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PubMed2005

Aggregation of methylene blue in frozen aqueous solutions studied by absorption spectroscopy.

The paper presents a qualitative as well as quantitative spectroscopic study of methylene blue (MB) aggregation that occurs upon freezing the aqueous solutions over a wide concentration range. The Gaussian curve analysis and the multivariate curve resolution-alternating least squares method were used to determine the number and concentration of chemical species responsible for the overlaying absorption visible spectra measured. The results show the extent of aggregation for the concentrations above 10(-7) mol L(-1), being dependent on the freezing rate and the initial concentration. While the local concentration of MB at the grain boundaries of polycrystalline ice increased by approximately 3 orders of magnitude upon fast freezing at 77 K compared to the liquid phase, the concentration raised at least by 6 orders of magnitude upon slow freezing at 243 K. Since enhancement of the local concentration of solutes plays an important role in (photo)chemical transformations in solid aqueous media, this work helps to understand how the initial conditions control the course of the process. The results are relevant in other interdisciplinary fields, such as environmental chemistry, cosmochemistry, or geochemistry.

Journal Article↗

PubMed1994

Gap detection and the precedence effect in young and old adults.

Thresholds for detecting a gap between two Gaussian-enveloped (standard deviation = 0.5 ms), 2-kHz tones were determined in young and old listeners. The gap-detection thresholds of old adults were more variable and about twice as large as those obtained from young adults. Moreover, gap-detection thresholds were not correlated with audiometric thresholds in either group. Estimates of the width of the temporal window of young subjects, based on the detection of a gap between two tone pips, were smaller than those typically obtained when a relatively long duration pure tone is interrupted [Moore et al., J. Acoust. Soc. Am. 85, 1266-1275 (1989)]. Because the amount of time it takes to recover from an adapting stimulus is likely to affect gap detection thresholds [Glasberg et al., J. Acoust. Soc. Am. 81, 1546-1556 (1987)], smaller estimates of temporal window size would be expected in this paradigm if the amount of adaptation produced by the first tone pip was negligible. The larger gap-detection thresholds of old subjects indicate that they may have larger temporal windows than young subjects. The lack of correlation between audiometric and gap-detection thresholds indicates that this loss of temporal acuity is not related to the degree of sensorineural hearing loss. In a second experiment on the precedence effect using the same subjects, a Gaussian-enveloped tone was presented over earphones to the left ear followed by the same tone pip presented to the right ear. To more realistically approximate a sound field situation, the tone pip presented to each ear was followed 0.6 ms later by an attenuated version presented to the contralateral ear. The delay between the left- and right-ear tone-pips was varied and the transition point between hearing a single tone on the left, and hearing two such sounds in close succession (one coming from the left and the other from the right) was determined. The transition point in this experiment did not differ between young and old subjects nor were these transition points correlated with gap-detection thresholds. These results indicate that monaural temporal acuity and binaural echo suppression may be based on different processes.

Acoustic Stimulation↗

PubMed1995

Automatic analysis of cortical signals recorded with voltage-sensitive dyes using a forward-backward non-linear filtering technique and deconvolution.

A method for automatically analyzing cortical signals recorded with voltage-sensitive dyes and a photodiode array is described. First, a forward-backward non-linear filtering technique is used to eliminate the background noise and preserve the fast transients of the signals. Then the filtered signals are deconvoluted from their maximal values by using a gaussian function. The different components of the signals can be identified and characterized by their respective latencies, amplitudes, plateau durations, and slopes. These parameters can be used for subsequent statistical analysis. This automated method is much faster than a manual analysis because of the large number of responses that are optically recorded. Moreover, it can be easily applied to different experimental protocols and to other signals such as field potentials.

Animals↗

PubMed2002

Adaptive filtering of evoked potentials with radial-basis-function neural network prefilter.

