scschmatart/208Wed, 06 Apr 2016 06:43:12 PDTThe radial motion of massive particles in the equatorial plane of aKerr black hole is considered. Screening of the Hawking radiation and shielding of the Penrose process are examined (both inside and outside the ergosphere) and their effect on the evaporation of the black hole is studied. In particular, the locus and width of a classically forbidden region and their dependence on the particle’s angular momentum and energy is analysed. Tunneling of particles between the boundaries of this region is considered and the transmission coefficient determined.
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Eamon Mc CaugheyThe Neural Dynamics of Somatosensory Processing and Adaptation Across Childhood: a High-Density Electrical Mapping Study
http://arrow.dit.ie/scschmatart/207
http://arrow.dit.ie/scschmatart/207Wed, 23 Mar 2016 05:27:59 PDT
Young children are often hyperreactive to somatosensory inputs hardly noticed by adults, as exemplified by irritation to seams or labels in clothing. The neurodevelopmental mechanisms underlying changes in sensory reactivity are not well understood. Based on the idea that neurodevelopmental changes in somatosensory processing and/or changes in sensory adaptation might underlie developmental differences in somatosensory reactivity, high-density electroencephalography was used to examine how the nervous system responds and adapts to repeated vibrotactile stimulation over childhood. Participants aged 6–18 yr old were presented with 50-ms vibrotactile stimuli to the right wrist over the median nerve at 5 blocked interstimulus intervals (ranging from 7 to 1 per second). Somatosensory evoked potentials (SEPs) revealed three major phases of activation within the first 200 ms, with scalp topographies suggestive of neural generators in contralateral somatosensory cortex. Although overall SEPs were highly similar for younger, middle, and older age groups (6.1–9.8, 10.0 –12.9, and 13.0 –17.8 yr old), there were significant age-related amplitude differences in initial and later phases of the SEP. In contrast, robust adaptation effects for fast vs. slow presentation rates were observed that did not differ as a function of age. A greater amplitude response in the later portion of the SEP was observed for the youngest group and may be related to developmental changes in responsivity to somatosensory stimuli. These data suggest the protracted development of the somatosensory system over childhood, whereas adaptation, as assayed in this study, is largely in place by 7 years of age.
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Neha Uppal et al.Hamiltonian Formulation for Wave-Current Interactions in Stratified Rotational Flows
http://arrow.dit.ie/scschmatart/206
http://arrow.dit.ie/scschmatart/206Mon, 14 Mar 2016 09:27:56 PDT
We show that the Hamiltonian framework permits an elegant formulation of the nonlinear governing equations for the coupling between internal and surface waves in stratified water flows with piecewise constant vorticity.
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Adrian Constantin et al.Second Order Approximation For Heat Conduction: Dissipation Principle and Free Energies
http://arrow.dit.ie/scschmatart/205
http://arrow.dit.ie/scschmatart/205Mon, 08 Feb 2016 06:31:41 PST
In the context of new models of heat conduction, the second-order approximation of Tzou’s theory, derived by Quintanilla and Racke, has been studied recently by two of the present authors, where it was proved equivalent to a fading memory material. The importance of determining free energy functionals for such materials, and indeed for any material with memory, is emphasized. Because the kernel does not satisfy certain convexity restrictions that allow us to obtain various traditional free energies for materials with fading memory, it is necessary to restrict the study to the minimum and related free energies, which do not require these restrictions. Thus, the major part of this work is devoted to deriving an explicit expression for the minimum free energy. Simple modifications of this expression also give an intermediate free energy and the maximum free energy for the material. These derivations differ in certain important respects from earlier work on such free energies.
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Giovambattista Amendola et al.Evaporation Screening, Scattering, and Quantum Tunneling Near the Horizons of Schwarzschild and Reissner-Nordstroem Black Holes
http://arrow.dit.ie/scschmatart/204
http://arrow.dit.ie/scschmatart/204Thu, 28 Jan 2016 01:13:11 PST
For the radial motion of massive particles with large angular momenta in Schwarzschild geometry and that of massive charged particles with large angular momenta or energy in a particular range in Reissner-Nordstroem geometry, there exist classically forbidden regions on the outside of the respective event horizons which scatter certain in-falling geodesics or screen some of the black holes' evaporation by reflecting the emitted particles back into the black holes. Quantum tunneling across this forbidden regions is studied.
