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PubMed2001

A competitive-layer model for feature binding and sensory segmentation.

We present a recurrent neural network for feature binding and sensory segmentation: the competitive-layer model (CLM). The CLM uses topographically structured competitive and cooperative interactions in a layered network to partition a set of input features into salient groups. The dynamics is formulated within a standard additive recurrent network with linear threshold neurons. Contextual relations among features are coded by pairwise compatibilities, which define an energy function to be minimized by the neural dynamics. Due to the usage of dynamical winner-take-all circuits, the model gains more flexible response properties than spin models of segmentation by exploiting amplitude information in the grouping process. We prove analytic results on the convergence and stable attractors of the CLM, which generalize earlier results on winner-take-all networks, and incorporate deterministic annealing for robustness against local minima. The piecewise linear dynamics of the CLM allows a linear eigensubspace analysis, which we use to analyze the dynamics of binding in conjunction with annealing. For the example of contour detection, we show how the CLM can integrate figure-ground segmentation and grouping into a unified model.

Brain↗
PubMed1989

[Dynamic computer models of xenobiotics].

On the basis of the physical substance transport in the circulatory system a dynamic pharmacokinetic model is developed. The method describes the behaviour of xenobiotics and their metabolites by direct simulation of the transport and the distribution in the bloodstream and tissues by suitable algorithms in discrete steps. The cardiovascular system represented by a 2-(or 3-)dimensional array is connected to other kinetic relevant components, likewise represented by arrays of suitable dimensions and acting as transport-, storage, distribution-, exchange- and reaction areas. (Bio-)chemical transformation is accounted for by introducing an identical new set of arrays, one set for each metabolite. The overall characterisation of a model needs a great variety of coefficients, thus creating a highly flexible tool for the study of kinetic processes. The result being a time- and position-dependent discrete distribution of substance reflects the heterogeneity and complexity of a real biological system. The compartmental (deterministic and stochastic) models are of course a subset of the DVM.

Algorithms↗
PubMed2000

Dose limits for astronauts.

Radiation exposures to individuals in space can greatly exceed natural radiation exposure on Earth and possibly normal occupational radiation exposures as well. Consequently, procedures limiting exposures would be necessary. Limitations were proposed by the Radiobiological Advisory Panel of the National Academy of Sciences/National Research Council in 1970. This panel recommended short-term limits to avoid deterministic effects and a single career limit (of 4 Sv) based on a doubling of the cancer risk in men aged 35 to 55. Later, when risk estimates for cancer had increased and were recognized to be age and sex dependent, the NCRP, in Report No. 98 in 1989, recommended a range of career limits based on age and sex from 1 to 4 Sv. NCRP is again in the process of revising recommendations for astronaut exposure, partly because risk estimates have increased further and partly to recognize trends in limiting radiation exposure occupationally on the ground. The result of these considerations is likely to be similar short-term limits for deterministic effects but modified career limits.

Astronauts↗
PubMed2000

How to quantify deterministic and random influences on the statistics of the foreign exchange market

It is shown that price changes of the U.S. dollar-German mark exchange rates upon different delay times can be regarded as a stochastic Marcovian process. Furthermore, we show how Kramers-Moyal coefficients can be estimated from the empirical data. Finally, we present an explicit Fokker-Planck equation which models very precisely the empirical probability distributions, in particular, their non-Gaussian heavy tails.

Journal Article↗
PubMed1998

Radiation protection recommendations as applied to the disposal of long-lived solid radioactive waste. ICRP Publication 81.

