493 research outputs found
Bistable Gradient Networks II: Storage Capacity and Behaviour Near Saturation
We examine numerically the storage capacity and the behaviour near saturation
of an attractor neural network consisting of bistable elements with an
adjustable coupling strength, the Bistable Gradient Network (BGN). For strong
coupling, we find evidence of a first-order "memory blackout" phase transition
as in the Hopfield network. For weak coupling, on the other hand, there is no
evidence of such a transition and memorized patterns can be stable even at high
levels of loading. The enhanced storage capacity comes, however, at the cost of
imperfect retrieval of the patterns from corrupted versions.Comment: 15 pages, 12 eps figures. Submitted to Phys. Rev. E. Sequel to
cond-mat/020356
Topology and Computational Performance of Attractor Neural Networks
To explore the relation between network structure and function, we studied
the computational performance of Hopfield-type attractor neural nets with
regular lattice, random, small-world and scale-free topologies. The random net
is the most efficient for storage and retrieval of patterns by the entire
network. However, in the scale-free case retrieval errors are not distributed
uniformly: the portion of a pattern encoded by the subset of highly connected
nodes is more robust and efficiently recognized than the rest of the pattern.
The scale-free network thus achieves a very strong partial recognition.
Implications for brain function and social dynamics are suggestive.Comment: 2 figures included. Submitted to Phys. Rev. Letter
Good methods for coping with missing data in decision trees
We propose a simple and effective method for dealing with missing data in decision trees used for classification. We call this approach 'missingness incorporated in attributes' (MIA). It is very closely related to the technique of treating 'missing' as a category in its own right, generalizing it for use with continuous as well as categorical variables. We show through a substantial data-based study of classification accuracy that MIA exhibits consistently good performance across a broad range of data types and of sources and amounts of missingness. It is competitive with the best of the rest (particularly, a multiple imputation EM algorithm method; EMMI) while being conceptually and computationally simpler. A simple combination of MIA and EMMI is slower but even more accurate
Integrated Generation of High-dimensional Entangled Photon States and Their Coherent Control
We demonstrate the generation of high-dimensional entangled photon pairs with a Hilbert-space dimensionality larger than 100 from an on-chip nonlinear microcavity, and introduce a coherent control scheme using standard telecommunications components
MGMT-independent temozolomide resistance in pediatric glioblastoma cells associated with a PI3-kinase-mediated HOX/stem cell gene signature
Sensitivity to temozolomide is restricted to a subset of glioblastoma patients, with the major determinant of resistance being a lack of promoter methylation of the gene encoding the repair protein DNA methyltransferase MGMT, although other mechanisms are thought to be active. There are, however, limited preclinical data in model systems derived from pediatric glioma patients. We screened a series of cell lines for temozolomide efficacy in vitro, and investigated the differential mechanisms of resistance involved. In the majority of cell lines, a lack of MGMT promoter methylation and subsequent protein overexpression were linked to temozolomide resistance. An exception was the pediatric glioblastoma line KNS42. Expression profiling data revealed a coordinated upregulation of HOX gene expression in resistant lines, especially KNS42, which was reversed by phosphoinositide 3-kinase pathway inhibition. High levels of HOXA9/HOXA10 gene expression were associated with a shorter survival in pediatric high-grade glioma patient samples. Combination treatment in vitro of pathway inhibition and temozolomide resulted in a highly synergistic interaction in KNS42 cells. The resistance gene signature further included contiguous genes within the 12q13-q14 amplicon, including the Akt enhancer PIKE, significantly overexpressed in the KNS42 line. These cells were also highly enriched for CD133 and other stem cell markers. We have thus shown an in vitro link between phosphoinositide 3-kinase-mediated HOXA9/HOXA10 expression, and a drug-resistant, progenitor cell phenotype in MGMT-independent pediatric glioblastoma.Cancer Research UK (C1178/A10294, C309/A2187, C309/A8274), the Oak
Foundation (L. Marshall), and La Fondation de France (N. Gaspar). We
acknowledge NHS funding to the NIHR Biomedical Research Centre.
P. Workman is a Cancer Research UK Life Fello
Surface superconducting states and paramagnetism in mesoscopic superconductors
In the framework of the Ginzburg-Landau equation, the temperature dependence
of the upper critical field of small ring-like superconductors is studied. At
equilibrium small parts of the phase diagram show paramagnetism for width /
radius ratios below 0.85. Their number and extension increase with the size of
the hole. In these regions, only the inner part of the ring shows a positive
magnetic moment. The order parameter density profile appears to change, when
crossing a first order transition line, which separates different angular
momentum values, and we clarify the relationship between the localization of
superconductivity nucleation and paramagnetism of those samples.Comment: 11 pages, 9 figure
Electron-Hole Correlations and Optical Excitonic Gaps in Quantum-Dot Quantum Wells: Tight-Binding Approach
Electron-hole correlation in quantum-dot quantum wells (QDQW's) is
investigated by incorporating Coulomb and exchange interactions into an
empirical tight-binding model. Sufficient electron and hole single-particle
states close to the band edge are included in the configuration to achieve
convergence of the first spin-singlet and triplet excitonic energies within a
few meV. Coulomb shifts of about 100 meV and exchange splittings of about 1 meV
are found for CdS/HgS/CdS QDQW's (4.7 nm CdS core diameter, 0.3 nm HgS well
width and 0.3 nm to 1.5 nm CdS clad thickness) which have been characterized
experimentally by Weller and co-workers [ D. Schooss, A. Mews, A. Eychmuller,
H. Weller, Phys. Rev. B, 49, 17072 (1994)]. The optical excitonic gaps
calculated for those QDQW's are in good agreement with the experiment.Comment: 3 figures, to appear in Phys.Rev.
