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At least 289 records · Page 16Linked to original sources

PubMed1989

Testing between the TRACE model and the fuzzy logical model of speech perception.

The TRACE model of speech perception (McClelland & Elman, 1986) is contrasted with a fuzzy logical model of perception (FLMP) (Oden & Massaro, 1978). The central question is how the models account for the influence of multiple sources of information on perceptual judgment. Although the two models can make somewhat similar predictions, the assumptions underlying the models are fundamentally different. The TRACE model is built around the concept of interactive activation, whereas the FLMP is structured in terms of the integration of independent sources of information. The models are tested against test results of an experiment involving the independent manipulation of bottom-up and top-down sources of information. Using a signal detection framework, sensitivity and bias measures of performance can be computed. The TRACE model predicts that top-down influences from the word level influence sensitivity at the phoneme level, whereas the FLMP does not. The empirical results of a study involving the influence of phonological context and segmental information on the perceptual recognition of a speech segment are best described without any assumed changes in sensitivity. To date, not only is a mechanism of interactive activation not necessary to describe speech perception, it is shown to be wrong when instantiated in the TRACE model.

Adult↗

PubMed1993

Modeling of normal tissue response to radiation: the critical volume model.

PURPOSE: A model for calculating normal tissue complication probability in response to therapeutic doses of radiation is presented. METHODS AND MATERIALS: The model which we call the "critical volume model" is based on a concept of functional subunits defined either structurally (e.g., nephrons) or functionally, and an assumption that normal tissue complication probability is fully determined by the number or fraction of surviving functional subunits composing an organ or tissue. The essential features of the model are that it takes into account variations in tissue radiosensitivity and architecture of an organ for a single patient and for a patient population, and predicts the normal tissue complication probability under conditions of 3-dimensional inhomogeneity of the dose distribution. The model can be used for Integral Response, or "parallel," organs (where all functional subunits are performing the same function in parallel and the output of the organ is the sum of the outputs of the functional subunits and for Critical Element, or "serial," organs (where damage to one functional subunit results in an expression of damage for the whole organ). The model combines into one compact scheme new concepts and several ideas and models which have been previously developed by other investigators. RESULTS: The behavior of the model is presented and discussed for the example of the kidney, with clinical nephritis as the functional endpoint. CONCLUSIONS: The model has the potential to be a useful tool for evaluation and optimization of 3-dimensional treatment plans for a variety of types of normal tissues.

Humans↗

PubMed1993

Variability of physiologically based pharmacokinetic (PBPK) model parameters and their effects on PBPK model predictions in a risk assessment for perchloroethylene (PCE).

When used in the risk assessment process, the output from physiologically based pharmacokinetic (PBPK) models has usually been considered as an exact estimate of dose, ignoring uncertainties in the parameter values used in the model and their impact on model predictions. We have collected experimental data on the variability of key parameters in a PBPK model for tetrachloroethylene (PCE) and have used Monte Carlo analysis to estimate the resulting variability in the model predictions. Blood/air and tissue/blood partition coefficients and the interanimal variability of these data were determined for tetrachloroethylene (PCE). The mean values and variability for these and other published model parameters were incorporated into a PBPK model for PCE and a Monte Carlo analysis (n = 600) was performed to determine the effect on model predicted dose surrogates for a PCE risk assessment. For a typical dose surrogate, area under the blood time curve for metabolite in the liver (AUCLM), the coefficient of variation was 25% and the mean value for AUCLM was within a factor of two of the maximum and minimum values generated in the 600 simulations. These calculations demonstrate that parameter uncertainty is not a significant potential source of variability in the use of PBPK models in risk assessment. However, we did not in this study consider uncertainties as to metabolic pathways, mechanism of carcinogenicity, or appropriateness of dose surrogates.

Administration, Inhalation↗

PubMed2004

Comparison between the predictions of a Gaussian plume model and a Lagrangian particle dispersion model for annual average calculations of long-range dispersion of radionuclides.

