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At least 577 records · Page 32Linked to original sources

PubMedDate not supplied

Turfgrass thatch effects on pesticide leaching: a laboratory and modeling study.

Process-based models are frequently used to assess the water quality impacts of turfgrass management emanating from proposed or existing golf courses. Thatch complicates the prediction of pesticide transport because surface-applied pesticides must pass through an organic-rich layer before entering the soil. This study was conducted to (i) compare the use of a linear equilibrium model (LEM) and two-site nonequilibrium (2SNE) model to predict pesticide transport through soil and thatch + soil columns, and (ii) evaluate thatch effects on pesticide transport through soil columns with a volume-averaging approach. Pesticide breakthrough curves were obtained for soil and thatch + soil columns from a 1 cm h(-1) flux applied one day after applying triclopyr (3,5,6-trichloro-2-pyridinyloxyacetic acid) and carbaryl (1-napthyl-methyl carbamate). Pesticide and bromide transport parameters indicated that nonequilibrium processes were affecting pesticide transport. Columns containing zoysiagrass (Zoysia japonica Steud.) thatch had lower triclopyr and carbaryl leaching losses than did soil-only columns, although total reductions attributable to thatch did not exceed 15% of the applied pesticide. When laboratory-based retardation factors were used, the 2SNE model explained 88 to 93% of the variability for triclopyr and 70 to 94% of the variability for carbaryl. Laboratory-based retardation factors performed well in a 2SNE model to predict the peak concentration and tailing behavior of triclopyr and carbaryl with a volume-averaging approach. These results suggest that separate representation of the thatch layer in process-based models is not a prerequisite to obtain reasonable estimates of pesticide transport under steady state flow conditions.

Environment Design↗

PubMed2005

A diameter increment model for Crimean pine (Pinus nigra Arnold) and Calabrian pine (Pinus brutia Ten.) stands in Isparta region, Turkey.

The purpose of this research was to test the precision of a diameter increment model for the estimation of future periodic diameter increment. Individual trees of Crimean pine (Pinus nigra Arnold) and Calabrian pine (Pinus brutia Ten.) located in both natural and plantation stands were selected. For that reason, normal closed canopy, pure, even-aged and undisturbed stands were examined. In 2002, plots were sampled in three natural and three plantation stands in Isparta region of Turkey. The number of sampling points in sample plots ranged from 19 to 55. In each sampling point, a subject tree and six competitors were selected. In each sampling point, subject tree and competitor trees were stem mapped (x and y coordinate system), and diameter (dbh), total height, age, and 10-yrs radial increment recorded. The predictors of a distance dependent diameter increment model were chosen that included tree level (diameter (d), competition index (CI), and age (t)) and stand level (basal area (G), and site index (SI)) characteristics as well as their transformations. The best fit index of the regression model was pursued in trials with variable combinations. The models explained 65%, 60%, 68% and 50% of the variation in individual tree diameter increment of Crimean pine and Calabrian pine for both natural and plantations stands, respectively. These models can be estimated diameter increment of individual trees at highly significant level (p<0.001).

Age Factors↗

PubMed1991

A model for asymmetric septum formation during sporulation in Bacillus subtilis.

Many differentiation processes in both prokaryotes and eukaryotes begin with an asymmetric division, producing 'daughter' cells that differ in size and developmental fate. This is particularly obvious in the well-studied prokaryotic life cycles of Caulobacter and Bacillus. In no system, however, is the mechanism of asymmetric division understood. Here I propose a model for the mechanism of asymmetric division during sporulation in Bacillus subtilis. The model explains both the timing and asymmetric localization of spore-septum formation. It also explains the morphological phenotypes of various asporogenous (spo) mutants.

Bacillus subtilis↗

PubMedDate not supplied

Job satisfaction and organizational attachment of nurses holding doctoral degrees.

