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At least 649 records · Page 36Linked to original sources

PubMed2000

Spectroscopic properties of protochlorophyllide analyzed in situ in the course of etiolation and in illuminated leaves.

The spectroscopic properties of photoactive (i.e. flash-transformable) and nonphotoactive protochlorophyll(ide)s (Pchl(ide)) were reinvestigated during the development of bean leaves in darkness. Two phases in the process of Pchl(ide) accumulation were apparent from quantitative measurements of pigment content: a lag phase (first week) during which photoactive Pchl(ide) accumulated faster than nonphotoactive Pchl(ide); and a fast phase (second week), showing parallel accumulation of both types of Pchl(ide). 'Flashed-minus-dark' absorbance difference spectra recorded in situ at 77 K showed that P650-655 was the predominant form of photoactive protochlorophyllide regardless of developmental stage. Quantitative analysis of energy migration processes between the Pchl(ide) forms showed the existence of energy transfer units containing a 1:8 ratio of nonphotoactive and photoactive Pchl(ide)s during development. Gaussian deconvolution of in situ 77 K fluorescence spectra indicated that the 633 nm band of nonphotoactive Pchl(ide) was made of four bands, at 625, 631, 637 and 643 nm, whose relative amplitudes only slightly changed during development. The emission band of photoactive Pchlide was also analyzed using the same method. Three components were found at 644, 652 and 657 nm. The emission band of P650-655 included the last two components, which become predominant only in fully etiolated plants. Photoactive Pchlide with an emission maximum at 653 nm was detected in the light during development of leaves of photoperiodically grown plants.

Darkness↗

PubMed2001

Beats, kurtosis and visual coding.

Techniques adapted from standard higher-order statistical methods are applied to natural-image data in an attempt to discover exactly what makes 'wavelet' representations of natural scenes sparse. Specifically, this paper describes a measure known as the phase-only second spectrum, a fourth-order statistic which quantifies harmonic beat interactions in data, and uses it to show that there are statistical consistencies in the phase spectra of natural scenes. The orientation-averaged phase-only second spectra of natural images appear to show power-law behaviour rather like image power spectra, but with a spectral exponent of approximately -1 instead of -2. They also appear to display a similar form of scale-invariance. Further experimental results indicate that the form of these spectra can account for the observed sparseness of bandpass-filtered natural scenes. This implies an intimate relationship between the merits of sparse neural coding and the exploitation of non-Gaussian 'beats' structures by the visual system.

Algorithms↗

PubMed1979

Automated detection of EEG artifacts during sleep: preprocessing for all-night spectral analysis.

This paper describes a simple artifact detection algorithm which can be used when large amounts of EEG data are to be automatically processed via spectral analysis techniques in a general purpose digital computer, and visual inspection of each EEG epoch becomes an impossible task. The technique is based on a chi-square (chi(2)) goodness-of-fit test to a Gaussian distribution (CSQ), and it was applied to EEG epochs each 30 sec long. This test proved to be very sensitive to non-stationarities in the EEG amplitude distribution for a particular epoch, and it produced a large value for the chi(2) coefficient when an artifact was present. EEG epochs that gave rise to chi(2) coefficients of value larger than a heuristically determined minimum were discarded from further analysis. The above technique enabled efficient data reduction and reliable automatic off-line processing of 50 nights of sleep EEG via spectral techniques.

Action Potentials↗

PubMed2003

Spatial frequency processing in inferred PC- and MC-pathways.

The goal of this study was to investigate the role of inferred parvocellular (PC) and magnocellular (MC) pathways in spatial contrast sensitivity. Localized, spatially narrow-band patterns (sixth derivatives of Gaussians, D6s) were presented at various peak spatial frequencies. When the D6 appeared on a pulsed luminance pedestal (Pulsed-Pedestal Paradigm), the spatial contrast sensitivity showed a band-pass shape with good contrast sensitivity at medium spatial frequencies. When the D6 appeared on a steady luminance pedestal (Steady-Pedestal Paradigm), the spatial contrast sensitivity showed a low-pass shape with decreased sensitivity at high spatial frequencies. The band-pass CSF was interpreted as reflecting PC-pathway mediation; the lower spatial frequency region of the low-pass CSF as reflecting MC-pathway mediation.

Contrast Sensitivity↗

PubMed2004

Development and evaluation of MRI based Bayesian image reconstruction methods for PET.

