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Initial Differences (initial + difference)
Selected AbstractsDeviations in the emergence of representations: a neuroconstructivist framework for analysing developmental disordersDEVELOPMENTAL SCIENCE, Issue 1 2000Andrew Oliver A common way of studying developmental disorders is to adopt a static neuropsychological deficit approach, in which the brain is characterized in terms of a normal brain with some parts or ,modules' impaired. In this paper we outline a neuroconstructivist approach in which developmental disorders are viewed as alternative developmental trajectories in the emergence of representations within neural networks. As a concrete instantiation of the assumptions underlying this general approach, we present a number of simulations in an artificial neural network model. The representations that emerge under different architectural, input and developmental timing conditions are then analysed within a multi-dimensional state space. We explore alternative developmental trajectories in these simulations, demonstrating how initial differences in the same parameter can lead to very different outcomes, and conversely how different starting states can sometimes result in similar end states (phenotypes). We conclude that the assumptions of the neuroconstructivist approach are likely to be more appropriate for analysing developmental deviations in complex dynamic neural networks, such as the human brain. [source] CROSS-GENERATIONAL ENVIRONMENTAL EFFECTS AND THE EVOLUTION OF OFFSPRING SIZE IN THE TRINIDADIAN GUPPY POECILIA RETICULATAEVOLUTION, Issue 2 2006Farrah Bashey Abstract The existence of adaptive phenotypic plasticity demands that we study the evolution of reaction norms, rather than just the evolution of fixed traits. This approach requires the examination of functional relationships among traits not only in a single environment but across environments and between traits and plasticity itself. In this study, I examined the interplay of plasticity and local adaptation of offspring size in the Trinidadian guppy, Poecilia reticulata. Guppies respond to food restriction by growing and reproducing less but also by producing larger offspring. This plastic difference in offspring size is of the same order of magnitude as evolved genetic differences among populations. Larger offspring sizes are thought to have evolved as an adaptation to the competitive environment faced by newborn guppies in some environments. If plastic responses to maternal food limitation can achieve the same fitness benefit, then why has guppy offspring size evolved at all? To explore this question, I examined the plastic response to food level of females from two natural populations that experience different selective environments. My goals were to examine whether the plastic responses to food level varied between populations, test the consequences of maternal manipulation of offspring size for offspring fitness, and assess whether costs of plasticity exist that could account for the evolution of mean offspring size across populations. In each population, full-sib sisters were exposed to either a low- or high-food treatment. Females from both populations produced larger, leaner offspring in response to food limitation. However, the population that was thought to have a history of selection for larger offspring was less plastic in its investment per offspring in response to maternal mass, maternal food level, and fecundity than the population under selection for small offspring size. To test the consequences of maternal manipulation of offspring size for offspring fitness, I raised the offspring of low- and high-food mothers in either low- or high-food environments. No maternal effects were detected at high food levels, supporting the prediction that mothers should increase fecundity rather than offspring size in noncompetitive environments. For offspring raised under low food levels, maternal effects on juvenile size and male size at maturity varied significantly between populations, reflecting their initial differences in maternal manipulation of offspring size; nevertheless, in both populations, increased investment per offspring increased offspring fitness. Several correlates of plasticity in investment per offspring that could affect the evolution of offspring size in guppies were identified. Under low-food conditions, mothers from more plastic families invested more in future reproduction and less in their own soma. Similarly, offspring from more plastic families were smaller as juveniles and female offspring reproduced earlier. These correlations suggest that a fixed, high level of investment per offspring might be favored over a plastic response in a chronically low-resource environment or in an environment that selects for lower reproductive effort [source] Effects of a hexokinase II deletion on the dynamics of glycolysis in continuous cultures of Saccharomyces cerevisiaeFEMS YEAST RESEARCH, Issue 2 2002Jasper A. Diderich Abstract In glucose-limited aerobic chemostat cultures of a wild-type Saccharomyces cerevisiae and a derived hxk2 null strain, metabolic fluxes were identical. However, the concentrations of intracellular metabolites, especially fructose 1,6-bisphosphate, and hexose-phosphorylating activities differed. Interestingly, the hxk2 null strain showed a higher maximal growth rate and higher Crabtree threshold dilution rate, revealing a higher oxidative capacity for this strain. After a pulse of glucose, aerobic glucose-limited cultures of wild-type S. cerevisiae displayed an overshoot in the intracellular concentrations of glucose 6-phosphate, fructose 6-phosphate, and fructose 1,6-bisphosphate before a new steady state was established, in contrast to the hxk2 null strain which reached a new steady state without overshoot of these metabolites. At low dilution rates the overshoot of intracellular metabolites in the wild-type strain coincided with the immediate production of ethanol after the glucose pulse. In contrast, in the hxk2 null strain the production of ethanol started gradually. However, in spite of the initial differences in ethanol production