17,454 research outputs found
Synchronization and Redundancy: Implications for Robustness of Neural Learning and Decision Making
Learning and decision making in the brain are key processes critical to
survival, and yet are processes implemented by non-ideal biological building
blocks which can impose significant error. We explore quantitatively how the
brain might cope with this inherent source of error by taking advantage of two
ubiquitous mechanisms, redundancy and synchronization. In particular we
consider a neural process whose goal is to learn a decision function by
implementing a nonlinear gradient dynamics. The dynamics, however, are assumed
to be corrupted by perturbations modeling the error which might be incurred due
to limitations of the biology, intrinsic neuronal noise, and imperfect
measurements. We show that error, and the associated uncertainty surrounding a
learned solution, can be controlled in large part by trading off
synchronization strength among multiple redundant neural systems against the
noise amplitude. The impact of the coupling between such redundant systems is
quantified by the spectrum of the network Laplacian, and we discuss the role of
network topology in synchronization and in reducing the effect of noise. A
range of situations in which the mechanisms we model arise in brain science are
discussed, and we draw attention to experimental evidence suggesting that
cortical circuits capable of implementing the computations of interest here can
be found on several scales. Finally, simulations comparing theoretical bounds
to the relevant empirical quantities show that the theoretical estimates we
derive can be tight.Comment: Preprint, accepted for publication in Neural Computatio
Quantum transport of slow charge carriers in quasicrystals and correlated systems
We show that the semi-classical model of conduction breaks down if the mean
free path of charge carriers is smaller than a typical extension of their
wavefunction. This situation is realized for sufficiently slow charge carriers
and leads to a transition from a metallic like to an insulating like regime
when scattering by defects increases. This explains the unconventional
conduction properties of quasicrystals and related alloys. The conduction
properties of some heavy fermions or polaronic systems, where charge carriers
are also slow, present a deep analogy.Comment: 4 page
Trilingual conversations: a window into multicompetence
A recurrent theme in the literature on trilingual language use is the question of whether there is a specific âtrilingual competence.â In this paper we consider this question in the light of codeswitching patterns in two dyadic trilingual conversations between a mother and daughter conducted in (Lebanese) Arabic, French, and English. Quantitative and qualitative analysis of codeswitching in both conversants shows that, despite the fact that both subjects are fluent in all three languages, uses of switching are significantly different for mother and daughter across a number of features, including relative frequency of different switch types, and the incidence of hybrid constructions involving items from two or more languages. The subjects appear to display qualitatively distinct profiles of competence in the trilingual mode. This in turn leads to the conclusion that the facts of trilingual language use are best characterized in terms of âmulticompetenceâ (Cook, 1991). The paper concludes with some further reflections on the uniqueness of trilingual language use (an âold chestnutâ in
trilingualism research, cf. Klein, 1995)
An assessment of the benefits of yellow Sigatoka (Mycosphaerella musicola) control in the Queensland Northern Banana Pest Quarantine Area
