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PubMed2005

A possible mechanism of zero-crossing detection using the concept of the extended classical receptive field of retinal ganglion cells.

The extended classical receptive field (ECRF) of retinal ganglion cells has been modelled as a combination of three zero-mean Gaussians at three different scales that has been shown to be equivalent to a Biharmonic or Bi-Laplacian of Gaussian filter. It has also been shown that the ECRF can be approximated by a combination of Laplacian of Gaussian (LoG) and the Dirac-delta function. Zero-crossings detected with this operator are more informative than those detected by the traditional filters like LoG or Difference of Gaussians (DoG) that had been devised using the classical receptive field of the ganglion cells. We have also explained that such an additional information processing is not in contradiction with the recent experimental findings on the physiology of retinal ganglion cells.

Animals↗

PubMed1990

Evolution of sexual preferences in quantitative characters.

An analysis of equilibria and dynamics of the means, variances, and covariances of female mating preference for a quantitative male secondary sexual character following a Gaussian model is presented. For many combinations of viability and sexual selection parameters the evolving Gaussian distribution of phenotypes can diverge. The results on the cases of convergence and their limiting forms suggest some reinterpretations of Fisher's "runaway" process of sexual selection. One possibility is to interpret Fisher's postulated "initial advantage not due to female preference" as a shift in viability selection where runaway evolution occurs if the mean preferred trait evolves beyond its new viability optimum (due to sexual selection). This definition is contrasted with situations in which the new viability optimum is undershot. The quantitative and qualitative conclusions differ from models that approximate genetic covariance evolution involving a constant covariance.

Biological Evolution↗

PubMed2004

Chemical coupling between ammonia, acid gases, and fine particles.

The concentrations of inorganic aerosol components in the fine particulate matter (PM(fine)< or =2.5 microm) consisted of primarily ammonium, sodium, sulfate, nitrate, and chloride are related to the transfer time scale between gas to particle phase, which is a function of the ambient temperature, relative humidity, and their gas phase constituent concentrations in the atmosphere. This study involved understanding the magnitude of major ammonia sources; and an up-wind and down-wind (receptor) ammonia, acid gases, and fine particulate measurements; with a view to accretion gas-to-particle conversion (GTPS) process in an agricultural/rural environment. The observational based analysis of ammonia, acid gases, and fine particles by annular denuder system (ADS) coupled with a Gaussian dispersion model provided the mean pseudo-first-order k(S-1) between NH(3) and H(2)SO(4) aerosol approximately 5.00 (+/-3.77)x10(-3) s(-1). The rate constant was found to increase as ambient temperature, wind speed, and solar radiation increases, and decreases with increasing relative humidity. The observed [NH(3)][HNO(3)] products exceeded values predicted by theoretical equilibrium constants, due to a local excess of ammonia concentration.

Aerosols↗

PubMed2004

A novel algorithm for non-adiabatic direct dynamics using variational Gaussian wavepackets.

In a recent paper (G. Worth, P. Hunt and M. Robb, J. Phys. Chem. A, 2003, 107, 621), we used surface hopping direct dynamics calculations to study the molecular dynamics of the butatriene radical cation in the X/A manifold, which is coupled by a conical intersection. Here, we present the first direct dynamics calculations using a novel algorithm, again using this ideal test system. The algorithm, which is based on the powerful multi-configuration time-dependent Hartree (MCTDH) wavepacket propagation method, uses a variational basis of coupled frozen Gaussian functions that optimally represent the evolving nuclear wavepacket at all times. Each Gaussian function follows a "quantum trajectory", along which the potential surface is evaluated by quantum chemistry calculations. As far fewer Gaussian functions are needed than classical trajectories in a semi-classical method, the number of quantum chemical calculations is drastically reduced. A crucial point in direct dynamics. To validate the method, initial calculations have been made using an analytic model Hamiltonian, where it is shown to reproduce the main features of the state population transfer with 8-16 basis functions per state. Coupled to the GAUSSIAN quantum chemistry program, the method is then shown to provide a feasible direct dynamics algorithm for the description of this non-adiabatic process.

Journal Article↗

PubMed2004

Functional characterization of linear delay Langevin equations.

We present an exact functional characterization of linear delay Langevin equations driven by any noise structure defined through its characteristic functional. This method relies on the possibility of finding an explicitly analytical expression for each realization of the delayed stochastic process in terms of those of the driving noise. General properties of the transient dissipative dynamics are analyzed. The corresponding interplay with a color Gaussian noise is presented. As a full application of our functional method we study a model for population growth with non-Gaussian fluctuations: the Gompertz model driven by multiplicative white shot noise.

