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Results for “Gaussian process”

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PubMed1986

Modelling of the control of heart rate by breathing using a kernel method.

The process of the breathing (input) to the heart rate (output) of man is considered for system identification by the input-output relationship, using a mathematical model expressed as integral equations. The integral equation is considered and fixed so that the identification method reduces to the determination of the values within the integral, called kernels, resulting in an integral equation whose input-output behaviour is nearly identical to that of the system. This paper uses an algorithm of kernel identification of the Volterra series which greatly reduces the computational burden and eliminates the restriction of using white Gaussian input as a test signal. A second-order model is the most appropriate for a good estimate of the system dynamics. The model contains the linear part (first-order kernel) and quadratic part (second-order kernel) in parallel, and so allows for the possibility of separation between the linear and non-linear elements of the process. The response of the linear term exhibits the oscillatory input and underdamped nature of the system. The application of breathing as input to the system produces an oscillatory term which may be attributed to the nature of sinus node of the heart being sensitive to the modulating signal the breathing wave. The negative-on diagonal seems to cause the dynamic asymmetry of the total response of the system which opposes the oscillatory nature of the first kernel related to the restraining force present in the respiratory heart rate system. The presence of the positive-off diagonal of the second-order kernel of respiratory control of heart rate is an indication of an escape-like phenomenon in the system.

Heart Rate↗

PubMed2006

Image reconstruction for positron emission tomography using fuzzy nonlinear anisotropic diffusion penalty.

Iterative algorithms such as maximum likelihood-expectation maximization (ML-EM) become the standard for the reconstruction in emission computed tomography. However, such algorithms are sensitive to noise artifacts so that the reconstruction begins to degrade when the number of iterations reaches a certain value. In this paper, we have investigated a new iterative algorithm for penalized-likelihood image reconstruction that uses the fuzzy nonlinear anisotropic diffusion (AD) as a penalty function. The proposed algorithm does not suffer from the same problem as that of ML-EM algorithm, and it converges to a low noisy solution even if the iteration number is high. The fuzzy reasoning instead of a nonnegative monotonically decreasing function was used to calculate the diffusion coefficients which control the whole diffusion. Thus, the diffusion strength is controlled by fuzzy rules expressed in a linguistic form. The proposed method makes use of the advantages of fuzzy set theory in dealing with uncertain problems and nonlinear AD techniques in removing the noise as well as preserving the edges. Quantitative analysis shows that the proposed reconstruction algorithm is suitable to produce better reconstructed images when compared with ML-EM, ordered subsets EM (OS-EM), Gaussian-MAP, MRP, TV-EM reconstructed images.

Algorithms↗

PubMed2005

A spectral framework for sperm shape characterization.

A novel methodology for characterization of animal sperm shape involving the use of a spectral approach to multiscale curvature estimation is proposed. By using the derivative property of the Fourier transform, allied to Gaussian smoothing, accurate estimates of the curvature along the sperm contour can be obtained in such a way that the curvature peaks corresponding to the sperm head vertices can be effectively identified. The measurements derived from such a processing, namely the width of the basal region of the head, the centralization of tail implantation, and the multiscale bending energy, provide valuable resources for fertility and phylogenetic studies.

Animals↗

PubMed2003

Discriminant snakes for 3D reconstruction of anatomical organs.

In this work a new statistic deformable model for 3D segmentation of anatomical organs in medical images is proposed. A statistic discriminant snake performs a supervised learning of the object boundary in an image slice to segment the next slice of the image sequence. Each part of the object boundary is projected in a feature space generated by a bank of Gaussian filters. Then, clusters corresponding to different boundary pieces are constructed by means of linear discriminant analysis. Finally, a parametric classifier is generated from each contour in the image slice and embodied into the snake energy-minimization process to guide the snake deformation in the next image slice. The discriminant snake selects and classifies image features by the parametric classifier and deforms to minimize the dissimilarity between the learned and found image features. The new approach is of particular interest for segmenting 3D images with anisotropic spatial resolution, and for tracking temporal image sequences. In particular, several anatomical organs from different imaging modalities are segmented and the results compared to expert tracings.

Algorithms↗

PubMed1990

Effects of direction and magnitude of horizontal disparities on binocular unmasking.