This study attempts to validate the applicability of a simple Gaussian dispersion model for predicting long-range dispersion of continuous releases from an industrial site, by comparison with a Lagrangian particle dispersion model. The United Kingdom Meteorological Office model NAME has been used to predict annual average concentrations of radionuclides over Western Europe, resulting from discharges from the British Nuclear Fuels (BNFL) site at Sellafield, UK. The results are compared here to calculations performed using a conventional Gaussian type of dispersion model, PLUME. The results of the Gaussian model were compared at 14 locations within Western Europe at long range (up to 1700 km). The differences in predictions between the models were explained readily by differences in the way dispersion and deposition processes are represented in the two models. However, differences are generally small compared to the expected precision of the models. The implementation of environmental processes in NAME is more complete and realistic than in PLUME, and as such the results from this model may be considered more realistic. However, given that PLUME is much simpler to use, and appears to over-estimate, rather than under-estimate, environmental concentrations, its use for radiological assessments appears appropriate.

Air Movements↗

PubMed2004

Can social psychological models be used to promote bicycle helmet use among teenagers? A comparison of the Health Belief Model, Theory of Planned Behavior and the Locus of Control.

PROBLEM: The bicycle helmet use rate is still low among teenagers despite the cumulating evidence that bicycle helmets can prevent cyclists from serious injuries and death. The objective of this study was to investigate the usefulness of the Health Belief Model (HBM; Health Education Monographs, 2 (1974) (1), Theory of Planned Behavior (TPB; Ajzen, I. (1988). Attitudes, personality and behavior. Open University Press, Milton Keynes) and Locus of Control model (LC; Psychological Monographs, (1966) (80) in understanding the intention to use bicycle helmet use among bicycle helmet owners. METHOD: Data were collected at two schools in Helsinki, Finland. Students (N=965) completed a questionnaire including three social psychological models applied to helmet use. Models were compared by structural equation modeling techniques. SUMMARY: Results showed that the TPB and LC model fitted the data well, whereas fit of the HBM model was lower than the fit of TPB and LC models. All components of TPB and external LC orientation were significantly related to the intention to use a helmet. TPB together with LC model provide a promising theoretical framework for helmet use promotion campaigns. Practical suggestions for future bicycle helmet campaigns were provided.

Adolescent↗

PubMed2006

The fear-avoidance model of chronic pain: validation and age analysis using structural equation modeling.

The cognitive-behavioral, fear-avoidance (FA) model of chronic pain (Vlaeyen JWS, Kole-Snijders AMJ, Boeren RGB, van Eek H. Fear of movement/(re)injury in chronic low back pain and its relation to behavioral performance. Pain 1995a;62:363-72) has found broad empirical support, but its multivariate, predictive relationships have not been uniformly validated. Applicability of the model across age groups of chronic pain patients has also not been tested. Goals of this study were to validate the predictive relationships of the multivariate FA model using structural equation modeling and to evaluate the factor structure of the Tampa Scale of Kinesiophobia (TSK), levels of pain-related fear, and fit of the FA model across three age groups: young (< or =40), middle-aged (41-54), and older (> or =55) adults. A heterogeneous sample of 469 chronic pain patients provided ratings of catastrophizing, pain-related fear, depression, perceived disability, and pain severity. Using a confirmatory approach, a 2-factor, 13-item structure of the TSK provided the best fit and was invariant across age groups. Older participants were found to have lower TSK fear scores than middle-aged participants for both factors (FA, Harm). A modified version of the Vlaeyen JWS, Kole-Snijders AMJ, Boeren RGB, van Eek H (Fear of movement/(re)injury in chronic low back pain and its relation to behavioral performance. Pain 1995a;62:363-72.) FA model provided a close fit to the data (chi(2)(29)=42.0, p>0.05, GFI=0.98, AGFI=0.97, CFI=0.99, RMSEA=0.031 (90% CI 0.000-0.050), p close fit=0.95). Multigroup analyses revealed significant differences in structural weights for older vs. middle-aged participants. For older chronic pain patients, a stronger mediating role for pain-related fear was supported. Results are consistent with a FA model of chronic pain, while indicating some important age group differences in this model and in levels of pain-related fear. Longitudinal testing of the multivariate model is recommended.

Adolescent↗

PubMed2005

Modelling of two-stage anaerobic digestion using the IWA Anaerobic Digestion Model No. 1 (ADM1).

