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At least 991 records · Page 55Linked to original sources

PubMed2003

The application of the convective diffusion model and the film equilibrium model to surfactant-facilitated dissolution of gliclazide.

Gliclazide is practically insoluble in water, and has low dissolution rate. Therefore, it was of interest to improve its dissolution rate using anionic and cationic surfactants. The intrinsic dissolution rates of gliclazide in solutions of sodium dodecyl sulfate (SDS) and in solutions of tetradecyltrimethyl ammonium bromide (TDTMAB) were measured using the rotating disk method to study the convective diffusion transport of drug-loaded micelles. Two different approaches were applied to the experimental data; the convective diffusion model and the film equilibrium model. The two approaches are based on the same fundamental assumptions differing only in their interpretation of the diffusional boundary layer. The results obtained from the film equilibrium model were less satisfactory, and in case of TDTMAB the model was inapplicable (negative diffusion coefficient). While excellent results were obtained from the convective diffusion model. The free solute diffusion coefficient (D(s)) obtained experimentally was 2.47 x 10(-5) cm(2)/s, and the diffusion coefficient of the drug-loaded SDS micelle (D(sm)) estimated was 1.74 x 10(-6) cm(2)/s. The drug-loaded SDS micelle radius was 14 A. The thickness of the diffusional boundary layer was 54 and 22 microm for the free solute and the drug-loaded SDS micelle, respectively. TDTMAB showed lower effect in improving the dissolution rate of gliclazide than SDS. The drug-loaded TDTMAB micelle diffusion coefficient was 1.03 x 10(-6) cm(2)/s. The radius of the drug-loaded TDTMAB micelle and the boundary layer thickness were 24 A and 19 microm, respectively.

Diffusion↗

PubMed2002

Model-based interpretation of cardiac beats by evolutionary algorithms: signal and model interaction.

This paper presents a new approach for cardiac beat interpretation, based on a direct integration between a model and observed ECG signals. Physiological knowledge is represented by means of a semi-quantitative model of the cardiac electrical activity. The interpretation of cardiac beats is formalized as an optimization problem, by minimizing an error function defined between the model's output and the observations. Evolutionary algorithms (EAs) are used as the search technique in order to obtain the set of model parameters reproducing at best the observed phenomena. Examples of model adaptation to three different kinds of cardiac beats are presented. Preliminary results show the potentiality of this approach to reproduce and explain complex pathological disorders and to better localize their origin.

Algorithms↗

PubMed2005

Progressive systematic underestimation of reaction energies by the B3LYP model as the number of C-C bonds increases: why organic chemists should use multiple DFT models for calculations involving polycarbon hydrocarbons.

[reaction: see text] Computational studies of three different reaction types involving hydrocarbons (homolytic C-C bond breaking of alkanes, progressive insertions of triplet methylene into C-H bonds of ethane, and [2+2] cyclizations of methyl-substituted alkenes to form polymethylcyclobutanes) show that the B3LYP model consistently underestimates the reaction energy, even when extremely large basis sets are employed. The error is systematic and cumulative, such that the reaction energies of reactions involving hydrocarbons with more than 4-6 C-C bonds are predicted quite poorly. Energies are underestimated for slightly and highly methyl-substituted cyclic and acyclic hydrocarbons, so the errors do not arise from structural issues such as steric repulsion or ring strain energy. We trace the error associated with the B3LYP approach to its consistent underestimation of the C-C bond energy. Other DFT models show this problem to lesser extents, while the MP2 method avoids it. As a consequence, we discourage the use of the B3LYP model for reaction energy calculations for organic compounds containing more than four carbon atoms. We advocate use of a collection of pure and hybrid DFT models (and ab initio models where possible) to provide computational "error bars".

Journal Article↗

PubMed1993

A correlation between the permeability characteristics of a series of peptides using an in vitro cell culture model (Caco-2) and those using an in situ perfused rat ileum model of the intestinal mucosa.

