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At least 937 records · Page 52Linked to original sources
PubMed1994

Dimensionality alterations of the hippocampal electroencephalographic activity following the induction of long-term potentiation in rats.

In the present in vivo experimental study the complexity alterations of the hippocampal electroencephalographic (EEG) activity were investigated prior and during the maintenance phase of long-term potentiation (LTP), using analytical methods based on the recent concepts of deterministic chaos. LTP was induced in the hippocampal dentate hilus after stimulation of the medial perforant path of the rat's brain. During the experimental procedure hippocampal EEG epochs were recorded prior and after the induction of LTP. Dimensionality computations performed on these epochs showed a maintained relative reduction in the correlation dimension during the maintenance phase of LTP. This result might suggest that different functional states of the brain are governed by different degrees of complexity and that the altered efficacy in the information process, as it is achieved by the induction of LTP, modifies the spontaneous EEG activity of the potentiated hippocampal area in a plastic manner.

Animals↗
PubMed2001

Nonlinear dynamics and chaos theory: concepts and applications relevant to pharmacodynamics.

The theory of nonlinear dynamical systems (chaos theory), which deals with deterministic systems that exhibit a complicated, apparently random-looking behavior, has formed an interdisciplinary area of research and has affected almost every field of science in the last 20 years. Life sciences are one of the most applicable areas for the ideas of chaos because of the complexity of biological systems. It is widely appreciated that chaotic behavior dominates physiological systems. This is suggested by experimental studies and has also been encouraged by very successful modeling. Pharmacodynamics are very tightly associated with complex physiological processes, and the implications of this relation demand that the new approach of nonlinear dynamics should be adopted in greater extent in pharmacodynamic studies. This is necessary not only for the sake of more detailed study, but mainly because nonlinear dynamics suggest a whole new rationale, fundamentally different from the classic approach. In this work the basic principles of dynamical systems are presented and applications of nonlinear dynamics in topics relevant to drug research and especially to pharmacodynamics are reviewed. Special attention is focused on three major fields of physiological systems with great importance in pharmacotherapy, namely cardiovascular, central nervous, and endocrine systems, where tools and concepts from nonlinear dynamics have been applied.

Biopharmaceutics↗
PubMed2002

Coherent properties of a two-level system based on a quantum-dot photodiode.

Present-day information technology is based mainly on incoherent processes in conventional semiconductor devices. To realize concepts for future quantum information technologies, which are based on coherent phenomena, a new type of 'hardware' is required. Semiconductor quantum dots are promising candidates for the basic device units for quantum information processing. One approach is to exploit optical excitations (excitons) in quantum dots. It has already been demonstrated that coherent manipulation between two excitonic energy levels--via so-called Rabi oscillations--can be achieved in single quantum dots by applying electromagnetic fields. Here we make use of this effect by placing an InGaAs quantum dot in a photodiode, which essentially connects it to an electric circuit. We demonstrate that coherent optical excitations in the quantum-dot two-level system can be converted into deterministic photocurrents. For optical excitation with so-called pi-pulses, which completely invert the two-level system, the current is given by I = fe, where f is the repetition frequency of the experiment and e is the elementary charge. We find that this device can function as an optically triggered single-electron turnstile.

Journal Article↗
PubMed2005

Phase-space beam summation analysis of rough surface waveguide.

A Gaussian beam summation (GBS) formulation is introduced for a doubly rough boundary waveguide, wherein the coherent and incoherent scattered fields are decomposed into a discrete phase-space summation of Gaussian beams (GB) that emanate from the rough surfaces in all directions. The scheme involves deterministic GB propagators and stochastic GB-to-GB (GB2GB) scattering matrices for the coherent and incoherent fields, where each scattered beam is propagated inside the waveguide and is scattered again from the rough boundaries. The GB2GB matrices are calculated from the statistical moments of the scattering amplitude, which are given either analytically or empirically. An analytical and numerical example for a waveguide with weak boundary roughness is presented and discussed. The formulation reveals explicitly the phase-space footprint of the stochastic multiple scattering process at the rough boundaries, thus providing a cogent physical interpretation and an effective mathematical representation to the field. The formulation also accommodates the receiver's pattern in the same phase-space format. Bistatic reverberations inside a rough surface waveguide as a function of the range and of the source and the receiver directions are thus examined as an implementation example.