Journal Article↗

PubMed1980

Applicability of white-noise nonlinear system analysis to the peripheral auditory system.

A functional series is capable of describing the input-output relationship of a general class of systems, linear or nonlinear. Wiener and his associates demonstrated methods by which these functionals could be measured from the response of the system to white noise. This paper is a critique of the application of this functional series approach to the measurement of the input-output relationship of the peripheral auditory system. The Wiener functionals are shown to be dependent on the parameters of the white noise used to measure them. The approximation of the output of a system by a truncated Wiener series is shown to have unpredictable error properties when inputs other than white Gaussian noise are used. Estimates of the Wiener functionals using the white-noise approach do not converge when the system has a point process output (e.g., spike train output).

Humans↗

PubMed2003

Freezing transition and correlated motion in a quasi-two-dimensional colloid suspension.

Recent experiments have demonstrated that the deviation of the single-particle displacement distribution from Gaussian form in a dense quasi-two-dimensional colloid suspension is a result of heterogenous dynamics that involves cooperative motions of neighboring colloid particles [J. Chem. Phys. 47, 9142 (2001)]. In this paper, we report the results of molecular dynamics (MD) simulations of a quasi-two-dimensional assembly of nearly hard-sphere colloid particles. The colloid-colloid interaction we use is short ranged and everywhere repulsive; it is related to the Marcus-Rice (MR) and modified MR interactions used in a previous study [Phys. Rev. E 58, 7529 (1998)]. As is the case for those systems, the one we study supports liquid, hexatic, and solid phases. Our calculations show that the deviation of the single-particle displacement distribution from Gaussian form is present in the liquid phase, and that a sharp increase in its magnitude occurs at the liquidus density and extends into the crystalline phase. For densities greater than the liquidus density we find three dynamical relaxation processes that include, at intermediate times, a slowing down in the rate of growth of the diffusive displacement of a particle due to the cage effect. As the density increases toward the solidus density, the dependence of the mean squared displacement on time, at intermediate times, changes from sublinear to zero. The onset of the long-time relaxation mode corresponds to the time at which the deviation of the particle displacement distribution from Gaussian form is a maximum. At this time, which increases exponentially with the density, the self-part of the van Hove function exhibits multiple maxima with respect to r while the distinct part of the van Hove function is a maximum at the origin, thereby signaling jump dynamics. At long times the particle mean square displacement has diffusive character at all densities including solid phase densities. A remarkable feature of our findings is the continuity of character of the particle displacement from the liquid phase through the hexatic phase and into the solid phase. Cooperative jumps that lead to diffusive process in crystals can be explained by a mechanism that involves many such correlated hops in random locations and random directions (but along the crystallographic axes) thereby generating effective random walk behavior. We argue that the collective motion we have found is generated by superpositions of instantaneous normal mode vibrations along diffusive paths. The diffusive paths are along the directions with strong bond orientation correlation, and start to grow in amplitude rapidly on entry into the hexatic phase.

Biophysical Phenomena↗

PubMed1995

Spatial filtering and the Zöllner-Judd geometrical illusion: further studies.

In a geometrical figure in which long vertical lines are each crossed by a series of short oblique lines, an illusory effect is obtained such that the orientations of the long lines are perceived as nonvertical and shifted away from the orientation of the oblique lines (the Zöllner illusion). In addition, the vertical separation between the crossing (oblique) lines is perceived as less than that if the crossing lines are horizontal (the Judd illusion). It has previously been shown that these two effects are closely related, and a single-process account has been proposed in which both effects are explained by a computational model involving band-pass spatial filtering of the figure by means of difference-of-Gaussians (DOG) filters. Two arguments are presented against the latter account. First, in an opposite-contrast-polarity figure with, for example, white vertical lines and black crossing lines on a mid-grey background, the peaks in the DOG filter output are such as to predict the reversal of the Zöllner-Judd effects. It is shown by demonstration that this prediction is disconfirmed, and that the normal effects are obtained. Second, it is shown that the normal Zöllner-Judd effects are obtained in the absence of the long vertical lines, and in the presence of anomalous contours. The latter effects are also in contradiction to the band-pass-filtering model. These findings are discussed in relation to a dual-process account of the Zöllner-Judd effects.

Contrast Sensitivity↗

PubMed2006

Ejection decisions by strike pilots: an extreme value interpretation.