In an attempt to establish an in vitro/in situ correlation of intestinal permeability data, the permeability coefficients (Papp) for a series of model peptides, which were determined using an in situ perfused rat ileum model, were compared to the permeability coefficients (Pmono) determined using an in vitro cell culture model (Caco-2). The model peptides, which were all blocked on the N-terminal (acetyl, Ac) and the C-terminal (amide, NH2) ends, consisted of D-phenylalanine (F) residues (e.g., AcFNH2, AcFFNH2, AcFFFNH2). To alter the degree of hydrogen bonding potential, the nitrogens of the amide bonds were sequentially methylated [e.g., AcFF(Me)FNH2, AcF(Me)F(Me)FNH2, Ac(Me)F(Me)FNH2, Ac(Me)F(Me)F(Me)]. These peptides were shown not to be metabolized in the in situ perfused rat ileum system. The results of the transport experiments showed that there were poor correlations between the apparent permeability coefficients (Papp) determined in an in situ perfused rat ileum model and the octanol-water partition coefficients (r = 0.60) or the hydrogen bonding numbers (r = 0.63) of these peptides. However, good correlations were observed between the in situ Papp values for these peptides and their partition coefficients in heptane-ethylene glycol (r = 0.96) and the differences in their partition coefficients between octanol-water and isooctane-water (r = 0.86). These results suggest that lipophilicity may not be the major factor in determining the intestinal permeability of these peptides and that hydrogen bonding potential may be a major contributing factor. These results suggest that lipophilicity may not be the major factor in determining the intestinal permeability of these peptides and that hydrogen bonding potential may be a major contributing factor.(ABSTRACT TRUNCATED AT 250 WORDS)

Amino Acid Sequence↗

PubMed1993

Coping modeling problem solving versus mastery modeling: effects on adherence, in-session process, and skill acquisition in a residential parent-training program.

This trial compared two approaches used to introduce parenting skills in a residential staff training program. Fifty staff were randomly assigned to: mastery modelling in which videotaped models demonstrated new skills, coping modelling problem solving (CMPS) in which participants formulated their own solutions to the errors depicted by videotaped models, or a waiting-list control group. In both, leaders used modelling, role playing, and homework projects to promote mastery and transfer of new skills. The skills of all groups improved, but CMPS participants attended more sessions, were late to fewer sessions, completed more homework, engaged in more cooperative in-session interaction, rated the program more positively, and reported higher job accomplishment scores. These data suggest that CMPS allowing participants to formulate their own solutions may enhance adherence and reduce the resistance observed in more didactic programs.

Adaptation, Psychological↗

PubMed2005

Model inference or model selection: discussion of Klugkist, Laudy, and Hoijtink (2005).

I. Klugkist, O. Laudy, and H. Hoijtink (2005) presented a Bayesian approach to analysis of variance models with inequality constraints. Constraints may play 2 distinct roles in data analysis. They may represent prior information that allows more precise inferences regarding parameter values, or they may describe a theory to be judged against the data. In the latter case, the authors emphasized the use of Bayes factors and posterior model probabilities to select the best theory. One difficulty is that interpretation of the posterior model probabilities depends on which other theories are included in the comparison. The posterior distribution of the parameters under an unconstrained model allows one to quantify the support provided by the data for inequality constraints without requiring the model selection framework.

Analysis of Variance↗

PubMed1989

Modeling versus modeling plus evoked production training: a comparison of two language intervention methods.

The effectiveness of two language intervention procedures, modeling versus modeling plus evoked production, was compared using a single-subject alternating treatments design. Four language-disordered children (5:5-6:11) participated in the study, consisting of four baseline sessions and 10-14 training sessions. There was no marked difference in the effectiveness of the modeling procedure and the modeling plus evoked production procedure as measured by performance on generalization probes following each training session. In fact, the highest levels of accuracy attained under the two training procedures were quite comparable. There was a tendency, however, for the modeling plus evoked production treatment to result in a slightly more stable pattern of acquisition for 3 of the subjects. Although steps were taken to minimize multiple-treatment interference, the possibility of these effects must be considered, and caution should be taken in interpreting these findings to mean that these two intervention methods would produce equivalent results when administered independently.

Age Factors↗

PubMed2004

Modeling benzene with single-site potentials from ab initio calculations: a step toward hybrid models of complex molecules.

