742 research outputs found
Diffuse Gamma Rays: Galactic and Extragalactic Diffuse Emission
Full text link"Diffuse" gamma rays consist of several components: truly diffuse emission
from the interstellar medium, the extragalactic background, whose origin is not
firmly established yet, and the contribution from unresolved and faint Galactic
point sources. One approach to unravel these components is to study the diffuse
emission from the interstellar medium, which traces the interactions of high
energy particles with interstellar gas and radiation fields. Because of its
origin such emission is potentially able to reveal much about the sources and
propagation of cosmic rays. The extragalactic background, if reliably
determined, can be used in cosmological and blazar studies. Studying the
derived "average" spectrum of faint Galactic sources may be able to give a clue
to the nature of the emitting objects.Comment: 32 pages, 28 figures, kapproc.cls. Chapter to the book "Cosmic
Gamma-Ray Sources," to be published by Kluwer ASSL Series, Edited by K. S.
Cheng and G. E. Romero. More details can be found at
http://www.gamma.mpe-garching.mpg.de/~aws/aws.htm
Stratorotational instability in MHD Taylor-Couette flows
Full text linkThe stability of dissipative Taylor-Couette flows with an axial stable
density stratification and a prescribed azimuthal magnetic field is considered.
Global nonaxisymmetric solutions of the linearized MHD equations with toroidal
magnetic field, axial density stratification and differential rotation are
found for both insulating and conducting cylinder walls. Flat rotation laws
such as the quasi-Kepler law are unstable against the nonaxisymmetric
stratorotational instability (SRI). The influence of a current-free toroidal
magnetic field depends on the magnetic Prandtl number Pm: SRI is supported by
Pm > 1 and it is suppressed by Pm \lsim 1. For too flat rotation laws a smooth
transition exists to the instability which the toroidal magnetic field produces
in combination with the differential rotation. This nonaxisymmetric azimuthal
magnetorotational instability (AMRI) has been computed under the presence of an
axial density gradient. If the magnetic field between the cylinders is not
current-free then also the Tayler instability occurs and the transition from
the hydrodynamic SRI to the magnetic Tayler instability proves to be rather
complex. Most spectacular is the `ballooning' of the stability domain by the
density stratification: already a rather small rotation stabilizes magnetic
fields against the Tayler instability. An azimuthal component of the resulting
electromotive force only exists for density-stratified flows. The related
alpha-effect for magnetic SRI of Kepler rotation appears to be positive for
negative d\rho/dz <0.Comment: 10 pages, 13 figures, submitted to Astron. Astrophy
Mutual information rate and bounds for it
Get PDFThe amount of information exchanged per unit of time between two nodes in a
dynamical network or between two data sets is a powerful concept for analysing
complex systems. This quantity, known as the mutual information rate (MIR), is
calculated from the mutual information, which is rigorously defined only for
random systems. Moreover, the definition of mutual information is based on
probabilities of significant events. This work offers a simple alternative way
to calculate the MIR in dynamical (deterministic) networks or between two data
sets (not fully deterministic), and to calculate its upper and lower bounds
without having to calculate probabilities, but rather in terms of well known
and well defined quantities in dynamical systems. As possible applications of
our bounds, we study the relationship between synchronisation and the exchange
of information in a system of two coupled maps and in experimental networks of
coupled oscillators
Weak pairwise correlations imply strongly correlated network states in a neural population
Get PDFBiological networks have so many possible states that exhaustive sampling is
impossible. Successful analysis thus depends on simplifying hypotheses, but
experiments on many systems hint that complicated, higher order interactions
among large groups of elements play an important role. In the vertebrate
retina, we show that weak correlations between pairs of neurons coexist with
strongly collective behavior in the responses of ten or more neurons.
