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PubMed2002

Models of skeletal muscle to explain the increase in passive stiffness in desmin knockout muscle.

Absence of desmin in skeletal muscle was found to induce an increase in passive stiffness. The present study aimed at developing rheological models of passive muscle to explain this stiffening. Models were elaborated by using experimental data depicting muscle viscoelastic behaviour. The experimental protocol included stepwise extension tests applied on control and desmin knockout soleus muscles from mice. Linear and non-linear models were composed of elastic and viscous elements. They were constructed with the aim at taking the presence or absence of desmin into account by simulating desmin as an elastic element. Furthermore, associated adaptation of connective tissues in absence of desmin was modelled as an additional elastic element. Differences in passive behaviour induced by absence of desmin were predicted by using a linear model and a non-linear one. The non-linear model was selected because: (1) it is able to predict experimental viscoelastic kinetics accounting for the increase in passive stiffness in muscles lacking desmin, (2) its design is consistent with morphological data, and (3) stiffness characteristics of its elements are in accordance with the literature. Finally, this modelling approach demonstrates that both absence of desmin and adaptation of connective tissue are required to explain the increase in passive stiffness in desmin knockout muscles.

Animals↗

PubMed2003

Non-linear material models for tracheal smooth muscle tissue.

The aim of this study is to investigate the hyperelastic material models to describe the non-linear stress-strain behavior of tracheal smooth muscle tissue. Specifically, the goal is to validate the material model with experimental data using different finite element models and discuss the trends in stress-strain behavior of smooth muscle tissue. Both 2D and 3D finite element analyses were carried out to estimate the stress-strain behavior of the smooth muscle tissue. The results obtained indicate that the developed Ogden material model is valid and useful in explaining the stress strain behavior of tracheal smooth muscle tissue under different conditions. Finite element simulation results of the stress-strain behavior in the transverse direction are presented.

Animals↗

PubMed1990

Comments on a time-dependent version of the linear-quadratic model.

The accuracy and interpretation of the "LQ + time" model (E = D(alpha + beta d) - gamma T) are discussed. Evidence is presented, based on data in the literature, that this model does not accurately describe the changes in isoeffect dose occurring with protraction of the overall treatment time during fractionated irradiation of the lung. This lack of fit of the model explains, in part, the surprisingly large values of gamma/alpha that have been derived from experimental lung data. The large apparent time factors for lung suggested by the model are also partly explained by the fact that gamma T/alpha, despite having units of dose, actually measures the influence of treatment time on the effect scale, not the dose scale, and is shown to consistently overestimate the change in total dose. The unusually high values of alpha/beta that have been derived for lung using the model (approximately 5 Gy) are shown to be influenced by the method by which the model was fitted to data. Reanalyses of the data using a more statistically valid regression procedure produce estimates of alpha/beta more typical of those usually cited for lung (approximately 3 Gy). Most importantly, published isoeffect data from lung indicate that the true deviation from the linear-quadratic (LQ) model is nonlinear in time, instead of linear, and also depends on other factors such as the effect level and the size of dose per fraction. Thus, we do not advocate the use of the "LQ + time" expression as a general isoeffect model.

Animals↗

PubMed1993

Comparison of three methods of estimating odds ratios from a job exposure matrix in occupational case-control studies.

A job exposure matrix consists of jobs on one axis and substances on the other, with the matrix elements describing the likelihood of an individual's exposure to a substance in a given job. This can be used in case-control studies to infer exposures of subjects whose jobs are known. The simplest form of job exposure matrix contains binary entries, but it is also possible to envisage continuous variables describing the probability of exposure in the job (probabilistic matrix). In such a case, the user has various options for transforming and analyzing the data, including the following: 1) transform to binary variables and analyze as conventional binary exposure variables; 2) leave as continuous variables and analyze using logistic regression; 3) leave as continuous variables and analyze using a linear model. Simulations were carried out to compare the ability of the three methods to estimate odds ratios under 36 experimental conditions. The linear model produced unbiased estimates, the logistic model produced somewhat biased estimates at high odds ratios, and the transformation to a binary variable produced systematically low estimates in most experimental circumstances. With the linear and logistic models, the odds ratio estimators had similar precision when the bias of the latter was not too great. The authors conclude that the linear model permits optimal use of a probabilistic matrix in an epidemiologic study and hope that these results will encourage the development of job exposure matrices containing probabilities rather than dichotomies.

