660 research outputs found
Learning during visual search in children with attentional and learning problems: a trial to trial evaluation of RT and ERP measures.
Trial to trial Event Related Potentials (ERPs) were recorded from children with attentional problems (APs), learning problems (LPs), and from children without these problems (NCs). The task required the subjects to memorize two figures and to selectively respond to their occurrence in a series of stimuli. Stimuli consisted of a display of figures at eight locations in a circle, whereby the targets were presented at a random or a fixed location. Learning from knowledge of prior displays was possible only in the fixed condition. Learning during presentation of the fixed series was manifest in several components. A Slow Wave (SW) difference between series, initially not present, developed within seven trials, and thus corresponded to the rapidity with which the reaction times (RTs) decreased over trials. A larger occipital SW difference was discovered in AP children and a larger frontocentral one in LP children compared to normals. The latency of this SW and the P300 difference between series were delayed with about 200 msec in APs compared to NCs. The task difference in the earliest component, the P120, that increased after behavioral task acquisition was completed, was seen in normal children only. This probably reflected feature-specific (location) attentional demands, that decreased slowly in normal children when the task became more predictable following a number of trials. Task differences of the N200, possibly reflecting covert orienting of attention, were initially smaller in APs and LPs than those of NCs, but they increased in APs (and in LPs more slowly) over trials. Differences were found for parietal amplitudes of the P300 in LPs and NCs, but not for APs. We concluded that AP children show early deficits that could originate from a limited capacity in focussing attention, which in turn prolongs stimulus evaluation. All subsequent processes are delayed by a similar amount of time. In addition, the relatively small frontocentral ERP's of the AP group suggest diminished frontal functioning. Problems in task acquisition and a prolonged process of memory updating might be induced by the slow adaptation to task differences in LP's, and delayed parietal SWs during task acquisition together with a marked frontocentral distribution and no RT difference
On the speed of convergence to stationarity of the Erlang loss system
We consider the Erlang loss system, characterized by servers, Poisson arrivals and exponential service times, and allow the arrival rate to be a function of We discuss representations and bounds for the rate of convergence to stationarity of the number of customers in the system, and display some bounds for the total variation distance between the time-dependent and stationary distributions. We also pay attention to time-dependent rates
Determination of the CP Violating Phase by a Sum Over Common Decay Modes to and
To help the difficult determination of the angle of the unitarity
triangle, Aleksan, Dunietz and Kayser have proposed the modes of the type
, common to and . We point out that it is possible
to gain in statistics by a sum over all modes with ground state mesons in the
final state, i.e. , , , .
The delicate point is the relative phase of these different contributions to
the dilution factor of the time-dependent asymmetry. Each contribution to
is proportional to a product where
denotes form factors and decay constants. Within a definite phase
convention, lattice calculations do not show any change in sign when
extrapolating to light quarks the form factors and decay constants. Then, we
can show that all modes contribute constructively to the dilution factor,
except the -wave , which is small. Quark model arguments
based on wave function overlaps also confirm this stability in sign. By summing
over all these modes we find a gain of a factor 6 in statistics relatively to
. The dilution factor for the sum is remarkably stable for
theoretical schemes that are not in very strong conflict with data on or extrapolated from semileptonic charm form factors, giving
, always close to .Comment: 22 pages, LPTHE Orsay 94/03, DAPNIA/SPP/94-2
The Magnetic Field of the Solar Corona from Pulsar Observations
We present a novel experiment with the capacity to independently measure both
the electron density and the magnetic field of the solar corona. We achieve
this through measurement of the excess Faraday rotation due to propagation of
the polarised emission from a number of pulsars through the magnetic field of
the solar corona. This method yields independent measures of the integrated