BACKGROUND: An explanation is offered for the asymmetry of Receiver Operating Characteristic (ROC) curves obtained from pilots' decisions to eject. The curves can be fitted by a Gaussian model with unequal variances; however, that model does not provide a ready interpretation of the shape of the obtained ROCs. HYPOTHESIS: In an emergency, a pilot receives information from many parallel sources and will decide to eject if any one value from these sources exceeds the criterion for ejection. This process can be described by the extreme maximum value model. RESULTS: When fitted to published asymmetrical ROC curves for the decision to eject, the extreme maximum value model with one parameter provides a better fit, according to Akaike and Baysean information criteria, than the unequal variance Gaussian model with two parameters. DISCUSSION: In an emergency a pilot's decision to eject depends on whether any one value from several sources provides sufficiently strong evidence to exceed the criterion for ejection. A decision based on this policy is described by the extreme value model, which, therefore, offers a credible explanation of why the obtained ROCs are asymmetrical in shape.

Accidents, Aviation↗

PubMed1997

Computer analysis of two-dimensional electrophoresis gels: a new segmentation and modeling algorithm.

The complexity of the spot patterns of two-dimensional electrophoresis gels made it necessary to use image processing techniques to analyze the gels. An important issue in the analysis is the detection and quantification of the protein spots. In this paper we describe a new technique to segment and model the different spots. For the segmentation of a gel into its different spot regions we apply a watershed technique, which is robust and efficient. For the quantification of the spots, a new spot model is constructed, based on diffusion principles. Besides the advantage of having a physical interpretation, the model is demonstrated to be superior to the commonly used Gaussian models.

Algorithms↗

PubMed2003

An intensity consistent filtering approach to the analysis of deformation tensor derived maps of brain shape.

Deformation tensor morphometry makes use of the derivatives of spatial transformations between anatomies, to provide highly localized volumetric maps of relative anatomical size. The analysis of such maps, however, has the challenge of describing the data in a way that allows the spatial scale and extent of the local shape properties to match those induced by the disease process being studied. This study examines an approach to the spatial filtering of transformation Jacobian maps created in multisubject studies of brain anatomy, which constrains the filter neighborhood within common structural boundaries present in the spatially normalized image data. The filtering incorporates information derived from the spatial normalization process, using a statistical framework to introduce a measure of uncertainty in local regional intensity correspondence following spatial normalisation. The proposed filtering approach is compared to the use of spatially invariant Gaussian filtering in the analysis of Jacobian determinant maps of brain shape and shape change in Alzheimer's disease and normal aging. Results show significantly improved delineation of fine scale patterns of shape difference (in cross-sectional studies) and shape change (from multiple serial magnetic resonance imaging studies).

Age Factors↗

PubMed1982

Retinal processing of intrinsic ad extrinsic noise.

1. Recordings were made from single ganglion cells in the isolated retinas of goldfish. Statistics of the maintained discharge were computed for steady-state firing in the dark, in the presence of a steady-adapting light confined essentially to the center of the receptive field, and in the presence of the same adapting light modulated by Gaussian white noise. 2. Steady adapting lights effected a significant initial change in firing rate and affected sensitivities to test probes. After a new minutes, however, the rate returned to a moderate level. There was no consistent relationship between firing in the dark and illuminated conditions, nor was there a consistent relationship between the values of the coefficient of variation of firing in the dark and in light, although some individual cells showed changes in these properties when illuminated. 3. There was no consistent change in the organization of the firing in dark and in steady light. Neither the normalized autocovariances nor the serial correlograms, indicators of short-term order, were significantly affected by light. Similarly, plots of the standard deviation of rate versus sample duration, an indicator of long-term structure, were not altered by illumination. The intrinsic noise thus appears unaffected by retinal illumination. 4. Gaussian modulation of an adapting light (extrinsic noise) increased the coefficient of variation; it also dramatically increased mean firing rate. This effect was observed in both red-on and red-off cells; it indicates a nonlinearity that is present even in the spatially linear X-like neurons. 5. Gaussian modulation of an adapting light made the short-duration values of the normalized autocovariances slightly more positive. Similarly, modulation made the first two or three coefficients of the serial correlogram more positive; this indicates low pass filtering of the extrinsic noise. 6. Gaussian modulation of an adapting light had no effect on the plots of standard deviation of rate versus sample duration other than simple translation due to increased variability. Since the form of these plots in the absence of modulation implies high-pass filtering of the intrinsic noise, the extrinsic noise must also be processed by the same (or a similar) high-pass filter. 7. A model is proposed in which extrinsic signals are band pass filtered by a low-pass and a high-pass filter in cascade. The intrinsic variability evident in the maintained discharge is apparently injected between the two filters. Alternative possibilities are discussed.