Extensive ab initio calculations at the MP2/6-31G* level have been carried out to sample the energy surface for the interactions of the benzene dimers. This database has been used to parameterize two anisotropic single-site models, meant to be used as building blocks in hybrid models of complex, liquid crystal forming molecules. A quadrupolar Gay-Berne (GBQIII) and an S-function (SF) Corner potentials have been obtained in this way. Their ability to reproduce, qualitatively at least, the phase diagram as well as energetic and structural properties of benzene has been tested with Monte Carlo simulations and compared with previous literature potentials, GBQI [S. Gupta et al., Mol. Phys. 65, 961 (1988)] and GBQII [T. R. Walsh, Mol. Phys. 100, 2867 (2002)]. It turned out that GBQI showed no melting transition in the temperature range explored (100-400 K), while GBQII underwent a phase transition from solid to gas, with no liquid phase. Conversely, both models parameterized on our database of ab initio interaction energies (GBQIII and SF) gave rise to a stable liquid phase. Melting has been observed between 100 and 150 K (GBQIII) and in the range 300-350 K (SF), i.e., substantially below and slightly above the experimental value at ambient pressure, 278 K. The description of the crystal structure of benzene at atmospheric pressure is also in better agreement with experimental data if the SF model is used, while positional correlations in the liquid are better described by the GBQIII potential. The S-function potential is also computationally more convenient. These results could be useful in the semirealistic modeling of more complex molecules.

Journal Article↗

PubMed1985

Mathematical models of central pattern generators in locomotion: III. Interlimb model for the cat.

Possible neural connective patterns and functions with respect to interlimb coordination are studied theoretically with a mathematical model of the central pattern generating system for cat locomotion. Activities in populations of neurons controlling limb joint flexors and extensors in all four limbs are represented by a system of nonlinear differential equations. Solutions of the system for various parameter values simulate various gaits of the cat. The model is shown to be capable of generating all gaits of the cat and accounting for corresponding phase changes in interlimb coordination. The model also exhibits smooth changes of gait, and smooth initiation and termination of stepping. Further, within each limb, muscle sequencing, step cycle phases, and flexor-extensor interactions can be studied. The model suggests that one of the simplest mechanisms for a central command system to change the gait is via inhibition of specific interlimb propriospinal pathways. In a final section, properties of both proposed single limb and interlimb models are reviewed with specific reference to planning future experimental and theoretical studies.

Journal Article↗

PubMed1991

Using generalized linear models (GLMs) to model errors in motor performance.

Because of differences in design factors, experiments in human motor performance sometimes produce a wide range in variability or consistency in a subject's individual errors. These differences in variation often lead to heterogeneity in the variance-covariance matrices between group factors, which prohibits the use of repeated-measures (RM) ANOVA or MANOVA techniques to analyze the error data. Provided certain conditions are met, however, each subject's individual errors can be collapsed into the summary error measures, constant error (CE) and variable error (VE), which can still provide a more than adequate description of the subjects' performance. This article proposes the appropriate conditions and the corresponding generalized linear models (GLMs) with which a subject's individual errors, recorded in short-term motor memory research, can be combined into the summary measures, CE and VE, which can be analyzed subsequently as the dependent variables in the experimental design. The CE scores can be modeled using GLMs without requiring the assumption of homogeneity of variances. Similarly, the VE scores can be modeled as a GLM, using a log-linear regression model that assumes a gamma distribution, rather than using a traditional analysis of variance (ANOV A) model that assumes an inappropriate normal error distribution for these scores. An example reveals that the analysis of VE scores, unlike the analysis of CE scores, is able to differentiate between group practice methods. These differences tend to be underestimated by traditional ANOVA methods, however. Differences between the ANOV A and GLM analyses of the VE scores are further clarified by simulation. Based on differences like those observed in the example, when simulated VE scores were analyzed, assuming both a normal and a gamma distribution, the power of the gamma tests was found to be superior to the normal analyses in all but a small range of cases, in which such differences were found to be negligible. Hence, it is only by declaring the VE scores to have a GLM with a gamma distribution that the anticipated group practice differences can be properly identified.

Journal Article↗

PubMed2002

Application of conditional moment tests to model checking for generalized linear models.

Generalized linear models (GLMs) are increasingly being used in daily data analysis. However, model checking for GLMs with correlated discrete response data remains difficult. In this paper, through a case study on marginal logistic regression using a real data set, we illustrate the flexibility and effectiveness of using conditional moment tests (CMTs), along with other graphical methods, to do model checking for generalized estimation equation (GEE) analyses. Although CMTs provide an array of powerful diagnostic tests for model checking, they were originally proposed in the econometrics literature and, to our knowledge, have never been applied to GEE analyses. CMTs cover many existing tests, including the (generalized) score test for an omitted covariate, as special cases. In summary, we believe that CMTs provide a class of useful model checking tools.