The aim of the study presented was to implement a process model to simulate the dynamic behaviour of a pilot-scale process for anaerobic two-stage digestion of sewage sludge. The model implemented was initiated to support experimental investigations of the anaerobic two-stage digestion process. The model concept implemented in the simulation software package MATLAB/Simulink is a derivative of the IWA Anaerobic Digestion Model No.1 (ADM1) that has been developed by the IWA task group for mathematical modelling of anaerobic processes. In the present study the original model concept has been adapted and applied to replicate a two-stage digestion process. Testing procedures, including balance checks and 'benchmarking' tests were carried out to verify the accuracy of the implementation. These combined measures ensured a faultless model implementation without numerical inconsistencies. Parameters for both, the thermophilic and the mesophilic process stage, have been estimated successfully using data from lab-scale experiments described in literature. Due to the high number of parameters in the structured model, it was necessary to develop a customised procedure that limited the range of parameters to be estimated. The accuracy of the optimised parameter sets has been assessed against experimental data from pilot-scale experiments. Under these conditions, the model predicted reasonably well the dynamic behaviour of a two-stage digestion process in pilot scale.

Anaerobiosis↗

PubMed2002

Optimal temperature input design for estimation of the square root model parameters: parameter accuracy and model validity restrictions.

As part of the model building process, parameter estimation is of great importance in view of accurate prediction making. Confidence limits on the predicted model output are largely determined by the parameter estimation accuracy that is reflected by its parameter estimation covariance matrix. In view of the accurate estimation of the Square Root model parameters, Bernaerts et al. have successfully applied the techniques of optimal experiment design for parameter estimation [Int. J. Food Microbiol. 54 (1-2) (2000) 27]. Simulation-based results have proved that dynamic (i.e., time-varying) temperature conditions characterised by a large abrupt temperature increase yield highly informative cell density data enabling precise estimation of the Square Root model parameters. In this study, it is shown by bioreactor experiments with detailed and precise sampling that extreme temperature shifts disturb the exponential growth of Escherichia coli K12. A too large shift results in an intermediate lag phase. Because common growth models lack the ability to model this intermediate lag phase, temperature conditions should be designed such that exponential growth persist even though the temperature may be changing. The current publication presents (i) the design of an optimal temperature input guaranteeing model validity yet yielding accurate Square Root model parameters, and (ii) the experimental implementation of the optimal input in a computer-controlled bioreactor. Starting values for the experiment design are generated by a traditional two-step procedure based on static experiments. Opposed to the single step temperature profile, the novel temperature input comprises a sequence of smaller temperature increments. The structural development of the temperature input is extensively explained. High quality data of E. coli K12 under optimally varying temperature conditions realised in a computer-controlled bioreactor yield accurate estimates for the Square Root model parameters. The latter is illustrated by means of the individual confidence intervals and the joint confidence region.

Bioreactors↗

PubMed2003

A circulatory model for calculating non-steady-state glucose fluxes. Validation and comparison with compartmental models.

This study presents a circulatory model of glucose kinetics for application to non-steady-state conditions, examines its ability to predict glucose appearance rates from a simulated oral glucose load, and compares its performance with compartmental models. A glucose tracer bolus was injected intravenously in rats to determine parameters of the circulatory and two-compartment models. A simulated oral glucose tolerance test was performed in another group of rats by infusing intravenously labeled glucose at variable rates. A primed continuous intravenous infusion of a second tracer was given to determine glucose clearance. The circulatory model gave the best estimate of glucose appearance, closely followed by the two-compartment model and a modified Steele one-compartment model with a larger total glucose volume. The standard one-compartment model provided the worst estimate. The average relative errors on the rate of glucose appearance were: circulatory, 10%; two-compartment, 13%; modified one-compartment, 11%; standard one-compartment, 16%. Recovery of the infused glucose dose was 93+/-2, 94+/-2, 92+/-2 and 85+/-2%, respectively. These results show that the circulatory model is an appropriate model for assessing glucose turnover during an oral glucose load.

Animals↗

PubMed2003

Modeling coupled interactions of carbon, water, and ozone exchange between terrestrial ecosystems and the atmosphere. I: model description.

