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At least 1,099 records · Page 61Linked to original sources

PubMed2003

Simultaneous measurements of particle backscattering and extinction coefficients and wind velocity by lidar with a Mach-Zehnder interferometer: principle of operation and performance assessment.

The development of remote-sensing instruments that can be used to monitor several parameters at the same time is important for the study of complex processes such as those that control climate and environment. In this paper the performance of a new concept of lidar receiver that allows for the direct measurement of aerosol and cloud optical properties simultaneously with wind velocity is investigated. This receiver uses a Mach-Zehnder interferometer. Two different configurations, either with four photometric output channels or with fringe imaging on a multichannel detector, are studied. Analytical expressions of the statistical errors are given under the assumption of Gaussian signal spectra. It is shown that similar accuracies can be achieved for both configurations. Performance modeling of the retrieval of semitransparent cloud optical scattering properties and wind velocity was done at different operation wavelengths for a Nd:YAG laser source. Results for such a lidar system onboard an aircraft flying at an altitude of 12 km show that for semitransparent clouds the best results were obtained at 355 nm, with relative standard deviations of 0.5% and 5% for the backscatter and extinction coefficients, respectively, together with a velocity accuracy of 0.2 ms(-1). The accuracy of optical properties retrieved for boundary layer aerosols are comparable, whereas the velocity accuracy is decreased to 1 ms(-1). Finally, an extrapolation to a large 355-nm spaceborne lidar shows accuracies in the range from 2.5% to 5% for the backscatter coefficient and from 10% to 15% for the extinction coefficient together with a vertical wind speed accuracy of better than 0.5 ms(-1) for semitransparent clouds and boundary layer, with a vertical resolution of 500 m and a 100 shot averaging.

Journal Article↗

PubMed2005

Effects of global electrosensory signals on motion processing in the midbrain of Eigenmannia.

Wave-type weakly electric fish such as Eigenmannia produce continuous sinusoidal electric fields. When conspecifics are in close proximity, interaction of these electric fields can produce deficits in electrosensory function. We examined a neural correlate of such jamming at the level of the midbrain. Previous results indicate that neurons in the dorsal layers of the torus semicircularis can (1) respond to jamming signals, (2) respond to moving electrosensory stimuli, and (3) receive convergent information from the four sensory maps of the electrosensory lateral line lobe (ELL). In this study we recorded the intracellular responses of both tuberous and ampullary neurons to moving objects. Robust Gaussian-shaped or sinusoidal responses with half-height durations between 55 ms and 581 ms were seen in both modalities. The addition of ongoing global signals with temporal-frequencies of 5 Hz attenuated the responses to the moving object by 5 dB or more. In contrast, the responses to the moving object were not attenuated by the addition of signals with temporal frequencies of 20 Hz or greater. This occurred in both the ampullary and tuberous systems, despite the fact that the ampullary afferents to the torus originate in a single ELL map whereas the tuberous afferents emerge from three maps.

Analysis of Variance↗

PubMed1986

Numerical considerations for interface reflections in medical ultrasound.

The original concept of the echo mechanism in diagnostic medical ultrasound--of step impedance discontinuities--is investigated in terms of the numerical implications of more refined models. The effect of a discontinuity in the attenuation coefficients is found to have as much significance as the impedance discontinuity at low reflection interfaces. An approximate analytical model is developed for spatially varying changes in the impedance. Numerical calculations are presented for reflections from delta-function and Gaussian-envelope RF pulses for two different models: an impedance gradient, and a connective tissue layer. The models used are well documented in acoustics textbooks, but the numerical results for typical tissue parameters show that a wide variety of interface structures may give rise to reflected amplitudes in the same range as that determined by the step impedance model. It is suggested that experimental investigation of interface structures may increase our understanding of the tissue: ultrasound interaction in diagnostic processes.

Humans↗

PubMed2006

Nonlinear spatial-temporal prediction based on optimal fusion.

The problem of spatial-temporal signal processing and modeling has been of great interest in recent years. A new spatial-temporal prediction method is presented in this paper. An optimal fusion scheme based on fourth-order statistic is first employed to combine the received signals at different spatial domains. The fused signal is then used to construct a spatial-temporal predictor by a support vector machine. It is shown theoretically that the proposed method has an improved performance even in non-Gaussian environments. To demonstrate the practicality of this spatial-temporal predictor, we apply it to model real-life radar sea scattered signals. Experimental results show that the proposed method can provide a more accurate model for sea clutter than the conventional methods.

Algorithms↗

PubMed2006

[Analysis of T cell receptor BV dominant usage and CDR3 sequences during acute exacerbation in patients with chronic hepatitis B].