Journal Article↗
PubMed1999

Numerical fourier transform spectroscopy of EMG half-waves: fragmentary-decomposition-based approach to nonstationary signal analysis.

A nonstationary signal analysis technique is introduced, which regards an oscillatory physiological signal as a sum of its fragments, presented in the form of a fragmentary decomposition (FD). The virtue of FD is that it is free of the necessity to choose a priori the basis functions intended for signal analysis or synthesis. FD uses an unchanged signal fragment between adjacent zero-crossings, as a natural basis function called the half-wave function (HWF). To show that such a function is a physically meaningful object, Fourier transform methods were employed, supported by the similar basis function (SBF) algorithm, which provides the means for numerical Fourier transform spectroscopy of separate half-waves and their frequency domain description in terms of both amplitude and phase. The application of this method to parameter identification of 751 EMG half-waves from the eye blink EMG records of ten normal subjects showed that HWF's frequency domain image represents a Gaussian distribution, which applies over a defined range of relative frequencies. This empirical evidence shows that HWFs are produced by a specific system of first-order nonlinear differential equations, whose dependency on a number of random factors is characteristic of deterministic chaos. The particular form of solutions indicates that statistical regularities relevant to the central limit theorem are likely to underlie the genesis of the mass potentials studied. FD shows potential utility in a range of nonstationary physiological signals.

Adolescent↗
PubMed1998

Stochastic kinetic analysis of developmental pathway bifurcation in phage lambda-infected Escherichia coli cells.

Fluctuations in rates of gene expression can produce highly erratic time patterns of protein production in individual cells and wide diversity in instantaneous protein concentrations across cell populations. When two independently produced regulatory proteins acting at low cellular concentrations competitively control a switch point in a pathway, stochastic variations in their concentrations can produce probabilistic pathway selection, so that an initially homogeneous cell population partitions into distinct phenotypic subpopulations. Many pathogenic organisms, for example, use this mechanism to randomly switch surface features to evade host responses. This coupling between molecular-level fluctuations and macroscopic phenotype selection is analyzed using the phage lambda lysis-lysogeny decision circuit as a model system. The fraction of infected cells selecting the lysogenic pathway at different phage:cell ratios, predicted using a molecular-level stochastic kinetic model of the genetic regulatory circuit, is consistent with experimental observations. The kinetic model of the decision circuit uses the stochastic formulation of chemical kinetics, stochastic mechanisms of gene expression, and a statistical-thermodynamic model of promoter regulation. Conventional deterministic kinetics cannot be used to predict statistics of regulatory systems that produce probabilistic outcomes. Rather, a stochastic kinetic analysis must be used to predict statistics of regulatory outcomes for such stochastically regulated systems.

Bacteriophage lambda↗
PubMedDate not supplied

[Molecular mechanisms of major radiobiological sequels of the effect of ionizing radiation on mammals].

The modern knowledge about the molecular bases of reproductive cell death, apoptosis, necrosis and disturbance of the cell reproduction, the main causes of radiation sickness and radiation death of mammals and men was considered. It was marked, that these deterministic effects of ionizing radiation as the stochastic effects (mutagenesis and carcinogenesis) have an common molecular basis that most full opened up by study of apoptosis and including in complex genetical and biochemical phenomena of DNA repair.

Animals↗
PubMed2005

Cancer self remission and tumor stability-- a stochastic approach.

The paper aims to express the spontaneous regression and progression of a malignant tumor system as a prey--predator like system. The model is a three dimensional deterministic system, consisting of tumor cells, hunting predator cells and resting predator cells. Local stability analysis is performed along with numerical simulations to support the analytical findings. Moreover, the deterministic model is extended to a stochastic one allowing random fluctuations around the positive interior equilibrium. The stochastic stability properties of the model are investigated both analytically and numerically. The thresholds obtained from our study may be helpful to control the malignant tumor growth.

