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Search Results: 1 - 10 of 223099 matches for " Julián Ramírez-Villegas "
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A Gap Analysis Methodology for Collecting Crop Genepools: A Case Study with Phaseolus Beans
Julián Ramírez-Villegas,Colin Khoury,Andy Jarvis,Daniel Gabriel Debouck,Luigi Guarino
PLOS ONE , 2012, DOI: 10.1371/journal.pone.0013497
Abstract: The wild relatives of crops represent a major source of valuable traits for crop improvement. These resources are threatened by habitat destruction, land use changes, and other factors, requiring their urgent collection and long-term availability for research and breeding from ex situ collections. We propose a method to identify gaps in ex situ collections (i.e. gap analysis) of crop wild relatives as a means to guide efficient and effective collecting activities.
Drought Tolerance in Wild Plant Populations: The Case of Common Beans (Phaseolus vulgaris L.)
Andrés J. Cortés, Fredy A. Monserrate, Julián Ramírez-Villegas, Santiago Madri?án, Matthew W. Blair
PLOS ONE , 2013, DOI: 10.1371/journal.pone.0062898
Abstract: Reliable estimations of drought tolerance in wild plant populations have proved to be challenging and more accessible alternatives are desirable. With that in mind, an ecological diversity study was conducted based on the geographical origin of 104 wild common bean accessions to estimate drought tolerance in their natural habitats. Our wild population sample covered a range of mesic to very dry habitats from Mexico to Argentina. Two potential evapotranspiration models that considered the effects of temperature and radiation were coupled with the precipitation regimes of the last fifty years for each collection site based on geographical information system analysis. We found that wild accessions were distributed among different precipitation regimes following a latitudinal gradient and that habitat ecological diversity of the collection sites was associated with natural sub-populations. We also detected a broader geographic distribution of wild beans across ecologies compared to cultivated common beans in a reference collection of 297 cultivars. Habitat drought stress index based on the Thornthwaite potential evapotranspiration model was equivalent to the Hamon estimator. Both ecological drought stress indexes would be useful together with population structure for the genealogical analysis of gene families in common bean, for genome-wide genetic-environmental associations, and for postulating the evolutionary history and diversification processes that have occurred for the species. Finally, we propose that wild common bean should be taken into account to exploit variation for drought tolerance in cultivated common bean which is generally considered susceptible as a crop to drought stress.
Una revisión de modelos de atención visual Bottom-up neurobiológicamente inspirados
Juan F. Ramírez-Villegas,David F. Ramírez-Moreno
El Hombre y la Máquina , 2010,
Abstract: La atención visual es un proceso desarrollado sobre gran parte del área de procesamiento visual primario. El procesamiento cerebral provoca la focalización selectiva de los objetos del campo visual que compiten por procesamiento, esta competencia se encuentra mediada por características primarias de los objetos como el contraste de color, el contraste de intensidad y el contraste de orientación. En esta revisión se procura establecer la conexión de los odelos de atención visual bottom-up disponibles en la literatura, sus arquitecturas y bases matemáticas, con la evidencia neurobiológica sobre el fenómeno de atención visual establecida hasta la actualidad. Adicionalmente, se ilustra una implementación propia de un modelo de atención visual bottom-up y se discuten algunos pormenores sobre el proceso de atención o mecanismo top-down cuya dinámica complementa la función del proceso de pre-atención o bottom-up.
Desarrollo de la atención visual en escenas naturales: un modelo bottom-up basado en mapa de prominencia con dinámica realista
JUAN F. RAMíREZ-VILLEGAS,DAVID F. RAMíREZ-MORENO
El Hombre y la Máquina , 2011,
Abstract: Este trabajo propone un modelo de atención visual para el análisis de escenas dinámicas que complementa el modelo de Itti & Koch (2000) [Itti, L., & Koch, C. (2000). A saliency-based search mechanism for overt and covert shifts of visual attention. Vision Research, 40, 1484-1506] modificado por de Brecht & Saiki (2006). A neural network implementation of a saliency map model. Neural Networks, 19, 1467-1474]. Este modelo incluye dos novedades principales, primero, el cómputo de la prominencia de movimiento por una red neuronal con dinámica realista y su respuesta en el tiempo, y segundo, para la etapa de extracción de características visuales primarias, el cálculo del mapa de color a partir del modelo de Itti & Koch (2000) con la inclusión de modelos de células rojo-cian y verde-magenta en el sistema estándar de color doble-oponente. Las respuestas transitorias de la red a objetos en movimiento así como el tiempo que le toma al sistema alcanzar el estado estable, se encuentra en un intervalo biológicamente plausible. Los mapas de color modificados produjeron mejores respuestas que los mapas de color reportados anteriormente en la literatura a una significancia estadística alta, en adicción, la exactitud del modelo en la predicción de las fijaciones del ojo humano se incrementó.