Surprisingly, we find that this collective behavior is described quantitatively
by models that capture the observed pairwise correlations but assume no higher
order interactions. These maximum entropy models are equivalent to Ising
models, and predict that larger networks are completely dominated by
correlation effects. This suggests that the neural code has associative or
error-correcting properties, and we provide preliminary evidence for such
behavior. As a first test for the generality of these ideas, we show that
similar results are obtained from networks of cultured cortical neurons.Comment: Full account of work presented at the conference on Computational and
Systems Neuroscience (COSYNE), 17-20 March 2005, in Salt Lake City, Utah
(http://cosyne.org
Evaluating psychometric properties of the Emotional Eating Scale Adapted for Children and Adolescents (EES-C) in a clinical sample of children seeking treatment for obesity: a case for the unidimensional model.
Get PDFBackgroundThe Emotional Eating Scale - Adapted for Children and Adolescents (EES-C) assesses children's urge to eat in response to experiences of negative affect. Prior psychometric studies have demonstrated the high reliability, concurrent validity, and test-retest reliability of theoretically defined subconstructs among non-clinical samples of children and adolescents who were primarily healthy weight; however, no psychometric studies exist investigating the EES-C among clinical samples of children with overweight/obesity (OW/OB). Furthermore, studies conducted in different contexts have suggested a discordant number of subconstructs of emotions related to eating. The purpose of this study was to evaluate the validity of the EES-C in a clinical sample of children seeking weight-loss treatment.MethodUsing a hierarchical bi-factor approach, we evaluated the validity of the EES-C to measure a single general construct, a set of two separate correlated subconstructs, or a hierarchical arrangement of two constructs, and determined reliability in a clinical sample of treatment-seeking children with OW/OB aged 8-12 years (N = 147, mean age = 10.4 years.; mean BMI z = 2.0; female = 66%; Hispanic = 32%, White and other = 68%).ResultsComparison of factor-extraction methods suggested a single primary construct underlying EES-C in this clinical sample. The bi-factor indices provided clear evidence that most of the reliable variance in the total score (90.8 for bi-factor model with three grouping factors and 95.2 for bi-factor model with five grouping factors) was attributed to the general construct. After adjusting for relationships with the primary construct, remaining correlations among sets of items did not suggest additional reliable constructs.ConclusionResults suggest that the primary interpretive emphasis of the EES-C among treatment-seeking children with overweight or obesity should be placed on a single general construct, not on the 3- or 5- subconstructs as was previously suggested
Rapid intraoperative insulin assay: a novel method to differentiate insulinoma from nesidioblastosis in the pediatric patient
Get PDFIntroduction: Hyperinsulinism is the most common cause of recurrent and persistent hypoglycemia in infancy and childhood. Causes can include nesidioblastosis, pancreatic islet cell tumors such as insulinoma, and associations with multiple endocrine neoplasia syndromes. Although new, improved imaging techniques have allowed for more precise preoperative localization of insulinomas, the differentiation of nesidioblastosis and insulinoma, particularly in children, can be challenging. To improve intraoperative localization and confirmation of successful resection of insulinoma, a novel hormonal assay, the rapid intraoperative insulin assay, is reported for the first time in a pediatric patient. This intraoperative radioimmunoassay for insulin yields results within several minutes and confirms complete resection of insulinoma. Case description: We present a case of pancreatic insulinoma in a child with symptoms of severe hypoglycemia, causing seizures. The insulinoma was enucleated laparoscopically, and rapid intra-operative insulin assay used to determine the success of the procedure. Discussion and evaluation: This rapid intra-operative test provides a valuable adjunct for determining complete excision in complicated cases of recurrent or questionable insulinoma. Although not a common problem, for pediatric patients in whom the diagnosis is not clear, this test may provide a novel approach to confirming disease. Conclusion: We propose the use of this assay in facilitating intra-operative resection and confirmation of complete excision in pediatric patients. This population may especially benefit from this novel assay to confirm complete resection and to differentiate multiple etiologies of hyperinsulinism
Stimulus-dependent maximum entropy models of neural population codes
Get PDFNeural populations encode information about their stimulus in a collective
fashion, by joint activity patterns of spiking and silence. A full account of
this mapping from stimulus to neural activity is given by the conditional
probability distribution over neural codewords given the sensory input. To be
able to infer a model for this distribution from large-scale neural recordings,
we introduce a stimulus-dependent maximum entropy (SDME) model---a minimal
extension of the canonical linear-nonlinear model of a single neuron, to a
pairwise-coupled neural population. The model is able to capture the
single-cell response properties as well as the correlations in neural spiking
due to shared stimulus and due to effective neuron-to-neuron connections. Here
we show that in a population of 100 retinal ganglion cells in the salamander
retina responding to temporal white-noise stimuli, dependencies between cells
play an important encoding role. As a result, the SDME model gives a more
accurate account of single cell responses and in particular outperforms
uncoupled models in reproducing the distributions of codewords emitted in
response to a stimulus. We show how the SDME model, in conjunction with static
maximum entropy models of population vocabulary, can be used to estimate
information-theoretic quantities like surprise and information transmission in
a neural population.Comment: 11 pages, 7 figure
Cosmic rays and molecular clouds
Full text linkThis paper deals with the cosmic-ray penetration into molecular clouds and
with the related gamma--ray emission. High energy cosmic rays interact with the
dense gas and produce neutral pions which in turn decay into two gamma rays.
