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At least 253 records · Page 14Linked to original sources
PubMed1994

Genetic information and ecosystem health: arguments for the application of chaos theory to identify boundary conditions for ecosystem management.

To meet the demands for goods and services of an exponentially growing human population, global ecosystems will come under increasing human management. The hallmark of successful ecosystem management will be long-term ecosystem stability. Ecosystems and the genetic information and processes which underlie interactions of organisms with the environment in populations and communities exhibit behaviors which have nonlinear characteristics. Nonlinear mathematical formulations describing deterministic chaos have been used successfully to model such systems in physics, chemistry, economics, physiology, and epidemiology. This approach can be extended to ecotoxicology and can be used to investigate how changes in genetic information determine the behavior of populations and communities. This article seeks to provide the arguments for such an approach and to give initial direction to the search for the boundary conditions within which lies ecosystem stability. The identification of a theoretical framework for ecotoxicology and the parameters which drive the underlying model is a critical component in the formulation of a prioritized research agenda and appropriate ecosystem management policy and regulation.

Ecosystem↗
PubMed1999

Red queen dynamics, competition and critical points in a model of RNA virus quasispecies.

RNA viruses offer a unique opportunity for the study of evolution at the molecular level. Recent experiments involving clonal populations of RNA viruses have shown that competition among virus strains of approximately equal relative fitness can result in the eventual competitive exclusion of one of the species. As competition proceeds in time, both the winners and the losers exhibited absolute gains in fitness, consistent with the "Red Queen" hypothesis of evolution. Further experiments involving closely related evolving quasispecies revealed a highly predictable nonlinear behavior suggesting a deterministic component in the underlying quasispecies dynamics. This is apparently in contradiction with the standard view of RNA virus evolution as a highly unpredictable process. In this paper we present a simple model which allows previous hypothesis to be tested and provides an interpretation for the observed experimental results.

Cloning, Molecular↗
PubMed1999

[Short-term dynamics of intertidal microphytobenthos biomass. Mathematical model].

We formulate a deterministic mathematical model to describe the dynamics of the microphytobenthos of intertidal mudflats. It is 'minimal' because it only takes into account the essential processes governing the functioning of the system: the autotrophic production, the active upward and downward migrations of epipelic microalgae, the saturation of the mud surface by a biofilm of diatoms and the global net loss rates of biomass. According to the photic environment of the benthic diatoms inhabiting intertidal mudflats, and to their migration rhythm, the model is composed of two sub-systems of ordinary differential equations; they describe the simultaneous evolution of the biomass 'S' concentrated in the mud surface biofilm--the photic layer--and of the biomass 'F' diluted in the topmost centimetre of the mud--the aphotic layer. Qualitatively, the model solutions agree fairly well with the in situ observed dynamics of the S + F biomass. The study of the mathematical properties of the model, under some simplifying assumptions, shows the convergence of solutions to a stable cyclic equilibrium, whatever the frequencies of the physical synchronizers of the production. The sensitivity analysis reveals the necessity of a better knowledge of the processes of biomass losses, which so far are uncertain, and may further vary in space and time.

Biomass↗
PubMed1998

Current status of HBV vaccine escape variants--a mathematical model of their epidemiology.

We present a deterministic model of the possible emergence of a vaccine escape variant of hepatitis B virus (HBV). The model identifies the key unknowns determining this process: the protection afforded by the current vaccines against particular HBV variants; the infectiousness of these variants; and the current prevalence of individuals infectious with the variants (each factor relative to wild-type). By making pessimistic assumptions about these unknowns we show that even a highly infectious variant, under a vaccine programme that affords no protection against the variant, would still take decades to emerge. Thus the current low prevalence of variants is not evidence for the cross-reactivity of the current vaccines or for a lack of infectiousness in the variants. As any vaccine failure will be inapparent for decades it may be sensible to recommend vaccine modifications now rather than later.

Hepatitis B Vaccines↗
PubMed1997

Spirals, chaos, and new mechanisms of wave propagation.

