Entropic determination of the phase transition in a coevolving opinion-formation model.

We study an opinion formation model by the means of a coevolving complex network where the vertices represent the individuals, characterized by their evolving opinions, and the edges represent the interactions among them. The network adapts to the spreading of opinions in two ways: not only connected agents interact and eventually change their thinking but an agent may also rewire one of its links to a neighborhood holding the same opinion as his. The dynamics, based on a global majority rule, depends on an external parameter that controls the plasticity of the network. We show how the information entropy associated to the distribution of group sizes allows us to locate the phase transition between a phase of full consensus and another, where different opinions coexist. We also determine the minimum size of the most informative sampling. At the transition the distribution of the sizes of groups holding the same opinion is scale free.

Elevación de creatinina sérica como indicador de necrosis pancreática / Elevation of serum creatinine as an indicator of pancreatic necrosis

La Pancreatitis Aguda (PA) es una patología que se autolimita en el 80% de los casos; estos casos en general evolucionan hacia la recuperación total. Su evolución puede ser de leve a severa. La forma grave varía desde un 10% a un 25% y se asocia con falla orgánica y/o complicaciones locales como necrosis pancreática. En Venezuela, la PA Severa es un importante problema de salud pública, encontrándose entre las primeras 25 causas de muerte. Este estudio plantea la utilización de una prueba de laboratorio ampliamente disponible, de fácil uso e interpretación, para pronosticar la aparición de complicaciones como necrosis pancreática. Objetivo: Determinar la utilidad de la creatinina sérica como factor predictivo de necrosis pancreática en pancreatitis aguda. Materiales y Metodos: Estudio de tipo analítico, transversal y retrospectivo. Se revisaron las historias clínicas de los pacientes que ingresaron al hospital “Dr. Miguel Pérez Carreño”, con diagnóstico de pancreatitis aguda entre 2008 y 2009. Se registró creatinina sérica y se relacionó con la clasificación de severidad tomográfica según Balthazar. Resultados: La población estuvo conformada por 50 casos, de éstos se excluyeron 4, por embarazo o enfermedad renal crónica. Treinta de sexo femenino (65%) y 16 masculino (35%). Edades comprendidas entre 18 a 77 años, con media de 40,2. La estancia hospitalaria media fue de 8,74 días. Del total de 46 pacientes, presentó Balthazar A 63% (n=29), B 17,39% (n=8), C 14,04% (n= 6) y D 6,5% (n=3). No se obtuvo ningún E. Al aplicar un análisis de varianza se observó relación estadística significativa directamente proporcional de la creatinina sérica de ingreso (p=0,001) y de las 48 horas (p=0,001) con el Balthazar y el hematocrito. Conclusiones: La evaluación de los niveles y variaciones de creatinina sérica son de utilidad para predecir la aparición de necrosis pancreática en pacientes con pancreatitis aguda.

Acute Pancreatitis (AP) is a self-limited pathology in 80% of the cases; these cases generally evolve towards total recovery. Its evolution can be mild or severe. The severe form varies from a 10% to a 25%, and is associated with organ failure and/or local complications as pancreatic necrosis. In Venezuela severe AP is an important public health problem, being in the first 25 causes of death. The present study proposes the use of a widely available laboratory test, of easy use and interpretation, to predict the appearance of complications as pancreatic necrosis. Objective: To determine the usefulness of the serum creatinine as predictive factor of pancreatic necrosis in acute pancreatitis. Materials and Methods: An analytic, transversal and retrospective study. Clinical histories of patients admitted to the “Dr. Miguel Perez Carreño” hospital with a diagnosis of acute pancreatitis between years 2008 and 2009 were reviewed. Serum creatinine was registered and compared according to the Balthazar classification of tomographic severity. Results: The sample was comprised by 50 cases, from which 4 were excluded, due to pregnancy or chronic renal disease. Thirty were female (65%) and 16 male (35%). Between the ages of 18 and 77, with mean age of 40,2. The median hospital stay was 8,74 days. From total of 46 patients, 63% had Balthazar A (n=29); B 17.39% (n=8); C 14,04% (n= 6) and D 6.5% (n=3). No Balthazar E was obtained. When applying a variance analysis, a significant statistical relation was observed, directly proportional to the serum creatinine upon admission (p=0,001) and the 48 hours (p=0,001) with the Balthazar and hematocrit. Conclusions: The evaluation of the levels and variations of serum creatinine is a useful tool for predicting the appearance of pancreatic necrosis in patients with acute pancreatitis.