A new biophysical model (FORFLUX) is presented to study the simultaneous exchange of ozone, carbon dioxide, and water vapor between terrestrial ecosystems and the atmosphere. The model mechanistically couples all major processes controlling ecosystem flows trace gases and water implementing recent concepts in plant eco-physiology, micrometeorology, and soil hydrology. FORFLUX consists of four interconnected modules-a leaf photosynthesis model, a canopy flux model, a soil heat-, water- and CO2- transport model, and a snow pack model. Photosynthesis, water-vapor flux and ozone uptake at the leaf level are computed by the LEAFC3 sub-model. The canopy module scales leaf responses to a stand level by numerical integration of the LEAFC3model over canopy leaf area index (LAI). The integration takes into account (1) radiative transfer inside the canopy, (2) variation of foliage photosynthetic capacity with canopy depth, (3) wind speed attenuation throughout the canopy, and (4) rainfall interception by foliage elements. The soil module uses principles of the diffusion theory to predict temperature and moisture dynamics within the soil column, evaporation, and CO2 efflux from soil. The effect of soil heterogeneity on field-scale fluxes is simulated employing the Bresler-Dagan stochastic concept. The accumulation and melt of snow on the ground is predicted using an explicit energy balance approach. Ozone deposition is modeled as a sum of three fluxes- ozone uptake via plant stomata, deposition to non-transpiring plant surfaces, and ozone flux into the ground. All biophysical interactions are computed hourly while model projections are made at either hourly or daily time step. FORFLUX represents a comprehensive approach to studying ozone deposition and its link to carbon and water cycles in terrestrial ecosystems.

Air Pollution↗

PubMed2001

Can dose-response models predict reliable normal tissue complication probabilities in radical radiotherapy of urinary bladder cancer? The impact of alternative radiation tolerance models and parameters.

PURPOSE: To analyze the consequences of selecting alternative normal tissue complication probability (NTCP) models and parameters for evaluation of radiotherapy of urinary bladder cancer. METHODS AND MATERIALS: Treatment plans of 24 bladder cancer patients referred to radical 4-field conformal radiotherapy were analyzed. Small intestinal and rectal NTCPs were determined using both the probit and relative seriality models with several sets of published radiation tolerance parameters. Various combinations of NTCP models and parameters were applied to find the prescription dose in individual patients as well as to estimate the benefit of the conformal radiotherapy setup. RESULTS: Different risk estimates were predicted from the two NTCP models, even when the same clinical radiation tolerance doses were fitted into the two models. The demonstrated variability translated into significant deviations (7-10 Gy) in the recommended prescription doses. Even if it was possible to discriminate between a 2-field plan and the 4-field conformal setup using a given complication model and set of tolerance parameters, the estimated benefit of the conformal treatment in terms of permitted dose escalation varied with as much as 10-12 Gy between the different NTCP models/parameters used. CONCLUSION: Different NTCP models and tolerance parameters might propose different answers to important clinical questions in radiotherapy treatment of bladder cancer, such as dose prescription and scoring of rival treatment plans. We therefore recommend that the variability caused by tolerance parameter uncertainty and model selection should be taken into account in dose-response modeling of radiotherapy treatment.

Aged↗

PubMed2000

A reduced compartmental model of the mitral cell for use in network models of the olfactory bulb.

We have developed two-, three- and four-compartment models of a mammalian olfactory bulb mitral cell as a reduction of a complex 286-compartment model [1]. A minimum of three compartments, representing soma, secondary (basal) dendrites and the glomerular tuft of the primary dendrite, is required to adequately reproduce the behaviour of the full model over a broad range of firing rates. Adding a fourth compartment to represent the shaft of the primary dendrite gives a substantial improvement. The reduced models exhibit behaviours in common with the full model which were not used in fitting the model parameters. The reduced models run 75 or more times faster than the full model, making their use in large, realistic network models of the olfactory bulb practical.

Ion Channels↗

PubMed2001

Peripheral link model as an alternative for pharmacokinetic-pharmacodynamic modeling of drugs having a very short elimination half-life.