The banana leaf spotting disease yellow Sigatoka is established and actively controlled in Australia through intensive chemical treatments and diseased leaf removal. In the State of Queensland, the State government imposes standards for de-leafing to minimise the risk of the disease spreading in 6 banana pest quarantine areas. Of these, the Northern Banana Pest Quarantine Area is the most significant in terms of banana production. Previous regulations imposed obligations on owners of banana plants within this area to remove leaves from plants with visible spotting on more than 15 per cent of any leaf during the wet season. Recently, this leaf disease threshold has been lowered to 5 per cent. In this paper we examine the likely impact this more-costly regulation will have on the spread of the disease. We estimate that the average net benefit of reducing the diseased leaf threshold is only likely to be $1.4 million per year over the next 30 years, expressed as the annualised present value of tightened regulation. This result varies substantially when the timeframe of the analysis is changed, with shorter time frames indicating poorer net returns from the change in protocols. Overall, the benefit of the regulation change is likely to be minor
Data assimilation for hydraulic state estimation of a development project
International audienceAssurer la sĂ»retĂ© hydraulique sur les amĂ©nagements du RhĂŽne est la prioritĂ© pour la Compagnie Nationale du RhĂŽne. Cela signifie qu'il est indispensable de respecter les consignes d'exploitation sur les amĂ©nagements. Pour la rĂ©gulation automatique des amĂ©nagements, le calcul de la commande prĂ©dictive s'appuie sur les mesures en cote en certains points caractĂ©ristiques ainsi que sur les dĂ©bits aux ouvrages. De ce fait, lors de pannes de capteurs ou bien lors de forts phĂ©nomĂšnes transitoires (tels que des arrĂȘts ou dĂ©marrages de turbines), la rĂ©gulation est perturbĂ©e. Suite Ă la prĂ©sente Ă©tude, la solution proposĂ©e pour corriger les observations et mettre Ă jour les lignes d'eau intervenant dans le calcul des commandes, est la mise en place d'une mĂ©thode d'assimilation de donnĂ©es de type filtre de Kalman. Cette mĂ©thode est optimale dans le sens oĂč elle assure la minimisation de l'erreur d'estimation. Mais cela n'est pas suffisant pour certaines applications industrielles. C'est pourquoi les notions de dĂ©tectabilitĂ© et de convergence ont Ă©tĂ© approfondies. Cette Ă©tude a permis de dĂ©terminer des conditions suffisantes qui assurent une erreur de l'estimation convergente vers une moyenne nulle. Pour tester la mise en place d'une telle mĂ©thode, celle-ci a Ă©tĂ© implĂ©mentĂ©e sur l'outil de test industriel appelĂ© " plateforme de simulation ". Des exemples de scĂ©narios habituellement difficiles pour les rĂ©gulations automatiques, telles que la perte de capteur et l'arrĂȘt brutal d'une turbine sont prĂ©sentĂ©s. Ils permettent de dĂ©montrer l'apport de la solution proposĂ©e. / Ensure the safety of development projects of the RhĂŽne is the priority of the Compagnie Nationale du RhĂŽne. That means it is essential to respect the water levels at some set points along the river. To regulate its development projects, the computation of the predictive control is based on measurements at the characteristic points as well as flows at the hydraulic facilities. Thus, when failures of sensors occur or during strong transients (such as stopping or starting the turbines), the regulation process can be disturbed. Following this study, the new solution suggested to detect sensor defaults, to correct the observations and to update the water profiles involved in the calculation of control actions, is based on a data assimilation method (Kalman Filter). This method is optimal because it ensures minimizing the estimation error. But this property is not sufficient for our industrial application. This is why the notions of detectability and convergence have been studied into detail. This study has identified sufficient conditions that ensure the error of the estimate to converge towards a nil average. In order to test and validate such a method, it has been implemented into the industrial test tool called "simulation platform." Examples of usually difficult scenarios simulating a loss of sensor and an unexpected stopping of a turbine are presented in this document. It allows demonstrating the benefits of the presented solution
The Chandra Iron-L X-Ray Line Spectrum of Capella
An analysis of the iron L-shell emission in the publicly available spectrum
of the Capella binary system, as obtained by the High Energy Transmission
Grating Spectrometer on board the Chandra X-ray Observatory, is presented. The
atomic-state model, based on the HULLAC code, is shown to be especially
adequate for analyzing high-resolution x-ray spectra of this sort. Almost all
of the spectral lines in the 10 - 18 Angstrom wavelength range are identified.
