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At least 793 records · Page 44Linked to original sources
PubMed1995

A new formalism for temporal modeling in medical decision-support systems.

We present a new mathematical formalism, which we call modifiable temporal belief networks (MTBNs) that extends the concept of an ordinary belief network (BN) to incorporate a dynamic causal structure and explicit temporal semantics. An important feature of MTBNs is that they allow portions of the model to be abstract and portions of it to be temporally explicit. We show how this property can lead to substantial knowledge acquisition and computational complexity savings. In addition to temporal modeling, the language of MTBNs can be an important analytical tool, as well as temporal language for causal discovery.

Decision Making, Computer-Assisted↗
PubMed2005

Physiological control of diving behaviour in the Weddell seal Leptonychotes weddelli: a model based on cardiorespiratory control theory.

Despite being obligate air breathers, many species of marine mammal are capable of spending most of their lives submerged in water. How they do this has been a subject of intense interest to physiologists for over a century, yet we still do not have a detailed understanding of the physiological mechanisms underlying this behaviour. What are the proximate mechanisms that trigger the 'decisions' to submerge and return to the surface? The present study proposes a model intended to address this question, based on fundamental concepts of cardiorespiratory control. Two basic hypotheses are examined by computer simulation, using a mathematical model of the mammalian cardiorespiratory control system with parameter values for an adult Weddell seal: (1) that the control of diving can be considered to be a respiratory control problem, and (2) that dives are initiated and maintained by disfacilitation of respiratory drive, not inhibition. Computer simulations confirmed the plausibility of these hypotheses. Simulated diving behaviour and physiological responses (ventilation, cardiac output, blood and tissue gas tensions) were consistent with published data from freely diving Weddell seals. Dives up to the estimated aerobic dive limit (ADL, 18-25 min) could be simulated without the need for active inhibition of breathing in this model. This theoretical analysis suggests that the most important physiological adjustments occur during the surface interval phase of the dive cycle and include hyperventilation accompanied by high cardiac output, appropriate regulation of cerebral blood flow and central chemoreceptor threshold shifts. During dives, cardiac output, distribution of peripheral blood flow, splenic contraction and peripheral chemoreflex drives were found to modulate physiological and behavioural responses, but were not essential for simulated dives to occur. The main conclusion from this study is that the central chemoreceptor may be an important mechanism involved in the regulation of diving behaviour, implying that CO2, not O2, is the key regulatory variable in this model. This model includes and extends the ADL concept and suggests an explicit mechanism by which the respiratory control system may play a central role in the regulation of diving behaviour. It is likely that respiratory mechanisms are an important component of a hierarchical behavioural control system and further studies are required to test the qualitative and quantitative validity of the model.

Animals↗
PubMed1990

Mathematics achievement of children in China and the United States.

First and fifth graders in Beijing and Chicago were given a battery of mathematics test. Whether tested with problems requiring solely computation or with ones requiring application of knowledge about mathematics, American children's performance was consistently inferior to that of Chinese children. Interviews with American children suggested that they like mathematics, believe they are doing well in mathematics, and do not perceive mathematics as a difficult subject. American children's poor performance appears to be attributable, in part, to low motivation for devoting more attention to mathematics. Low standards held by American parents for academic achievement and lower interest in teaching mathematics by American teachers appear to contribute to American children's poor performance.

Achievement↗
PubMedDate not supplied

[Ventricular fibrillation: the current methods for analysing the degree of irregularity of the process].

Ventricular fibrillation has traditionally been described as "chaotic" and in recent years there has been discussions that fibrillation may be an instance of deterministic chaos in the context of nonlinear dynamical systems theory. The current paper summarizes modern methods of mathematical analysis of the degree of electrical irregularities of the heart during VF. The traditional methods of Fourier analysis of electrocardiographic data as well as concepts of chaos theory--fractal dimension, entropy, reconstruction of attractors and some new methods such as spatial coherence have been considered. The results are discussed in context of mathematical models and hypothesis of mechanisms of VF.

Animals↗
PubMed2002

The electrophysiology of gap junctions and gap junction channels and their mathematical modelling.

In most tissues of vertebrates, gap junctions control the exchange of ions and small molecules between adjacent cells, thus co-ordinating the cellular activities. The application of the dual voltage-clamp method to cell pair preparations enables one to elucidate the electrical properties of gap junctions and gap junction channels. The conductive and kinetic data obtained at the multichannel and single channel level led to a generalised concept for the operation of gap junction channels. Based on the biological data gained in this way, a mathematical model has been developed. This model is versatile and allows to simulate the electrophysiological behaviour of different types of vertebrate gap junctions.

Animals↗
PubMed2005

Attaining exponential convergence for the flux error with second- and fourth-order accurate finite-difference equations. I. Presentation of the basic concept and application to a pure diffusion system.