OBJECTIVES: To understand the role cellular immunology plays in the pathogenesis of chronic hepatitis B (CHB) through analysis of T cell receptor (TCR) beta chain variable region gene (BV) family dominant usage and beta chain complementarity determining region3 (CDR3) sequences of peripheral blood mononuclear cells of the patients. METHODS: TCR BV families were amplified by inverse polymerase chain reaction (RT-PCR), and the dominant usage of BV families and CDR3 repertoire were analyzed by immunoscope technology for 8 CHB patients during their acute exacerbations and for 4 healthy blood donors who served as controls. The clonality of the T cells suspected by immunoscope was further confirmed by CDR3 sequencing. RESULTS: The TCR BV CDR3 repertoire of the 4 healthy blood donors showed a Gaussian distribution. In the 8 CHB patients, however, the clonal expansion of T cells showed different TCR BV families with each patient. The T cells of the clonal expansion shared different CDR3 sequences. CONCLUSION: The peripheral blood T cells of CHB patients during their acute exacerbation showed significantly a clonal expansion and their T cell clonal expansion may be stimulated by several HBV epitopes. These results indicate that cellular immunology is involved in the pathogenesis of the liver inflammation process of CHB.

Adult↗

PubMed1986

Receptive fields and the representation of visual information.

Receptive fields in the retina indicate the first measurements taken over the (discrete) visual image. Why are they circular surround with an excitatory/inhibitory structure? We hypothesize that this provides a representation of the visual information in a form suitable for transmission over the optic nerve, a rather limited channel, that can then be extended into a variety of representations at the cortex. These cortical representations span a range of sizes and functionally separate positive and negative contrast data, precisely as is required for further processing. Our scheme is both physiologically and psychophysically plausible. In particular, we derive an explicit formula for constructing large receptive fields from small ones, and introduce the notion of de-blurring to derive interpolation filters for hyperacuity. A mathematical requirement of our scheme is a form of separation between positive and negative contrast data, a nonlinearity that we predict will agree with observations. Furthermore, the mathematics that we utilize are more naturally applicable to physiological models based on Gaussian than are (Fourier) spatial frequencies.

Animals↗

PubMed1983

[Effect of temperature on the myocardial chrono- and inotropism of warm-blooded animals].

The influence of temperature on interval-force characteristics of rabbit ventricular myocardium was studied in conditions of random Gaussian sequences stimulation intervals. The interval-force relationships were described with Volterra series and cross-correlation function. Decrease of temperature causes inverse influence of Tn and Tn-1 intervals of the contraction force: at 35 degrees C coefficient of cross-correlation rho (0) of Tn-Pn equals 0.1 and coefficient of cross-correlation Tn-1-Pn rho (+1) equals -0.75; at 20 degrees C rho (0) = 0.8; rho (+1) = -0.15. Index Q10 for the process responsible for extrasystolic potentiation is about 3.0 and Q10 for restitutions process equals 1.5. At high temperature calcium seems to enter the myoplasm from extracellular pool, to be absorbed by sarcoplasmic reticulum and released in next cycle contraction. At low temperature, a part of calcium entering from that pool directly activates contractions at the same cycle.

Animals↗

PubMedDate not supplied

Quadriceps muscles activation in anterior knee pain during isokinetic exercise.

The aim of this paper is to study the femoral quadriceps muscles activation in patients suffering from anterior knee pain (AKP) during an isokinetic exercise. AKP is a common pathological condition, associated to an abnormal motion of the patella. It possibly depends on a muscular or structural imbalance. A lack of synergy in the quadriceps muscles results in a dynamic misalignment of the patella, which in turn produces pain. A quantitative analysis of the muscle activation strategy is important for an objective measurement of the knee functionality in that it helps to diagnose and monitor the rehabilitative treatment. Surface electromyography (EMG) from the three superficial muscles of the femoral quadriceps during a concentric isokinetic exercise has been analysed. A group of AKP patients has been compared with a control group of healthy subjects. Ensemble average of the EMG linear envelopes reveals significant modifications in Vastus Medialis activity in AKP patients. In order to quantify the synergy of the muscles, different parameters have been associated to EMG linear envelopes significant features. A decomposition in gaussian pulses and an asymmetry coefficient have been utilised. The results relative to these concise parameters highlight an appreciable delay and a modification in the activation of the Vastus Medialis in AKP patients.

Adolescent↗

PubMed2006

Looking for Thom's biomarkers with proteomics.

In recent years, large numbers of putative disease biomarkers have been identified. Combinations of protein biomarkers have been proposed to overcome the lack of single, magic-bullet identifiers of disease conditions. The number of biomarkers in a panel must be kept small to avoid the combinatorial explosion that requires very large, uneconomical sample cohorts for validation. Recent results on high sensitivity blood-based diagnostic proteomics (Godovac-Zimmermann, J et al., J. Proteome Res. 2006) suggest that the keys to identifying useful panels include judicious application of physiological knowledge to choose appropriate combinations of local, tissue/disease markers and global, systemic markers and to use very high sensitivity protein detection. Biomarkers that show non-Gaussian landscapes reminiscent of Rene Thom's multiple, stable-state landscapes seem to have the greatest predictive value for breast cancer (Godovac-Zimmermann, J. et al., J. Proteome Res. 2006).

