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PubMed1998

Application of the extremum stack to neurological MRI.

The extremum stack, as proposed by Koenderink, is a multiresolution image description and segmentation scheme which examines intensity extrema (minima and maxima) as they move and merge through a series of progressively isotropically diffused images known as scale space. Such a data-driven approach is attractive because it is claimed to be a generally applicable and natural method of image segmentation. The performance of the extremum stack is evaluated here using the case of neurological magnetic resonance imaging data as a specific example, and means of improving its performance proposed. It is confirmed experimentally that the extremum stack has the desirable property of being shift-, scale-, and rotation-invariant, and produces natural results for many compact regions of anatomy. It handles elongated objects poorly, however, and subsections of regions may merge prematurely before each region is represented as a single node. It is shown that this premature merging can often be avoided by the application of either a variable conductance-diffusing preprocessing step, or more effectively, the use of an adaptive variable conductance diffusion method within the extremum stack itself in place of the isotropic Gaussian diffusion proposed by Koenderink.

Brain↗

PubMed1999

Development of receptoral responses in pigmented and albino guinea-pigs (Cavia porcellus).

We describe the postnatal development of the electroretinogram (ERG) receptoral response in the guinea pig. In addition, the time course and nature of maturation was compared between albino and pigmented strains to consider the role that melanogenesis might have in this process. Electroretinograms were collected on groups of albino and pigmented animals from postnatal day (PD) PD1 to PD60. A-wave amplitudes and implicit times were extracted from filtered data (0-75 Hz). Receptoral components were modelled using the delayed gaussian model of Hood and Birch [1] fitted as an ensemble to the raw data. Guinea pigs show saturated amplitudes (RmP3) that are 50% of adult values at birth, these mature by PD12. Receptoral delay (t(d)) also undergoes some postnatal maturation, while phototransduction gain (log S) is adult-like at birth. Albino animals had significantly (p<0.05) larger RmP3 and log S across all ages. Guinea pigs have significant postnatal development in their receptoral response. Maturation of RmP3 implies a postnatal increase in rod outer segment length. Whereas the adult values of log S implies a mature phototransduction process at birth. We argue that the likely cause for the larger log S of albino eyes is compatible with theories of increased levels of internal light. Whereas the larger RmP3, even after allowing for increased light effectiveness, may reflect a lower ocular resistance in albino eyes due to their lower levels of melanin. Furthermore, decreased RmP3 and log S with age is observed in the pigmented group only and is consistent with increased ocular resistance due to melanin development in this strain.

Albinism, Oculocutaneous↗

PubMed1990

A stochastic model for inter-keypress times in a typing task.

A mathematical model for the inter-keypress times (IKT) in a typing task is proposed. The model, which includes a diffusion process terminated by a single response threshold, was evaluated using data obtained from typists. The differences in performance for successive cross-hand and within-hand keypresses were examined using IKT distributions and hazard functions, and it was shown that the empirical hazard functions could be fit by the theoretical hazard function derived from the convolution of normal and inverse Gaussian random variables. Some possible applications of the model for the evaluation of fatigue and strategic effects in typing are suggested.

Adult↗

PubMed2005

Transport and recombination in organic light-emitting diodes studied by electrically detected magnetic resonance.

We have used electrically detected magnetic resonance (EDMR) to study a series of multilayer organic devices based on aluminum (III) 8-hydroxyquinoline (Alq3). These devices were designed to identify the microscopic origin of different spin-dependent processes, i.e. hopping and exciton formation. The EDMR signal in organic light-emitting diodes (OLEDs) based on Alq3 is only observed when the device is electroluminescent and is assigned to spin-dependent exciton formation. It can be decomposed in at least two Gaussians: one with peak-to-peak line (deltaH(PP)) of 1.6 mT and another with deltaH(PP) of 2.0 to 3.4 mT, depending on bias and temperature. The g-factors of the two components are barely distinguishable and close to 2.003. The broad line is attributed to the resonance in Alq3 anions, while the other line is attributed to cationic states. These attributions are supported by line shape and its electrical-field dependence of unipolar Alq3-based diodes, where hopping process related to dication and dianion formation is observed. In these unipolar devices, it is shown that the signal coming from spin-dependent hopping occurs close to organic semiconductor/metal interfaces. The sign of the magnetic-resonance-induced conductivity change is dominated by charge injection rather than charge mobility. Our results indicate that the probability of singlet exciton formation in our OLEDs is smaller than 25%.