Adaptation, Physiological↗

PubMed2001

Temporal resolution of orientation-based texture segregation.

We analysed the temporal-frequency characteristics of two functional processes involved in orientation-based texture segregation: local orientation coding and subsequent orientation-contrast coding. Two texture images, in which each micropattern was rotated by 90 degrees, were alternated at various temporal frequencies. A micropattern was a second-derivative (D2) of a Gaussian that loses orientation information when temporally fused with the orthogonal D2 pattern. We measured the upper temporal-frequency limits for localising the target region whose mean orientation differed from the background by 90 degrees or by 45 degrees. If the temporal limit of the texture perception is determined by the most sluggish processing stage, the temporal limit for the 90 degrees texture should be determined by local orientation coding or by orientation-contrast coding, depending on which stage has the lower temporal precision. On the other hand, the 45 degrees texture should always be segregated below the temporal limit of local orientation coding regardless of the temporal limit of orientation-contrast coding. We found that the temporal limit for the 90 degrees texture was slightly higher than that for the 45 degrees texture under spatial conditions appropriate for texture segregation. Moreover, an orientation-noise analysis of segregation performance for a wide range of temporal frequencies revealed that the temporal-frequency sensitivities for the two textures were nearly identical. These results imply that the temporal limit for orientation-based texture segregation depends only on that of local orientation coding. This conclusion further suggests that the potential temporal resolution of orientation-contrast coding is not lower than that of local orientation coding, which would imply that the orientation-contrast coding is unlikely to be mediated by sluggish neural processes.

Contrast Sensitivity↗

PubMed2005

Partial least squares based decomposition of five spectrally overlapping factors.

The development of a multi-sensory fiber-optic based fluence rate probe (MSP) for light monitoring and dosimetry during photodynamic therapy (PDT) created the need for a robust multivariate signal analysis algorithm capable of quantifying the intensity of five component spectra, representing the sensors, which display a large degree of spectral overlap. Partial least squares (PLS) analysis, as an option for such an analysis algorithm, was evaluated through simulations in the presence of three types of noise, which experimentally may limit the accuracy of PLS quantification of component spectra contributions. Random, or white noise, background was varied over a range of 0-15% relative intensity. A non-random (Gaussian) background vector was simulated, as an experimentally relevant spectral interferent, over a range of 0-7% relative intensity. Spectral variation was modeled by a spectral shift of +/-1-5 nm. Procedures for selecting preferred combinations of fluorophores, with minimum possible spectral overlap, were developed. To illustrate the importance of this selection process, PLS performance results were compared for two possible combinations of five fluorophores, as well as for the combination of three fluorophores currently in experimental use with MSPs. Experimentally anticipated worst-case quantifications were identified for all three types of simulated noise (5% random background, 0.5% Gaussian background, and +/-2 nm spectral shift). The effects of single and combined types of noise were evaluated by independently varying each type of simulated noise over the experimentally relevant range. The mean percentage error in determining the fluorophore contributions and hence quantification of the fluence rate were compared with the required performance standard of better than 10% error for optical power meters in medical use. The PLS algorithm provided an accuracy of 7 +/- 2% for five fluorophores and 3.3 +/- 0.8% for three fluorophores, indicating that PLS is an appropriate choice for this application.

Algorithms↗

PubMed2002

Peak deconvolution in one-dimensional chromatography using a two-way data approach.

A deconvolution methodology for overlapped chromatographic signals is proposed. Several single-wavelength chromatograms of binary mixtures, obtained in different runs at diverse concentration ratios of the individual components, were simultaneously processed (multi-batch approach), after being arranged as two-way data. The chromatograms were modelled as linear combinations of forced peak profiles according to a polynomially modified Gaussian equation. The fitting was performed with a previously reported hybrid genetic algorithm with local search, leaving all model parameters free. The approach yielded more accurate solutions than those found when each experimental chromatogram was fitted independently to the peak model (single-batch approach). The improvement was especially significant for those chromatograms where the peaks were severely affected by the tails of the preceding compounds. Peak shifts among chromatograms, which are a usual source of non-bilinearity, were modelled in a continuous domain instead of in a discrete way, which avoided some drawbacks associated with latent variable methods. An experimental design involving simulated chromatograms was applied to check the method performance. Five main factors affecting the deconvolution were examined: concentration pattern, chromatographic resolution, number of batches and replicates, and noise level, which were evaluated using first- and second-order figures of merit. The method was also tested on three real samples containing compounds showing different overlap. Four multi-batch deconvolution methods were considered differing in the nature of the processed information and kind of peak matching among chromatograms. In all cases, the multi-batch deconvolution yielded better performance than the single-batch approach.

