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Results for “Nonlinear Dynamics”

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At least 1,207 records · Page 67Linked to original sources

PubMed2005

Automatic control strategy for step feed anoxic/aerobic biological nitrogen removal process.

Control of sludge age and mixed liquid suspended solids concentration in the activated sludge process is critical for ensuring effective wastewater treatment. A nonlinear dynamic model for a step-feed activated sludge process was developed in this study. The system is based on the control of the sludge age and mixed liquor suspended solids in the aerator of last stage by adjusting the sludge recycle and wastage flow rates respectively. The simulation results showed that the sludge age remained nearly constant at a value of 16 d in the variation of the influent characteristics. The mixed liquor suspended solids in the aerator of last stage were also maintained to a desired value of 2500 g/m3 by adjusting wastage flow rates.

Bioreactors↗

PubMed2005

Robust interval-based regulation for anaerobic digestion processes.

A robust regulation law is applied to the stabilization of a class of biochemical reactors exhibiting partially known highly nonlinear dynamic behavior. An uncertain environment with the presence of unknown inputs is considered. Based on some structural and operational conditions, this regulation law is shown to exponentially stabilize the aforementioned bioreactors around a desired set-point. This approach is experimentally applied and validated on a pilot-scale (1 m3) anaerobic digestion process for the treatment of raw industrial wine distillery wastewater where the objective is the regulation of the chemical oxygen demand (COD) by using the dilution rate as the manipulated variable. Despite large disturbances on the input COD and state and parametric uncertainties, this regulation law gave excellent performances leading the output COD towards its set-point and keeping it inside a pre-specified interval.

Bacteria, Anaerobic↗

PubMedDate not supplied

[Mathematical modeling of the mechanisms of locomotory pattern formation under epidural spinal cord stimulation with consideration of peripheral feedback].

The mechanisms of nervous regulation of locomotory activity of the spinal cord and participation of afferent peripheral feedback from lower limb muscles in the formation of locomotory patterns were investigated. The set of electromyograms of lower-limb muscle groups recorded in experiments on mesencephalic cats with application of electric epidural stimulations of lumbar segments of the spinal cord is described by a nonlinear dynamic model constructed on the basis of the Van-der-Pol equation with the compelling member. The conditions of occurrence of the regime of self-oscillations were investigated depending on the parameters of external influence. A modified equation was proposed, which takes into account the role of the afferent feedback and delay between the beginning of stimulation and muscle reaction. The conformity of the mathematical model with experimental data was shown, which makes possible its use both for the description of the mechanism of locomotory pattern formation under epidural spinal cord stimulation and the choice of optimum stimulation conditions.

Animals↗

PubMed2005

Infection transmission science and models.

Infection transmission systems circulate infection through complex contact patterns related to both contact patterns and patterns of factors that affect the risk of transmission given contact. The nonlinear dynamics of infection transmission cause these patterns to make big differences in population infection levels. A science of infection transmission system analysis is needed to focus on those details that affect the control of infection transmission. This science must have a strong theoretical base because there is little chance that a dominantly data based approach not using mechanistic models of transmission will have any predictive value. The theoretical base should be built on linked transmission system models that are focused on making needed inferences for both building the theoretical base and making infection control decisions. The linking of different models is needed for a strategy of inference robustness assessment that is designed to find the model that is simple enough to effectively analyze the transmission system but not so simple that realistic violation of simplifying assumptions will change an inference. Types of models that should be used in such linked analyses include deterministic and stochastic compartmental models, discrete individual models with individual event histories but structured mass action mixing, network models that provide more detail as to who has contact with whom, and intermediate model forms such as correlation models that address some aspects of contact details while preserving the flexibility of deterministic compartmental models to structure mixing and analyze the system. While transmission system science is currently weak in regards both to its data base and its theory base, many things are now coming together that could make this science flourish. On the data side these include greater ability to detect infectious agent sequences in the environment and greater ability to sequence and genetically relate agents identified at different sites in the transmission system. On the theory sides, new model construction and model analysis methods are providing new potential to use the new sources of data. Also new parameter estimation methods provide new potential to combine models and data in effective analytic strategies.

Communicable Diseases↗

PubMedDate not supplied

[Detection of cardiac pathologies by dimensional characteristics of RR-intervals of the electrocardiograms].

On the basis of the study of electrocardiograms by nonlinear dynamics methods, we consider the possibility to reveal pathologies of patients with different heart failures. Two parameters were analyzed: the correlation dimension and embedding dimension of RR-intervals of electrocardiorgams. Some restrictions of this method and the ways of their solutions are discussed. It was found that the proposed methods are suitable for the initial diagnosis.