Attempts to obtain estimates of pharmacokinetic-pharmacodynamic (PK-PD) parameters for mivacurium with traditional central link models were unsuccessful in many patients. We hypothesized that a link model with the peripheral compartment would be more appropriate for mivacurium in view of its extremely rapid plasma clearance and its potential elimination by tissue pseudocholinesterases. For validation purposes, the peripheral link model was applied to other neuromuscular blocking agents (NMBA), i.e., atracurium and doxacurium which have respectively an intermediate and a long elimination half-life. Assuming peripheral elimination in PK-PD modeling was investigated but found to have no impact on the estimation of PK-PD parameters. Our results indicate that, for drugs having intermediate and long elimination half-lives, EC50 values are similar with either the central or peripheral link model. For mivacurium, a peripheral link model enables PK-PD modeling in all subjects, with more precision in the PK-PD parameter estimates and a better fitting of the effect data when compared to the central link model. For these reasons, a peripheral link model should be preferred for mivacurium.

Adolescent↗

PubMed2004

Multi-formalism modelling and simulation: application to cardiac modelling.

Cardiovascular modelling has been a major research subject for the last decade. Different cardiac models have been developed at a cellular level as well as at the whole organ level. Most of these models are defined by a comprehensive cellular modelling using continuous formalisms or by a tissue-level modelling often based on discrete formalisms. Nevertheless, both views still suffer from difficulties that reduce their clinical applications: the first approach requires heavy computational resources while the second one is not able to reproduce certain pathologies. This paper presents an original methodology trying to gather advantages from both approaches, by means of a hybrid model mixing discrete and continuous formalisms. This method has been applied to define a hybrid model of cardiac action potential propagation on a 2D grid of endocardial cells, combining cellular automata and a set of cells defined by the Beeler-Reuter model. For simulations under physiological and ischemic conditions, results show that the action potential propagation as well as electrogram reconstructions are consistent with clinical diagnosis. Finally, the advantage of the proposed approach is discussed within the frame of cardiac modelling and simulation.

Action Potentials↗

PubMed2004

Compartmental modeling of transdermal iontophoretic transport: I. In vitro model derivation and application.

PURPOSE: The objective of this study was to develop a family of compartmental models to describe in a strictly quantitative manner the transdermal iontophoretic transport of drugs in vitro. METHODS: Two structurally different compartmental models describing the in vitro transport during iontophoresis and one compartmental model describing the in vitro transport in post-iontophoretic period are proposed. These models are based on the mass transfer from the donor compartment to the acceptor compartment via the skin as an intermediate compartment. In these models, transdermal iontophoretic transport is characterized by 5 parameters: 1) kinetic lag time (tL), 2) steady-state flux during iontophoresis (Jss), 3) skin release rate constant (K(R)), 4) the first-order rate constant of the iontophoretic driving force from the skin to the acceptor compartment (I1), and 5) passive flux in the post-iontophoretic period (Jpas). The developed models were applied to data on the iontophoretic transport in human stratum corneum in vitro of R-apomorphine after pretreatment with phosphate buffered saline pH 7.4 (PBS) and after pretreatment with surfactant (SFC), as well as the iontophoretic transport of 0.5 mg ml(-1) rotigotine at pH 5 (RTG). RESULTS: All of the proposed models could be fitted to the transport data of PBS, SFC, and RTG groups both during the iontophoresis and in the post-iontophoretic period. The incorporation of parameter I1 failed to improve the fitting performance of the model. This might indicate a negligible contribution of iontophoretic driving force to the mass transfer in the direction from the skin to the acceptor compartment, although it plays an important role in loading the skin with the drug. The estimated values of Jss of PBS, SFC, and RTG were identical (p > 0.05) to the values obtained with the diffusion lag time method. Moreover, time required to achieve steady-state flux can be estimated based on the parameter tL and the reciprocal value of parameter K(R). In addition, accumulation of drug molecules in the skin is reflected in a reduction of the value of the K(R) parameter. CONCLUSIONS: The developed in vitro models demonstrated their strength and consistency to describe the drug transport during and post-iontophoresis.

Administration, Cutaneous↗

PubMed1991

Model of kinetic behavior of deoxyglucose in heterogeneous tissues in brain: a reinterpretation of the significance of parameters fitted to homogeneous tissue models.