Glassy properties of frustrated arrays of nonlinear devices.

We study linear arrays of different number of quartic oscillators shaped in the form of a ring when Gaussian noise (temperature) is added. Frustration is introduced through periodic boundary conditions and repulsive, directional interactions between neighboring oscillators. We show that these systems have similar dynamic properties than the arrays of fluxgates magnetometers. We find that there is a critical number of oscillators separating the regimes arising for systems with few and many oscillators and show that they reach an optimum ordering for a nonvanishing temperature. We also find that they have a relaxation process with an infinite mean life that is typical of glassy systems.

Two classes of bipartite networks: nested biological and social systems.

Bipartite graphs have received some attention in the study of social networks and of biological mutualistic systems. A generalization of a previous model is presented, that evolves the topology of the graph in order to optimally account for a given contact preference rule between the two guilds of the network. As a result, social and biological graphs are classified as belonging to two clearly different classes. Projected graphs, linking the agents of only one guild, are obtained from the original bipartite graph. The corresponding evolution of its statistical properties is also studied. An example of a biological mutualistic network is analyzed in detail, and it is found that the model provides a very good fitting of all the main statistical features. The model also provides a proper qualitative description of the same features observed in social webs, suggesting the possible reasons underlying the difference in the organization of these two kinds of bipartite networks.

A model for the emergence of adaptive subsystems.

We investigate the interaction of learning and evolution in a changing environment. A stable learning capability is regarded as an emergent adaptive system evolved by natural selection of genetic variants. We consider the evolution of an asexual population. Each genotype can have 'fixed' and 'flexible' alleles. The former express themselves as synaptic connections that remain unchanged during ontogeny and the latter as synapses that can be adjusted through a learning algorithm. Evolution is modelled using genetic algorithms and the changing environment is represented by two optimal synaptic patterns that alternate a fixed number of times during the 'life' of the individuals. The amplitude of the change is related to the Hamming distance between the two optimal patterns and the rate of change to the frequency with which both exchange roles. This model is an extension of that of Hinton and Nowlan in which the fitness is given by a probabilistic measure of the Hamming distance to the optimum. We find that two types of evolutionary pathways are possible depending upon how difficult (costly) it is to cope with the changes of the environment. In one case the population loses the learning ability, and the individuals inherit fixed synapses that are optimal in only one of the environmental states. In the other case a flexible subsystem emerges that allows the individuals to adapt to the changes of the environment. The model helps us to understand how an adaptive subsystem can emerge as the result of the tradeoff between the exploitation of a congenital structure and the exploration of the adaptive capabilities practised by learning.

Thermal treatment of the minority game.

We study a cost function for the aggregate behavior of all the agents involved in the minority game (MG) or the bar attendance model (BAM). The cost function allows us to define a deterministic, synchronous dynamic that yields results that have the main relevant features than those of the probabilistic, sequential dynamics used for the MG or the BAM. We define a temperature through a Langevin approach in terms of the fluctuations of the average attendance. We prove that the cost function is an extensive quantity that can play the role of an internal energy of the many-agent system while the temperature so defined is an intensive parameter. We compare the results of the thermal perturbation to the deterministic dynamics and prove that they agree with those obtained with the MG or BAM in the limit of very low temperature.

Stochastic resonant memory storage device.