Conditions under which binocular unmasking (BU), as an analogue of binaural unmasking, occurs have been explored. Observers were to detect through a stereoscope a Gabor signal in patches of two-dimensional broadband gaussian noise surrounded by a frame of uniform noise. The right-eye gaussian field was displaced relative to the left eye so that it appeared either in front of or behind the frame. Performance when signal disparity was equal to that of the noise--a condition functionally equivalent to monocular processing--was compared to that obtained when signal disparity was zero--a case in which BU should occur. Enhanced signal detectability of up to 12 dB and of nearly constant magnitude was observed in the latter condition when uncrossed disparities of up to 67.60 min visual angle and display durations of 1 s were employed. Signal detectability declined appreciably with increasing disparity (both crossed and uncrossed) when display duration was reduced to 90 ms, thus preventing the occurrence of compensatory vergence eye movements. It is suggested that BU effects may result from a process of linear summation of monocular inputs.

Adult↗

PubMed1994

A decentralized process for finding equilibria given by linear equations.

I present a decentralized process for finding the equilibria of an economy characterized by a finite number of linear equilibrium conditions. The process finds all equilibria or, if there are none, reports that, in a finite number of steps at most equal to the number of equations. The communication and computational complexity compare favorably with other decentralized processes. The process may also be interpreted as an algorithm for solving a distributed system of linear equations. Comparisons with the Linpack program for LU (lower and upper triangular decomposition of the matrix of the equation system, a version of Gaussian elimination) are presented.

Journal Article↗

PubMed2004

Reliable biological communication with realistic constraints.

Communication in biological systems must deal with noise and metabolic or temporal constraints. We include these constraints into information theory to obtain the distributions of signal usage corresponding to a maximal rate of information transfer given any noise structure and any constraints. Generalized versions of the Boltzmann, Gaussian, or Poisson distributions are obtained for linear, quadratic and temporal constraints, respectively. These distributions are shown to imply that biological transformations must dedicate a larger output range to the more probable inputs and less to the outputs with higher noise and higher participation in the constraint. To show the general theory of reliable communication at work, we apply these results to biochemical and neuronal signaling. Noncooperative enzyme kinetics is shown to be suited for transfer of a high signal quality when the input distribution has a maximum at low concentrations while cooperative kinetics for near-Gaussian input statistics. Neuronal codes based on spike rates, spike times or bursts have to balance signal quality and cost-efficiency and at the network level imply sparseness and uncorrelation within the limits of noise, cost, and processing operations.

Action Potentials↗

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↗

PubMed2004

Exact joint density-current probability function for the asymmetric exclusion process.

We study the asymmetric simple exclusion process with open boundaries and derive the exact form of the joint probability function for the occupation number and the current through the system. We further consider the thermodynamic limit, showing that the resulting distribution is non-Gaussian and that the density fluctuations have a discontinuity at the continuous phase transition, while the current fluctuations are continuous. The derivations are performed by using the standard operator algebraic approach and by the introduction of new operators satisfying a modified version of the original algebra.

Journal Article↗

PubMedDate not supplied

Thin structure segmentation and visualization in three-dimensional biomedical images: a shape-based approach.

This paper presents a shape-based approach in extracting thin structures, such as lines and sheets, from three-dimensional (3D) biomedical images. Of particular interest is the capability to recover cellular structures, such as microtubule spindle fibers and plasma membranes, from laser scanning confocal microscopic (LSCM) data. Hessian-based shape methods are reviewed. A synthesized linear structure is used to evaluate the sensitivity of the multiscale filtering approach in extracting closely positioned fibers. We find that the multiscale approach tends to fuse lines together, which makes it unsuitable for visualizing mouse egg spindle fibers. Single-scale Gaussian filters, balanced between sensitivity and noise resistance, are adopted instead. In addition, through an ellipsoidal Gaussian model, the eigenvalues of the Hessian matrix are quantitatively associated with the standard deviations of the Gaussian model. Existing shape filters are simplified and applied to LSCM data. A significant improvement in extracting closely positioned thin lines is demonstrated by the resultant images. Further, the direct association of shape models and eigenvalues makes the processed images more understandable qualitatively and quantitatively.

Algorithms↗

PubMed1997

A nonisometric kinetic model for smooth muscle.