It is a well-known phenomenon called superconvergence in the mathematical literature that the error level of an integral quantity can be much smaller than the magnitude of the local errors involved in the computation of this quantity. When discretizing an integrated form of Fick's second law of diffusion the local errors reflect the accuracy of individual concentration points while the integral quantity has the physical meaning of the flux. This article demonstrates how an extraordinary fast exponential convergence towards zero can be achieved for the simulated flux error on the basis of finite-difference approximations that are only second-order (Box 2 method) or fourth-order (Box 4 method) accurate as far as the level of local errors is concerned.

Journal Article↗
PubMed2003

Testimony of the correlation between DHEA and bioavailable testosterone using a biochromatographic concept: effect of two salts.

In a previous paper (C. André et al., submitted to J. Chromatogr. B) a mathematical model based on the Langmuir theory was developed to visualize the competition effect between testosterone and deshydroepiandrosterone (DHEA) for their identical human serum albumin (HSA) binding cavity. In this work, the thermodynamic mechanisms of (i) the binding of two hormones, DHEA and testosterone to HSA and (ii) the testosterone displacement of its HSA binding cavity by DHEA was studied by biochromatography. The Na+ cation effect used as physico-chemical marker of these binding processes was clearly described. The Gibbs free energy value (DeltaGo ) of the displacement equilibrium was always negative demonstrating that DHEA well displaced testosterone of its HSA binding cavity. The thermodynamic data also showed that this displacement equilibrium was enthalpically controlled. Moreover, the effect of (Mg2+) concentration (x') on the two binding mechanisms was analyzed. It appeared that for old men with a deficit of testosterone, Mg(2+) supplementation during treatment with DHEA can increased the free testosterone concentration and its biological effect. All these results must be confirmed by in vivo test.

Biological Availability↗
PubMed2001

Computer-assisted drug development (CADD): an emerging technology for designing first-time-in-man and proof-of-concept studies from preclinical experiments.

Computer-assisted drug development (CADD) is an emerging technology for accelerating drug development based on the integration of mathematical modelling and simulation. This methodology provides a knowledge-based decisional tool on alternative development strategies based on the evaluation of potential risks on drug safety, and the definition of experimental design of new trials with expected power and probability of success. An example of CADD implementation is presented to design the first-time-in-man (FTIM) and the proof-of-concept (PoC) study of a new CNS compound. The final objective of the example presented is not necessarily to supply a success story of a correct prediction of human data from animal studies but to define a credible strategy suitable to design FTIM and PoC studies using preclinical data without the support of any human in vivo information. Rhesus monkey and human PK were initially estimated using allometric scaling on data collected in dogs, cynomolgus monkeys and rats. A PK/PD model was derived from a study conducted in rodent and validated by comparing the model predicted response to the one observed in a PET experiment conducted in rhesus monkey. The final PK/PD model, incorporating potential variability and uncertainty on scaled human prediction together with a receptor affinity adjustment derived from in vitro binding studies, was used to design the first-time-in-man and the proof-of-concept study.

Animals↗
PubMed1976

[Dysfunctions of space perception and spatial ability in schizophrenia (author's transl)].

Thirty-seven acute schizophrenics and 25 normal subjects were investigated with regard to their spatial perception, representation, and their spatial ability, Schizophrenics showed significantly poorer performances (in terms of quantity) in those variables, which characterized the ocular apprehension of structures and wholes, the relation of parts to a whole, or the analysis of a whole in its various components as well as the availability of visual engrams, partly in the sense of spatial representation, and the practical realization of imagined spatial configurations. On the other hand, in ocular apprehension, interpretation, and when necessary the practical realization of two-dimensional represented spatial symbols and configurations and perspective abilities there was only statistical trend for quantitatively poorer performances in schizophrenics compared with healthy persons. Factor analysis of the intercorrelation matrices for 14 variables separated out the same 4 factors in each group for space perception and spatial ability. In spite of the demonstrated "quantitative" differences for the single variables between the two groups a marked similarity in a "qualitative" sense was found, when the two-factor structures were compared mathematically (similarity coefficient: 0.664). In order to specify these differences, 56 schizophrenics were classified as paranoid or nonparanoid and investigated in the above-mentioned manner. While these groups did not show any significant quantitative difference concerning their performances at variable level, they showed entirely different structures using factor analysis. Spatial perception of the paranoid schizophrenics seemed less structured and their perceptual conception less systematical. On the other hand, when factor structures from nonparanoids and healthy controls were compared mathematically, both groups showed a marked similarity (0.783). An attempt was made to relate these results to other findings of literature and to theories of cognitive and perceptual dysfunctions of schizophrenics.

Adult↗