We show that an extended system operating in the regime of stochastic resonance can act as a short-term memory device. The system under study is a ring of overdamped bistable oscillators coupled directionally, being each also subject to an external source of Gaussian white noise (the noise sources are independent). A single oscillator is driven by an external periodic force, assumed to act only over the time that the signal takes to traverse the whole ring. A traveling wave is then found to be transmitted several times along the ring with a small damping, provided that the driven oscillator operates in a regime close to stochastic resonance. If noise is suppressed from any oscillator of the chain, the traveling wave is immediately damped. The ring is thus found to act as a short-term memory device in which the stored information (one bit, corresponding to the presence or absence of the external driving) is sustained by noise during a characteristic time T(mem).

Dynamical quenching and annealing in self-organization multiagent models.

We study the dynamics of a generalized minority game (GMG) and of the bar attendance model (BAM) in which a number of agents self-organize to match an attendance that is fixed externally as a control parameter. We compare the usual dynamics used for the minority game with one for the BAM that makes a better use of the available information. We study the asymptotic states reached in both frameworks. We show that states that can be assimilated to either thermodynamic equilibrium or quenched configurations can appear in both models, but with different settings. We discuss the relevance of the parameter G that measures the value of the prize for winning in units of the fine for losing. We also provide an annealing protocol by which the quenched configurations of the GMG can progressively be modified to reach an asymptotic equilibrium state that coincides with the one obtained with the BAM.

A model for the interaction of learning and evolution.

We present a simple model in order to discuss the interaction of the genetic and behavioral systems throughout evolution. This considers a set of adaptive perceptrons in which some of their synapses can be updated through a learning process. This framework provides an extension of the well-known Hinton and Nowlan model by blending together some learning capability and other (rigid) genetic effects that contribute to the fitness. We find a halting effect in the evolutionary dynamics, in which the transcription of environmental data into genetic information is hindered by learning, instead of stimulated as is usually understood by the so-called Baldwin effect. The present results are discussed and compared with those reported in the literature. An interpretation is provided of the halting effect.

Fault tolerance in noise-enhanced propagation.

As a mock-up of synaptic transmission between neurons, we revisit a problem that has recently risen the interest of several authors: the propagation of a low-frequency periodic signal through a chain of one-way coupled bistable oscillators, subject to uncorrelated additive noise. On a numerical study performed in the optimal range of noise intensity for which essentially undamped transmission along the chain has been reported, we focus on the outcome of feeding with noise all the nodes but the central one. A (moderate) critical value of the coupling between oscillators is found such that whereas below it the propagation can be considered to be interrupted at the "dead neuron," it is reestablished above it. Thus, one of the distinguishing features of synaptic transmission, namely, a fault-tolerant behavior that enhances reliability at the expense of efficiency, arises here as an emergent property of the system.

An adaptive Boolean automation to model circadian cycles.

We propose a Boolean cellular automation to model an artificial adaptive living organism in order to investigate the development of cyclic vital functions during a simulated evolutionary process. The organism is endowed with a basic architecture consisting of several sensor (input), motor (output) and processing Boolean gates whose connectivity pattern is adapted with a genetic algorithm. Cyclic searching behaviors develop that are tuned to the spatial distribution of "food". Under additional assumptions we also find that internal pacemakers can develop to adapt plastically to the alternance of "light" an "darkness". These pacemakers coexist with a "free running" regime in which the circadian cycles persist and even in the absence of external periodic stimuli.

[Neuroendocrine cell tumor of the stomach (carcinoid). Clinico-pathological experience with 4 patients and demonstration of its ultrastructure]. / Tumor de células neuroendocrinas del estómago (carcinoide). Experiencia clínico patológica en cuatro pacientes y demostración de su ultraestructura.

Neuroendocrine cell tumours of the stomach (carcinoid) are uncommon, low potential for malignancy, lesions grow slowly and produce hormonal substances in contrast with others malignant neoplasias of the gastrointestinal tract. Early diagnosis is important because the curative treatment could not be done in advanced stage with disseminated metastases. In small lesions less than 2 cm without invasion to lymph node, the local wedge resection is reasonable and definitive curative treatment. Neuroendocrine differentiation, in doubtful cases, becomes established by a characteristic silver affinity, by the ultrastructurally observed presence of "endocrine" secretion granules, and by immunohistochemically detectable biogenic amines, neurohormonal peptides and "panNE markers".