20,082 research outputs found
Modelling spatial recall, mental imagery and neglect
We present a computational model of the neural mechanisms in the pari-etal and temporal lobes that support spatial navigation, recall of scenes and imagery of the products of recall. Long term representations are stored in the hippocampus, and are associated with local spatial and object-related features in the parahippocampal region. Viewer-centered representations are dynamically generated from long term memory in the parietal part of the model. The model thereby simulates recall and im-agery of locations and objects in complex environments. After parietal damage, the model exhibits hemispatial neglect in mental imagery that rotates with the imagined perspective of the observer, as in the famous Milan Square experiment [1]. Our model makes novel predictions for the neural representations in the parahippocampal and parietal regions and for behavior in healthy volunteers and neuropsychological patients.
Covariant approach to equilibration in effective field theories
The equilibration of two coupled reservoirs is studied using a Green function
approach which is suitable for future development with the closed time path
method. The problem is solved in two parameterizations, in order to demonstrate
the non-trivial issues of parameterization in both the intermediate steps and
the interpretation of physical quantities. We use a covariant approach to find
self-consistent solutions for the statistical distributions as functions of
time. We show that by formally introducing covariant connections, one can
rescale a slowly varying non-equilibrium theory so that it appears to be an
equilibrium one, for the purposes of calculation. We emphasize the importance
of properly tracking variable redefinitions in order to correctly interpret
physical quantities.Comment: 11 pages, Late
Smarter task assignment or greater effort: the impact of incentives on team performance
We use an experiment to study the impact of team-based incentives, exploiting rich data from personnel records and management information systems. Using a triple difference design, we show that the incentive scheme had an impact on team performance, even with quite large teams. We examine whether this effect was due to increased effort from workers or strategic task reallocation. We find that the provision of financial incentives did raise individual performance but that managers also disproportionately reallocated efficient workers to the incentivised tasks. We show that this reallocation was the more important contributor to the overall outcome
Brane Gases on K3 and Calabi-Yau Manifolds
We initiate the study of Brane Gas Cosmology (BGC) on manifolds with
non-trivial holonomy. Such compactifications are required within the context of
superstring theory in order to make connections with realistic particle
physics. We study the dynamics of brane gases constructed from various string
theories on background spaces having a K3 submanifold. The K3 compactifications
provide a stepping stone for generalising the model to the case of a full
Calabi-Yau three-fold. Duality symmetries are discussed within a cosmological
context. Using a duality, we arrive at an N=2 theory in four-dimensions
compactified on a Calabi-Yau manifold with SU(3) holonomy. We argue that the
Brane Gas model compactified on such spaces maintains the successes of the
trivial toroidal compactification while greatly enhancing its connection to
particle physics. The initial state of the universe is taken to be a small, hot
and dense gas of p-branes near thermal equilibrium. The universe has no initial
singularity and the dynamics of string winding modes allow three spatial
dimensions to grow large, providing a possible solution to the dimensionality
problem of string theory.Comment: 26 pages; Significant revisions: review material truncated;
presentation improve
Coriolis force corrections to g-mode spectrum in 1D MHD model
The corrections to g-mode frequencies caused by the presence of a central
magnetic field and rotation of the Sun are calculated. The calculations are
carried out in the simple one dimensional magnetohydrodynamical model using the
approximations which allow one to find the purely analytical spectra of
magneto-gravity waves beyond the scope of the JWKB approximation and avoid in a
small background magnetic field the appearance of the cusp resonance which
locks a wave within the radiative zone. These analytic results are compared
with the satellite observations of the g-mode frequency shifts which are of the
order one per cent as given in the GOLF experiment at the SoHO board. The main
contribution turns out to be the magnetic frequency shift in the strong
magnetic field which obeys the used approximations. In particular, the fixed
magnetic field strength 700 KG results in the mentioned value of the frequency
shift for the g-mode of the radial order n=-10. The rotational shift due to the
Coriolis force appears to be small and does not exceed a fracton of per cent,
\alpha_\Omega < 0.003.Comment: RevTeX4, 9 pages, 4 eps figures; accepted for publication in
Astronomy Reports (Astronomicheskii Zhurnal
On Bouncing Brane-Worlds, S-branes and Branonium Cosmology
We present several higher-dimensional spacetimes for which observers living
on 3-branes experience an induced metric which bounces. The classes of examples
include boundary branes on generalised S-brane backgrounds and probe branes in
D-brane/anti D-brane systems. The bounces we consider normally would be
expected to require an energy density which violates the weak energy condition,
and for our co-dimension one examples this is attributable to bulk curvature
terms in the effective Friedmann equation. We examine the features of the
acceleration which provides the bounce, including in some cases the existence