Estilos de aprendizaje y metodologías de ense anza en estudiantes de obstetricia Learning styles and teaching methodologies in obstetric students
E. Villalobos-Alarcón,M. Guerrero-Faquíez,R. Pérez-Villegas,A. Avenda?o-Veloso
Educación Médica , 2009,
Abstract: Introducción. Si consideramos que todos los estudiantes no son iguales y aprenden de manera diferente, es necesario identificar sus estilos de aprendizaje (EA) y las metodologías que faciliten el proceso ense anza-aprendizaje. Este estudio pretende diagnosticar los EA predominante en estudiantes de obstetricia y correlacionarlos con las metodologías educativas aplicadas. Sujetos y métodos. Se realizó un estudio observacional, correlacional y transversal, durante el período 2006-2007, en 72 estudiantes de obstetricia de la Facultad de Medicina, Universidad de Concepción, Chile. Las variables estudiadas y correlacionadas fueron estilo de aprendizaje y metodologías de ense anza. Se utilizó el Cuestionario de EA Honey-Alonso (CHAEA), y se realizó una encuesta de satisfacción de las metodologías que facilitaron el aprendizaje. Para el análisis estadístico se utilizó la prueba chi al cuadrado, y la correlación entre ambas variables se obtuvo con el coeficiente de rho de Spearman. Resultados. Los estilos predominantes fueron el reflexivo y teórico (p < 0,0001). Las metodologías de ense anzas que más facilitaron el aprendizaje fueron el uso de modelos y la clase expositiva. No se encontró correlación estadística entre los EA y las metodologías de ense anza utilizadas. Conclusión. La determinación de los EA en los estudiantes de la salud es fundamental para seleccionar las metodologías de ense anza más adecuadas. Introduction. Considering that all students are not equal and learn differently, is necessary to identify their learning styles and methodologies that facilitate the teaching-learning process. This study aims to diagnose learning styles prevalent in Obstetric students and correlation with educational methodologies applied. Subjects and methods. Observational and cross-correlation study, during the period 2006-2007 in 72 Obstetric students, Faculty of Medicine, University of Concepción, Chile. The variables were learning style and teaching methodologies. The instrument used was the questionnaire learning styles Honey-Alonso (CHAEA) and satisfaction survey methodologies that helped the learning. The statistical differences were conducted using chi-square test and the correlation between two variables was obtained with the coefficient of Spearman's rho. Results. The dominant were reflexive and theoretical styles (p < 0,0001). Methodologies for more facilitated learning were the use of models and the expositive class. No correlation was found between statistical learning styles and teaching methodologies used. Conclusion. The identification of learning styl
Calidad de vida en ni?os con leucemia linfoblástica aguda durante la inducción a la remisión mediante el PedsQL Cancer Module?
Castillo-Martínez, Io Daiela;Juárez-Villegas, Luis Enrique;Palomo-Colli, Miguel ángel;Medina-Sansón, Aurora;Zapata-Tarrés, Marta;
Boletín médico del Hospital Infantil de México , 2009,
Abstract: introduction. survival of children with acute lymphoblastic leukemia (all) is 80% in accordance with actual protocols. we ignore quality of life (qol) during these chronic treatments, especially in our institution. the aim of this pilot study was to measure qol in stable children with all during the first part of treatment (induction therapy) with pedsql cancer module?. methods. we made two measurements in children with recent diagnosis of all and determined changes in the qol between the beginning and the end of induction therapy. we included 26 patients from 2 to 18 years of age with all, at 2 weeks and 2 months after diagnosis, and divided them into four groups: 2-4, 5-7, 8-12, and 13-18 years of age. results. in the second measurement, we observed a better qol in relation to an adaptation process in the child and remission of symptoms. conclusions. pedsql cancer module? was a useful instrument for measuring qol and detected changes in children with all during induction therapy.
Estilos de aprendizaje y metodologías de ense?anza en estudiantes de obstetricia
Villalobos-Alarcón,E.; Guerrero-Faquíez,M.; Pérez-Villegas,R.; Avenda?o-Veloso,A.; Ceballos-Morales,A.; Ortiz-Contreras,J.; Parra-Ponce,C.;
Educación Médica , 2009, DOI: 10.4321/S1575-18132009000100007
Abstract: introduction. considering that all students are not equal and learn differently, is necessary to identify their learning styles and methodologies that facilitate the teaching-learning process. this study aims to diagnose learning styles prevalent in obstetric students and correlation with educational methodologies applied. subjects and methods. observational and cross-correlation study, during the period 2006-2007 in 72 obstetric students, faculty of medicine, university of concepción, chile. the variables were learning style and teaching methodologies. the instrument used was the questionnaire learning styles honey-alonso (chaea) and satisfaction survey methodologies that helped the learning. the statistical differences were conducted using chi-square test and the correlation between two variables was obtained with the coefficient of spearman's rho. results. the dominant were reflexive and theoretical styles (p < 0,0001). methodologies for more facilitated learning were the use of models and the expositive class. no correlation was found between statistical learning styles and teaching methodologies used. conclusion. the identification of learning styles in student health is essential to select the most appropriate teaching methodologies.
