Cortical free association dynamics: distinct phases of a latching network

A Potts associative memory network has been proposed as a simplified model of macroscopic cortical dynamics, in which each Potts unit stands for a patch of cortex, which can be activated in one of S local attractor states. The internal neuronal dynamics of the patch is not described by the model, rather it is subsumed into an effective description in terms of graded Potts units, with adaptation effects both specific to each attractor state and generic to the patch. If each unit, or patch, receives effective (tensor) connections from C other units, the network has been shown to be able to store a large number p of global patterns, or network attractors, each with a fraction a of the units active, where the critical load p_c scales roughly like p_c ~ (C S^2)/(a ln(1/a)) (if the patterns are randomly correlated). Interestingly, after retrieving an externally cued attractor, the network can continue jumping, or latching, from attractor to attractor, driven by adaptation effects. The occurrence and duration of latching dynamics is found through simulations to depend critically on the strength of local attractor states, expressed in the Potts model by a parameter w. Here we describe with simulations and then analytically the boundaries between distinct phases of no latching, of transient and sustained latching, deriving a phase diagram in the plane w-T, where T parametrizes thermal noise effects. Implications for real cortical dynamics are briefly reviewed in the conclusions

Gauge fields in a string-cigar braneworld

In this work we investigate the properties of an Abelian gauge vector field in a thin and in a smoothed string-like braneworld, the so-called string-cigar model. This thick brane scenario satisfies the regularity conditions and it can be regarded as an interior and exterior string-like solution. The source undergoes a geometric Ricci flow which is connected to a variation of the bulk cosmological constant. The Ricci flow changes the width and amplitude of the massless mode at the brane core and recover the usual thin string-like behavior at large distances. By numerical means we obtain the Kaluza-Klein (KK) spectrum for both the thin brane and the string-cigar. It turns out that both models exhibit a mass gap between the massless and the massive modes and between the high and the low mass regimes. The KK modes are smooth near the brane and their amplitude are enhanced by the string-cigar core. The analogue Schr\"odinger potential is also tuned by the geometric flow.Comment: The discussion about the Kaluza-Klein spectrum of the gauge field was improved. Numerical analysis was adapted to the conventional notation on Kaluza-Klein number. Some graphics were modified for considering other notation. Results unchanged. References added. Corrected typos. 17 pages. 6 figures. To match version to appears in Physics Letters

A 450-day light curve of the radio afterglow of GRB 970508: Fireball calorimetry

We report on the results of an extensive monitoring campaign of the radio afterglow of GRB 970508, lasting 450 days after the burst. The spectral and temporal radio behavior indicate that the fireball has undergone a transition to sub-relativistic expansion at t~100 days. This allows us to perform "calorimetry" of the explosion. The derived total energy, ~5\times 10^{50} erg, is well below the ~5\times 10^{51} erg inferred under the assumption of spherical symmetry from gamma-ray and early afterglow observations. A natural consequence of this result, which can also account for deviations at t<100 days from the spherical relativistic fireball model predictions, is that the fireball was initially a wide-angle jet of opening angle ~30 degrees. Our analysis also allows to determine the energy fractions carried by electrons and magnetic field, and the density of ambient medium surrounding the fireball. We find that during the sub-relativistic expansion electrons and magnetic field are close to equipartition, and that the density of the ambient medium is ~1/cm^3. The inferred density rules out the possibility that the fireball expands into a strongly non-uniform medium, as would be expected, e.g., in the case of a massive star progenitor.Comment: 33 pages, including 7 figures, submitted to Ap

Effective Schroedinger dynamics on ϵ \epsilon -thin Dirichlet waveguides via Quantum Graphs I: star-shaped graphs

We describe the boundary conditions at the vertex that one must choose to obtain a dynamical system that best describes the low-energy part of the evolution of a quantum system confined to a very small neighbourhood of a star-shaped metric graph.Comment: in memory of Pierre Duclo

Levantamento de fungos causadores de podridões de colmo em milho na região Centro Oeste do Brasil.

As podridões de colmo estão entre as mais importantes doenças da cultura do milho. Vários são os patógenos causadores de podridões de colmo em plantas de milho, incluindo fungos e bactérias. Entre os principais, destacam-se Fusarium verticillioides, Fusarium graminearum, Colletotrichum graminicola, Stenocarpella macrospora, Stenocarpella maydis e Macrophomina phaseolina. O presente trabalho teve como objetivo realizar um levantamento da incidência de fungos causadores de podridões de colmo em milho na região Centro-Oeste do Brasil. Foram realizadas coletas de amostras nos estados de Goiás, Mato Grosso e Mato Grosso do Sul. Durante as coletas das amostras foi possível verificar uma elevada incidência de plantas apresentando sintomas de podridão de colmo em, praticamente, todas as regiões. Dentre os patógenos detectados, as maiores frequências foram verificadas para Fusarium spp. Nigrospora spp. e Stenocarpella spp., com freqüência de 45,7, 38,3 e 29,9%, respectivamente. Coletotrichum graminicola e Rhizoctonia spp. apresentaram frequência de 5,8 e 3,4%. Os demais fungos apresentaram menos de 3% de ocorrência nos isolamentos. Esses resultados demonstram a grande variabilidade de fungos envolvidos com as podridões de colmo na cultura do milho no Centro-Oeste do Brasil. Revelam, também, a elevada ocorrência de fungos do gênero Fusarium spp. e Stenocarpella spp., os quais predominaram nas amostras analisadas no presente trabalho. Os trabalhos de monitoramento da ocorrência de fungos causadores de podridões de colmo são fundamentais para o desenvolvimento e a adoção das melhores estratégias de controle desta doença na cultura do milho.bitstream/item/145936/1/bol-133.pd

Recomendações para o controle químico da mancha branca do milho.

bitstream/item/50969/1/circ-167.pd

Metodologia para avaliação da reação de genótipos de milho à Fusarium verticillioides em casa de vegetação.

bitstream/item/76761/1/circ-179.pd

Recomendação para o controle químico da antracnose foliar do sorgo.

bitstream/item/63207/1/circ-171.pd

Composite fluxbranes with general intersections

Generalized composite fluxbrane solutions for a wide class of intersection rules are obtained. The solutions are defined on a manifold which contains a product of n Ricci-flat spaces M_1 x ... x M_n with 1-dimensional M_1. They are defined up to a set of functions H_s obeying non-linear differential equations equivalent to Toda-type equations with certain boundary conditions imposed. A conjecture on polynomial structure of governing functions H_s for intersections related to semisimple Lie algebras is suggested. This conjecture is valid for Lie algebras: A_m, C_{m+1}, m > 0. For simple Lie algebras the powers of polynomials coincide with the components of the dual Weyl vector in the basis of simple roots. Explicit formulas for A_1 + ... + A_1 (orthogonal), "block-ortogonal" and A_2 solutions are obtained. Certain examples of solutions in D = 11 and D =10 (II A) supergravities (e.g. with A_2 intersection rules) and Kaluza-Klein dyonic A_2 flux tube, are considered.Comment: 19 pages, Latex, 1 reference (on a pioneering paper of Gibbons and Wiltshire) and two missing relations are added Published: Class. Quantum Grav. 19 (2002) 3033-304