This makes molecular clouds potential sources of gamma rays, especially if they
are located in the vicinity of a powerful accelerator that injects cosmic rays
in the interstellar medium. The amplitude and duration in time of the
cosmic--ray overdensity around a given source depend on how quickly cosmic rays
diffuse in the turbulent galactic magnetic field. For these reasons, gamma-ray
observations of molecular clouds can be used both to locate the sources of
cosmic rays and to constrain the properties of cosmic-ray diffusion in the
Galaxy.Comment: To appear in the proceedings of the San Cugat Forum on Astrophysics
2012, 27 pages, 10 figure
General Analysis of Antideuteron Searches for Dark Matter
Full text linkLow energy cosmic ray antideuterons provide a unique low background channel
for indirect detection of dark matter. We compute the cosmic ray flux of
antideuterons from hadronic annihilations of dark matter for various Standard
Model final states and determine the mass reach of two future experiments
(AMS-02 and GAPS) designed to greatly increase the sensitivity of antideuteron
detection over current bounds. We consider generic models of scalar, fermion,
and massive vector bosons as thermal dark matter, describe their basic features
relevant to direct and indirect detection, and discuss the implications of
direct detection bounds on models of dark matter as a thermal relic. We also
consider specific dark matter candidates and assess their potential for
detection via antideuterons from their hadronic annihilation channels. Since
the dark matter mass reach of the GAPS experiment can be well above 100 GeV, we
find that antideuterons can be a good indirect detection channel for a variety
of thermal relic electroweak scale dark matter candidates, even when the rate
for direct detection is highly suppressed.Comment: 44 pages, 15 Figure
Consequences of converting graded to action potentials upon neural information coding and energy efficiency
Get PDFInformation is encoded in neural circuits using both graded and action potentials, converting between them within single neurons and successive processing layers. This conversion is accompanied by information loss and a drop in energy efficiency. We investigate the biophysical causes of this loss of information and efficiency by comparing spiking neuron models, containing stochastic voltage-gated Na+ and K+ channels, with generator potential and graded potential models lacking voltage-gated Na+ channels. We identify three causes of information loss in the generator potential that are the by-product of action potential generation: (1) the voltage-gated Na+ channels necessary for action potential generation increase intrinsic noise and (2) introduce non-linearities, and (3) the finite duration of the action potential creates a ‘footprint’ in the generator potential that obscures incoming signals. These three processes reduce information rates by ~50% in generator potentials, to ~3 times that of spike trains. Both generator potentials and graded potentials consume almost an order of magnitude less energy per second than spike trains. Because of the lower information rates of generator potentials they are substantially less energy efficient than graded potentials. However, both are an order of magnitude more efficient than spike trains due to the higher energy costs and low information content of spikes, emphasizing that there is a two-fold cost of converting analogue to digital; information loss and cost inflation
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