Cardiovascular Diseases↗

PubMedDate not supplied

Recovery of postural control after cerebral concussion: new insights using approximate entropy.

CONTEXT: The return-to-play decision after sport-related cerebral concussion depends in part on knowing when an athlete has fully recovered postural control after injury. OBJECTIVE: To describe the postconcussion recovery of postural control using approximate entropy (ApEn), a regularity statistic from nonlinear dynamics. DESIGN: Retrospective case series analysis. SETTING: Sports medicine research laboratory. PATIENTS OR OTHER PARTICIPANTS: Collegiate athletes from whom center-of-pressure and symptom data were collected at preseason, less than 48 hours after injury, and 48 to 96 hours after injury. MAIN OUTCOME MEASURE(S): Approximate entropy values reflecting the amount of randomness contained in center-of-pressure oscillations were calculated for anterior-posterior (AP) and medial-lateral (ML) time series. Equilibrium scores reflecting the amplitude of center-of-pressure AP oscillations were used to indicate postural stability. The number and severity of symptoms were described. RESULTS: Compared with the healthy preseason state, ApEn values for the AP and ML time series generally declined immediately after injury in both steady and unsteady injured athletes. At 48 to 96 hours after injury, ApEn values for the ML time series remained significantly depressed (mean difference compared with preseason = -0.268, standard error = 0.072), even among athletes whose initial postural instability had resolved. We found few significant relationships between changes in ApEn values and changes in symptoms before and after injury. CONCLUSIONS: The effects of cerebral concussion on postural control appear to persist for longer than 3 to 4 days, even among athletes with no signs of unsteadiness. Our results may reflect changes in neurophysiologic or mechanical constraints on postural control. Approximate entropy provides a theoretically distinct, valuable measurement alternative that may prove useful for reducing uncertainty in the return-to-play decision.

Journal Article↗

PubMed2007

Paradigms lost?

Thomas Kuhn (1957, 1962) explored the issues of paradigm shifts, scientific revolutions, and the relationship between them. Written before the techniques and practices of the complexity sciences were developed, Kuhn described what he termed the Copernican Revolution as the last scientific revolution signifying a paradigmatic shift in society. We will explore whether New Science approaches in nonlinear dynamics and complexity research signify postmodern science perspectives, and examine the role of New Science in what may be the on-going evolution of the next paradigm shift.

Journal Article↗

PubMed1987

[Guanylate cyclase from human blood platelets].

Human blood platelets were disrupted by ultrasonication, and the guanylate cyclase activity was determined in the 105,000 g supernatant. The guanylate cyclase preparation obtained in the absence of dithiothreitol (DTT) was characterized by a nonlinear dynamics of cGMP synthesis during incubation at 37 degrees C. The use of 0.2 mM DTT during platelet ultrasonication stabilized the guanylate cyclase reaction and did not influence the enzyme activity. With a rise in DTT concentration up to 2 mM the guanylate cyclase activity diminished. Sodium nitroprusside stimulated the enzyme; this effect was enhanced in the presence of DTT. The maximum guanylate cyclase activity was revealed at 4 mM Mn2+ or Mg2+ and with 1 mM GTP. In the presence of Mn2+ the enzyme activity was higher than with Mg2+. The apparent Km values for GTP in the presence of 4 mM Mn2+ and Mg2+ was 30 and 200 microM, respectively. At GTP/cation ratio of 1:4 the Km values for Mn2+ and Mg2+ were nearly the same (249 and 208 microM, respectively). It was assumed that besides being involved in the formation of the GTP-substrate complex, Mn2+ exerts a strong influence on guanylate cyclase by oxidizing the SH-groups of the enzyme.

Blood Platelets↗

PubMed1984

[Deterministic and stochastic identification of neurophysiologic systems].

The paper deals with deterministic and stochastic identification methods applied to the concrete neurophysiological systems. The deterministic identification was carried out for the system: efferent fibres-muscle. The obtained transition characteristics demonstrated dynamic nonlinearity of the system. Identification of the neuronal model and the "afferent fibres-synapses-neuron" system in mollusc Planorbis corneus was carried out using the stochastic methods. For these purpose the Wiener method of stochastic identification was expanded for the case of pulse trains as input and output signals. The weight of the nonlinear component in the Wiener model and accuracy of the model prediction were quantitatively estimated. The results obtained proves the possibility of using these identification methods for various neurophysiological systems.

Animals↗

PubMed1982

[Transformation of frequency-modulated afferent activity by spinal motor neurons in the cat].