Evoked potentials (EPs) are time-varying signals typically buried in relatively large background noise. To extract the EP more effectively from noise, we had previously developed an approach using an adaptive signal enhancer (ASE) (Chen et al., 1995). ASE requires a proper reference input signal for its optimal performance. Ensemble- and moving window-averages were formerly used with good results. In this paper, we present a new method to provide even more effective reference inputs for the ASE. Specifically, a Gaussian radial basis function neural network (RBFNN) was used to preprocess raw EP signals before serving as the reference input. Since the RBFNN has built-in nonlinear activation functions that enable it to closely fit any function mapping, the output of RBFNN can effectively track the signal variations of EP. Results confirmed the superior performance of ASE with RBFNN over the previous method.

Algorithms↗

PubMed2006

Hidden Markov models for wavelet-based blind source separation.

In this paper, we consider the problem of blind source separation in the wavelet domain. We propose a Bayesian estimation framework for the problem where different models of the wavelet coefficients are considered: the independent Gaussian mixture model, the hidden Markov tree model, and the contextual hidden Markov field model. For each of the three models, we give expressions of the posterior laws and propose appropriate Markov chain Monte Carlo algorithms in order to perform unsupervised joint blind separation of the sources and estimation of the mixing matrix and hyper parameters of the problem. Indeed, in order to achieve an efficient joint separation and denoising procedures in the case of high noise level in the data, a slight modification of the exposed models is presented: the Bernoulli-Gaussian mixture model, which is equivalent to a hard thresholding rule in denoising problems. A number of simulations are presented in order to highlight the performances of the aforementioned approach: 1) in both high and low signal-to-noise ratios and 2) comparing the results with respect to the choice of the wavelet basis decomposition.

Algorithms↗

PubMed2004

Modeling of esophageal bolus flow by functional data analysis of scintigrams.

Peristaltic forces cause a topographic, time-dependent distribution of bolus mass during its esophageal transport. A two-dimensional spatial-temporal pattern (profile) of local transit times is constructed by computer-based double compression of scintigraphic images sampled from whole swallows. Reconstruction by Gaussian bands and modeling this pattern discloses transient ellipsoidal bolus structures. The structures studied in 10 healthy volunteers present highly reproducible quantitative parameters for marking a region-specificity of transit times, which is related to the known region-selectivity of esophageal functions. Correlation of bolus flow with the dynamics of peristalsis is essential for understanding the complex mechanisms of esophageal transport as well as for diagnostic discrimination of disturbances of bolus flow.

Adult↗

PubMed2006

Factors influencing flip angle mapping in MRI: RF pulse shape, slice-select gradients, off-resonance excitation, and B0 inhomogeneities.

To understand the various effects that influence actual flip angles, and correct for these effects, it is important to precisely quantify the MRI parameters (such as T1, T2, and perfusion). In this paper actual flip angle maps are calculated using a conventional gradient-echo (GRE) sequence with different radiofrequency (RF) pulse shapes (Gaussian, sinc, and truncated-sinc), slice-selection gradients, off-resonance excitations, and B0 field inhomogeneities. The experimental results demonstrate that RF pulse shapes significantly affect the flip angle distribution and calibration factors. Off-resonance RF excitations, B0 nonuniformities, and slice-selection gradients can lead to degradations in the signal intensities of the images used to map the flip angle, and potentially introduce a bias and increased variance in the measured flip angles.

Algorithms↗

PubMed2006

Examination of cancer in mouse models using high-frequency quantitative ultrasound.

Two mouse models of mammary cancer (a carcinoma and sarcoma) were examined using quantitative ultrasound (QUS). Scatterer property estimates, i.e., the average scatterer diameter (ASD) and average acoustic concentration (AAC), were estimated from regions-of-interest (ROIs) inside the tumors. Initially, the spherical Gaussian model was used over an analysis bandwidth of 10 to 25 MHz to obtain ASD and AAC estimates. ASD estimates were 31.7 +/- 9.36 microm and 31.0 +/- 7.20 microm for the carcinomas and sarcomas, respectively. AAC estimates were 6.77 +/- 8.75 dB[mm(-3)] and 9.87 +/- 9.24 dB[mm(-3)], respectively. The initial ASD and AAC estimates did not yield statistically significant differences between the two kinds of tumors (p = 0.83, 0.86 for the ASD and AAC estimates, respectively). However, optical photomicrographs revealed distinct morphologic differences between the tumors. F-tests on the average power spectra from the tumors revealed statistically significant differences between the spectra over the range of 16 to 25 MHz. ASD and AAC estimates using the spherical Gaussian model were then obtained over the new analysis bandwidth of 16 to 25 MHz. The new ASD estimates were 42.1 +/- 4.01 microm and 32.1 +/- 3.81 microm for the carcinomas and sarcomas, respectively. The new AAC estimates were 16.4 +/- 17.1 dB[mm(-3)] and 36.4 +/- 11.9 dB[mm(-3)], respectively. Statistically significant differences were observed for both the ASD and AAC estimates when using the new analysis bandwidth. Structural differences between the tumors were revealed by both QUS and optical photomicrographs.