electron density, via dispersion of the pulsed signal and the magnetic field,
via the amount of Faraday rotation. In principle this allows the determination
of the integrated magnetic field through the solar corona along many lines of
sight without any assumptions regarding the electron density distribution. We
present a detection of an increase in the rotation measure of the pulsar
J18012304 of approximately 160 \rad at an elongation of 0.95 from
the centre of the solar disk. This corresponds to a lower limit of the magnetic
field strength along this line of sight of . The lack of
precision in the integrated electron density measurement restricts this result
to a limit, but application of coronal plasma models can further constrain this
to approximately 20mG, along a path passing 2.5 solar radii from the solar
limb. Which is consistent with predictions obtained using extensions to the
Source Surface models published by Wilcox Solar ObservatoryComment: 16 pages, 4 figures (1 colour): Submitted to Solar Physic
Experimental and theoretical aspects of the formation of radical cations from tripyrrolidinobenzenes and their follow-up reactions
Tripyrrolidinobenzene radical cations(1*+), obtained from the corresponding arenes by oxidation with silver nitrate, are specially stabilized and thus allow specific reaction pathways of arene radical cations to be investigated separately and individually. Radical cations 1*+ ,for instance, generated under exclusion of oxygen, undergo dimerization to 2, or they abstract hydrogen from the solvent to form 3. In a pure oxygen atmosphere, the O2 reaction products 6 and 7 are formed, respectively, either exclusively or together with 2 and 3. Kinetic measurements give the following order of reactivity for these individual processes: reaction with O2 > dimerization. > H-abstraction from solvent. The changes in the product spectrum upon modification of the reaction conditions are in accord with the kinetic results. The dimeric u complexes 2 show surprisingly facile dissociation into two radical cations, two (1*+)with a much higher dissociation rate for the alkyl derivatives 2b-d than for 2a. Dissociation is enhanced substantially by light or in the presence of π donors. Individual product formation, rate of reactions of the radical cations 1*+, and photochemical cleavage of the dimeric σ complexes 2 can be rationalized, by qualitative and quantitative MO considerations, in terms of their relative frontier orbital energies
The intra-tumoural stroma in patients with breast cancer increases with age
PURPOSE: The tumour microenvironment in older patients is subject to changes. The tumour-stroma ratio (TSR) was evaluated in order to estimate the amount of intra-tumoural stroma and to evaluate the prognostic value of the TSR in older patients with breast cancer (≥ 70 years).METHODS: Two retrospective cohorts, the FOCUS study (N = 619) and the Nottingham Breast Cancer series (N = 1793), were used for assessment of the TSR on haematoxylin and eosin stained tissue slides.RESULTS: The intra-tumoural stroma increases with age in the FOCUS study and the Nottingham Breast Cancer series (B 0.031, 95% CI 0.006-0.057, p = 0.016 and B 0.034, 95% CI 0.015-0.054, p CONCLUSIONS: The intra-tumoural stroma increases with age. This might be the result of an activated tumour microenvironment. The TSR did not validate as an independent prognostic parameter in patients ≥ 70 years in contrast to young women with breast cancer as published previously.Surgical oncolog
Hadronic final states in deep-inelastic scattering with Sherpa
We extend the multi-purpose Monte-Carlo event generator Sherpa to include
processes in deeply inelastic lepton-nucleon scattering. Hadronic final states
in this kinematical setting are characterised by the presence of multiple
kinematical scales, which were up to now accounted for only by specific
resummations in individual kinematical regions. Using an extension of the
recently introduced method for merging truncated parton showers with
higher-order tree-level matrix elements, it is possible to obtain predictions
which are reliable in all kinematical limits. Different hadronic final states,
defined by jets or individual hadrons, in deep-inelastic scattering are
analysed and the corresponding results are compared to HERA data. The various
sources of theoretical uncertainties of the approach are discussed and
quantified. The extension to deeply inelastic processes provides the
opportunity to validate the merging of matrix elements and parton showers in
multi-scale kinematics inaccessible in other collider environments. It also
allows to use HERA data on hadronic final states in the tuning of hadronisation
models.Comment: 32 pages, 22 figure
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