Case-Control Studies↗

PubMed1994

A cross-national investigation of diet and bladder cancer.

The existence of a large unexplained portion of attributable risk, and the marked variation in bladder cancer rates globally, have stimulated an interest in the role of nutrition in cancer of the urinary bladder. For this cross-national comparison study, we had complete data available for 50 countries. Using stepwise regression followed by general linear modelling, age-truncated (45-74 years), world-standardised, sex-specific bladder cancer mortality rates were regressed on an array of nutritional and socioeconomic independent variables in an effort to identify important predictors of bladder cancer mortality. Separate principal components analyses were used to summarise the nutritional and the socioeconomic (SES) variables. In the stepwise analyses, using food scores expressed in kcal/day per capita (as opposed to the nutritional components), total fat consistently entered the model first, and explained the greatest share of variability (R2) for both males and females. General linear models were fitted that included total fat, tobacco, alcohol, the three SES components (comprising seven socioeconomic predictors) and two food categories found significant in stepwise modelling, roots/tubers and vegetable oil. The R2 values were 0.84 for male rates and 0.77 for female rates, meaning that these study factors account for 84% of bladder cancer mortality in men and 77% in women. Substitution of the nutritional components for the foods resulted in general linear models with slightly higher R2 values (0.85 for males, 0.77 for females), but with attenuated fat effects. Results are discussed in light of biological plausibility.

Aged↗

PubMed1991

Methodological issues in the study of divergent views of the family.

A wealth of methodologies, both qualitative and quantitative, are available for conducting research on divergent views in the family. While the pathways for investigating divergent views of the family are clearer for those who chose more quantitative methods (as current literature reflects this tradition), qualitative methods may serve to clarify the process through which divergent views occur and are maintained within the family, both on an individual and on a dyadic or triadic level. In terms of the analysis of data from quantitative designs, divergent views of the family can quite easily be explored by viewing the family as a system and the individual family members as components of that system. This approach minimizes problems arising from having sampled families and included more than one family member from each. In addition, the organization of the data as family (rather than individual) records enables the researcher to directly examine divergences between family members by comparing ratings by different family members on the same construct. The issues involved in such analyses were discussed, first, in terms of the reliability of difference scores and then, in more detail, in terms of alternative regression-based models that estimate the effects of differences on a dependent variable. Although differences between variables are reputed to be (and often are) unreliable, the conditions under which these scores are likely to be reliable are the conditions found when examining divergent family views. Also, the reliability of difference variables can be treated directly. The discussion of regression models indicated the statistical equivalence (or in some instances nonequivalence) of models containing difference variables to alternative models not containing difference variables. Although algebraically equivalent, these alternative linear models provide different views of the same information. When moving from linear models to nonlinear models, however, researchers are cautioned to choose each specific model carefully since the nonlinear terms containing difference variables are actually more complex than they at first appear. The current volume speaks to the importance of variation in family members' views for the adjustment and well-being of adolescents. This chapter has taken on the question of how best to look at these divergences, and how different methods and statistical techniques may yield similar or different information regarding divergent views. The diversity in research methods and analytical strategies creates a challenging task for the investigator, and the continued exploration of questions regarding the implications of divergent views in the family should enhance our knowledge of how and why views diverge, as well as what divergences mean to family members.

Data Collection↗

PubMed1996

Interactions between memory scanning and visual scanning in display monitoring.