Animals↗

PubMed1992

Sodium-23 and potassium-39 nuclear magnetic resonance relaxation in eye lens. Examples of quadrupole ion magnetic relaxation in a crowded protein environment.

Single and multiple quantum nuclear magnetic resonance (NMR) spectroscopic techniques were used to investigate the motional dynamics of sodium and potassium ions in concentrated protein solution, represented in this study by cortical and nuclear bovine lens tissue homogenates. Both ions displayed homogeneous biexponential magnetic relaxation behavior. Furthermore, the NMR relaxation behavior of these ions in lens homogenates was consistent either with a model that assumed the occurrence of two predominant ionic populations, "free" and "bound," in fast exchange with each other or with a model that assumed an asymmetric Gaussian distribution of correlation times. Regardless of the model employed, both ions were found to occur in a predominantly "free" or "unbound" rapidly reorienting state. The fraction of "bound" 23Na+, assuming a discrete two-site model, was approximately 0.006 and 0.017 for cortical and nuclear homogenates, respectively. Corresponding values for 39K+ were 0.003 and 0.007, respectively. Estimated values for the fraction of "bound" 23Na+ or 39K+ obtained from the distribution model (tau C greater than omega L-1) were less than or equal to 0.05 for all cases examined. The correlation times of the "bound" ions, derived using either a two-site or distribution model, yielded values that were at least one order of magnitude smaller than the reorientational motion of the constituent lens proteins. This observation implies that the apparent correlation time for ion binding is dominated by processes other than protein reorientational motion, most likely fast exchange between "free" and "bound" environments. The results of NMR visibility studies were consistent with the above findings, in agreement with other studies performed by non-NMR methods. These studies, in combination with those presented in the literature, suggest that the most likely role for sodium and potassium ions in the lens appears to be the regulation of cell volume by affecting the intralenticular water chemical potential.

Animals↗

PubMed1996

Single nuclear pores visualized by confocal microscopy and image processing.

How nuclear pore complexes, mediating the transport of nucleic acids, proteins, and metabolites between cell nucleus and cytoplasm, are arranged in the nuclear envelope is essentially unknown. Here we describe a method combining high-resolution confocal imaging with image processing and pattern recognition to visualize single nuclear pore complexes (120 nm diameter), determine their relative positions with nanometer accuracy, and analyze their distribution in situ. The method was tested by means of a model system in which the very same sample areas could be imaged by confocal and electron microscopy. It was thus found that single fluorescent beads of 105 nm nominal diameter could be localized with a lateral accuracy of <20 nm and an axial accuracy of approximately 20 nm. The method was applied to digitonin-permeabilized 3T3 cells, whose nuclear pore complexes were fluorescently labeled with the anti-nucleoporin antibody mAb414. Stacks of optical sections were generated by confocal imaging at high resolution. Herein the nuclear pore complexes appeared as bright diffraction-limited spots whose centers were localized by fitting them by three-dimensional gaussians. The nearest-neighbor distribution function and the pair correlation function were calculated and found to agree well with those of randomly distributed hard cylinders of 138 +/- 17 nm diameter, but not with those of randomly distributed points or nonrandomly distributed cylinders. This was supported by a cluster analysis. Implications for the direct observation of the transport of single particles and molecules through individual nuclear pore complexes are discussed.

3T3 Cells↗

PubMed1994

A performance study of 3D reconstruction algorithms for positron emission tomography.

