SEARCH · PubMed Health

Results for “Gaussian process”

Explore indexed PubMed citations for clinical trials, systematic reviews and public health research. Read source abstracts and follow each citation to its original PubMed record.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 1,243 records · Page 69Linked to original sources

PubMed2002

Numerical study of comparison of vorticity and passive vectors in turbulence and inviscid flows.

The nonlinear vortex stretching in incompressible Navier-Stokes turbulence is compared with a linear stretching process of passive vectors (PVs). In particular, we pay special attention to the difference of these processes under long and short time evolutions. For finite time evolution, we confirm our previous finding that the stretching effect of vorticity is weaker than that of general passive vectors for a majority of the initial conditions with the same energy spectra. The above difference can be explained qualitatively by examining the Biot-Savart formula. In order to see to what extent infinitesimal time development explains the above difference, we examine the probability density functions (PDFs) of the stretching rates of the passive vectors in the vicinity of a solution of Navier-Stokes equations. It is found that the PDFs are found to have a Gaussian distribution, suggesting that there are equally many PVs that stretched less and more than the vorticity. This suggests the importance of the vorticity-strain correlation built up over finite time in turbulence. We also discuss the case of Euler equations, where the dynamics of the Jacobian matrix relating the physical and material coordinates is examined numerically. A kind of alignment problem associated with the Cauchy-Green tensor is proposed and studied using the results of numerical simulations. It is found that vorticity tends to align itself with the most compressing eigenvector of the Cauchy-Green tensor. A two-dimensional counterpart of active-passive comparison is briefly studied. There is no essential difference between stretching of vorticity gradients and that of passive scalar gradients and a physical interpretation is given to it.

Journal Article↗

PubMed2002

Natural discriminant analysis using interactive Potts models.

Natural discriminant analysis based on interactive Potts models is developed in this work. A generative model composed of piece-wise multivariate gaussian distributions is used to characterize the input space, exploring the embedded clustering and mixing structures and developing proper internal representations of input parameters. The maximization of a log-likelihood function measuring the fitness of all input parameters to the generative model, and the minimization of a design cost summing up square errors between posterior outputs and desired outputs constitutes a mathematical framework for discriminant analysis. We apply a hybrid of the mean-field annealing and the gradient-descent methods to the optimization of this framework and obtain multiple sets of interactive dynamics, which realize coupled Potts models for discriminant analysis. The new learning process is a whole process of component analysis, clustering analysis, and labeling analysis. Its major improvement compared to the radial basis function and the support vector machine is described by using some artificial examples and a real-world application to breast cancer diagnosis.

Journal Article↗

PubMed1989

Relation of spatial and temporal frequencies to vernier acuity.

Vernier thresholds were measured with a pair of vertical sinusoidal gratings of one and a half cycles as targets. The amplitude was weighted by a one-dimensional Gaussian and contrast was set one log unit above contrast threshold. The vernier thresholds were estimated with the method of constant stimuli. Temporal frequency effects were introduced by movement of the vernier targets. It was found that vernier thresholds expressed in phase angle were unchanged in the effective range of spatial frequencies provided that the temporal frequency and the visibility were unchanged, and that thresholds deteriorated by increasing the temporal frequency. It is suggested that the detection of relative phase may be involved in the discrimination of vernier offsets and that it may be mediated by a sustained unit. Three possible types of mechanisms, edge-localization processes, orientation-selective units and phase-sensitive units, were considered in relation to vernier acuity.

Contrast Sensitivity↗

PubMed2006

Analytically exact correction scheme for signal extraction from noisy magnitude MR signals.

An analytically exact method is proposed to extract the signal intensity and the noise variance simultaneously from noisy magnitude MR signals. This method relies on a fixed point formula of signal-to-noise ratio (SNR) and a correction factor. The correction factor, which is a function of SNR, establishes a fundamental link between the variance of the magnitude MR signal and the variance of the underlying Gaussian noise in the two quadrature channels. A more general but very similar method is developed for parallel signal acquisitions with multiple receiver coils. In the context of MR imaging, the proposed method can be carried out on a pixel-by-pixel basis if the mean and the standard deviation of the magnitude signal are available.

Algorithms↗

PubMed2003

A numeric study of the noise-induced tremor in a mathematical model of the stretch reflex.