of positive acceleration without event horizons, and we give a geometrical
interpretation for it. We discuss the stability of the solutions from the point
of view of both the brane and the bulk. Some of our examples appear to be
stable from the bulk point of view, suggesting the possible existence of stable
bouncing cosmologies within the brane-world framework.Comment: 35 pages, 7 figures, JHEP style. Title changed and references adde
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Statistical issues in Mendelian randomization: use of genetic instrumental variables for assessing causal associations
Mendelian randomization is an epidemiological method for using genetic variation to estimate the causal effect of the change in a modifiable phenotype on an outcome from observational data. A genetic variant satisfying the assumptions of an instrumental variable for the phenotype of interest can be used to divide a population into subgroups which differ systematically only in the phenotype. This gives a causal estimate which is asymptotically free of bias from confounding and reverse causation. However, the variance of the causal estimate is large compared to traditional regression methods, requiring large amounts of data and necessitating methods for efficient data synthesis. Additionally, if the association between the genetic variant and the phenotype is not strong, then the causal estimates will be biased due to the “weak instrument” in finite samples in the direction of the observational association. This bias may convince a researcher that an observed association is causal. If the causal parameter estimated is an odds ratio, then the parameter of association will differ depending on whether viewed as a population-averaged causal effect or a personal causal effect conditional on covariates. We introduce a Bayesian framework for instrumental variable analysis, which is less susceptible to weak instrument bias than traditional two-stage methods, has correct coverage with weak instruments, and is able to efficiently combine gene–phenotype–outcome data from multiple heterogeneous sources. Methods for imputing missing genetic data are developed, allowing multiple genetic variants to be used without reduction in sample size. We focus on the question of a binary outcome, illustrating how the collapsing of the odds ratio over heterogeneous strata in the population means that the two-stage and the Bayesian methods estimate a population-averaged marginal causal effect similar to that estimated by a randomized trial, but which typically differs from the conditional effect estimated by standard regression methods. We show how these methods can be adjusted to give an estimate closer to the conditional effect. We apply the methods and techniques discussed to data on the causal effect of C-reactive protein on fibrinogen and coronary heart disease, concluding with an overall estimate of causal association based on the totality of available data from 42 studies.This work was supported by the U.K. Medical Research Council [grant number U.1052.00.001]
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Estimating and contextualizing the attenuation of odds ratios due to non collapsibility
The odds ratio is a measure commonly used for expressing the association between an exposure and a binary outcome. A feature of the odds ratio is that its value depends on the choice of the distribution over which the probabilities in the odds ratio are evaluated. In particular, this means that an odds ratio conditional on a covariate may have a different value from an odds ratio marginal on the covariate, even if the covariate is not associated with the exposure (not a confounder). We define the individual and population odds ratios as the ratio of the odds of the outcome for a unit increase in the exposure respectively for an individual in the population, and for the whole population, in which case the odds are averaged across the population. The attenuation of conditional, marginal and population odds ratios from the individual odds ratio is demonstrated in a realistic simulation exercise. The degree of attenuation differs in the whole population and in a case-control sample, and the property of invariance to outcome-dependent sampling is only true for the individual odds ratio. The relevance of the non-collapsibility of odds ratios in a range of methodological areas is discussed.Stephen Burgess is supported by the Wellcome Trust (grant number 100114). No specific funding was received for the writing of this manuscript.This is the author accepted manuscript. The final version is available from Taylor & Francis via http://dx.doi.org/10.1080/03610926.2015.100677
Prepontine non-giant neurons drive flexible escape behavior in zebrafish
Many species execute ballistic escape reactions to avoid imminent danger. Despite fast reaction times, responses are often highly regulated, reflecting a trade-off between costly motor actions and perceived threat level. However, how sensory cues are integrated within premotor escape circuits remains poorly understood. Here, we show that in zebrafish, less precipitous threats elicit a delayed escape, characterized by flexible trajectories, which are driven by a cluster of 38 prepontine neurons that are completely separate from the fast escape pathway. Whereas neurons that initiate rapid escapes receive direct auditory input and drive motor neurons, input and output pathways for delayed escapes are indirect, facilitating integration of cross-modal sensory information. These results show that rapid decision-making in the escape system is enabled by parallel pathways for ballistic responses and flexible delayed actions and defines a neuronal substrate for hierarchical choice in the vertebrate nervous system
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