We have modeled the nonisometric contractile dynamics of smooth muscle by modifying a four-state model of actin and myosin bonds originally proposed by Hai and Murphy to simulate the isometric contractions of vertebrate smooth muscle. The model includes a latch bridge, which cycles more slowly than regular cross bridges. We generalized this model to represent the calcium-regulated processes of vertebrate and invertebrate smooth muscles. We added length dynamics by assuming length-dependent bonding and unbonding rates for the cross bridges. The calculation of the cross-bridge length distribution was simplified by assuming a Gaussian distribution, as first done by Zahalak for skeletal muscle. To test the performance of this model, we simulated isometric and nonisometric responses of different kinds of smooth muscle, including vascular smooth muscle, airway smooth muscle, molluscan catch muscle (anterior byssus retractor muscle), and Aplysia I(2) muscle. The model captures the economical force maintenance property at the later stages of isometric muscle contraction and responses to imposed lengthening and shortening movements.

Actomyosin↗

PubMedDate not supplied

[Definition of normal values in medicine].

The definition of the normal values of quantitative characters is discussed. Three distinct types of normality: statistical, biological and epidemiological, are examined. A method postulating the presence of several Gaussian components, one of which represents the truly normal population, in a heterogeneous population, is proposed for the statistical analysis of frequency distribution. Examples are given of the application of a relatively simple and effective graphical-numerical process. In the light of the preliminary results, systematic adoption of the process is suggested, not only for the definition of normal values in the sense of biological normality, but also in the comparison of patients and healthy subjects, whereby epidemiological normality can be defined by means of discriminant analysis. Lastly, the possibility of defining normal values for groups of variables is considered.

Biometry↗

PubMed1996

Motility of rhG-CSF-induced neutrophils in patients undergoing chemotherapy: evidence for inhibition detected by image analysis.

The motility of circulating neutrophils from seven patients affected by intermediate and high-grade non-Hodgkin's lymphoma was investigated before and after rhG-CSF administration (5 micrograms/kg/d for 5 d subcutaneously) in the course of chemotherapy. Random motility and bacterial lipopolysaccharide-induced chemotaxis were studied by the micropore filter technique in a Boyden chamber. These functions were evaluated by a very sensitive technique, based on a computer-assisted image processing system, capable of giving several parameters about the kinetics of cell migration. Along with a significant increase in neutrophil number, a significant decrease both in random and stimulated motility was found. The kinetics of cell migration showed that the cells maintained the typical gaussian pattern of random motility. On the contrary, neutrophils were found to have lost the typical stimulated migration peak. These findings are consistent with a rhG-CSF-induced impairment of the directional movement, rather than of the ability of moving at random. These effects were found in patients who, in the same experimental conditions, had displayed an enhanced phagocytosis and phagocytosis-associated chemiluminescence along with an enhanced CD32 expression, not due to an aspecific cell manipulation. Two hypotheses may be taken into account: (i) an increased adhesiveness due to a direct or an indirect activity of the cytokine; (ii) an abnormality in the cytoskeleton maturation and/or rearrangement during the accelerated bone marrow transit of myeloid cells. These findings emphasize that rh-GCSF administration can modulate several functions which play an important role in host defence, and suggest the utility of carrying out further studies to investigate the optimum dosage both to correct neutrophil number and preserve neutrophil functional activities.

Adult↗

PubMed2004

Adaptive stochastic resonance in noisy neurons based on mutual information.

Noise can improve how memoryless neurons process signals and maximize their throughput information. Such favorable use of noise is the so-called "stochastic resonance" or SR effect at the level of threshold neurons and continuous neurons. This paper presents theoretical and simulation evidence that 1) lone noisy threshold and continuous neurons exhibit the SR effect in terms of the mutual information between random input and output sequences, 2) a new statistically robust learning law can find this entropy-optimal noise level, and 3) the adaptive SR effect is robust against highly impulsive noise with infinite variance. Histograms estimate the relevant probability density functions at each learning iteration. A theorem shows that almost all noise probability density functions produce some SR effect in threshold neurons even if the noise is impulsive and has infinite variance. The optimal noise level in threshold neurons also behaves nonlinearly as the input signal amplitude increases. Simulations further show that the SR effect persists for several sigmoidal neurons and for Gaussian radial-basis-function neurons.

Algorithms↗

PubMed2006

Photophysical processes involved within the anisole-thioindoxyl dyad system.