The present publication deals with the radiological protection of members of the public following the disposal of long-lived solid radioactive waste using the 'concentrate and retain' strategy. It covers options including shallow land burial and deep geological disposal. The recommendations made in this report apply to new disposal facilities.The main protection issue concerns exposure that may or may not occur in the far future, i.e. a situation of potential exposure. Constrained optimisation is the central approach to evaluating the radiological acceptability of a waste disposal system. In this context optimisation of protection is a judgmental process with social and economic factors being taken into account and should be conducted in a structured essentially qualitative way.Two broad categories of exposure situations have to be considered: natural processes and human intrusion. Application of the radiological protection criteria to these two categories of exposure situations is different.In the first case, assessed doses or risks arising from natural processes should be compared with a constraint of no more than about 0.3 mSv per year or its risk equivalent of around 10(-5) per year. With regard to human intrusion, understood here as inadvertent human intrusion, the consequences from one or more plausible stylised scenarios should be considered in order to evaluate the resilience of the repository to such events. The Commission considers that in circumstances where human intrusion could lead to doses to those living around the site sufficiently high that intervention on current criteria would almost always be justified, reasonable efforts should be made at the repository development stage to reduce the probability of human intrusion or to limit its consequences. In this respect, the Commission has previously advised that an existing(1) annual dose of around 10 mSv per year may be used as a generic reference level below which intervention is not likely to be justifiable. Conversely, an existing(1) annual dose of around 100 mSv per year may be used as a generic reference level above which intervention should be considered almost always justifiable. Similar considerations apply in situations where the thresholds for deterministic effects in relevant organs are exceeded.The conclusion of the report is that in the Commission's view, provided reasonable measures have been taken both to satisfy the constraint for natural processes and to reduce the probability or the consequences of inadvertent human intrusion, and technical and managerial principles have been followed, then radiological protection requirements can be considered to have been complied with.

Journal Article↗
PubMed2000

Atrial heart fibrillation modeling: a tool for adequate choice of signal processing functions.

Framingham Study showed that Atrial Fibrillation (AF) exposes the patient to an increased stroke risk and to low cardiac output. It is well known that the mechanism of Atrial Fibrillation is reentry. In order to facilitate AF's studies, we propose a deterministic model that can be used for cardiac signal characterization. The model is based on a linear combination of several "Atrial Flutter-like signals. We obtained AF-close aspects. To demonstrate its usefulness we performed a power spectrum transform (i.e. scalar Fast Fourier Transform) of this model. Thanks to the model we show that this transform cannot be used as is, and may induce errors in results interpretation. We also show based on the same example how to overcome the limitation with help of the model. We expect that this model will help in mathematical transforms relevance evaluation. This may lead to new findings in AF classification, and to improvement in patients' treatment, follow-up and survival.

Atrial Fibrillation↗
PubMed1998

Kleinian guilt, determinism, and free will: implications for clinical theory and treatment.

The purpose of this discussion of teleological and deterministic concepts in clinical theory and treatment was not to offer definitive answers to a timeless philosophical problem. Indeed, a solution to the problem of free will in a deterministic science eludes even the greatest thinkers (cf. Howard and Conway, 1987). Following Feyerabend (1984), the purpose was instead to question one of the "general and lasting boundary conditions" (p.3)-causal explanations-introduced by Enlightenment-era philosophers of science to "protect" (p.3) science. I have attempted to demonstrate the logical inconsistencies of Klein's theory of etiology of infantile guild using both a teleological perspective that questions the assumption of causal explanations imposed on a scientific inquiry of human experience, and a deterministic perspective that affirms this assumption. When taken to their logical conclusions, both perspectives reduce to mutually irreconcilable theoretical and clinical absurdities. As as clinician I myself have chosen to work within a deterministic framework. I have found that perceiving the patient deterministically allows me to empathize with even the most unspeakable intrapsychic experiences. I share with my patient the antecedent conditions that efficiently cause our common experiences. It is this empathy for our share human condition-in al its transcendence, frailty, and profanity-communicated through interpretation that seems to give birth to the healing process (Goodman, 1991). This discussion doubtless also contains inconsistencies. Nevertheless, it contributes to the ongoing discourse of clinical theory and treatment because it draws attention to the enormous, yet still largely implicit, debt that clinical theory and treatment owe to philosophy. It is a great irony that the discipline that has historically put scientific inquiry in shackles also has the power to set it free. Whether teleology is the philosophical perspective to set clinical theory and treatment free, however, seems unlikely. In choosing to discuss Klein's theory of infantile guilt, I have taken seriously R.J. Rychlak and Rychlak's (1990) implicit support (cf. p.48) for a teleology that functions throughout the life span, et find it difficult to imagine both the unconscious and conscious minds with freely premising capacities-as if human being were actually two persons in one. One the other hand, reducing human beings to the status of Hobbesian machines-as the deterministic perspective seems to imply-presents a similarly unattractive alternative. To understand the nature of human suffering and the process of healing it, however, one thing is certain: A philosophical discourse of clinical theory and treatment must continue.