Journal Article↗

PubMed2006

Selection of procedures in mental subtraction.

Adults solved simple subtraction problems (e.g., 16 - 9). Half of the 32 participants provided immediate self-reports of their solution processes on each problem. Performance was analyzed using both traditional descriptive statistics (i.e., means, standard deviations, and percentage of errors) and with statistics derived from fitting the ex-Gaussian distributions to latencies (i.e., mu and tau). The results support the view that ex-Gaussian analyses can be useful in exploring patterns of procedure selection that relate both to characteristics of the stimuli (e.g., problem size) and to characteristics of the participants (e.g., arithmetic skill). More generally, the results provide further evidence that adults use a variety of procedures to solve simple subtraction problems and that these choices are related to patterns of performance on more complex problems that require calculation.

Choice Behavior↗

PubMed2004

Resonant activation in the presence of nonequilibrated baths.

We study the generic problem of the escape of a classical particle over a fluctuating barrier under the influence of non-Gaussian noise mimicking the effects of nonequilibrated bath. The model system is described by a Langevin equation with two independent noise sources, one of which stands for the dichotomous process and the other describes external driving by alpha-stable noise. Our attention focuses on the effect of the structure of stable noises on the mean escape time and on the phenomenon of resonant activation. Possible physical interpretation of the occurrence of Lévy noises and the relevance of the model for chemical kinetics is briefly discussed.

Journal Article↗

PubMed1998

Automatic setup deviation measurements with electronic portal images for pelvic fields.

The purpose of this work was to develop a fully automatic tool for the detection of setup deviation for small pelvic field using, in external beam radiotherapy, an electronic portal imaging device (EPID). The algorithm processes electronic portal images of prostate cancer patients. No fiducial points or user interventions are needed. Deviation measurements are based on bone edge detection performed with Laplacian of a Gaussian (LoG) operator. Two bone edge images are then correlated, one of which is a reference image taken as the first fraction image for the purpose of this study. The electronic portal images (EPI) also show band artefacts which are removed using the morphological top-hat transform. The algorithm was first validated with 59 phantom images acquired in clinical treatment conditions with known displacements. The algorithm was then validated with 79 clinical images where bone contours were delineated manually. For the phantom images, the setup deviations were measured with a absolute mean error of 0.59 mm and 0.47 mm with a standard deviation of 0.64 mm and 0.42 mm, horizontally and vertically, respectively. A second validation was performed using clinical prostate cancer images. The measured patient displacements have an absolute mean error of 0.48 mm and 1.41 mm with a standard deviation of 0.58 mm and 1.30 mm in the X and Y directions, respectively. The algorithm execution time on a SUN workstation is 5 s. This algorithm shows good potential as a setup deviation measurement tool in clinical practice. The possibility of using this algorithm combined with decision rules based on statistical observations is very promising.

Algorithms↗

PubMed1984

Intensity perception. XIII. Perceptual anchor model of context-coding.