Many real-world tasks require the simultaneous performance of memory scanning of several memorized items and visual scanning of several physically separated sources of information in the visual field. This paper reports a study that was conducted to quantify the possible interactions between memory scanning and visual scanning. A quantitative model was derived to integrate Sternberg's linear model of memory scanning and Neisser's linear model of visual scanning. The derived model was tested through two experiments. The experiments simulated a process controller's task of monitoring an array of instrument meters to detect if any of them indicated a system error. The subjects were required to keep a number of items in their working memory (the definition of errors) and search through an organized array of instrument meters to decide whether any of the meters carried an item that matched any of the memorized items in their working memory. Two experimental factors were investigated in both experiments: the number of memorized items and the number of circles needed to be searched. These defined memory scanning demand and visual scanning demand respectively. Experiment 1 employed a different set of memorized items for each experimental trial, whereas experiment 2 employed the same set for all the trials that had the same experimental condition. The experiments identified both the strengths and the limitations of the derived model. Implications for human-machine interface design and human performance modelling are discussed.

Humans↗

PubMed1987

The relative biological effectiveness of 670 MeV/A neon as a function of depth in water for a tissue model.

Linear energy transfer (LET infinity) spectra of identified charge fragments and primaries, produced by nuclear interactions of 670 MeV/A neon in water, were measured along the unmodulated Bragg curve of the neon beam. The relative biological effectiveness (RBE) values for spermatogonial cell killing, as reported on the basis of weight loss assay of mouse testes irradiated with beams of approximately constant single LET infinity, were summed over the particle LET infinity spectra to obtain an effective RBE for each charged-particle species, as a function of water absorber thickness. The resultant values of effective RBE were combined to obtain an effective RBE for the mixed radiation field. The RBE calculated in this way was compared with experimental RBEs obtained for spermatogonial cell killing in the mixed radiation field produced by neon ions traversing a thick water absorber. Discrepancies of 10-40% were observed between the calculated RBE and the RBE measured in the mixed radiation field. Part of this discrepancy can be attributed to undetected low-Z fragments, whose contribution is not included in the calculation, leading to an overestimated value for the calculated RBE. On the other hand, calculated values 10% greater than the measured RBE are explained as track structure effects due to the higher radial ionization density near neon tracks relative to the ionization density near the silicon tracks used to fit the RBE vs LET infinity data.

Animals↗

PubMed2000

Applying artificial neural network models to clinical decision making.

Because psychological assessment typically lacks biological gold standards, it traditionally has relied on clinicians' expert knowledge. A more empirically based approach frequently has applied linear models to data to derive meaningful constructs and appropriate measures. Statistical inferences are then used to assess the generality of the findings. This article introduces artificial neural networks (ANNs), flexible nonlinear modeling techniques that test a model's generality by applying its estimates against "future" data. ANNs have potential for overcoming some shortcomings of linear models. The basics of ANNs and their applications to psychological assessment are reviewed. Two examples of clinical decision making are described in which an ANN is compared with linear models, and the complexity of the network performance is examined. Issues salient to psychological assessment are addressed.

Adolescent↗

PubMed2006

Individual differences in the onset of tense marking: a growth-curve analysis.

The purpose of this study was to explore individual differences in children's tense onset growth trajectories and to determine whether any within- or between-child predictors could account for these differences. Twenty-two children with expressive vocabulary abilities in the low-average to below-average range participated. Sixteen children were at risk for specific language impairment (SLI), and 6 children had low-average language abilities. Spontaneous language samples, obtained at 3-month intervals between 2;0 and 3;0, were analyzed to examine change in a cumulative productivity score for 5 tense morphemes: third person singular present, past tense, copula BE, auxiliary BE, and auxiliary DO. Hierarchical linear modeling was used to model intercept and linear growth at 30 months and quadratic growth overall. A growth model that included mean length of utterance (MLU) and MLU growth better explained within-child productivity score growth trajectories than a parallel model with vocabulary and vocabulary growth. Significant linear growth in productivity scores remained even after a control for MLU was in place. When between-child predictors were added in the final conditional model, only positive family history approached statistical significance, improving the overall estimation of the model's growth parameters. The findings support theoretical models of language acquisition that claim relative independence of tense marking from other more general aspects of vocabulary development and sentence length. The trends for family history are also consistent with proposals implicating faulty genetic mechanisms underlying developmental language disorders. Systematic use of familial risk data is recommended in future investigations examining the relationship between late-talking children and children at risk for SLI.

Age Factors↗

PubMed1985

General treatment of linear mammillary models.