A mathematical model of the stretch reflex for the cat soleus muscle is presented. The time-delay differential equations of the model are solved using the fourth-order Runge-Kutta algorithm, introducing a Gaussian-noise term to simulate the environmental noise. The muscle response dynamics are then studied under various levels of average muscle activation. Finally, the feasibility of explaining the so-called physiological tremor from the properties of the stretch reflex mechanisms is discussed by comparing our results with reported experimental evidence.

Animals↗

PubMed2005

Statistical encoding model for a primary motor cortical brain-machine interface.

A number of studies of the motor system suggest that the majority of primary motor cortical neurons represent simple movement-related kinematic and dynamic quantities in their time-varying activity patterns. An example of such an encoding relationship is the cosine tuning of firing rate with respect to the direction of hand motion. We present a systematic development of statistical encoding models for movement-related motor neurons using multielectrode array recordings during a two-dimensional (2-D) continuous pursuit-tracking task. Our approach avoids massive averaging of responses by utilizing 2-D normalized occupancy plots, cascaded linear-nonlinear (LN) system models and a method for describing variability in discrete random systems. We found that the expected firing rate of most movement-related motor neurons is related to the kinematic values by a linear transformation, with a significant nonlinear distortion in about 1/3 of the neurons. The measured variability of the neural responses is markedly non-Poisson in many neurons and is well captured by a "normalized-Gaussian" statistical model that is defined and introduced here. The statistical model is seamlessly integrated into a nearly-optimal recursive method for decoding movement from neural responses based on a Sequential Monte Carlo filter.

Algorithms↗

PubMed1994

Temporal pattern discrimination and speech recognition under electrical stimulation.

Discrimination of temporal patterns has been suggested as a relevant process in speech recognition by subjects with normal hearing [Sorkin, J. Acoust. Soc. Am. 87, 1695-1701 (1990)]. This paper investigates whether performance of Nucleus multichannel cochlear implant subjects on a temporal pattern discrimination task is an efficient and valid psychophysical measure of speech recognition ability. Stimuli consisted of temporal sequences defined by twelve 35-ms tones and eleven randomly generated temporal gaps separating the tones. A fixed-level same/different paradigm was used to measure the discriminability of these sequences as a function of their average correlation across a block of trials. On each trial, the "standard" sequence was generated randomly by drawing gap durations from a Gaussian distribution. The gaps of the comparison sequence were generated in a similar fashion with a specified average correlation with the gaps of the first sequence. Performance of implanted and normal hearing subjects decreased monotonically with increasing average sequence correlation. However, performance across implanted subjects ranged from that observed for acoustically stimulated subjects with audiometrically normal hearing to levels near chance. Comparing these data with measures of speech recognition in the same subjects, we have found that performance on standard speech recognition tests correlates with ability to discriminate among such random temporal patterns.

Adult↗

PubMedDate not supplied

[Two-parameter flow fluorescence analysis of human chromosomes. Quantitative processing].

A procedure for the quantitative analysis of the results of fluorimetric studies on human chromosomes in a flow is described. The procedure enables one to simultaneously follow two parameters by using two fluorochromes, one of which is selective to the GC-, and the other, to AT base pairs of DNA. Thus, it becomes possible to derive from the experimentally obtained distributions an information on the relative DNA content in individual chromosomes, differences in DNA content between homologous chromosomes, the relative number of chromosomes of each type in the sample tested, as well as on the percent ratio of AT and GC base pairs in a particular chromosome. In addition, the procedure enables one to assign, with a high degree of accuracy, the peaks on experimental distributions to the objects of analysis chromosomes. The analysis cutting off the extreme combines the method of components with the least square method, as applied to the peaks of the Gaussian shape. The first step involves the filtration of the starting experimental data from contaminating signals from both the parts of degraded chromosomes and stained cytoplasm fragments, and from chromosome aggregates. The procedure is realized as a package of programs for IBM-compatible PC/AT computers (486 and later versions), which permits one to perform a comprehensive analysis of two-parameter distributions of chromosome fluorescence signals to solve a number of problems, such as the identification of chromosome aberrations in cell lines tested, quantitative comparison of distinguishing features of homologous chromosomes and whole chromosome sets.