This paper investigates the statistical and systematic accuracy of five three-dimensional reconstruction algorithms for multi-ring PET scanners operated without septa: the reprojection method, the direct Fourier reconstruction, the FAVOR algorithm, and the single-slice and multi-slice rebinning algorithms. Simulated data of a uniform cylinder, of Gaussian sources, and of spherical sources are used to compare respectively the noise properties, the modulation transfer function, and the recovery coefficients of the algorithms. Brain scans reconstructed with the different algorithms are compared by calculating the linear regression of the mean values within regions of interest. The most significant observations are a slight loss of transaxial resolution with the reprojection algorithm in the external slices of the scanner, and increased noise in the images reconstructed using multi-slice rebinning.

Algorithms↗

PubMed1999

Psychometric functions for gap detection in a yes-no procedure.

To examine models of temporal resolution and to investigate the decision processes underlying the detection of a brief pause in a bandpass noise, psychometric functions for gap detection were measured at octave frequencies from 0.25 to 8 kHz. Three normal listeners were tested using a constant-stimulus procedure with a cued Yes-No paradigm. The Minimum Detectable Gap (MDG) estimated from the midpoint of the psychometric functions decreased systematically with increasing frequency. The slopes of the psychometric functions generally increased as the test frequency increased up to 2 kHz, but remained constant at the higher frequencies. Two models were investigated: an energy-detector model and a loudness-detector model. Both consisted of auditory filtering, a nonlinearity, and short-term integration. In the energy-detector model, the nonlinearity was a square law. In the loudness-detector model, it was a compressive power law. Using the usual Gaussian approximations, the energy-detector model fails at low frequencies because the probability distributions of short-term energy differ from Gaussian distributions. The probability distributions of short-term loudness closely follow Gaussian distributions. The loudness-detector model predicts the frequency dependence of the MDG quite accurately, except at 0.25 kHz. It also predicts psychometric functions that resemble the data at low frequencies, but the predicted slopes increase much less with frequency than the measured slopes. This result may indicate that the onset response to the trailing marker of the gap provides an important cue for detection of gaps with durations exceeding the MDG.

Adult↗

PubMed1992

Temporal factors in the discrimination of coherent motion.

When observers view the relative movements of a pair of bars defined by the difference of spatial Gaussian functions (DOGs), they can accurately discriminate coherent movements over a range of temporal frequencies and temporal asynchronies. Of particular interest is the fact that performance accuracy is maintained even when the two bars differ in spatial-frequency content and contrast. On each trial, observers viewed two brief presentation intervals in which a pair of vertically oriented DOGs moved randomly back and forth within a restricted range. During one observation interval, both elements moved in the same direction and by the same magnitude (correlated), and in the other interval, the movements were independent (uncorrelated). Temporal asynchronies were introduced by delaying the displacement of the right bar relative to that of the left bar in each interval. Observers were able to discriminate correlated versus uncorrelated movements up to a 45-60-msec temporal delay between the two elements' relative displacements. If motion processing is accomplished by mechanisms operating over multiple spatial and temporal scales, the visual system's tolerance of temporal delays among correlated signals may facilitate their space-time integration, thereby capitalizing on the perceptual utility of coherent-motion information for image segmentation and interpolating surface structure from the movements of spatially separated features.

Adult↗

PubMed2001

Reconstruction of individual absorbed doses by tooth enamel on the base of non-linear simulation of their EPR-spectra.

Model of separate spectral lines composing EPR-spectra of tooth enamel can be described with fine accuracy by integral of convolution of three distribution functions: Lorenzian, Gaussian and anisotropy. Simulation of spectra was done by the method of optimisation of non-linear parameters combined with the Gauss method of exclusion for linear parameters to obtain the minimum of the sum of the squares of the differences between the experimental EPR-spectrum and its model. The final result was the deconvolution of complex EPR-spectrum into its components (the background and radiation-induced signals) with the following reconstruction of the individual absorbed dose.

Anisotropy↗

PubMed2001

Time-frequency analysis of temporomandibular joint (TMJ) clicking sounds using radially Gaussian Kernels.