In our preliminary theory of intensity resolution [e.g., see N. I. Durlach and L. D. Braida, J. Acoust. Soc. Am. 46, 372-383 (1969)], two modes of memory operation are postulated: the trace mode and the context-coding mode. In this paper, we present a revised model of the context-coding mode which describes explicitly a process by which sensations are coded relative to the context and which predicts a resolution edge effect [L. D. Braida and N. I. Durlach, J. Acoust. Soc. Am. 51, 483-502 (1972); J. E. Berliner, L. D. Braida, and N. I. Durlach, J. Acoust. Soc. Am. 61, 1256-1267 (1977)]. The sensation arising from a given stimulus presentation is coded by determining its distance from internal references or perceptual anchors. The noise in this process, combined with the sensation noise, constitutes the limitation on resolution in the model. In the revised model the probability density functions of the decision variable are not precisely Gaussian (and cannot be expressed analytically in closed form). This paper outlines the predictions of the model for one-interval paradigms and for fixed-level two-interval paradigms and derives estimates of the values of model parameters.

Discrimination Learning↗

PubMed1991

Computerized methods for analyzing two-dimensional agarose gel electropherograms.

Previous methods interpret zonal or polydisperse gel patterns of two-dimensional Serwer-type gels in terms of size and free mobility (surface net charge density). These two parameters have been determined for each component without quantitatively measuring the abundance of the components. The present study advances these previous methods by determining the relative concentration of each component by computer evaluation of densitometrically analyzed gel patterns. Suitable procedures and their underlying algorithms are presented. The mathematical routines are implemented in a user-friendly software package, called GelFit and designed for a Macintosh personal computer. The program input consists of digitized images of gel staining patterns exemplified by those obtained from electrophoresis of native subcellular-sized particles. The data are processed through the following steps: (i) Noise reduction and calibration. (ii) Geometrical transformation of the pattern onto a rectangular size/free mobility coordinate system using rationales of the extended Ogston model. (iii) Analysis of the transformed image to determine density maxima, density profiles along iso-free-mobility or iso-size lines, curve fitting of one-dimensional profiles or two-dimensional surfaces using Gaussian functions and curve stripping of surfaces to determine the possible number of particle populations.

Algorithms↗

PubMed2001

Comparing protein structures: a Gaussian-based approach to the three-dimensional structural similarity of proteins.

This study describes a new method for comparing three-dimensional protein structures based on an optimal alignment of their steric fields. The method is based upon the use of spherical Gaussian functions located on individual atoms. This representation generates a flexible description of the underlying fold geometry of proteins that can be adjusted by changing the 'width' of the Gaussians. Reducing the width sharpens the representation and leads to a more 'atomlike' description; increasing the width creates a fuzzier representation that preserves the general shape features of the chain fold but with a consequent loss in atomic resolution. The width used in this study is based upon the features of individual atoms and provides a representation that is quite robust with respect to the variety of geometric features typically encountered in the alignment process. In addition, a post-alignment analysis is performed that generates sequence alignments from the corresponding structure alignments. An example, based on five mammalian and fungal matrix metalloproteinase crystal structures (human fibroblast collagenase, neutrophil collagenase, stromelysin, astacin, and adamalysin), illustrates a number of features of the Gaussian-based approach.

Amino Acid Sequence↗

PubMed2005

Physiological effects of indomethacin and celecobix: an S-transform laser Doppler flowmetry signal analysis.

Conventional signal processing typically involves frequency selective techniques which are highly inadequate for nonstationary signals. In this paper, we present an approach to perform time-frequency selective processing of laser Doppler flowmetry (LDF) signals using the S-transform. The approach is motivated by the excellent localization, in both time and frequency, afforded by the wavelet basis functions. Suitably chosen Gaussian wavelet functions are used to characterize the subspace of signals that have a given localized time-frequency support, thus enabling a time-frequency partitioning of signals. In this paper, the goal is to study the influence of various pharmacological substances taken by the oral way (celecobix (Celebrex), indomethacin (Indocid) and placebo) on the physiological activity behaviour. The results show that no statistical differences are observed in the energy computed from the time-frequency representation of LDF signals, for the myogenic, neurogenic and endothelial related metabolic activities between Celebrex and placebo, and Indocid and placebo. The work therefore proves that these drugs do not affect these physiological activities. For future physiological studies, there will therefore be no need to exclude patients having taken cyclo-oxygenase 1 inhibitions.