It is shown that, for the most part, these lines can be attributed to emission
from L-shell iron ions in the Capella coronae. Possibilities for electron
temperature diagnostics using line ratios of Fe16+ are demonstrated. It is
shown that the observed iron-L spectrum can be reproduced almost entirely by
assuming a single electron temperature of kTe= 600 eV. This temperature is
consistent with both the measured fractional ion abundances of iron and with
the temperature derived from ratios of Fe16+ lines. A volume emission measure
of 1053 cm-3 is calculated for the iron L-shell emitting regions of the Capella
coronae indicating a rather small volume of 1029 cm3 for the emitting plasma if
an electron density of 1012 cm-3 is assumed.Comment: Accepted to Ap
Sensitivity to the initial state of interacting ultracold bosons in disordered lattices
We study the dynamics of a nonlinear one-dimensional disordered system
obtained by coupling the Anderson model with the Gross-Pitaevskii equation. An
analytical model provides us with a single quantity globally characterizing the
localization of the system. This quantity obeys a scaling law with respect to
the width of the initial state, which can be used to characterize the dynamics
independently of the initial state.Comment: 10 pages, 12 figures, revtex4, submited to PR
High-Resolution X-Ray Spectroscopy of the Accretion Disk Corona Source 4U 1822-37
We present a preliminary analysis of the X-ray spectrum of the accretion disk
corona source, 4U 1822-37, obtained with the High Energy Transmission Grating
Spectrometer onboard the Chandra X-ray Observatory. We detect discrete emission
lines from photoionized iron, silicon, magnesium, neon, and oxygen, as well as
a bright iron fluorescence line. Phase-resolved spectroscopy suggests that the
recombination emission comes from an X-ray illuminated bulge located at the
predicted point of impact between the disk and the accretion stream. The
fluorescence emission originates in an extended region on the disk that is
illuminated by light scattered from the corona.Comment: 12 pages, 6 figures; Accepted for publication in ApJ Letter
The 511 keV emission from positron annihilation in the Galaxy
The first gamma-ray line originating from outside the solar system that was
ever detected is the 511 keV emission from positron annihilation in the Galaxy.
Despite 30 years of intense theoretical and observational investigation, the
main sources of positrons have not been identified up to now. Observations in
the 1990's with OSSE/CGRO showed that the emission is strongly concentrated
towards the Galactic bulge. In the 2000's, the SPI instrument aboard ESA's
INTEGRAL gamma-ray observatory allowed scientists to measure that emission
across the entire Galaxy, revealing that the bulge/disk luminosity ratio is
larger than observed in any other wavelength. This mapping prompted a number of
novel explanations, including rather "exotic ones (e.g. dark matter
annihilation). However, conventional astrophysical sources, like type Ia
supernovae, microquasars or X-ray binaries, are still plausible candidates for
a large fraction of the observed total 511 keV emission of the bulge. A closer
study of the subject reveals new layers of complexity, since positrons may
propagate far away from their production sites, making it difficult to infer
the underlying source distribution from the observed map of 511 keV emission.
However, contrary to the rather well understood propagation of high energy
(>GeV) particles of Galactic cosmic rays, understanding the propagation of low
energy (~MeV) positrons in the turbulent, magnetized interstellar medium, still
remains a formidable challenge. We review the spectral and imaging properties
of the observed 511 keV emission and we critically discuss candidate positron
sources and models of positron propagation in the Galaxy.Comment: 62 pages, 35 figures. Review paper to appear in Reviews of Modern
Physic
Conductance and localization in disordered wires: role of evanescent states
This paper extends an earlier analytical scattering matrix treatment of
conductance and localization in coupled two- and three Anderson chain systems
for weak disorder when evanescent states are present at the Fermi level. Such
states exist typically when the interchain coupling exceeds the width of
propagating energy bands associated with the various transverse eigenvalues of
the coupled tight-binding systems. We calculate reflection- and transmission
coefficients in cases where, besides propagating states, one or two evanescent
states are available at the Fermi level for elastic scattering of electrons by
the disordered systems. We observe important qualitative changes in these
coefficients and in the related localization lengths due to ineffectiveness of
the evanescent modes for transmission and reflection in the various scattering
channels. In particular, the localization lengths are generally significantly
larger than the values obtained when evanescent modes are absent. Effects
associated with disorder mediated coupling between propagating and evanescent
modes are shown to be suppressed by quantum interference effects, in lowest
order for weak disorder
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