Journal Article↗

PubMed2000

Application of process-based models to forest management: experience with PROMOD, a simple plantation productivity model.

Process-based growth models are rarely used as a basis for decisions in forest management. Reasons for this are discussed and the development and application of PROMOD, a simple process-based plantation productivity model designed to meet the needs of managers, is described. In particular,PROMOD is intended for screening prospective plantation sites on the basis of readily available input data, including site latitude, soil characteristics, and long-term climate data. Originally developed for use with Eucalyptus globulus Labill., PROMOD has since been parameterized also for Eucalyptus nitens (Deane & Maiden) Maiden and Pinus radiata D. Don. We discuss the adaptation of PROMOD to new environmental conditions and species; the sensitivity of the model's predictions to changes in model structure, the values of its physiological parameters and the accuracy and resolution of site survey information; and the use of the model both in assessing the profitability of plantation irrigation in semi-arid regions, and for generating broad-area productivity maps.

Journal Article↗

PubMed2002

An isolated, antegrade, perfused, peroneal nerve anterior tibialis muscle model in the rat: a novel model developed to study the factors governing the time course of action of neuromuscular blocking agents.

BACKGROUND: A model of an antegrade, perfused, isolated rat peroneal nerve anterior tibial muscle was developed to study potentially important factors governing the time course of action of (nondepolarizing) neuromuscular blocking agents such as concentration, blood flow, and temperature. The model allows observation of the effects of selective changes in these factors. METHODS: The authors isolated the anterior tibial muscle and cannulated the anterior tibial artery and vein, providing a way for single-pass perfusion with blood from a donor rat. A force transducer was connected to the tibialis anterior muscle and a stimulator was connected to the tibial nerve. The influence of intrinsic potency (EC90) and muscle blood flow rate on the time course of pancuronium and rocuronium was investigated. RESULTS: The model remained stable for at least 4 h with respect to twitch height, muscle structure and function, and blood chemistry. Doubling the muscle-blood flow resulted in a significantly faster onset and offset for both pancuronium and rocuronium. Trebling the intrinsic potency (EC90) was not associated with significant changes in the time course of action of the relaxants. CONCLUSION: The authors developed and validated a model that allows us to study biophase kinetics of neuromuscular blocking agents in the anterior tibial muscle of the rat. In this model, muscle-blood flow rather than EC90 appears to predominantly determine the onset and offset time of nondepolarizing muscle relaxants.

Androstanols↗

PubMed2001

VO(2) slow component: to model or not to model?

PURPOSE: The purpose of this study was to compare several techniques often used in the literature for measuring the amplitude of the slow component of oxygen uptake kinetics. METHODS: Eight healthy male volunteer cyclists performed two identical bouts of square wave cycle ergometry, from a VO(2) of 60% of the lactic acid threshold (LAT) to 30% of the difference between LAT and VO(2) peak. Predetermined intervals (3--6 and 3--10 min) were chosen to reflect those often used in the literature, namely 3-6 min and 3 min to the end of exercise. Several procedures were used to estimate the 3, 6, and 10-min VO(2) values (20-s averaging, 60-s averaging, and mono-exponential modeling). These were compared with the modeled slow component amplitude using a two-phase model with independent time delays: VO(2)(t) = B VO(2) + A(1)(1 -- e(-(t-TD1)/tau(1)) + A(2)(1 -- e(-(t-TD2)/tau(2)). CONCLUSIONS: The results showed a significant underestimation for all methods of slow component amplitude estimation (P < 0.05) when compared with the actual (modeled) amplitude. In so far as research on oxygen uptake kinetics is used to understand the underlying physiology, it is imperative that the components of the kinetics be determined accurately. The use of a predetermined time frame for estimation of the amplitude of the slow component is not supported by this study. Future investigations should consider these results and make every effort to model the underlying response.

Adult↗

PubMed2001

Comparative study of an Eden model for the irreversible growth of spins and the equilibrium Ising model.