Administration, Oral↗

PubMed2006

First passage time densities in resonate-and-fire models.

Motivated by the dynamics of resonant neurons we discuss the properties of the first passage time (FPT) densities for non-Markovian differentiable random processes. We start from an exact expression for the FPT density in terms of an infinite series of integrals over joint densities of level crossings, and consider different approximations based on truncation or on approximate summation of this series. Thus the first few terms of the series give good approximations for the FPT density on short times. For rapidly decaying correlations the decoupling approximations perform well in the whole time domain. As an example we consider resonate-and-fire neurons representing stochastic underdamped or moderately damped harmonic oscillators driven by white Gaussian or by Ornstein-Uhlenbeck noise. We show that approximations reproduce all qualitatively different structures of the FPT densities: from monomodal to multimodal densities with decaying peaks. The approximations work for the systems of whatever dimension and are especially effective for the processes with narrow spectral density, exactly when Markovian approximations fail.

Action Potentials↗

PubMed2002

Autoregressive processes with exponentially decaying probability distribution functions: applications to daily variations of a stock market index.

We consider autoregressive conditional heteroskedasticity (ARCH) processes in which the variance sigma(2)(y) depends linearly on the absolute value of the random variable y as sigma(2)(y) = a+b absolute value of y. While for the standard model, where sigma(2)(y) = a + b y(2), the corresponding probability distribution function (PDF) P(y) decays as a power law for absolute value of y-->infinity, in the linear case it decays exponentially as P(y) approximately exp(-alpha absolute value of y), with alpha = 2/b. We extend these results to the more general case sigma(2)(y) = a+b absolute value of y(q), with 0 < q < 2. We find stretched exponential decay for 1 < q < 2 and stretched Gaussian behavior for 0 < q < 1. As an application, we consider the case q=1 as our starting scheme for modeling the PDF of daily (logarithmic) variations in the Dow Jones stock market index. When the history of the ARCH process is taken into account, the resulting PDF becomes a stretched exponential even for q = 1, with a stretched exponent beta = 2/3, in a much better agreement with the empirical data.

Journal Article↗

PubMed2003

[Bispectral analysis does not differentiate between anaesthesia EEG and a linear random process].

Bispectral analysis of the electroencephalogram (EEG) has been used to monitor depth of anaesthesia. In the majority of publications this has involved the use of the so called BIS-Index TM (Aspect Medical Systems, Inc.). The exact relationship of this index to such bispectral parameters as the bispectrum and bicoherence has not yet been reported. If the EEG is considered as a linear random process, bicoherence is trivial, i.e. it is independent of the EEG frequency. The aim of this study was to determine the proportion of EEG epochs with non-trivial bicoherence during isoflurane/N20 anaesthesia. We reanalyzed 25.5 hours of digitalised EEG signal from 9 patients undergoing gynaecological surgery. The test developed by Hinich for Gaussian distribution and linearity was then applied. The test was validated using various synthetic surrogate data: Gaussian random data, the z-component of the Lorenz attractor, the phase randomized EEG and the phase randomized z-component of the Lorenz attractor. The percentage of epochs (8.192 s, 1024 data points) with non-trivial bicoherence was: Lorenz data 95.4%, phase randomized Lorenz data 9.4%, synthetic Gaussian data 14.8%, original EEG 9.1%, phase randomized EEG 5.1%. The original EEG data were not found to contain a higher percentage of epochs with non-trivial bicoherence than the phase randomized data and the synthetic Gaussian data. We conclude that bispectral analysis does not substantially add to the information obtained with other methods of quantitative EEG analysis.

Adult↗

PubMed1997

Investigation of filter sets for supervised pixel classification of cephalometric landmarks by spatial spectroscopy.