Biomarkers↗

PubMed2005

Building robust wavelet estimators for multicomponent images using Stein's principle.

Multichannel imaging systems provide several observations of the same scene which are often corrupted by noise. In this paper, we are interested in multispectral image denoising in the wavelet domain. We adopt a multivariate statistical approach in order to exploit the correlations existing between the different spectral components. Our main contribution is the application of Stein's principle to build a new estimator for arbitrary multichannel images embedded in additive Gaussian noise. Simulation tests carried out on optical satellite images show that the proposed method outperforms conventional wavelet shrinkage techniques.

Algorithms↗

PubMed2006

Evidence for elongated receptive field structure for mechanisms subserving stereopsis.

To study the spatial extent and shape of the binocular disparity mechanisms subserving depth perception, we employ the spatial summation paradigm of contrast threshold for front/back depth discrimination at a fixed binocular disparity. The stimuli were Gabor patches with disparity set at either 4 or 8 arcmin and spatial frequency set at an optimal value of 4 cy/deg. Contrast threshold was measured as a function of length and width of the Gabor patches to determine the aspect ratio of greatest efficiency. The space constant of the Gaussian envelope varied between 0.0375 degrees and 0.9 degrees in either vertical or horizontal directions, or both simultaneously. For vertical elongation of the Gabor patches, discrimination sensitivity improved by 4-6 dB for a doubling of the length of the Gabor patches, then reduced more slowly as the length further increased. However, extending the Gabor patches horizontally across cycles produced little or no sensitivity improvement. Instead, discrimination performance collapsed in a fashion that is incompatible with many models of disparity processing. The results imply that the main mechanisms subserving stereoscopic depth discrimination are vertically elongated for vertical-bar Gabors and encounter special difficulties integrating horizontal disparity information. Disparity discrimination sensitivity for very small targets was also much greater than predicted by the single-mechanism fit, implying the presence of a second, independent mechanism with a very small summation field, which may underlie the fine stereoscopic processing system.

Contrast Sensitivity↗

PubMed1998

Independent component analysis of natural image sequences yields spatio-temporal filters similar to simple cells in primary visual cortex.

Simple cells in the primary visual cortex process incoming visual information with receptive fields localized in space and time, bandpass in spatial and temporal frequency, tuned in orientation, and commonly selective for the direction of movement. It is shown that performing independent component analysis (ICA) on video sequences of natural scenes produces results with qualitatively similar spatio-temporal properties. Whereas the independent components of video resemble moving edges or bars, the independent component filters, i.e. the analogues of receptive fields, resemble moving sinusoids windowed by steady Gaussian envelopes. Contrary to earlier ICA results on static images, which gave only filters at the finest possible spatial scale, the spatio-temporal analysis yields filters at a range of spatial and temporal scales. Filters centred at low spatial frequencies are generally tuned to faster movement than those at high spatial frequencies.

Models, Biological↗

PubMed2003

Enhancing interaural-delay-based extents of laterality at high frequencies by using "transposed stimuli".

An acoustic pointing task was used to determine whether interaural temporal disparities (ITDs) conveyed by high-frequency "transposed" stimuli would produce larger extents of laterality than ITDs conveyed by bands of high-frequency Gaussian noise. The envelopes of transposed stimuli are designed to provide high-frequency channels with information similar to that conveyed by the waveforms of low-frequency stimuli. Lateralization was measured for low-frequency Gaussian noises, the same noises transposed to 4 kHz, and high-frequency Gaussian bands of noise centered at 4 kHz. Extents of laterality obtained with the transposed stimuli were greater than those obtained with bands of Gaussian noise centered at 4 kHz and, in some cases, were equivalent to those obtained with low-frequency stimuli. In a second experiment, the general effects on lateral position produced by imposed combinations of bandwidth, ITD, and interaural phase disparities (IPDs) on low-frequency stimuli remained when those stimuli were transposed to 4 kHz. Overall, the data were fairly well accounted for by a model that computes the cross-correlation subsequent to known stages of peripheral auditory processing augmented by low-pass filtering of the envelopes within the high-frequency channels of each ear.

Adult↗

PubMed2003

Pharmacokinetic strategies in deciphering atypical drug absorption profiles.