Humans↗
PubMed1980

On the stochastic modeling of tracer kinetics.

Virtually all of the mathematical models in current use to describe tracer kinetics are deterministic (non-stochastic). However, in this paper we suggest that the "real world" of tracer kinetics is stochastic, and we formulate a unified one-compartment model structure that incorporates multiple sources of stochasticity. The stochastic models are presented with various combinations of a probabilistic transfer mechanism, a random rate coefficient, and a random initial condition, and the mean value functions and covariances are derived for these models with both time-independent and time-varying rate coefficients. The covariances have unique forms that are helpful in model identification and many of the means are non-exponential functions. Outstanding among the conclusions are 1) a proof that a deterministic model is not always equivalent in an average sense to its stochastic counterpart; 2) the existence of many rich, practical, and realistic alternatives to the exponential decay function; and 3) the notion that models are identifiable from their covariance structure.

Kinetics↗
PubMed2002

SpikeCell: a deterministic spiking neuron.

We present a model of spiking neuron that emulates the output of the usual static neurons with sigmoidal activation functions. It allows for hardware implementations of standard feedforward networks, trained off-line with any classical learning algorithm (i.e. back-propagation and its variants). The model is validated on hand-written digits recognition, and image classification tasks. A digital architecture is proposed and evaluated. The area needed for implementing the spiking neuron on a chip is 10 times smaller than that for the corresponding static neuron. The accuracy of the network's output increases with time, and reaches that of the emulated static neural network after an adequate integration period. Single errors in the spike trains, or interruption of the relaxation process, due for example to irradiation in harsh environments, are harmless.

Action Potentials↗
PubMed2004

Fluctuation theorem for a deterministic one-particle system.

A Duffing oscillator is driven by a sum of N chaotic time series. These time series are solutions of the undriven Duffing equation. It is shown that N=1 is sufficient to render the fluctuation theorem [Phys. Rev. Lett. 74, 2694 (1995)]; J. Math. Phys. 41, 4061 (2000)]; Adv. Phys. 51, 1529 (2002)]] for the power J(tau) averaged within intervals of length tau . In particular, the probabilities p( J(tau) ) follow a nearly Gaussian distribution. Also, ln[p( J(tau) )/p(- J(tau) )] versus J(tau) can be fitted by strikingly linear functions, the slopes being proportional to tau for large tau . These results indicate that validity of the fluctuation theorem requires neither a many-particle system nor a stochastic process, which are requirements used in previous works.

Journal Article↗
PubMed2006

New technologies to reduce pediatric radiation doses.

X-ray dose reduction in pediatrics is particularly important because babies and children are very sensitive to radiation exposure. We present new developments to further decrease pediatric patient dose. With the help of an advanced exposure control, a constant image quality can be maintained for all patient sizes, leading to dose savings for babies and children of up to 30%. Because objects of interest are quite small and the speed of motion is high in pediatric patients, short pulse widths down to 4 ms are important to reduce motion blurring artifacts. Further, a new noise-reduction algorithm is presented that detects and processes signal and noise in different frequency bands, generating smooth images without contrast loss. Finally, we introduce a super-resolution technique: two or more medical images, which are shifted against each other in a subpixel region, are combined to resolve structures smaller than the size of a single pixel. Advanced exposure control, short exposure times, noise reduction and super-resolution provide improved image quality, which can also be invested to save radiation exposure. All in all, the tools presented here offer a large potential to minimize the deterministic and stochastic risks of radiation exposure.

Biotechnology↗
PubMedDate not supplied

Evolutionary-algorithm-based strategy for computer-assisted structure elucidation.

An evolutionary algorithm (EA) using a graph-based data structure to explore the molecular constitution space is presented. The EA implementation proves to be a promising alternative to deterministic approaches to the problem of computer-assisted structure elucidation (CASE). While not relying on any external database, the EA-guided CASE program SENECA is able to find correct solutions within calculation times comparable to that of other CASE expert systems. The implementation presented here significantly expands the size limit of constitutional optimization problems treatable with evolutionary algorithms by introducing novel efficient graph-based genetic operators. The new EA-based search strategy is discussed including the underlying data structures, component design, parameter optimization, and evolution process control. Typical structure elucidation examples are given to demonstrate the algorithm's performance.