Causality↗
PubMed2000

A first-passage-time analysis of the periodically forced noisy leaky integrate-and-fire model.

We present a general method for the analysis of the discharge trains of periodically forced noisy leaky integrate-and-fire neuron models. This approach relies on the iterations of a stochastic phase transition operator that generalizes the phase transition function used for the study of periodically forced deterministic oscillators to noisy systems. The kernel of this operator is defined in terms of the the first passage time probability density function of the Ornstein Uhlenbeck process through a suitable threshold. Numerically, it is computed as the solution of a singular integral equation. It is shown that, for the noisy system, quantities such as the phase distribution (cycle histogram), the interspike interval distribution, the autocorrelation function of the intervals, the autocorrelogram and the power spectrum density of the spike train, as well as the input-output cross-correlation and cross-spectral density can all be computed using the stochastic phase transition operator. A detailed description of the numerical implementation of the method, together with examples, is provided.

Action Potentials↗
PubMed2000

Connectance in Sorghum development: beyond the genotype-phenotype duality.

Connectance, the level of linkage between organs, was measured in different lines of Sorghum bicolor during their reproductive development. It was compared with expression of characters, their level of variability and their heritability. A negative relationship is observed between connectance and heritability. Further results indicate that connectance does not simply introduce a noise factor in expression of a pre-existing information, but that it is directly involved in phenotypic expression and plasticity. Connectance appears as partly determined by the nature and dynamics of the network of relationships. It is concluded that the phenotype is not restricted to the deterministic expression of a pre-existing program, the genotype. Morphogenesis also involves another dimension, self-organized, which confers reliability, stability and adaptability to the developmental processes. The complex interactions between these two dimensions and their evolutionary consequences are discussed.

Edible Grain↗
PubMed2001

Extremal principle for the steady-state selection in driven lattice gases with open boundaries.

This paper investigates the steady states of one-dimensional driven lattice gases with open boundary conditions. It shows how the extremal principle proposed recently by Popkov and Schütz can be modified to apply to more general cases. Monte Carlo simulations are presented for a one-dimensional totally asymmetric simple exclusion process with nearest neighbor repulsion under parallel update as an example. The simulations enable one to guess the exact phase diagram for this particular lattice gas with deterministic bulk dynamics, by fitting the data to analytic formulas, which appear to be exact in the thermodynamic limit.

Journal Article↗
PubMedDate not supplied

Cytosolic calcium oscillators: critical discussion and stochastic modelling.

In the last few years, an immense amount of experimental data on agonist-induced cytosolic Ca2+ oscillations has emphasized the necessity of theoretical models accounting for these phenomena. In the first part of the paper, a critical analysis of different minimal Ca2+ oscillator models is presented, which reveals that not all of the so far proposed mechanisms are capable to reproduce experimental data adequately. The second part of the paper is devoted to a computational method using a stochastic simulation algorithm which describes the time evolution of Ca2+ oscillations at the molecular level. In contrast to the deterministic formulation of the models presented so far, the stochastic treatment takes account of the inherent fluctuations of cytosolic Ca2+ in cellular subcompartments. In the macroscopic limit, the stochastic models display dynamics analogous to the deterministic ones.