Temporomandibular joint (TMJ) sounds and motion were recorded during two clinically-derived movements--simple jaw opening and jaw protrusion followed by opening--from ten patients. A new time-frequency method--radially Gaussian kernel distribution--was applied to classify the TMJ clicking sounds into six groups, type I to type VI, based on the time-frequency patterns of energy distribution. The number of clicks and percentage of each type were examined. Relations between the two movements were examined by the prevalence of each type. A detailed classification of TMJ clicking sounds is provided by time-frequency patterns and may provide a better understanding of temporomandibular disorders.

Adult↗

PubMed1991

The distribution of the intervals between neural impulses in the maintained discharges of retinal ganglion cells.

Simulated neural impulse trains were generated by a digital realization of the integrate-and-fire model. The variability in these impulse trains had as its origin a random noise of specified distribution. Three different distributions were used: the normal (Gaussian) distribution (no skew, normokurtic), a first-order gamma distribution (positive skew, leptokurtic), and a uniform distribution (no skew, platykurtic). Despite these differences in the distribution of the variability, the distributions of the intervals between impulses were nearly indistinguishable. These inter-impulse distributions were better fit with a hyperbolic gamma distribution than a hyperbolic normal distribution, although one might expect a better approximation for normally distributed inverse intervals. Consideration of why the inter-impulse distribution is independent of the distribution of the causative noise suggests two putative interval distributions that do not depend on the assumed noise distribution: the log normal distribution, which is predicated on the assumption that long intervals occur with the joint probability of small input values, and the random walk equation, which is the diffusion equation applied to a random walk model of the impulse generating process. Either of these equations provides a more satisfactory fit to the simulated impulse trains than the hyperbolic normal or hyperbolic gamma distributions. These equations also provide better fits to impulse trains derived from the maintained discharges of ganglion cells in the retinae of cats or goldfish. It is noted that both equations are free from the constraint that the coefficient of variation (CV) have a maximum of unity.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗

PubMed2001

Velocity estimation of spots in three-dimensional confocal image sequences of living cells.

BACKGROUND: The analysis of three-dimensional (3D) motion is becoming increasingly important in life cell imaging. A simple description of sometimes complex patterns of movement in living cells gives insight in the underlying mechanisms governing these movements. METHODS: We evaluate a velocity estimation method based on intensity derivatives in spatial and temporal domain from 3D confocal images of living cells. Cells of the sample contain intense spots throughout the cell nucleus. In simulations, we model these spots as Gaussian intensity profiles which are constant in intensity and shape. To quantify the quality of the estimated velocity, we introduce a reliability measure. RESULTS: For constant linear velocity, the velocity estimation is unbiased. For accelerated motion paths or when a neighboring spot disturbs the intensity profile, the method results are biased. The influence of the point-spread function on the velocity estimation can be compensated for by introducing anisotropic derivative kernels. The insight gained in the simulations is confirmed by the results of the method applied on an image sequence of a living cell with fluorescently labeled chromatin. CONCLUSIONS: With the velocity estimation method, a tool for estimating 3D velocity fields is described which is successfully applied to a living cell sequence. With the estimated velocity fields, motion patterns can be observed, which are a useful starting point for the analysis of dynamic processes in living cells.

Animals↗

PubMed1989

Morphometric analysis of hyperthermia-induced nuclear and nucleolar size modifications in a human melanoma cell line.

Although no prominent morphologic alterations were observed by light microscopy in a human melanoma cell line (M-14) exposed at 42 degrees C for 4 hr, the computerized image analysis demonstrated a significant enlargement of both nuclei and nucleoli. The mean nuclear and nucleolar areas were found to be 32% and 94% increased, respectively, in comparison with normal controls. The gaussian distribution of absolute frequency of the measures demonstrated that this nuclear and nucleolar enlargement was distributed among the whole cell population, taking place in clonogenic cells and in "sterilized" cells, ultimately doomed to die. This finding, not previously described in the context of the heat shock response, may be regarded as morphological evidence of an increase in the protein content of the nuclei of heat-treated cells, as determined by biochemical methods.

Cell Nucleolus↗