A model explaining job satisfaction and organizational attachment (commitment and intent to leave) was estimated for a national sample of nurses holding doctoral degrees and employed in academic and nonacademic setting. The purpose of the study was (a) to test a model from Price-Mueller that has been used primarily for nurses who do not have their doctorates and (b) to determine if the results of the test were consistent with expectations from the professions literature The results showed that although nursing is categorized as a semiprofession in the professions literature, the satisfaction and attachment of nurses holding doctorate degrees was explained by variables from arguments about numbers of the well-established professions, known as "true" professions.

Education, Nursing, Graduate↗

PubMed1989

Asymmetric velocity and acceleration profiles of human arm movements.

Displacement, velocity, acceleration and jerk (change of acceleration with time) were analyzed for arm flexion movement over a wide range of movement amplitudes and speeds. Relative time to peak velocity or relative duration of acceleration, k, was approximately 0.5 for the movements with intermediate speed (about 0.5 s in movement time), i.e., symmetric velocity and acceleration profiles. For the slow and ballistic movements, k shifted towards values below and above 0.5, respectively creating asymmetric profiles. Consistent k-dependence of movement time, peak velocity, maximum acceleration and maximum deceleration were observed. "Jerk cost", the square of the magnitude of jerk integrated over the entire movement, was calculated for each movement. A dynamic optimization technique to minimize jerk cost under the constraint on jerk input was applied to interpret the results, assuming that a major goal of skilled movements was to produce optimally smooth movements. The constrained minimum-jerk model explained speed-dependent asymmetry of the velocity and acceleration profiles. Jerk cost consumed by the movements with intermediate speed approximately satisfied minimum-cost criterion predicted by the model but was higher than the criterion for slow and ballistic movements. The results suggested that optimality criteria other than jerk cost also should be considered to predict movement profiles over the entire range of speeds.

Acceleration↗

PubMed2000

Triple-loop model of path control by head direction and place cells.

Arthropods as well as mammals are able to return straight home after a random search excursion under conditions that are designed to exclude all external cues. After a brief clarification of the terminology, two principal systems of information processing that can achieve this performance are introduced and analysed: Polar versus Cartesian path integration. The different demands and achievements of the two systems are confronted with neurophysiological findings on the functioning of the hippocampus, and with a recent comprehensive model of how the hippocampal place cells perform path integration. To connect the neurophysiological findings with the behavior of the animal, a new model is developed. It achieves three functionally diverse performances: maintenance and control of a compass direction, navigation by path integration, and formation of goals by connecting non-spatial features with their location. This is done by three interconnected feedback loops, set by a common reference variable. Their information-processing structure enables the animal not only to home but also to go straight from any stored goal to any other, without explicit representation of the distance between them, and without a topological arrangement of the store. The model explains behaviors not yet understood and predicts still undiscovered performances. Because it allows the isolation of orienting from storing functions yet also shows how they can be connected, the model may help to reconcile conflicting views on the function of the hippocampus.

Animals↗

PubMed2007

Prediction of continuous positive airway pressure in obstructive sleep apnea.

Continuous positive airway pressure (CPAP) prediction formulas can potentially simplify the treatment of obstructive sleep apnea (OSA). However, they can be difficult to derive and validate. We tested a statistical method to derive and validate a CPAP prediction formula using the same sample population. Seventy-six OSA patients underwent polysomnography and CPAP titration. Anthropometric measures, sleep parameters, and the Epworth sleepiness scale (ESS) were evaluated as predictors. All subsets regression was used to determine the optimum number of variables in the model. The Bayes information criterion was used to find the best-fit model. The model was then evaluated by a tenfold cross-validation procedure. Subjects were obese (BMI 31.3 +/- 5.4) and had significant daytime somnolence (ESS 11.9 +/- 5). Mean respiratory disturbance index (RDI) was 53.5 +/- 31.3. The ESS was not predictive of titrated CPAP. The best-fit model included three variables (CPAP(pred) = 30.8 + RDI x 0.03 - nadir saturation x 0.05 - mean saturation x 0.2). This model explained 67% of the variance. Our data and the literature suggest that a combination of two to three factors is predictive of titrated CPAP: RDI, oxyhemoglobin saturation, and obesity. Except for RDI, the specific factors vary in each population. A CPAP prediction formula that explains a high proportion of the titrated CPAP variance can be easily derived from parameters measured during the diagnostic work-up of OSA patients using a unique statistical model that allows derivation and validation of the formula in the same test population.