and dynamic behaviour of the intracellular metabolites, the steady-state fluxes after transition from glucose limitation to glucose excess were not significantly different in the wild-type strain and the hxk2 null strain at any dilution rate. [source] The initial phase of fracture healing is specifically sensitive to mechanical conditionsJOURNAL OF ORTHOPAEDIC RESEARCH, Issue 4 2003Petra Klein Abstract Interfragmentary movements affect the quality and quantity of callus formation. The mounting plane of monolateral external fixators may give direction to those movements. Therefore, the aim of this study was to determine the influence of the fixator mounting plane on the process of fracture healing. Identically configured fixators were mounted either medially or anteromedially on the tibiae of sheep. Interfragmentary movements and ground reaction forces were evaluated in vivo during a nine week period. Histomorphological and biomechanical parameters described the bone healing processes. Changing only the mounting plane led to a modification of interfragmentary movements in the initial healing phase. The difference in interfragmentary movements between the groups was only significant during the first post-operative period. However, these initial differences in mechanical conditions influenced callus tissue formation significantly. The group with the anteromedially mounted fixator, initially showing significantly more interfragmentary movements, ended up with a significantly smaller callus diameter and a significantly higher callus stiffness as a result of advanced fracture healing. This demonstrates that the initial phase of healing is sensitive to mechanical conditions and influences the course of healing. Therefore, initial mechanical stability of an osteosynthesis should be considered an important factor in clinical fracture treatment. © 2003 Orthopaedic Research Society. Published by Elsevier Science Ltd. All rights reserved. [source] Strong shifts in plant diversity and vegetation composition in grassland shortly after climatic changeJOURNAL OF VEGETATION SCIENCE, Issue 3 2008Maria-Teresa Sebastią Abstract Questions: Is plant diversity in mesic grassland ecosystems vulnerable in the short-term to extreme climate change events? How rapidly can responses in vegetation composition occur in perennial grasslands? Are the expected compositional changes related to rare species losses or to shifts in the relative abundance of the dominants? Location: Subalpine mesic grasslands on limestone in the Pyrenees. Methods: Transplanting turves from the upland, with cold-temperate climate, to a lowland location, with continental Mediterranean climate. Results: Transplanting led to decreased biodiversity and strong shifts in vegetation composition. Results from both permutation tests and traditional multivariate analysis suggested different trajectories of vegetation depending on the initial species pool. Vegetation showed a tendency to converge in composition in the lowland over time, independently of initial differences. Estimated changes in relative biomass of the five most abundant species between the upland and the lowland ranged from -89 to +96 %. The ensemble of all other species was reduced by 80%. The most dominant species in the upland, Festuca nigrescens, reduced its abundance in the lowland, shifting from having mainly positive to mainly negative associations with other species. Conclusions: Mesic grassland ecosystems in the Pyrenees showed strong shifts in plant diversity and composition after a short period of warming and drought, as a consequence of acute vulnerability of some dominant grasses, losses of rare species, and aggregate and trigger effects of originally uncommon forb species. [source] Detecting Technological Catch-Up in Economic ConvergenceMETROECONOMICA, Issue 2-3 2003Francesco Pigliaru We address the problem of measuring, in the absence of reliable indices of technology levels, how much of the convergence we observe is due to convergence in technology or in capital,labour ratios. We first develop a growth model where technology accumulation in lagging economies depends on their propensity to innovate and on technological spillovers, with convergence due both to capital-deepening and to technological diffusion. Then we study the transitional dynamics of the model to show how to discriminate empirically between the following three hypotheses: (i) convergence is due to capital-deepening with technology levels uniform across economies, as in Mankiw, Romer and Weil; (ii) convergence is due to capital-deepening with stationary differences in individual technologies, as in Islam; (iii) convergence is due to both technological catch-up and capital-deepening. Our main findings are as follows. First, we show that it may be difficult to distinguish between hypotheses (ii) and (iii) in cross-section or panel data. This problem has often been overlooked in the empirical literature on convergence. Second, we suggest how the problem can be overcome by noting that hypothesis (iii) does (and hypothesis (ii) does not) imply that the initial differences in technology levels may tend to decrease over time. A careful analysis of the fixed-effects estimates obtained by means of panel data methodology proposed in Islam should allow researchers to discriminate between the two competing hypotheses. [source] Investment dynamics in markets with endogenous demandTHE JOURNAL OF INDUSTRIAL ECONOMICS, Issue 2 2000Nikolaos Vettas I examine entry into markets where demand is an increasing function of past sales because of learning, networks, or fashion. Demand is initially unknown (with firms learning in Bayesian fashion) and grows endogenously over time. The competitive expansion path and the efficient/monopoly solution differ not only with respect to levels (the market's investment is too low), but also time patterns: externalities contribute to S-shaped diffusion. There is also path-dependence: small initial differences may determine whether the market will grow or not open. Policy arguments for subsidizing entry into new markets, especially in infant export industries, are examined. [source] | |||||||||||||