Transformation of the frequency-modulated afferent activity into postsynaptic shifts of the membrane potential in the spinal motoneurons and then into firing rates of evoked activity was studied in anesthetized cats. The phase of membrane potential oscillations evoked by activation of excitatory monosynaptic inputs practically coincided with the phase of a modulation signal in a wide frequency range. Phase lags of the oscillation were observed only at high frequencies, but even at 10.0 Hz the lag did not exceed 30-45 degrees. At the same time changes in the firing rates of the evoked activity always led the modulation signal. Transformation properties of the spinal motoneurons are discussed from the standpoint of nonlinear dynamic sensitivity of the central neurons.

Animals↗

PubMed1983

The physiological basis of mental images.

An important hypothesis in neurophysiology supposes that sensory information is encoded in spatial patterns of neural activity. A test of this hypothesis in rabbits reveals that the spatial pattern of encephalographic (EEG) activity of the olfactory bulb depends less on odor than on expectation of odor. Analysis of the structure and nonlinear dynamics of the olfactory system suggests that the EEG manifests a neural activity pattern that is an active filter corresponding to what ethologists call a "chemical search image." This first glimpse at the physical aspect of what may be a primitive form of mental image allows us to infer some of its properties. It is self-organized; input destabilizes the bulb, and the event runs a course that is shaped by the initial conditions, the sensory input, the chemical state of the bulb from centrifugal input, and the record of past input embedded in synaptic connections. The event is analog, but not an analog of input. Its intensity pattern exists in two dimensions but it is not a picture. Its bases in feedback and resonance make it connotative and ascriptive, not denotative or descriptive. The neural-mental image is an operator, not an operand. It gathers neurons by the millions into coherent activity and creates information, which it transmits to the next stage in the brain, thereby helping to shape behavior. It cannot be understood without reference to both what the subject does and what is done to it. Its function in the bulb is to regulate olfactory input with minimal and nonspecific centrifugal control.

Animals↗

PubMed1982

[Analysis of spinal neuron transformation of sinusoidal depolarizing activity into spike activity in the cat].

Impulse activity evoked in spinal neurons by sinusoidal depolarizing currents injected through intracellular microelectrode was studied in anesthetized cats. Logarithmic amplitude and phase frequency characteristics are plotted for the first harmonic component of the firing rate. Extreme values of the current were chosen within the linear zone of the firing rate dependence on the current intensity. Amplitude characteristics showed relatively small changes within a frequency range of 0.2 divided by 10.0 Hz, whereas the phase of output signal was found to lead the input one. Mean phase shifts were 32 degrees, 50 degrees and 83 degrees at frequencies of 1, 5 and 10.0 Hz, respectively. Transformation of depolarizing currents to neuronal firing rate is shown to possess pronounced nonlinear dynamic properties. Frequency characteristics of neuronal activity are discussed with respect to dynamic properties of the movement control system.

Action Potentials↗

PubMed1995

Cancer chronotherapy: is there a right time in the day to treat?

Cicadian variation in drug metabolism and tissue sensitivity to drugs impacts their activity and toxicity. A growing body of data suggests that therapy may be improved and toxicity reduced, by administering antineoplastic agents at carefully selected times of the day. This article gives background information that supports the predictable and exploitable nonlinear dynamic relationship between dose and effect that occurs each day, explores the ways in which clinical research must determine when to treat, wrestles with the question of the stability of circadian time structure in chronically ill cancer patients, and discusses the implications of the principles outlined to anticancer drug development.

Animals↗

PubMed1993

Circadian pharmacodynamics of anticancer therapies.

Circadian variation in drug metabolism and tissue sensitivity to drugs affects their activity and toxicity. A growing body of data suggests that therapy may be improved and toxicity reduced by administering antineoplastic agents at carefully selected times of the day. Here I briefly review molecular, cellular, and organismic time-keeping mechanisms as well as cytokinetic, pharmacokinetic, and pharmacodynamic data, which support the predictable and exploitable nonlinear dynamic relation between dose and effect that occurs each day.

Animals↗

PubMed1993

Commentary: development of perception-action systems and general principles of pattern formation.