Chromosomes, Human↗

PubMed2004

Automatic registration of brain magnetic resonance images based on Talairach reference system.

PURPOSE: To demonstrate a robust registration method of brain magnetic resonance (MR) images based on the Talairach reference system with automatic determinations of the fiducial points. MATERIALS AND METHODS: Eight specified landmark points of the Talairach reference system are determined after successfully extracting the midsagittal plane from three-dimensional MR imaging (MRI) data. Projection information of the image intensity is used to determine the midline of the cerebrum in axial and coronal view images, which is a necessary step for extraction of the midsagittal plane. To find the landmarks of anterior commissure (AC) and posterior commissure (PC) in the midsagittal plane, we adopt two-step shape matching that properly finds locations of the corpus callosum (CC), and then AC and PC, respectively. The shape matching is performed on the edge-enhanced midsagittal plane image to minimize dependency on image intensity variation. Other landmark points of the Talairach reference system are determined by fitting the intensity curve of the cutview with the Gaussian model. RESULTS: The proposed method automatically determines seven landmark points, except the inferior point (IP), and the brain MR images can be successfully registered with the Talairach reference system. CONCLUSION: The suggested registration method can be applied to any MR images for functional studies. It can also be applied to patients unless their brains are highly deformed or have a highly deformed CC.

Brain↗

PubMed2004

The shape of neural dependence.

The product-moment correlation coefficient is often viewed as a natural measure of dependence. However, this equivalence applies only in the context of elliptical distributions, most commonly the multivariate gaussian, where linear correlation indeed sufficiently describes the underlying dependence structure. Should the true probability distributions deviate from those with elliptical contours, linear correlation may convey misleading information on the actual underlying dependencies. It is often the case that probability distributions other than the gaussian distribution are necessary to properly capture the stochastic nature of single neurons, which as a consequence greatly complicates the construction of a flexible model of covariance. We show how arbitrary probability densities can be coupled to allow greater flexibility in the construction of multivariate neural population models.

Action Potentials↗

PubMed2001

Generalized approach for accelerated maximum likelihood based image restoration applied to three-dimensional fluorescence microscopy.

For deconvolution applications in three-dimensional microscopy we derived and implemented a generic, accelerated maximum likelihood image restoration algorithm. A conjugate gradient iteration scheme was used considering either Gaussian or Poisson noise models. Poisson models are better suited to low intensity fluorescent image data; typically, they show smaller restoration errors and smoother results. For the regularization, we modified the standard Tikhonov method. However, the generic design of the algorithm allows for more regularization approaches. The Hessian matrix of the restoration functional was used to determine the step size. We compared restoration error and convergence behaviour between the classical line-search and the Hessian matrix method. Under typical working conditions, the restoration error did not increase over that of the line-search and the speed of convergence did not significantly decrease allowing for a twofold increase in processing speed. To determine the regularization parameter, we modified the generalized cross-validation method. Tests that were done on both simulated and experimental fluorescence wide-field data show reliable results.

Algorithms↗

PubMed2005

Derivation of a closed form analytical expression for fluorescence recovery after photo bleaching in the case of continuous bleaching during read out.

Measurements of very slow diffusive processes in membranes, like the diffusion of integral membrane proteins, by fluorescence recovery after photo bleaching (FRAP) are hampered by bleaching of the probe during the read out of the fluorescence recovery. In the limit of long observation time (very slow diffusion as in the case of large membrane proteins), this bleaching may cause errors to the recovery function and thus provides error-prone diffusion coefficients. In this work we present a new approach to a two-dimensional closed form analytical solution of the reaction-diffusion equation, based on the addition of a dissipative term to the conventional diffusion equation. The calculation was done assuming (i) a Gaussian laser beam profile for bleaching the spot and (ii) that the fluorescence intensity profile emerging from the spot can be approximated by a two-dimensional Gaussian. The detection scheme derived from the analytical solution allows for diffusion measurements without the constraint of observation bleaching. Recovery curves of experimental FRAP data obtained under non-negligible read-out bleaching for native membranes (rabbit endoplasmic reticulum) on a planar solid support showed excellent agreement with the analytical solution and allowed the calculation of the lipid diffusion coefficient.

Animals↗

PubMed2006

Thoracic aortic compliance as a determinant of rupture of abdominal aortic aneurysms.