Drug absorption is a very complex process that manifests itself through potential interaction with a host of physicochemical and physiological variables. Some factors that may affect the absorption processes include presystemic metabolism/efflux, the "absorption window" along the gastrointestinal tract, disease states, demographics (gender, age, ethnicity), and biopharmaceutical classification of solid dosage forms. Despite the complexity of the absorption processes, the analysis of the absorption kinetic data is mostly empirical, and the assumption of first-order absorption is axiomatic. Nevertheless, we often encounter irregular drug absorption profiles (such as double-peak, absorption window-type absorption profiles, etc.) that cannot be satisfactorily described by a simple first-order absorption process. The selection of an inappropriate absorption model would result in the misspecification of the pharmacokinetic model and subsequent erroneous prediction of the dosing regimen. This article presents several pharmacokinetic strategies in analyzing typical and atypical absorption profiles. The atypical absorption profiles discussed in this article include parallel first-order absorption, mixed zero-order and first-order absorption, Weibull-type absorption, absorption window with or without Michaelis-Menton absorption, time-dependent absorption, and inverse Gaussian density absorption. In any event, intravenous drug concentration-time data are generally needed to avoid the ambiguousness in the absorption analyses.

Administration, Oral↗

PubMed1987

Inverse diffusion methods for data peak separation.

Previous methods for separation of overlapping data peaks include geometrical assessment and Fourier deconvolution. On the basis of inverse diffusion theory, we present new separation methods suitable for convenient programming and rapid calculation of Gaussian area contributions. Both continuum and discrete inverse diffusion models are described. Example computations are given for biological data: density gradient centrifugation, isoelectric focusing electrophoresis, and high-pressure liquid chromatography.

Centrifugation, Density Gradient↗

PubMed1996

A computer-based statistical pattern recognition for Doppler spectral waveforms of intracranial blood flow.

A computer-based statistical pattern recognition system has been developed for the analysis of transcranial Doppler (TCD) spectral waveforms of the intracranial middle cerebral artery with varying degrees of increased intracranial pressure. This system extracts multidimensional features from TCD waveforms and performs a cluster analysis of those features. The system can automatically recognize the pattern of spectral waveform and classify it as a normal, abnormal, or borderline subclass of TCD spectral waveform. An optimum decision function was generated based on the Bayes Gaussian classifier. The accuracy of the Bayes Gaussian model the spectral waveforms reaches 100% by estimating posterior probability and using the resubstituting method of estimating misclassification in the training TCD data.

Artificial Intelligence↗

PubMed1983

Changes of alanine-sodium co-transport during maturation of Xenopus laevis oocytes.

Maturation of full grown Xenopus oocytes is associated with a decrease of their capacity to transport amino acids and nucleosides. Nevertheless, the transport continues at a reduced rate until the time of metaphase arrest when the oocytes are ready for shedding. We have found that the L-alanine transport in unfertilized, shed oocytes is a saturable process which is Na+-dependent. The stoichiometrical ratio between sodium and alanine is 2:1. Comparison with the alanine uptake in full grown oocytes shows that during maturation Vmax is strongly reduced while Km and the stoichiometrical ratio of sodium and alanine is unaffected. Lowering the osmolarity of the shedding medium reduces the rate of uptake of alanine below the rates seen after shedding into isotonic media. The transport rates amongst the individual oocytes differ. They do not fit to a Gaussian distribution. In addition, in individual oocytes, there exists no correlation between the variations of the transport rates for amino acids and nucleosides, which are also taken up by the unfertilized, shed oocytes.

Alanine↗

PubMed1999

Fractal behavior in quantum statistical physics.

The properties of an ideal gas of spinless particles are investigated by using the path integral formalism. It is shown that the quantum paths exhibit a fractal character which remains unchanged in the relativistic domain provided the creation of new particles is avoided, and the Brownian motion remains the stochastic process associated with the quantum paths. These results are obtained by using a special representation of the Klein-Gordon wave equation. On the quantum paths the relation between velocity and momentum is not the usual one. The mean square value of the velocity depends on the time needed to define the velocity and its value shows the interplay between pure quantum effects and thermodynamics. The fractal character is also investigated starting from wave equations by analyzing the evolution of a Gaussian wave packet via the Hausdorff dimension. Both approaches give the same fractal character in the same limit. It is shown that the time that appears in the path integral behaves like an ordinary time, and the key quantity is the time interval needed for the thermostat to give to the particles a thermal action equal to the quantum of action. Thus, the partition function calculated via the path integral formalism also describes the dynamics of the system for short time intervals. For low temperatures, it is shown that a time-energy uncertainty relation is verified at the end of the calculations. The energy involved in this relation has not a thermodynamic meaning but results from the fact that the particles do not follow the equations of motion along the paths. The results suggest that the density matrix obtained by quantification of the classical canonical distribution function via the path integral formalism should not be totally identical to that obtained via the usual route.

Journal Article↗