Chromatography↗

PubMed2006

Optimizing UV laser focus profiles for improved MALDI performance.

Matrix assisted laser desorption/ionization (MALDI) applications, such as proteomics, genomics, clinical profiling and MALDI imaging, have created a growing demand for faster instrumentation. Since the commonly used nitrogen lasers have throughput and life span limitations, diode-pumped solid-state lasers are an alternative. Unfortunately this type of laser shows clear performance limitations in MALDI in terms of sensitivity, resolution and ease of use, for applications such as thin-layer sample preparations, acceptance of various matrices (e.g. DHB for glycopeptides) and MALDI imaging. While it is obvious that the MALDI process has some dependence on the characteristics of the laser used, it is unclear which features are the most critical in determining laser performance for MALDI. In this paper we show, for the first time, that a spatially structured laser beam profile in lieu of a Gaussian profile is of striking importance. This result enabled us to design diode-pumped Nd : YAG lasers that on various critical applications perform as well for MALDI as the nitrogen lasers and in some respects even better. The modulation of the beam profile appears to be a new parameter for optimizing the MALDI process. In addition, the results trigger new questions directing us to a better understanding of the MALDI process.

Algorithms↗

PubMed2005

Stochastic analysis to assess the spatial distribution of groundwater nitrate concentrations in the Po catchment (Italy).

A large database including temporal trends of physical, ecological and socio-economic data was developed within the EUROCAT project. The aim was to estimate the nutrient fluxes for different socio-economic scenarios at catchment and coastal zone level of the Po catchment (Northern Italy) with reference to the Water Quality Objectives reported in the Water Framework Directive (WFD 2000/60/CE) and also in Italian legislation. Emission data derived from different sources at national, regional and local levels are referred to point and non-point sources. While non-point (diffuse) sources are simply integrated into the nutrient flux model, point sources are irregularly distributed. Intensive farming activity in the Po valley is one of the main Pressure factors Driving groundwater pollution in the catchment, therefore understanding the spatial variability of groundwater nitrate concentrations is a critical issue to be considered in developing a Water Quality Management Plan. In order to use the scattered point source data as input in our biogeochemical and transport models, it was necessary to predict their values and associated uncertainty at unsampled locations. This study reports the spatial distribution and uncertainty of groundwater nitrate concentration at a test site of the Po watershed using a probabilistic approach. Our approach was based on geostatistical sequential Gaussian simulation used to yield a series of stochastic images characterized by equally probable spatial distributions of the nitrate concentration across the area. Post-processing of many simulations allowed the mapping of contaminated and uncontaminated areas and provided a model for the uncertainty in the spatial distribution of nitrate concentrations.

Agriculture↗

PubMed2000

Fine-structure enhancement--assessment of a simple method to resolve overlapping bands in spectra.

A simple mathematical procedure--fine-structure enhancement--has been assessed on its ability to resolve overlapping bands in spectra. Its advantages and limitations have been explored using synthetic and experimental spectra. Fine-structure enhancement involves smoothing the original spectrum, multiplying the smoothed spectrum with a weighting factor and subtracting this spectrum from the original spectrum. As a result, the fine-structure of the original spectrum is enhanced in the processed spectrum and bands that overlap in the original spectrum appear as distinct bands in the processed spectrum. To be resolved by fine-structure enhancement, Lorentzian lines have to be separated by more than their quarter width at half maximum, Gaussian lines by more than their half width at half maximum. A comparison of fine-structure enhancement and Fourier self-deconvolution shows that Fourier self-deconvolution has in theory a higher potential to resolve overlapping bands. However, this depends crucially on the correct choice of the parameters. In practice, when parameters commonly used are chosen for Fourier self-deconvolution, fine-structure enhancement leads to similar results. This is demonstrated at the example of the infrared absorbance spectrum of the protein papain, where the amide I band components could be resolved similarly with both methods. Thus, fine-structure enhancement seems to be a simple alternative to Fourier self-deconvolution that does not require specialised software.

Models, Chemical↗