Animals↗
PubMedDate not supplied

Statistical methods of detection of a periodic phenomenon in a short series. Example of application: a density series of nematode eggs - II.

Using the example of the statistical treatment of a series of density of nematode eggs, we give here a survey of the main methods of detection of a periodic phenomenon in a short series. The purpose of these methods, which may be ranked into 3 groups: smoothing methods, regression methods, autocorrelations, is to study non random characteristics of the process. We detected in this series and estimated a rhythm of period 12 h. We concluded that the regression methods are the most resourceful for detecting a deterministic periodic phenomenon but that it is useful to confirm the results using the other methods.

Animals↗
PubMed1989

Saltatory transitions are a naturally occurring property of evolving systems.

On the basis of paleological evidence, it has been suggested that biological evolution need not necessarily be characterized by gradual change. Rather, evolutionary history may display saltatory periods of rapid speciation alternating with periods of relative quiescence, the whole dynamic being called punctuated equilibria. The empirical evidence that has been presented in support of this hypothesis has been the object of a vigorous dispute. Mathematical investigations of complex models of biological evolution that contain random elements have demonstrated that these systems can display saltatory behavior. In this paper we address a more abstract question: can saltations occur in the evolution of very simple, deterministic mathematical systems that function in a constant environment? The answer appears to be yes. Saltations appear as a natural dynamical behavior in the evolution of simplistic information processing networks. We stress that these networks do not constitute a model of biological evolution. However, the appearance of saltations in such simple systems suggests that their appearance in a process as complex as biological evolution is not surprising.

Animals↗
PubMed1994

Dynamics of local neuronal networks: control parameters and state bifurcations in epileptogenesis.

The aim of this overview is to present evidence that local neuronal networks (LNNs) are functionally organized in such a way that they behave as dynamic non-linear systems that can exhibit multiple types of attractor and can present bifurcations between different attractors, depending on control parameters. To begin with, some of the theoretical concepts of non-linear dynamics and chaos are briefly presented. As a case study, we described the CA1 area of the hippocampus and the changes that the corresponding LNNs undergo during kindling epileptogenesis. During epileptic seizures, evidence exists for the presence of low-dimensional chaos, since the correlation dimension estimated from the corresponding EEG signals decreases dramatically from a large value, characteristic of the resting state, to a low value typical of deterministic chaos. We propose that, among other things, an important control parameter of the dynamics of this brain area is the balance between excitatory (E) and inhibitory (I) processes. We assume that this balance can be experimentally estimated by using a paired-pulse paradigm. Accordingly, we demonstrate that the paired-pulse response changes during kindling epileptogenesis in the sense that the E/I ratio increases in the course of the establishment of a kindled epileptogenic focus. This change in E/I leads to a shift in the operating point of the LNN moving it close to a bifurcation where a rapid state change takes place. In this way, the LNN dynamics can change more readily to the basin of attraction of a chaotic attractor than under normal conditions. This is in essence what makes the behavior of the LNN more sensitive to tetanus, and predicts the facilitated occurrence of epileptic seizures during kindling.

Animals↗
PubMed1994

Scientific methodology in temporomandibular disorders. Part III: Diagnostic reasoning.

Temporomandibular disorders (TMD), as a cluster of individual diseases and disorders, pose new intellectual challenges to the diagnostic skills of dentists. New technologies enable dentists to avail themselves of paraclinical data such that diagnosis can and should be disease specific or etiology specific. The importance of logic in diagnostic reasoning is discussed. Studies of the reasoning process of doctors with reputations for having good clinical judgement have resulted in protocols of diagnostic reasoning. Three specific strategies are presented and discussed-probabilistic, causal and deterministic. The difference between intellectual and managerial decisions are explained relative to utility of the strategies.