The diagnostic process of orthodontics requires the analysis of a cephalometric radiograph. Image landmarks on this two-dimensional lateral projection image of the patient's head are manually identified and spatial relationships are evaluated. This method is very time consuming. A reliable method for automatic computer landmark identification does not exist. Spatial Spectroscopy is a proposed method of automatic landmark identification on cephalometric radiographs, that decomposes an image by convolving it with a set of filters followed by a statistical decision process. The purpose of this paper is to discuss and test appropriate filter sets for the application of Spatial Spectroscopy for automatic identification of cephalometric radiographic landmarks. This study evaluated two different filter sets with 15 landmarks on fourteen images. Spatial Spectroscopy was able to consistently locate landmarks on all 14 cephalometric radiographs tested. The mean landmark identification error of 0.841 +/- 1.253 pixels for a Multiscale Derivative filter set and 0.912 +/- 1.364 pixels for an Offset Gaussian filter set was not significantly different. Furthermore, there were no significant differences between identification of individual landmarks for the Multiscale Derivative and the Offset Gaussian filter set (P > 0.05). These results suggest that Spatial Spectroscopy may be useful in landmark identification tasks.

Cephalometry↗

PubMed2006

Ylidic radical-molecule reactions.

The known reactions between a neutral radical and a neutral closed-shell sigma-type molecule are generally associated with the atom/group-abstraction (e.g., CN, OH reactions) or the carbenoid insertion processes (e.g., CH reactions). In this article, we describe a new type of neutral radical-molecule reaction that feature the nucleophilic addition and elimination mechanism based on the systematic Gaussian-3//MP2(full)/6-31G(d) investigations on the PH(3)CH+HX model reactions (X = CH(3), NH(2), OH, F, CH(2)F, CHF(2), CF(3), NHCH(3)/CH(2)NH(2), OCH(3)/CH(2)OH). The novel properties are found to benefit both from the radical and ylidic characters of PH(3)CH, and the type of reagent HX determines the nucleophilic addition reactivity. The present work is the first study on the chemical reactivity of the ylidic radicals.

Computer Simulation↗

PubMed2002

When and where was the most recent common ancestor?

The structured coalescent is investigated for single-locus, digenic samples in the diffusion limit of the unidimensional stepping-stone model for homogeneous, isotropic migration and random genetic drift. Let T denote the scaled time to the most recent common ancestor (MRCA) of the two genes, and let Z designate the scaled deviation of the position of the MRCA from the average position of the two genes. The joint probability density of T and Z is evaluated explicitly. Both the marginal and conditional distributions of T have infinite expectation, as does the marginal distribution of Z. Conditioned on T = tau, the distribution of Z is Gaussian with mean zero and variance 2tau. The main results are extended to anisotropic migration. The results establish the existence of and define in the diffusion limit a retrospective stochastic process for digenic samples in one spatial dimension.

Animals↗

PubMed1994

Lesion development in focused ultrasound surgery: a general model.

An analytical model has been constructed for the process of formation of thermal lesions in tissue, resulting from exposure to intense, highly focused ultrasound beams such as may be used in minimally invasive surgery. The model assumes a Gaussian approximation to beam shape in the focal region and predicts, for any such focal beam, the time delay to initiation of a lesion and the subsequent time course of growth of that lesion in lateral and axial dimensions, taking into account the effects of thermal diffusion and blood perfusion. The necessary approximations and assumptions of the model are considered. Comparison of predictions with experimentally measured data on excised pig liver indicate generally good agreement. Comparisons are also made of this theory with previously published data on exposure-time dependence of lesioning threshold intensity. Deficiencies are identified in existing practice for measuring and reporting acoustic exposures for focused ultrasound surgery, and the proposal is therefore made that a quantity that would be more satisfactory, from the viewpoints both of metrology and biophysical relevance, is the intensity spatially averaged over the area enclosed by the half-pressure-maximum contour in the focal plane, as determined under linear conditions, provisionally denoted as ISAL.

Acoustics↗