This paper deals with the mathematical analysis of time courses of absorption, distribution and elimination of drug in a body, which is of considerable value in developing dosage schedules to provide optimal therapeutic action and to reduce the unwanted side effects due to accumulation of drug in the body to a minimum. We consider a general n-compartment model, where elimination occurs from the central compartment which, in turn, is connected reversibly with all other compartments. This linear mammillary model can be used to study the kinetics of protein metabolism in organism. We use an optimization method to characterize the pharmacokinetic profiles of drug for a general n-compartment model. Results are compared to those obtained by making use of (a) SAAM program and (b) an asymptotic method.

Biometry↗

PubMed2004

Spectral-daylight recovery by use of only a few sensors.

Linear models have already been proved accurate enough to recover spectral functions. We have resorted to such linear models to recover spectral daylight with the response of no more than a few real sensors. We performed an exhaustive search to obtain the best set of Gaussian sensors with a combination of optimum spectral position and bandwidth. We also examined to what extent the accuracy of daylight estimation depends on the number of sensors and their spectral properties. A set of 2600 daylight spectra [J. Opt. Soc. Am. A 18, 1325 (2001)] were used to determine the basis functions in the linear model and also to evaluate the accuracy of the search. The estimated spectra are compared with the original ones for different spectral daylight and skylight sets of data within the visible spectrum. Spectral similarity, colorimetric differences, and integrated spectral irradiance errors were all taken into account. We compare our best results with those obtained by using a commercial CCD, revealing the CCD's potential as a daylight-estimation device.

Journal Article↗

PubMed2001

Determination of physical properties of bitumens by use of near-infrared spectroscopy with neural networks. Joint modelling of linear and non-linear parameters.

The fact that bitumens behave as non-Newtonian fluids results in non-linear relationships between their near-infrared (NIR) spectra and the physico-chemical properties that define their consistency (viz. penetration and viscosity). Determining such properties using linear calibration techniques [e.g. partial least-squares regression (PLSR)] entails the previous transformation of the original variables by use of non-linear functions and employing the transformed variables to construct the models. Other properties of bitumens such as density and composition exhibit linear relationships with their NIR spectra. Artificial neural networks (ANNs) enable modelling of systems with a non-linear property-spectrum relationship; also, they allow one to determine several properties of a sample with a single model, so they are effective alternatives to linear calibration methods. In this work, the ability of ANNs simultaneously to determine both linear and non-linear parameters for bitumens without the need previously to transform the original variables was assessed. Based on the results, ANNs allow the simultaneous determination of several linear and non-linear physical properties typical of bitumens.

Journal Article↗

PubMed2005

A comparison of models of force production during stimulated isometric ankle dorsiflexion in humans.

In this paper, we compare seven models on their ability to fit isometric muscle force. We stimulated the ankle dorsiflexors of eight subjects at seven ankle angles (85 degrees-120 degrees). Three different stimulation patterns (twitch, triangular, and random) were applied at all ankle angles. Four additional patterns (doublets, steady rates, "catch property," and walking-like) were applied at 95 degrees. Parameter values were optimized for each model at each angle. Parameters for the general linear model were calculated using a novel least-squares algorithm. A linear, second-order critically damped model gave the poorest fits (average root mean square (rms) error: 15 N). The models of Ding et al. (2002) and Bobet and Stein (1998) gave the best fits (average rms errors: 9.2 and 9.4 N). The other models (general linear second-order model, Wiener model, Zhou et al. (1995) model, general linear model) gave intermediate results. Results were similar at all ankle angles. We conclude that the Ding and Bobet-Stein models are the best overall for isometric contractions, that no linear model of any kind will give an error less than 9% of maximum force, and that the models tested are consistent across lengths.

Ankle Joint↗

PubMed2004

Extending the linear-quadratic model for large fraction doses pertinent to stereotactic radiotherapy.