Algorithms↗
PubMed1992

Extending the stochastic two-stage model of carcinogenesis to include self-regulation of the nonmalignant cell population.

One of the challenges of introducing greater biological realism into stochastic models of cancer induction is to find a way to represent the homeostatic control of the normal cell population over its own size without complicating the analysis too much to obtain useful results. Current two-stage models of carcinogenesis typically ignore homeostatic control. Instead, a deterministic growth path is specified for the population of "normal" cells, while the population of "initiated" cells is assumed to grow randomly according to a birth-death process with random immigrations from the normal population. This paper introduces a simple model of homeostatically controlled cell division for mature tissues, in which the size of the nonmalignant population remains essentially constant over time. Growth of the nonmalignant cell population (normal and initiated cells) is restricted by allowing cells to divide only to fill the "openings" left by cells that die or differentiate, thus maintaining the constant size of the nonmalignant cell population. The fundamental technical insight from this model is that random walks, rather than birth-and-death processes, are the appropriate stochastic processes for describing the kinetics of the initiated cell population. Qualitative and analytic results are presented, drawn from the mathematical theories of random walks and diffusion processes, that describe the probability of spontaneous extinction and the size distribution of surviving initiated populations when the death/differentiation rates of normal and initiated cells are known. The constraint that the nonmalignant population size must remain approximately constant leads to much simpler analytic formulas and approximations, flowing directly from random walk theory, than in previous birth-death models.(ABSTRACT TRUNCATED AT 250 WORDS)

Cell Death↗
PubMed1995

Active mutation in self-reproducing networks of machines and tapes.

Self-reproduction via description is discussed in a network model of machines and description tapes. Tapes consist of bit strings, which encode the machines' function A tape is replicated when it is read by adequate machines. Generally, a machine rewrites a tape without doing correct replication. The variation in a reproduced tape is taken as mutation. Because this mutation is caused by a machine's program, we call it active mutation. Which machine is translated from a given tape is dependent on what kind of a machine reads the tape. External noise is introduced in a machine's reading process to make errors. A new reaction pathway is induced by external noise via a machine's error action. We find that the induced pathways will be mimicked deterministically in an emerging core structure. This core structure will remain stable after turning off external noise. Low external noise develops a core structure of a minimal self-replicative loop. When external noise is elevated, a more complex network evolves. Machines containing a complex core network, which has been bred in high external noise, will actively rewrite tapes rather than just replicate them. Self-replication not as an individual but as a network now becomes important.

Biological Evolution↗
PubMed2000

Stochastic modelling of environmental variation for biological populations.

We examine stochastic effects, in particular environmental variability, in population models of biological systems. Some simple models of environmental stochasticity are suggested, and we demonstrate a number of analytic approximations and simulation-based approaches that can usefully be applied to them. Initially, these techniques, including moment-closure approximations and local linearization, are explored in the context of a simple and relatively tractable process. Our presentation seeks to introduce these techniques to a broad-based audience of applied modellers. Therefore, as a test case, we study a natural stochastic formulation of a non-linear deterministic model for nematode infections in ruminants, proposed by Roberts and Grenfell (1991). This system is particularly suitable for our purposes, since it captures the essence of more complicated formulations of parasite demography and herd immunity found in the literature. We explore two modes of behaviour. In the endemic regime the stochastic dynamic fluctuates widely around the non-zero fixed points of the deterministic model. Enhancement of these fluctuations in the presence of environmental stochasticity can lead to extinction events. Using a simple model of environmental fluctuations we show that the magnitude of this system response reflects not only the variance of environmental noise, but also its autocorrelation structure. In the managed regime host-replacement is modelled via periodic perturbation of the population variables. In the absence of environmental variation stochastic effects are negligible, and we examine the system response to a realistic environmental perturbation based on the effect of micro-climatic fluctuations on the contact rate. The resultant stochastic effects and the relevance of analytic approximations based on simple models of environmental stochasticity are discussed.

Animals↗