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Emil ProdanovCongruent Visual Speech Enhances Entrainment to Continuous Auditory Speech in Noise-free Conditions
http://arrow.dit.ie/scschmatart/203
http://arrow.dit.ie/scschmatart/203Tue, 19 Jan 2016 06:38:37 PST
Congruent audiovisual speech enhances our ability to comprehend a speaker, even in noise-free conditions. When incongruent auditory and visual information is presented concurrently, it can hinder a listener’s perception and even cause him or her to perceive information that was not presented in either modality. Efforts to investigate the neural basis of these effects have often focused on the special case of discrete audiovisual syllables that are spatially and temporally congruent, with less work done on the case of natural, continuous speech. Recent electrophysiological studies have demonstrated that cortical response measures to continuous auditory speech can be easily obtained using multivariate analysis methods. Here, we apply such methods to the case of audiovisual speech and, importantly, present a novel framework for indexing multisensory integration in the context of continuous speech. Specifically, we examine how the temporal and contextual congruency of ongoing audiovisual speech affects the cortical encoding of the speech envelope in humans using electroencephalography. We demonstrate that the cortical representation of the speech envelope is enhanced by the presentation of congruent audiovisual speech in noise-free conditions. Furthermore, we show that this is likely attributable to the contribution of neural generators that are not particularly active during unimodal stimulation and that it is most prominent at the temporal scale corresponding to syllabic rate (2– 6 Hz). Finally, our data suggest that neural entrainment to the speech envelope is inhibited when the auditory and visual streams are incongruent both temporally and contextually.
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Michael Crosse et al.Audiovisual Processing is Abnormal in Parkinson's Disease and Correlates with Freezing of Gait and Disease Duration
http://arrow.dit.ie/scschmatart/202
http://arrow.dit.ie/scschmatart/202Tue, 19 Jan 2016 06:38:28 PST
Background: Sensory and perceptual disturbances progress with disease duration in Parkinson’s disease (PD) and probably contribute to motor deficits such as bradykinesia and gait disturbances, including freezing of gait (FOG). Simple reaction time tests are ideal to explore sensory processing, as they require little cognitive processing. Multisensory integration is the ability of the brain to integrate sensory information from multiple modalities into a single coherent percept, which is crucial for complex motor tasks such as gait. 9 10 11 12 13 Objectives: The aims of this study were to: 1. Assess differences in unisensory (auditory and visual) and multisensory processing speed in people with PD and age-matched healthy controls. 2. Compare relative differences in unisensory processing in people with PD with disease duration and freezing of gait status taking into account the motor delays, which are invariably present in PD. 3. Compare relative differences in multisensory (audiovisual) processing between the PD cohort and age-matched controls. 14 15 16 17 Methods: 39 people with PD (23 with FOG) and 17 age-matched healthy controls performed a reaction time task in response to unisensory (auditory-alone, visual-alone) and multisensory (audiovisual) stimuli. 18 19 Results: The PD group were significantly slower than controls for all conditions compared with healthy controls but auditory reaction times were significantly faster than visual for the PD group only. These relative unisensory differences are correlated with disease duration and divide the PD group by FOG status, but these factors are co-dependent. Although multisensory facilitation occurs in PD, it is significantly less enhanced than in healthy controls. 20 21 22 23 Conclusion: There are significant unisensory and multisensory processing abnormalities in PD. The relative differences in unisensory processing are specific to PD progression, providing a link between these sensory abnormalities and a motor feature of PD. Sensory disturbances have previously been postulated to be central to FOG but this is the first study to predict audiovisual processing abnormalities using FOG status. The multisensory processing abnormalities are independent of disease duration and FOG status and may be a potential biomarker for the disease.