Ongoing clinical trials designed to explore the use of extracranial stereotactic radiosurgery (ESR) for different tumour sites use large doses per fraction (15, 20, 30 Gy or even larger). The question of whether the linear-quadratic (LQ) model is appropriate to describe radiation response for such large fraction doses has been raised and has not been answered definitively. It has been proposed that mechanism-based models, such as the lethal-potentially lethal (LPL) model, could be more appropriate for such large fraction/acute doses. However, such models are not well characterized with clinical data and they are generally not easy to use. The purpose of this work is to modify the LQ model to more accurately describe radiation response for high fraction/acute doses. A new parameter is introduced in the modified LQ (MLQ) model. The new parameter introduced is characterized based both on in vitro cell survival data of several human tumour cell lines and in vivo animal iso-effect curves. The MLQ model produces a better fit to the iso-effect data than the LQ model. For a high single dose irradiation, the prediction of the MLQ is consistent with that from the LPL model. Unlike the LPL model, the MLQ model retains the simplicity of the LQ model and uses the well-characterized alpha and beta parameters. This work indicates that the standard LQ model can lead to erroneous results when used to calculate iso-effects with large fraction doses, such as those used for ESR. We present a solution to this problem.

Animals↗

PubMed2003

Pressure inactivation kinetics of Yersinia enterocolitica ATCC 35669.

The survival curves of Yersinia enterocolitica ATCC 35669 inactivated by high hydrostatic pressure were obtained at four pressure levels (300, 350, 400, and 450 MPa) in sodium phosphate buffer (0.1 M, pH 7.0) and four pressure levels (350, 400, 450, 500 MPa) in UHT whole milk. Tailing was observed in all the survival curves. A linear model and three nonlinear models were fitted to these data and the performances of these models were compared. The linear regression model for survival curves at four pressure levels had regression coefficients (R2) values of 0.785-0.962 and mean square error (MSE) of 0.265-0.893. A residual plot strongly suggested that a linear regression function was not appropriate as there was strong curvature in the plotted data. The nonlinear regression model using the log-logistic had R2 values of 0.946-0.982 and MSE values of 0.110-0.320. The Weibull model had R2 values of 0.944-0.975 and MSE values of 0.153-0.349. These results indicated that both were better models to describe the pressure inactivation kinetics of Y. enterocolitica in milk and buffer. Among the three nonlinear models studied, the modified Gompertz model produced the poorest fit to data. The number of parameters of the log-logistic model was reduced from four to two so that the model was greatly simplified. The reduced log-logistic model still produced a fit comparable to the full model. Since pressure had no significant effect on the shape factors of the Weibull model at the pressure levels of 300-400 MPa for buffer and 400-500 MPa for milk, models were developed to predict survival curves of Y. enterocolitica at pressures different from the experimental pressures.

Animals↗

PubMedDate not supplied

Adaptive rival penalized competitive learning and combined linear predictor model for financial forecast and investment.

We propose a prediction model called Rival Penalized Competitive Learning (RPCL) and Combined Linear Predictor method (CLP), which involves a set of local linear predictors such that a prediction is made by the combination of some activated predictors through a gating network (Xu et al., 1994). Furthermore, we present its improved variant named Adaptive RPCL-CLP that includes an adaptive learning mechanism as well as a data pre-and-post processing scheme. We compare them with some existing models by demonstrating their performance on two real-world financial time series--a China stock price and an exchange-rate series of US Dollar (USD) versus Deutschmark (DEM). Experiments have shown that Adaptive RPCL-CLP not only outperforms the other approaches with the smallest prediction error and training costs, but also brings in considerable high profits in the trading simulation of foreign exchange market.

Algorithms↗

PubMed1986

Off-line removal of ocular artifacts from event-related potentials using a multiple linear regression model.

A method for correction of event-related potentials (ERP) and cortical DC shifts, disturbed by eyeblink and eye movement potentials, is described. The correcting algorithm employs a multiple linear regression model with random regressors which prevents an incorrect calculation of the propagation factor when both ocular potentials and event-related cortical potentials occur together. This propagation factor is calculated for each event-related EEG record. Segmentation of the record into 2.56 s time intervals guarantees, moreover, calculation of different propagation factors for eyeblinks and eye movements within a single trial. The correcting algorithm is executed off-line with the propagation factors calculated from the experimental data proper. A correction is carried out only when the EOG has a significant influence on ERP. The application of the procedure is illustrated by individual examples.

Blinking↗