Adult↗

PubMed1994

Modelling the combined effect of temperature and pH on the rate coefficient for bacterial growth.

An extension of a model of Davey (1989a) has been used to model data of Adams et al. (1991) describing the combined effect of temperature and pH on the rate coefficient for growth of Yersinia enterocolitica in liquid media acidified with four different acidulants: sulphuric, citric, lactic and acetic acids. The model explained between 97.8% and 99.1% of the variance in the results. This very good fit between the predictions and the observed data confirms pH can be incorporated into a model that had previously been applied to the combined effect of temperature and water activity, and to the effect of temperature alone. These findings, taken together with previously reported results, demonstrate the value of the model for predicting the combined effect of two environmental factors on the growth rate. Based on results reported in this paper and previously reported results, a generalised model is proposed for the combined effect of three or more environmental factors, that is amenable to testing.

Bacteria↗

PubMed1999

Hypothetical model for monitoring microbial growth by using capacitance measurements--a minireview.

Microbiological impedance devices are used routinely by food and manufacturing industries, and public health agencies to measure microbial growth and metabolism. In this paper a hypothetical model explaining the effects of microbial growth and metabolism on capacitance at electrode-medium interfaces, that can be supported by fundamental theories and principles of electrochemistry, is presented. This model provides a framework to interpret changes in capacitance during microbial growth and metabolism and can be used to generate and test hypotheses on factors (i.e., temperature, microbial cell density, microbial growth and medium conductivity) contributing to increases or decreases in capacitance.

Culture Media↗

PubMed1997

Cell-cycle sensitivity, recovery from radiation damage and a new paradigm for risk assessment.

Tikvah Alper's interest in science was broad, from scrapie to mammalian cells and cancer. Much of her own work focused on cell lethality, like that of many other radiobiologists, but this was natural because of the simplicity of the endpoint cell survival and its relevance to cancer therapy. Tikvah had broader interests, however, that included the effects of radiation on living systems in general like the induction of cancer and the cellular and molecular processes contributing to it. In this essay, some ideas are developed that lie in the mainstream of her interests. Starting with functional measures of the recovery or repair from radiation damage, a role for repair is illustrated in connection with mutagenesis and neoplastic transformation both discussed in the context of radiation-induced cancer. These topics are central to a model explaining the anomalous enhanced neoplastic transformation and cancer observed when low doses of a high-LET radiation are protracted in time. Under particular circumstances, the formalism of the model predicts application to protracted low-LET exposures as in the instance of repair-deficient target cells and sporadic breast cancer. The latter discussion leads to the proposal that the paradigm in current use for evaluating cancer risk should be broadened: from a simple dose-effect relation to one that includes cell kinetics (during protracted exposures), cell-cycle dependencies, and the influence of cellular repair or the lack thereof.

Animals↗

PubMed1995

Dynamic viscoelastic nonlinearity of lung parenchymal tissue.

To investigate the contribution of nonlinear tissue viscoelasticity to the dynamic behavior of lung, time and frequency responses of isolated parenchymal strips of degassed dog lungs were investigated. The strips were subjected to loading and unloading stretch steps for 60 s and to sinusoidal oscillations (0.03-3 Hz) of different stretch amplitudes (delta lambda = 0.05, 0.1, and 0.2) and at different operating stresses (T(o) = 0.5, 1, and 2 kPa). Elastance (E) increased linearly with the logarithm of frequency (approximately 10% per frequency decade), and resistance (R) decreased hyperbolically with frequency. Both E and R varied little with delta lambda but they increased proportionally with T(o). Hysteresivity (eta = R x 2 pi x frequency/E) ranged from 0.07 to 0.10. In agreement with the frequency response, the magnitude of the unit step response increased with T(o) and was higher when loading than when unloading, and the stress relaxation ratio (approximately 0.10) did not vary greatly with T(o) or with delta lambda. The time and frequency behavior of the strips were interpreted in terms of the quasilinear viscoelastic model of Navajas et al. (J. Appl. Physiol. 73:2681-2692, 1992). The model explains most of the dependencies of step and oscillatory responses on the measurement conditions, in particular the proportional dependence of E and R on T(o). According to the model, about two-thirds of energy dissipated during oscillation arises from tissue viscoelasticity. The remaining dissipated energy could be accounted for by plasticity. Thus the effect of nonlinear elasticity on the dynamic behavior of lung tissue can be empirically described by a simple quasilinear model characterized by only two parameters.