The relative importance of collagen and elastin in formation, expansion, and rupture of abdominal aortic aneurysms (AAAs) has been investigated extensively. Aortic compliance, which is a relevant component of cardiac afterload, is also determined by the relative amount of media proteins in large arteries as well as by pathological arterial processes. The objective of this study was to determine if thoracic aortic compliance was different in patients with ruptured AAAs compared to those undergoing elective AAA repair. The study was carried out in 43 patients with infrarenal AAAs in the postoperative period. The first group (A) included 17 patients undergoing emergency ruptured AAA repair. The second group (B) included 26 patients operated on for an AAA who underwent elective repair. Patients were studied by a noninvasive Doppler echocardiography. Pulse wave velocity (PWV) was determined in the descending thoracic aorta. Results show that patients with electively repaired AAAs had an accelerated pulse wave transmission, typical of an atherosclerotic aorta with a Gaussian distribution (PWV 9.26 m/sec +/- 1.27). In contrast, patients with ruptured aneurysms presented in a distribution with three peaks. A striking increase in aortic compliance (41% of patients with PWV<6 m/sec in group A vs. 3% of group B) was observed in patients with ruptured AAAs.

Aneurysm, Ruptured↗

PubMed2005

Cross-cultural effect on the brain revisited: universal structures plus writing system variation.

Recognizing printed words requires the mapping of graphic forms, which vary with writing systems, to linguistic forms, which vary with languages. Using a newly developed meta-analytic approach, aggregated Gaussian-estimated sources (AGES; Chein et al. [2002]: Psychol Behav 77:635-639), we examined the neuroimaging results for word reading within and across writing systems and languages. To find commonalities, we compiled 25 studies in English and other Western European languages that use an alphabetic writing system, 9 studies of native Chinese reading, 5 studies of Japanese Kana (syllabic) reading, and 4 studies of Kanji (morpho-syllabic) reading. Using the AGES approach, we created meta-images within each writing system, isolated reliable foci of activation, and compared findings across writing systems and languages. The results suggest that these writing systems utilize a common network of regions in word processing. Writing systems engage largely the same systems in terms of gross cortical regions, but localization within those regions suggests differences across writing systems. In particular, the region known as the visual word form area (VWFA) shows strikingly consistent localization across tasks and across writing systems. This region in the left mid-fusiform gyrus is critical to word recognition across writing systems and languages.

Brain↗

PubMed2002

Interplay of light antenna and excitation "energy reservoir" effects in a bichromophoric system based on ruthenium-polypyridine and pyrene units linked by a long and flexible poly(ethylene glycol) chain.

Steady-state and time-resolved spectroscopic properties of bichromophoric species containing [Ru(bpy)(3)](2+) and pyrene (pyr) units linked together by flexible poly(ethylene glycol) chains of variable length, [Ru(bpy)(2)(bpy-pyr)](PF(6))(2) (1) and [Ru(bpy)(2)(bpy-O6-pyr)](PF(6))(2) (2), have been investigated in acetonitrile solvent. The complexes were designed with the aim of examining the intercomponent energy-transfer processes taking place after light absorption at the two chromophores and the influence of the distance separation between them; in the case of complex 2, the linking chain in the extended conformation is as long as 21 A. Direct excitation of the pyrene unit (lambda(exc) = 410 nm) results in singlet-to-singlet energy transfer (an antenna effect) to the Ru-based component, (1)pyr --> (1)MLCT, which we analyze in terms of the Förster mechanism taking place with unit efficiency. Analysis of the time-resolved pyrene fluorescence reveals that the actual center-to-center distance separation (d(cc)) between the photoactive centers changes according to a Gaussian distribution, with an average d(cc) = 13.6 A (distribution width, a = 2.8 A) and 12 A (a = 10.2 A), for 1 and 2, respectively; this is ascribed to folding of the poly(ethylene glycol) linking chain. In O(2)-free solvent at room temperature, after population of the (1)MLCT level (which takes place either because of direct excitation by using lambda(exc) > 355 nm or via the "antenna" effect) and subsequent intersystem crossing localized at the Ru center, (1)MLCT --> (3)MLCT, a triplet-triplet thermal equilibration is established which involves the physically separated centers, (3)MLCT <--> (3)pyr, with K(eq) = 11 (the energy gap between the two levels is 480 cm(-1), as determined from luminescence data obtained at 77 K). As a consequence of this equilibrium, the (3)MLCT luminescence lifetime becomes tau(Ru) approximately 9 micros both in 1 and 2, i.e., 1 order of magnitude longer than for the unsubstituted [Ru(bpy)(3)](2+) luminophore. In air-equilibrated solvent, diffusional quenching by O(2) effectively depletes the (3)pyr level and only the forward (3)MLCT --> (3)pyr energy transfer step is observed with k(en) = 4 x 10(8) and 2 x 10(8) s(-1) for 1 and 2, respectively. As briefly discussed, reasons for the high rate constants observed for the various triplet-triplet steps may be traced back to the folding properties of the linking chains.