Stellar Orbital Studies in Normal Spiral Galaxies I: Restrictions to the Pitch Angle
A. Pérez-Villegas,B. Pichardo,E. Moreno
Physics , 2013, DOI: 10.1088/0004-637X/772/2/91
Abstract: We built a family of non-axisymmetric potential models for normal non-barred or weakly-barred spiral galaxies as defined in the simplest classification of galaxies: the Hubble sequence. For this purpose a three-dimensional self-gravitating model for spiral arms PERLAS is superimposed to the galactic axisymmetric potentials. We analyze the stellar dynamics varying only the pitch angle of the spiral arms, from 4$\deg$ to 40$\deg$, for an Sa galaxy, from 8$\deg$ to 45$\deg$, for an Sb galaxy, and from 10$\deg$ to 60$\deg$, for an Sc galaxy. Self-consistency is indirectly tested through periodic orbital analysis, and through density response studies for each morphological type. Based on ordered behavior, periodic orbits studies show that for pitch angles up to approximately $15\deg$, $18\deg$, and $20\deg$ for Sa, Sb and Sc galaxies, respectively, the density response supports the spiral arms potential, a requisite for the existence of a long-lasting large-scale spiral structure. Beyond those limits, the density response tends to "avoid" the potential imposed by mantaining lower pitch angles in the density response; in that case the spiral arms may be explained as transient features rather than long-lasting large-scale structures. In a second limit, from a phase space orbital study based on chaotic behavior, we found that for pitch angles larger than $\sim30\deg$, $\sim40\deg$ and $\sim50\deg$ for Sa, Sb, and Sc galaxies, respectively, chaotic orbits dominate all phase space prograde region that surrounds the periodic orbits sculpting the spiral arms and even destroying them. This result seems to be in good agreement with observations of pitch angles in typical isolated normal spiral galaxies.
Stellar Orbital Studies in Normal Spiral Galaxies II: Restrictions to Structural and Dynamical parameters on Spiral Arms
Angeles Pérez-Villegas,Bárbara Pichardo,Edmundo Moreno
Physics , 2015,
Abstract: Making use of a set of detailed potential models for normal spiral galaxies, we analyze the disk stellar orbital dynamics as the structural and dynamical parameters of the spiral arms (mass, pattern speed and pitch angle) are gradually modified. With this comprehensive study of ordered and chaotic behavior, we constructed an assemblage of orbitally supported galactic models and plausible parameters for orbitally self-consistent spiral arms models. We find that, to maintain orbital support for the spiral arms, the spiral arm mass, M$_{sp}$, must decrease with the increase of the pitch angle, $i$; if $i$ is smaller than $\sim10\deg$, M$_{sp}$ can be as large as $\sim7\%$, $\sim6\%$, $\sim5\%$ of the disk mass, for Sa, Sb, and Sc galaxies, respectively. If $i$ increases up to $\sim25\deg$, the maximum M$_{sp}$ is $\sim1\%$ of the disk mass independently in this case of morphological type. For values larger than these limits, spiral arms would likely act as transient features. Regarding the limits posed by extreme chaotic behavior, we find a strong restriction on the maximum plausible values of spiral arms parameters on disk galaxies beyond which, chaotic behavior becomes pervasive. We find that for $i$ smaller than $\sim20\deg$, $\sim25\deg$, $\sim30\deg$, for Sa, Sb, and Sc galaxies, respectively, M$_{sp}$ can go up to $\sim10\%$, of the mass of the disk. If the corresponding $i$ is around $\sim40\deg$, $\sim45\deg$, $\sim50\deg$, M$_{sp}$ is $\sim1\%$, $\sim2\%$, $\sim3\%$ of the mass of the disk. Beyond these values, chaos dominates phase space, destroying the main periodic and the neighboring quasi-periodic orbits.
The Contribution of Spiral Arms to the Thick Disk along the Hubble Sequence
L. A. Martinez-Medina,B. Pichardo,A. Pérez-Villegas,E. Moreno
Physics , 2015, DOI: 10.1088/0004-637X/802/2/109
Abstract: The first mechanism invoked to explain the existence of the thick disk in the Milky Way Galaxy, were the spiral arms. Up-to-date work summon several other possibilities that together seem to better explain this component of our Galaxy. All these processes must affect differently in distinct types of galaxies, but the contribution of each one has not been straightforward to quantify. In this work, we present a first comprehensive study of the effect of the spiral arms in the formation of thick disks, as going from early to late type disk galaxies, in an attempt to characterize and quantify this specific mechanism in galactic potentials. To this purpose, we perform numerical simulations of test particles in a three-dimensional spiral galaxy potential of normal spiral galaxies (from early to late types). By varying the parameters of the spiral arms we found that the vertical heating of the stellar disk becomes very important in some cases, and strongly depends on the galaxy morphology, pitch angle, arms mass and its pattern speed. The later the galaxy type, the larger is the effect on the disk heating. This study shows that the physical mechanism causing the vertical heating is different from simple resonant excitation. The spiral pattern induce chaotic behavior not linked necessarily to resonances but to direct scattering of disk stars, which leads to an increase of the velocity dispersion. We applied this study to the specific example of the Milky Way Galaxy, for which we have also added an experiment that includes the Galactic bar. From this study we deduce that the effect of spiral arms of a Milky-Way-like potential, on the dynamical vertical heating of the disk is negligible, unlike later galactic potentials for disks.
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