Animals↗

PubMed1989

Emotion, reason, and communication in coping with cancer.

Researchers have sought to identify which coping styles are most conducive to recovery from cancer and other serious illnesses. The results have been equivocal. It appears that what helps one patient does not necessarily help another. This article proposes a model explaining how emotional, cognitive, and environmental conditions influence the styles of coping used by cancer patients and the likelihood that particular styles will prove helpful to these patients. The model is based on the premise that coping does not occur in isolation from the emotional makeup, cognitive predispositions, and social environment of the patient. It describes how these processes interact to influence the protective effect of various coping styles.

Journal Article↗

PubMed2004

Geometrical computations explain projection patterns of long-range horizontal connections in visual cortex.

Neurons in primary visual cortex respond selectively to oriented stimuli such as edges and lines. The long-range horizontal connections between them are thought to facilitate contour integration. While many physiological and psychophysical findings suggest that collinear or association field models of good continuation dictate particular projection patterns of horizontal connections to guide this integration process, significant evidence of interactions inconsistent with these hypotheses is accumulating. We first show that natural random variations around the collinear and association field models cannot account for these inconsistencies, a fact that motivates the search for more principled explanations. We then develop a model of long-range projection fields that formalizes good continuation based on differential geometry. The analysis implicates curvature(s) in a fundamental way, and the resulting model explains both consistent data and apparent outliers. It quantitatively predicts the (typically ignored) spread in projection distribution, its nonmonotonic variance, and the differences found among individual neurons. Surprisingly, and for the first time, this model also indicates that texture (and shading) continuation can serve as alternative and complementary functional explanations to contour integration. Because current anatomical data support both (curve and texture) integration models equally and because both are important computationally, new testable predictions are derived to allow their differentiation and identification.

Animals↗

PubMed2005

A risk-based predictive tool to prevent accidental introductions of nonindigenous marine species.

Preventing the introduction of nonindigenous species (NIS) is the most efficient way to avoid the costs and impacts of biological invasions. The transport of fouling species on ship hulls is an important vector for the introduction of marine NIS. We use quantitative risk screening techniques to develop a predictive tool of the abundance and variety of organisms being transported by ocean-going yachts. We developed and calibrated an ordinal rank scale of the abundance of fouling assemblages on the hulls of international yacht hulls arriving in New Zealand. Fouling ranks were allocated to 783 international yachts that arrived in New Zealand between 2002 and 2004. Classification tree analysis was used to identify relationships between the fouling ranks and predictor variables that described the maintenance and travel history of the yachts. The fouling ranks provided reliable indications of the actual abundance and variety of fouling assemblages on the yachts and identified most (60%) yachts that had fouling on their hulls. However, classification tree models explained comparatively little of the variation in the distribution of fouling ranks (22.1%), had high misclassification rates (approximately 43%), and low predictive power. In agreement with other studies, the best model selected the age of the toxic antifouling paint on yacht hulls as the principal risk factor for hull fouling. Our study shows that the transport probability of fouling organisms is the result of a complex suite of interacting factors and that large sample sizes will be needed for calibration of robust risk models.

Animals↗

PubMed2006

Changes in stomatal conductance and net photosynthesis during phenological development in spring wheat: implications for gas exchange modelling.