The magnetic Eden model (MEM) [N. Vandewalle and M. Ausloos, Phys. Rev. E 50, R635 (1994)] with ferromagnetic interactions between nearest-neighbor spins is studied in (d+1)-dimensional rectangular geometries for d=1,2. In the MEM, magnetic clusters are grown by adding spins at the boundaries of the clusters. The orientation of the added spins depends on both the energetic interaction with already deposited spins and the temperature, through a Boltzmann factor. A numerical Monte Carlo investigation of the MEM has been performed and the results of the simulations have been analyzed using finite-size scaling arguments. As in the case of the Ising model, the MEM in d=1 is noncritical (only exhibits an ordered phase at T=0). In d=2 the MEM exhibits an order-disorder transition of second order at a finite temperature. Such transition has been characterized in detail and the relevant critical exponents have been determined. These exponents are in agreement (within error bars) with those of the Ising model in two dimensions. Further similarities between both models have been found by evaluating the probability distribution of the order parameter, the magnetization, and the susceptibility. Results obtained by means of extensive computer simulations allow us to put forward a conjecture that establishes a nontrivial correspondence between the MEM for the irreversible growth of spins and the equilibrium Ising model. This conjecture is certainly a theoretical challenge and its confirmation will contribute to the development of a framework for the study of irreversible growth processes.

Journal Article↗

PubMed2004

Critical dynamics of stochastic models with energy conservation (model C).

We calculate the field-theoretic functions of the generalized dynamical model C*, in which a conserved secondary density is coupled to a nonconserved complex order parameter, in two-loop order. We show that the fixed points in this extended model are equal to the fixed points obtained in model C with a real order parameter, which has been introduced by Halperin, Hohenberg, and Ma. Our results correct long-standing errors in the field-theoretic functions in model C published by several authors leading also to different fixed point values w* for the ratio of the two time scales involved. The stability regions of the fixed points, which remained partially unclear, considered in a "phase diagram"--whose axes are the spatial dimension d and number of order parameter components n--are now clarified. Especially an anomalous region found by previous authors, in which the scaling properties remained unsolved, does not exist. There are only two regions: one with a finite fixed point w* where the dynamical exponent z of the order parameter is z=2+alpha/nu and another region where w*=0 and z is equal to the model A value.

Journal Article↗

PubMed1997

Dual-route and connectionist models: a step towards a combined model.

Current models of word recognition are mainly constructed within the frameworks of either dual-route or connectionist theories. The most important test of a word recognition model is how it succeeds in accounting for various reading behaviors. In the present paper dual-route and connectionist word recognition models are briefly described and evaluated. As a further development of these models, a combined framework is proposed. An amalgamation of the two main types of models might give a more satisfactory account of various phenomena within word recognition.

Child↗

PubMed1994

Modeling the fine structure of the 2f1-f2 acoustic distortion product. II. Model evaluation.

In a previous article, a vector sum model was developed that successfully reproduces the ADP rippling pattern when a nonuniformity is introduced in the active damping factor, a parameter that is inversely related to the energy gain contribution of the outer hair cells [OHCs, Sun et al., J. Acoust. Soc. Am. 96, 2166-2174 (1994)]. Here, the mean of the damping factor is increased nonlinearly with input level, mimicking the saturation of the active feedback of the OHCs. The passive damping factor in the transmission line model was also nonlinearly increased with input level to reproduce the frequency shift of the peak of the traveling wave observed in experimental data. The resulting model simulates an ADP that is compatible with data from human subjects wherein the ADP fine structure does not saturate with level. Moreover, the model suggests that the shifting of the ADP pattern with level is a direct result of the peak shift of the traveling wave, thus implicating the nonlinear damping factors as the underlying basis of this phenomenon. The input/output (I/O) functions of the simulated ADP emissions at specific frequencies were also examined. The resulting functions show a variety of shapes, depending on the pattern of ADP fine structure around the I/O frequency, and the way the fine structure shifts as primary levels increase. These I/O functions are also similar to those observed in human subjects, even with regard to overall slopes which approximate one. Thus the model illustrates how cubic distortion generators coupled with damping on linearities can yield I/O function slopes on the order of one, rather than the expected three.(ABSTRACT TRUNCATED AT 250 WORDS)

Basilar Membrane↗