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Conor Fearon et al.Age-Related Sexual Dimorphism in Temporal Discrimination and in Adult-Onset Dystonia Suggests GABAergic Mechanisms
http://arrow.dit.ie/scschmatart/201
http://arrow.dit.ie/scschmatart/201Tue, 19 Jan 2016 06:38:05 PST
Background: Adult-onset isolated focal dystonia (AOIFD) presenting in early adult life is more frequent in men, whereas in middle age it is female predominant. Temporal discrimination, an endophenotype of adult-onset idiopathic isolated focal dystonia, shows evidence of sexual dimorphism in healthy participants. Objectives: We assessed the distinctive features of age-related sexual dimorphism of (i) sex ratios in dystonia phenotypes and (ii) sexual dimorphism in temporal discrimination in unaffected relatives of cervical dystonia patients. Methods: We performed (i) a meta-regression analysis of the proportion of men in published cohorts of phenotypes of adult-onset dystonia in relation to their mean age of onset and (ii) an analysis of temporal discrimination thresholds in 220 unaffected first-degree relatives (125 women) of cervical dystonia patients. Results: In 53 studies of dystonia phenotypes, the proportion of men showed a highly significant negative association with mean age of onset (p < 0.0001, pseudo-R2 = 59.6%), with increasing female predominance from 40 years of age. Age of onset and phenotype together explained 92.8% of the variance in proportion of men. Temporal discrimination in relatives under the age of 35 years is faster in women than men but the age-related rate of deterioration in women is twice that of men; after 45 years of age, men have faster temporal discrimination than women. Conclusion: Temporal discrimination in unaffected relatives of cervical dystonia patients and sex ratios in adult-onset dystonia phenotypes show similar patterns of age-related sexual dimorphism. Such age-related sexual dimorphism in temporal discrimination and adult-onset focal dystonia may reflect common underlying mechanisms. Cerebral GABA levels have been reported to show similar age-related sexual dimorphism in healthy participants and may be the mechanism underlying the observed age-related sexual dimorphism in temporal discrimination and the sex ratios in AOIFD.
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John S. Butler et al.The Minimum and Other Free Energies for Non-Linear Materials With Memory.
http://arrow.dit.ie/scschmatart/200
http://arrow.dit.ie/scschmatart/200Mon, 07 Dec 2015 07:03:41 PST
Expressions are obtained for free energies of materials with a certain type of non-linear constitutive relation. In particular, the minimum and related free energies are considered in some detail. Minimal states are defined for these materials, and it is shown that any free energy yielding a linear constitutive equation that is a functional of the minimal state has a counterpart in the non-linear case which is also a minimal state functional in this more general context. These results are explored for simple examples, including discrete spectrum materials.
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Murrough GoldenFree Energies and Minimal States for Scalar Linear Viscoelasticity
http://arrow.dit.ie/scschmatart/199
http://arrow.dit.ie/scschmatart/199Mon, 07 Dec 2015 06:50:55 PST
The concept of a minimal state was introduced in recent decades, based on earlier work by Noll. The property that a given quantity is a functional of the minimal state is of central interest in the present work. Using a standard representation of a free energy associated with a linear memory constitutive relation, a new condition, involving linear functionals, is derived which, if satisfied, ensures that the free energy is a functional of the minimal state. Using this result and recent work on constructing free energy functionals, it is shown that if the kernel of the rate of dissipation functional is given by sums of products, the associated free energy functional is a functional of the minimal state.
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Murrough GoldenOn the Dynamics of Internal Waves Interacting with the Equatorial Undercurrent
http://arrow.dit.ie/scschmatart/198
http://arrow.dit.ie/scschmatart/198Wed, 18 Nov 2015 08:07:41 PST
The interaction of the nonlinear internal waves with a nonuniform current with a specific form, characteristic for the equatorial undercurrent, is studied. The current has no vorticity in the layer, where the internal wave motion takes place. We show that the nonzero vorticity that might be occuring in other layers of the current does not affect the wave motion. The equations of motion are formulated as a Hamiltonian system.
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Alan Compelli et al.Vacuum Polarization on the Brane
http://arrow.dit.ie/scschmatart/197
http://arrow.dit.ie/scschmatart/197Thu, 08 Oct 2015 01:47:55 PDT
We compute the renormalized expectation value of the square of a massless, conformally coupled, quantum scalar field on the brane of a higher-dimensional black hole. Working in the AADD braneworld scenario, the extra dimensions are flat and we assume that the compactification radius is large compared with the size of the black hole. The four-dimensional on-brane metric corresponds to a slice through a higher-dimensional Schwarzschild-Tangherlini black hole geometry and depends on the number of bulk space-time dimensions. The quantum scalar field is in a thermal state at the Hawking temperature. An exact, closed-form expression is derived for the renormalized expectation value of the square of the quantum scalar field on the event horizon of the black hole. Outside the event horizon, this renormalized expectation value is computed numerically. The answer depends on the number of bulk space-time dimensions, with a magnitude which increases rapidly as the number of bulk space-time dimensions increases.
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Cormac Breen et al.