Journal Article↗
PubMed2006

Genetic effect of transportation infrastructure on roe deer populations (Capreolus capreolus).

Anthropogenic transportation infrastructure is a major factor of habitat fragmentation leading to genetic population fragmentation in wildlife. Assessing and understanding the impact of this deterministic factor on genetic diversity and divergence of populations is crucial to appraise the viability of wildlife populations in fragmented landscapes. In this study, the roe deer is used as an example species for the assessment of genetic differentiation of populations separated by an anthropogenic barrier. In order to detect genetic discontinuities, we screened 12 polymorphic microsatellites on 222 individuals out of 11 roe deer populations that were sampled on the east and the westside of a fenced motorway in Central Switzerland. The interaction between landscape structure and microevolutionary processes such as gene flow and drift were assessed and evaluated by different population genetic methods like F-statistics, Mantel test, spatial autocorrelation analyses, Monmonier algorithm, and principal component analysis in conjunction with geographic information system data (synthesis map). We revealed an influence of the transportation infrastructure on genetic divergence of the roe deer population examined, but no impact on genetic diversity was detected. Based on the achieved genetic findings, recommendations for management implementation were made.

Animal Migration↗
PubMed2005

Deterministic and stochastic features of rhythmic human movement.

The dynamics of rhythmic movement has both deterministic and stochastic features. We advocate a recently established analysis method that allows for an unbiased identification of both types of system components. The deterministic components are revealed in terms of drift coefficients and vector fields, while the stochastic components are assessed in terms of diffusion coefficients and ellipse fields. The general principles of the procedure and its application are explained and illustrated using simulated data from known dynamical systems. Subsequently, we exemplify the method's merits in extracting deterministic and stochastic aspects of various instances of rhythmic movement, including tapping, wrist cycling and forearm oscillations. In particular, it is shown how the extracted numerical forms can be analysed to gain insight into the dependence of dynamical properties on experimental conditions.

Forearm↗
PubMed1994

Modelling of parasite populations: gastrointestinal nematode models.

This paper surveys models of nematode parasites of veterinary importance. A distinction is drawn between generic models which are usually simple formulations applicable to whole classes of parasite and specific models which are often more complex and designed to address questions concerning a particular species. Most of the models considered employ a deterministic framework. Four main groups are considered: generic models of trichostrongylid infection of domestic ruminants, specific models of trichostrongylid infection of domestic ruminants, specific models of experimental laboratory infections of rodents, and a specific model of nematode infections in wildlife.

Animals↗
PubMed2006

Adaptive regularization network based neural modeling paradigm for nonlinear adaptive estimation of cerebral evoked potentials.

In this paper we report an adaptive regularization network (ARN) approach to realizing fast blind separation of cerebral evoked potentials (EPs) from background electroencephalogram (EEG) activity with no need to make any explicit assumption on the statistical (or deterministic) signal model. The ARNs are proposed to construct nonlinear EEG and EP signal models. A novel adaptive regularization training (ART) algorithm is proposed to improve the generalization performance of the ARN. Two adaptive neural modeling methods based on the ARN are developed and their implementation and performance analysis are also presented. The computer experiments using simulated and measured visual evoked potential (VEP) data have shown that the proposed ARN modeling paradigm yields computationally efficient and more accurate VEP signal estimation owing to its intrinsic model-free and nonlinear processing characteristics.

Algorithms↗
PubMed1990

Age structure, environmental fluctuations, and hermaphroditic sex allocation.

This paper studies sex allocation in an age-structured population of hermaphrodites living in a temporally fluctuating environment. The general condition for the evolutionary stable state (ESS) of allocation is derived for density-independent dynamics. This condition is used to determine the effect on the deterministic ESS of a dependence of survival rates on allocation. It is also used to identify the special conditions under which a stochastic ESS is given by a product rule and show how demographic structure and the correlation structure of vital rates determines the stochastic ESS.

Biological Evolution↗