Journal Article↗

PubMed1998

Registration of synthetic tomographic projection data sets using cross-correlation.

Tomographic registration, a method that makes possible accurate patient registration directly from projection data, consists of three processing steps: (i) manual coarse positioning, (ii) tomographic projection set acquisition, and (iii) computer mediated refined positioning. In the coarse positioning stage, the degree of patient alignment is comparable with that achieved with the standard radiotherapy set-up. However, the accuracy requirements are somewhat more relaxed in that meticulous alignment of the patient using external laser indicators is not necessary. Instead, tomographic projection sets are compared with planning CTs in order to achieve improved patient set-up. The projection sets are cross-correlated to obtain the best-fit translation and rotation offsets. The algorithm has been tested on synthetic data with the incorporation of varying amounts of Gaussian pseudo-random noise. These tests demonstrate the algorithm's stability and also confirm that alignment can be achieved with an accuracy of less than one projection pixel.

Algorithms↗

PubMed1998

A nonlinear spatially variant object-dependent system model for prediction of partial volume effects and scatter in PET.

Accurate quantitation of small lesions with positron emission tomography (PET) requires correction for the partial volume effect. Traditional methods that use Gaussian models of the PET system were found to be insufficient. A new approach that models the non-Gaussian object-dependent scatter was developed. The model consists of eight simple functions with a total of 24 parameters. Images of line and disk sources in circular and elliptical cylinders, and an anthropomorphic chest phantom were used to determine the parameter values. Empirical rules to determine these parameter values for various objects based on those for a reference object, a 21.5-cm circular cylinder, were also proposed. For seven spheroids and a 3.4-cm cylinder, pixel values predicted by the model were compared with the measured values. The model-to-measurement-ratio was 1.03+/-0.07 near the center of the spheroids and 0.99+/-0.03 near the center of the 3.4-cm cylinder. In comparison, the standard single Gaussian model had corresponding ratios of 1.27+/-0.09 and 1.24+/-0.03, respectively, and the corresponding ratios for a double Gaussian model were 1.13+/-0.09 and 1.05+/-0.01. Scatter fraction (28.5%) for a line source in the 21.5-cm cylinder was correctly estimated by our model. Because of scatter, we found that errors in the measurement of activity in spheroids with diameters from 0.6 to 3.4 cm were more significant than previously appreciated.

Algorithms↗

PubMed2005

Comparison and validation of tissue modelization and statistical classification methods in T1-weighted MR brain images.

This paper presents a validation study on statistical nonsupervised brain tissue classification techniques in magnetic resonance (MR) images. Several image models assuming different hypotheses regarding the intensity distribution model, the spatial model and the number of classes are assessed. The methods are tested on simulated data for which the classification ground truth is known. Different noise and intensity nonuniformities are added to simulate real imaging conditions. No enhancement of the image quality is considered either before or during the classification process. This way, the accuracy of the methods and their robustness against image artifacts are tested. Classification is also performed on real data where a quantitative validation compares the methods' results with an estimated ground truth from manual segmentations by experts. Validity of the various classification methods in the labeling of the image as well as in the tissue volume is estimated with different local and global measures. Results demonstrate that methods relying on both intensity and spatial information are more robust to noise and field inhomogeneities. We also demonstrate that partial volume is not perfectly modeled, even though methods that account for mixture classes outperform methods that only consider pure Gaussian classes. Finally, we show that simulated data results can also be extended to real data.

Adult↗