Gas exchange was measured from 1 month before the onset of anthesis until the end of grain filling in field-grown spring wheat, Triticum aestivum L., cv. Vinjett, in southern Sweden. Two g ( s ) models were parameterised using these data: one Jarvis-type multiplicative g ( s ) model (J-model), and one combined stomatal-photosynthesis model (L-model). In addition, the multiplicative g ( s ) model parameterisation for wheat used within the European Monitoring and Evaluation Programme (EMEP-model) was tested and evaluated. The J-model performed well (R (2)=0.77), with no systematic pattern of the residuals plotted against the driving variables. The L-model explained a larger proportion of the variation in g ( s ) data when observations of A (n) were used as input data (R (2)=0.71) compared to when A (n) was modelled (R (2)=0.53). In both cases there was a systematic model failure, with g (s) being over- and underestimated before and after anthesis, respectively. This pattern was caused by the non-parallel changes in g ( s ) and A (n) during plant phenological development, with A (n) both peaking and starting to decline earlier as compared to g ( s ). The EMEP-model accounted for 41% of the variation in g ( s ) data, with g ( s ) being underestimated after anthesis. We conclude that, under the climatic conditions prevailing in southern Scandinavia, the performance of the combined stomatal-photosynthesis approach is hampered by the non-parallel changes in g ( s ) and A (n), and that the phenology function of the EMEP-model, having a sharp local maximum at anthesis, should be replaced by a function with a broad non-limiting period after anthesis.

Carbon Dioxide↗

PubMed1988

General kinetic model for protein-mediated phospholipid transfer between membranes.

Phospholipid transfer protein catalyzes the transfer of phospholipids between bilayer membranes. A general model is developed for describing the kinetics of this process. While previous models derive detailed expressions only for the initial rate of transfer from donor to acceptor membranes, this model takes into account donor-to-donor, acceptor-to-acceptor, and acceptor-to-donor transfers, in addition to the usual donor-to-acceptor transfer. The apparent rate of transfer along any of these specific routes is given as the product of the total rate of transfer (the sum of the rates of transfer along all four routes) and a probability function uniquely defined for each route. The model explains adequately the effects of membrane concentration on phospholipid transfer activity as well as the consequences of varying membrane surface charge and size. Using bovine liver phosphatidylcholine transfer protein, the model is applied to the kinetic analysis of phosphatidylcholine transfer between two populations of small unilamellar vesicles. Rates of protein-catalyzed phosphatidylcholine transfer between vesicles with identical phosphatidic acid content (2 or 6 mol%) are determined experimentally as a function of total vesicle concentration to calculate apparent dissociation constants and maximum rates of transfer; apparent rates of transfer between various combinations of vesicles containing 2 or 6 mol% phosphatidic acid are then deduced from the derived velocity expression. Reasonably good agreement is seen between theoretical apparent rate-vesicle concentration relationships and those measured experimentally. The results support the general treatment of the kinetics of protein-mediated phospholipid transfer and permit an estimation of useful kinetic parameters.

Animals↗

PubMed2005

Mass transport model for semiconductor nanowire growth.

We present a mass transport model based on surface diffusion for metal-particle-assisted nanowire growth. The model explains the common observation that for III/V materials thinner nanowires are longer than thicker ones. We have grown GaP nanowires by metal-organic vapor phase epitaxy and compared our model calculations with the experimental nanowire lengths and radii. Moreover, we demonstrate that the Gibbs-Thomson effect can be neglected for III/V nanowires grown at conventional temperatures and pressures.

Journal Article↗

PubMed2005

Synthesis of the core structure of acutumine.

[reaction: see text] The tricyclic core of the bioactive natural product acutumine has been synthesized. Key steps include an oxidative phenolic coupling to form a masked o-benzoquinone, an anionic oxy-Cope rearrangement to construct an all-carbon quaternary center, and a Michael-type cyclization to form an amine-bearing quaternary carbon. The target compound exists in solution as an enol, in contrast to related compounds that are ketones. A model explaining these observations is presented.

Alkaloids↗