The chaos theory is based on the idea that phenomena that appear disordered and random may actually be produced by relatively simple deterministic mechanisms. The disordered (aperiodic) activation that characterizes a chaotic motion is reached through one of a few well-defined paths that are characteristic of nonlinear dynamical systems. Our group has been studying VF using computerized mapping techniques. We found that in electrically induced VF, reentrant wavefronts (spiral waves) are present both in the initial tachysystolic stage (resembling VT) and the later tremulous incoordination stage (true VF). The electrophysiological characteristics associated with the transition from VT to VF is compatible with the quasiperiodic route to chaos as described in the Ruelle-Takens theorem. We propose that specific restitution of action potential duration (APD) and conduction velocity properties can cause a spiral wave (the primary oscillator) to develop additional oscillatory modes that lead to spiral meander and breakup. When spiral waves begin to meander and are modulated by other oscillatory processes, the periodic activity is replaced by unstable quasiperiodic oscillation, which then undergoes transition to chaos, signaling the onset of VF. We conclude that VF is a form of deterministic chaos. The development of VF is compatible with quasiperiodic transition to chaos. These results indicate that both the prediction and the control of fibrillation are possible based on the chaos theory and with the advent of chaos control algorithms.

Animals↗
PubMed1991

An evaluation of sensory noise in the human visual system.

It is assumed that the activity of a visual channel may be represented as V(t) = g(t) + xi(t), where g(t) is the deterministic response of the channel due to the presentation of a stimulus and xi(t) is the trajectory of a wide-sense stationary Gauss process. The stimulus is detected if the event (V(t) greater than S for at least one t epsilon[0, T]) occurs. Two approximations for the probability of this event are proposed, and it is demonstrated how they may be employed to estimate (i) the value of the second spectral moment lambda 2 of the noise process xi t, where lambda 2 reflects the speed of the fluctuations of the trajectories xi(t), and (ii) the value of the internal threshold S. The commonly made assumption of peak--detection is shown to serve as a very good first approximation in particular if the channel is of transient type or--in case of detection by a channel of sustained type--if the stimulus durations are not too long.

Humans↗
PubMed1996

Dynamics of ongoing activity: explanation of the large variability in evoked cortical responses.

Evoked activity in the mammalian cortex and the resulting behavioral responses exhibit a large variability to repeated presentations of the same stimulus. This study examined whether the variability can be attributed to ongoing activity. Ongoing and evoked spatiotemporal activity patterns in the cat visual cortex were measured with real-time optical imaging; local field potentials and discharges of single neurons were recorded simultaneously, by electrophysiological techniques. The evoked activity appeared deterministic, and the variability resulted from the dynamics of ongoing activity, presumably reflecting the instantaneous state of cortical networks. In spite of the large variability, evoked responses in single trials could be predicted by linear summation of the deterministic response and the preceding ongoing activity. Ongoing activity must play an important role in cortical function and cannot be ignored in exploration of cognitive processes.

Animals↗
PubMed1991

[Synergetics--a possible hypothetical approach to better understanding of pathologic processes].

Modern understanding of the human disease cannot be reduced only on the disturbed biological processes of the living organism. Disease as the consequence of the disorders of a complex system needs in the same sense a complex view. It is necessary to take in consideration the theory of systems, the concept of layers and the idea of the deterministic chaos. A helpful clamp to coordinate these different views seems to be the synergetics, the teaching of the cooperation in complex systems. Synergetics investigate the mechanisms of the self-organization, too. The self-organization is of great significance in the phylogenetic and ontogenetic morphogenesis. Especially all diseases which are characterized by the disorders in the differentiation the processes forming "Gestalten" are to understand as processes of the disturbed self-organization. These refers the malformations, the malignant tumours and the chronic diseases. Self-organization being a system-internal mechanism is to understand as a process which determines the behaviour of complex organized living systems in confrontation with their environment.

Biological Evolution↗
PubMedDate not supplied

Random drug excess.

If a patient is treated in a hospital, drugs are administered at 'deterministic', known times. However, if the patient is not supervised the times of drug intake may be less 'deterministic' and less well known. It seems therefore worthwhile to make a theoretical experiment in which one assumes that drugs are applied according to a random process. In the present article it is proposed that shot noise models could be used to represent concentration curves in such situations. By means of Monte Carlo methods different measures of random drug excess are described. An included program may encourage readers to consider these methods and to perform computer experiments.

Drug Administration Schedule↗
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↗
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↗
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

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↗
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↗