EGFRvIII deletion mutations in pediatric high-grade glioma and response to targeted therapy in pediatric glioma cell lines
Purpose: The epidermal growth factor receptor (EGFR) is amplified and overexpressed
in adult glioblastoma, with response to targeted inhibition dependent on the underlying
biology of the disease. EGFR has thus far been considered to play a less important role
in pediatric glioma, although extensive data are lacking. We have sought to clarify the
role of EGFR in pediatric high-grade glioma (HGG).
Experimental Design: We retrospectively studied a total of 90 archival pediatric HGG
specimens for EGFR protein overexpression, gene amplification, and mutation and assessed the in vitro sensitivity of pediatric glioma cell line models to the small-molecule
EGFR inhibitor erlotinib.
Results: Amplification was detected in 11% of cases, with corresponding overexpression of the receptor. No kinase or extracellular domain mutations were observed; however, 6 of 35 (17%) cases harbored the EGFRvIII deletion, including two anaplastic
oligodendrogliomas and a gliosarcoma overexpressing EGFRvIII in the absence of gene
amplification and coexpressing platelet-derived growth factor receptor α. Pediatric glioblastoma cells transduced with wild-type or deletion mutant EGFRvIII were not rendered
more sensitive to erlotinib despite expressing wild-type PTEN. Phosphorylated receptor
tyrosine kinase profiling showed a specific activation of platelet-derived growth factor
receptor α/β in EGFRvIII-transduced pediatric glioblastoma cells, and targeted coinhibition with erlotinib and imatinib leads to enhanced efficacy in this model.
Conclusions: These data identify an elevated frequency of EGFR gene amplification and
EGFRvIII mutation in pediatric HGG than previously recognized and show the likely
necessity of targeting multiple genetic alterations in the tumors of these children.Cancer Research UK grants C1178/A10294, C309/A2187, and
C309/A8274; Oak Foundation (L. Marshall); La Fondation de France (N. Gaspar);
and Breakthrough Breast Cancer (J.S. Reis-Filho). We acknowledge NHS funding to the National Institute for Health Research Biomedical Research Centre
Keeping It Real: Revisiting a Real-Space Approach to Running Ensembles of Cosmological N-body Simulations
In setting up initial conditions for ensembles of cosmological N-body
simulations there are, fundamentally, two choices: either maximizing the
correspondence of the initial density field to the assumed fourier-space
clustering or, instead, matching to real-space statistics and allowing the DC
mode (i.e. overdensity) to vary from box to box as it would in the real
universe. As a stringent test of both approaches, I perform ensembles of
simulations using power law and a "powerlaw times a bump" model inspired by
baryon acoustic oscillations (BAO), exploiting the self-similarity of these
initial conditions to quantify the accuracy of the matter-matter two-point
correlation results. The real-space method, which was originally proposed by
Pen 1997 and implemented by Sirko 2005, performed well in producing the
expected self-similar behavior and corroborated the non-linear evolution of the
BAO feature observed in conventional simulations, even in the
strongly-clustered regime (sigma8 >= 1). In revisiting the real-space method
championed by Sirko 2005, it was also noticed that this earlier study
overlooked an important integral constraint correction to the correlation
function in results from the conventional approach that can be important in
LambdaCDM simulations with Lbox == Lbox / 10.
Rectifying this shows that the fourier space and real space methods are about
equally accurate and efficient for modeling the evolution and growth of the
correlation function, contrary to previous claims. An appendix provides a
useful independent-of-epoch analytic formula for estimating the importance of
the integral constraint bias on correlation function measurements in LambdaCDM
simulations.Comment: 28 pages, 7 figures, substantial improvements throughou
Evaluation of the impact of orally administered carbohydrates on postprandial blood glucose levels in different pre-clinical models
ABSTRACT We developed a pre-clinical model in which to evaluate the impact of orally administered carbohydrates on postprandial blood glucose levels. For this purpose, we compared the effects of different carbohydrates with well-established glycemic indexes. We orally administered (gavage) increasing amounts (0.2, 0.4, 0.6, 0.8, and 1.0 g/kg) of sucrose and lactose to rats which had been fasted for 6 h or 15 h, respectively. In part of the experiments we administered frutose (gavagem). Three different models were compared for measuring postprandial blood glucose levels: a) evaluation of interstitial glucose concentrations by using a real time continuous glucose monitoring system; b) evaluation of glucose levels in blood obtained from the rat tail; c) evaluation of serum glucose levels in blood collected after decapitation. Our results showed that blood obtained from the tails of 15-h fasted rats was the best model in which to evaluate the effect of carbohydrates on postprandial blood glucose levels
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