Our commentary on this special issue devoted to Developmental Biodynamics: Brain, Body, and Behavior Connections is divided into 3 main sections. The first section is an overview of the individual contributions. 5 major themes are identified: (1) inappropriateness of computational treatments of development and the need for more biologically and physically relevant treatments; (2) significance of tailoring muscular to nonmuscular forces in developing movement coordination; (3) importance of spontaneous movements as exploratory and formative mechanisms; (4) influences of action capabilities on the development of perception capabilities, and vice versa; (5) applications of methods and techniques of nonlinear dynamics to developmental processes. In the second section, we provide a synopsis of current ways of thinking about prototypical developmental processes, namely, pattern formation and pattern differentiation, in various classes of physical and biological systems. It is suggested that efforts to understand the progressive formation and differentiation of patterns in terms of very general principles provide a valuable resource of concepts and methods for students of child development. In the third section, hypotheses about the development of perception-action systems are generated from juxtaposing the themes and conjectures of this special issue with general principles of pattern formation. The hypotheses suggest the possibility of a pattern formation or dynamics approach to child development as an alternative to the conventional approaches emphasizing maturation (nativist), specific learning experiences (empiricist), cognitive stages (Piagetian), and strategies of encoding and retrieval (information processing).

Child Development↗

PubMedDate not supplied

[The induction of psychopathology by somatic disorders, medications, toxins and drugs after systemic disorders].

The purpose of this article is to review the somatic disorders, drugs and toxics more often related to psychopathology. It is also exposed the conceptual framework chosen for this task, based in the nonlinear dynamical systems theory and in a recently proposed subdivision of central nervous system. We intuitively suggest that most psychopathological syndromes could be correlated with the cerebral dysfunction conditioned by the self-organization of dissipative structures formed by hierarchical and diffuse subsystems interrelated neurons groups. Any perturbation which affect these subsystems in a brain region, and change enough their activation level and interaction pattern to make any of these interactions functionally insufficient, could begin the process.

Brain↗

PubMed1997

[Cortical variations of the dimension of correlation in children with attention deficit hyperactivity syndrome].

INTRODUCTION AND OBJECTIVE: The syndrome of deficient attention and hyperactivity affects approximately 5% of children of school age. Clinically it is characterized by deficient attention, impulsiveness and excessive motor activity. Developments in nonlinear dynamic theory have made it possible to think of modelling cerebral activity as a dynamic system of chaotic type. One the most widely standardized measurements is the dimension of correlation (D2). In this paper we study the electroencephalographic traces of a control group of 9 healthy children and another group of 19 children with the syndrome of deficient attention, at rest and whilst carrying out a visuomotor task (Test of perception of differences). MATERIAL AND METHODS: The dimension of correlation was estimated for both groups. Statistical comparison was made between the cortical distributions of the D2 between both types of recordings and between both groups. RESULTS: The children with lack of attention showed a larger number of activated cortical areas than those of the control group when carrying out the same task. However, during the resting state there were no differences in the D2 between the control children and the children with deficient attention. The indicates that both groups have the same basal level of activation. CONCLUSION: The dimension of correlation is a method which permits the demonstration of an increase in neuronal activity in the stimulated areas, in this case in the occipital region.

Adolescent↗

PubMed1998

Skin thermoregulation during local cooling in healthy volunteers and patients with systemic sclerosis--synchronous assessment of capillary red blood cell velocity, laser Doppler flux and skin temperature.

BACKGROUND: Patients with Raynaud's phenomenon due to systemic sclerosis exhibit a functional microangiopathy with endothelial cell damage. The aim of this study was to assess differences in the so-called nutritive and thermoregulative skin blood flow and to obtain further information by Fourier transformation and by nonlinear analysis of laser Doppler flux (LDF) time series. PATIENTS AND METHODS: A local cold stress test was performed in 10 patients and 10 age- and sex-matched healthy controls. Significant differences were detected between nutritive blood flow in the nailfold capillaries, assessed by capillary red blood cell velocity (CBV), and thermoregulatory blood flow, which was synchronously assessed by LDF. RESULTS: CBV was reduced during cooling, the drop being significantly larger in the patients than in the controls. In contrast, there was no significant difference in the decrease of LDF during cooling. The difference between the fall in CBV values and that in LDF was significantly more pronounced in patients. Further analysis of the LDF frequency spectrum by fast Fourier transformation revealed a significantly greater decrease in amplitude at the heart frequency level in healthy volunteers. A further analysis of the LDF signal revealed significant differences at rest and after cooling in fractal dimensions, suggesting an increased complexity of LDF signals in patients. SUMMARY: The patient's increased sensitivity towards local cold cn be observed best at the capillary level. But the changes of LDF in the frequency spectrum during cooling as calculated by fast Fourier transformation revealed significant differences. In addition differences in fractal dimensions of LDF time series suggest that an analysis of nonlinear dynamics may be a promising approach.

Adult↗