The photophysical properties and the nature of the photoinduced electron transfer (PET) reactions within a synthesized anisole (A)-thioindoxyl (T) dyad system (24MBTO) have been studied by electrochemical, steady-state, and time-resolved spectroscopic techniques. Computations on the dyad were performed both in gas phase as well as solvent environment by TD-DFT method with B3LYP density function. The geometry optimization calculation of 24MBTO was done by 6-311G(d,p) basis function set implemented in the Gaussian package. The theoretical values of singlet vertical excitation energies were found to correlate well with the experimentally observed ones. The electrochemical measurements indicate the possibility of occurrence of PET reactions within 24MBTO between the linked redox centers A and T. Both steady-state and time-resolved spectroscopic measurements on the novel synthesized 24MBTO dyad demonstrate the formations of the two types of isomeric species: Z- and E- forms, resulted from the charge separation reactions. From the detailed studies it reveals that the present thioaurone may behave as a versatile photoswitchable system. It has been hinted that the loss process (charge recombination) within 24MBTO could possibly be prevented by incorporating it within the hydrophobic cavity of beta-cyclodextrin (betaCD).

Journal Article↗

PubMed1998

Wide frequency range 31P relaxation in the ion conducting glass LiPO3.

Spin-lattice relaxation studies on the stationary 31P nucleus have been performed in order to investigate to which extent the dynamics of the mobile lithium ions are reflected in the behavior of the glassy network. The temperature dependence of the 31P relaxation, which is governed by the heteronuclear dipole-dipole interaction between lithium and phosphorus, can be described in terms of a Gaussian distribution of activation energies and that over a wide frequency range from about 34 kHz to 81 MHz. A relaxation rate maximum, which provides useful information about correlation times and activation energies of the lithium-ion diffusion process, could only be observed in the rotating frame relaxation measurements.

Lithium↗

PubMed2005

Determining the vibrational pattern via overtone cold spectra: C-H methyl stretches of propyne.

Vibrationally mediated photodissociation and photoacoustic (PA) spectroscopy were employed for studying the intramolecular dynamics of propyne initially excited to the first through fourth overtone of methyl C-H stretching modes. Room-temperature PA and jet-cooled action spectra, monitoring the absorption of the parent and the yield of the ensuing H photofragments, respectively, were obtained. The PA spectra exhibit mainly broad features, while the action spectra, due to inhomogeneous structure reduction, expose multiple peaks of recognizable shapes in the differing overtone manifolds. Symmetric rotor simulations of the band contours of the action spectra allowed retrieving of band origins and linewidths. The linewidths of the bands in each manifold enabled estimates for energy redistribution times out of the corresponding states to the bath states, the times ranging from 18+/-6 ps for two quanta of C-H excitation to subpicosecond for five quanta. The data were also analyzed in terms of a normal-mode model and a joint local-/normal-mode model. These models enabled determination of harmonic frequencies, anharmonicities, and interaction parameters reproducing the observed data in all monitored regions and provided spectral assignments. The measured Doppler profiles were well fitted by Gaussians with widths suggesting low average translational energies for the released H photofragments. These low energies and their similarities to those for dissociation of propyne isotopomers preexcited to acetylenic C-H stretches were ascribed to an indirect dissociation process occurring after internal conversion to the ground electronic state and isomerization to allene.

Journal Article↗

PubMed1990

A physical explanation of the lognormality of pollutant concentrations.

Investigators in different environmental fields have reported that the concentrations of various measured substances have frequency distributions that are lognormal, or nearly so. That is, when the logarithms of the observed concentrations are plotted as a frequency distribution, the resulting distribution is approximately normal, or Gaussian, over much of the observed range. Examples include radionuclides in soil, pollutants in ambient air, indoor air quality, trace metals in streams, metals in biological tissue, calcium in human remains. The ubiquity of the lognormal distribution in environmental processes is surprising and has not been adequately explained, since common processes in nature (for example, computation of the mean and the analysis of error) usually give rise to distributions that are normal rather than lognormal. This paper takes the first step toward explaining why lognormal distributions can arise naturally from certain physical processes that are analogous to those found in the environment. In this paper, these processes are treated mathematically, and the results are illustrated in a laboratory beaker experiment that is simulated on the computer.

Models, Theoretical↗