Effects of tissue heterogeneity on regional CMRglc (rCMRglc) calculated by use of the deoxyglucose (DG) method at 45 min following the pulse of DG were evaluated in simulation studies. A theoretical model was developed to describe the kinetics of DG uptake and metabolism in heterogeneous brain tissues. Rate constants were fitted to simulation data for mixed tissue and rCMRglc computed on the basis of this tissue heterogeneity model. The results were compared with those obtained by use of the original model of the DG method for homogeneous tissue, both without (3K model) and with (4K model) a term to describe an apparent loss of deoxyglucose-6-phosphate (DG-6-P). As a direct consequence of tissue heterogeneity, the effective rate constant for phosphorylation of DG, k3*, declined with time. To compensate for the time-changing k3*, estimates of the dephosphorylation rate constant, k4*, were artifactually high when the 4K model was used, even though no dephosphorylation of DG-6-P actually occurred. The present study demonstrates that the finding of a significant k4*, at least within 45 min following a pulse of DG, may not represent dephosphorylation at all, but rather the consequence of measuring radioactivity in a heterogeneous tissue and applying a model designed for a homogeneous tissue. Furthermore, the high estimates of k4* resulted in significant overestimation of rCMRglc. When rCMRglc was computed with the conventional single-scan or autoradiographic method at 45 min after a pulse of DG, the 3K and tissue heterogeneity models yielded values that were within 5% of the true weighted average value for the heterogeneous tissue as a whole. We conclude that the effects of tissue heterogeneity alone can give the appearance of product loss, even when none occurs, and that the use of the 4K model with the assumption of product loss in the 45-min experimental period recommended for the DG method may lead to overestimation of the rates of glucose utilization.

Brain↗

PubMedDate not supplied

An individual modelling tool for within and between lactation consecutive cases of clinical mastitis in the dairy cow: an approach based on a survival model.

Clinical mastitis in dairy cows has for many years been the subject of numerous epidemiological surveys to determine the main risk factors. In most cases this data has been analysed using a standard Poisson model without taking into consideration possible dependence between consecutive pathological events. These analyses have brought to light a great many potential risk factors without making it possible to clarify a certain amount of confusion surrounding the effects. The extension of an individual within a lactation model, considering dependence between clinical cases of mastitis within lactation so as to take into account inter-lactation dependence (which has already been published) is presented in the form of mixed distributions within the same survival model framework. By introducing new parameters, infection rate at calving and the identification of a higher exogenous infection rate indoors than at pasture, it is possible to take into consideration what had previously appeared to be a lactation stage factor, a calving month factor or even part of a parity factor. By considering these two types of dependence within the same model, it appears to be possible to obtain a simpler model in terms of the factors to be taken into account, and one that is based on generally acknowledged and easily understandable biological considerations. Lastly, a possible way of extending the model is to consider the dry period before calving and this is presented. This would make it possible to envisage developing a complete model of the animal's lifetime in the not-too-distant future. It is still necessary, however, to determine the farming system factors in the general sense of the term, which specifically affect one or the other of the different model parameters, before one can draw conclusions as to the potential extension of this type of model. A national survey is currently being carried out on approximately 600 French breeding farms that will help meet this last objective.

Animals↗

PubMed1998

Homology modeling, model and software evaluation: three related resources.

MOTIVATION: Homology modeling is rapidly becoming the method of choice for obtaining three-dimensional coordinates for proteins because genome projects produce sequences at a much higher rate than NMR and X-ray laboratories can solve the three-dimensional structures. The quality of protein models will not be immediately clear to novices and support with the evaluation seems to be needed. Expert users are sometimes interested in evaluating the quality of modeling programs rather than the quality of the models themselves. RESULTS: Three servers have been made available to the scientific community: a homology modeling server, a model quality evaluation server and a server that evaluates models built of proteins for which the structure is already known, thereby implicitly evaluating the quality of the modeling program. AVAILABILITY: The modeling-related servers and several structure analysis servers are freely available at http://swift.embl-heidelberg.de/servers/ CONTACT: gert.vriend@embl-heidelberg.de

Computational Biology↗