A maximum a posteriori algorithm, which incorporates correlated magnetic resonance images into the processing of positron emission tomography reconstruction with the aim of improving image quality was developed. The line site map from MRI a priori is made up of a modified Markov random field or Canny edge detector with Gaussian smoothing filter. It is used in the MAP algorithm by a weighted line site method. We evaluate and compare the performance of these reconstruction methods. The results show that the Bayesian methods produce reconstructed images with less noise and better spatial resolution than those produced by the maximum likelihood-expectation maximization method.

Algorithms↗

PubMedDate not supplied

Mechanisms of signal analysis and pattern perception in periodicity pitch.

Progress in the knowledge of auditory processing of complex sounds has been made through coordinated psychophysical, physiological and theoretical studies of periodicity pitch and combination tones. Periodicity pitch is the basis for human perception of musical notes and pitch of voiced speech. The mechanism of perception involves harmonic pattern recognition on the complex Fourier frequency spectra generated by auditory frequency analysis. Combination tones are perceptible distortion tones generated within the cochlea by nonlinear interaction of component stimulus tones. Perception of periodicity pitch is quantitatively accounted for by a two-stage process of frequency analysis subject to random errors and significant nonlinearities, followed by a pattern recognizer that operates very efficiently to measure the period of musical and speech sounds. The basic characteristic of the first stage is a Gaussian standard error function that quantifies the randomness in aural estimation of frequencies of component tones in a complex tone stimulus. Efficient aural measurement of neural spike intervals from the eighth nerve provides a physiological account for the psychophysical characteristic of aural frequency analysis with complex sounds. Although cochlear filtering is an essential stage in auditory frequency analysis, neural time following, rather than details of the filter characteristics, is the decisive factor in determining the precision of aural frequency measurement. It is likely that peripheral auditory coding is similar for sounds in periodicity pitch and in speech perception, although the 'second stage' representing central processing would differ.

Acoustic Stimulation↗

PubMed1999

Longitudinal data analysis (repeated measures) in clinical trials.

Longitudinal data is often collected in clinical trials to examine the effect of treatment on the disease process over time. This paper reviews and summarizes much of the methodological research on longitudinal data analysis from the perspective of clinical trials. We discuss methodology for analysing Gaussian and discrete longitudinal data and show how these methods can be applied to clinical trials data. We illustrate these methods with five examples of clinical trials with longitudinal outcomes. We also discuss issues of particular concern in clinical trials including sequential monitoring and adjustments for missing data. A review of current software for analysing longitudinal data is also provided. Published in 1999 by John Wiley & Sons, Ltd. This article is a US Government work and is the public domain in the United States.

Clinical Trials as Topic↗

PubMed1995

Deconvolution of STM images using entropy as a regularization functional.

The Maximum Entropy approach is applied to restore and sharpen scanning tunneling microscopy (STM) images with atomic resolution. Based on the STM theory of Tersoff and Hamann the process of data acquisition can be approximated by the convolution of a localised atomic density of states (i.e. narrow spots in the reconstruction) of the sample and a Gaussian resolution function which limits the resolution. In STM practice a good and robust estimation of the atomic core positions is necessary for different reasons, such as to be able to calculate the characteristics of the atomic lattice or to study non-periodicities.

Journal Article↗

PubMedDate not supplied

Coding of information in models of tuberous electroreceptors.

Weakly electric fish continuously emit a quasi-sinusoidal electric organ discharge (EOD) to probe their near environment (electrolocation). P-type tuberous receptors located on their skin respond to amplitude modulations of the EOD by varying their firing rate. These receptors, and the neuronal circuitry downstream from them, must encode and process low-frequency stimuli due to prey and obstacles and certain communication signals, as well as high-frequency communication signals emitted by other fish. We ultimately seek the biophysics that govern the encoding process, and in particular, the sensitivity to certain stimulus features. Since the pyramidal cells to which these receptors project can also be monitored, studies of weakly electric fish offer a great opportunity for deciphering the encoding/decoding problem. Here we briefly summarize our recent advances on this issue. We then present new results on the encoding properties and relative modeling advantages of two widely used classes of neuron models of electroreceptor activity: a leaky integrate-and-fire dynamical model, and a non-dynamical modulated stochastic point process model. The quality of encoding, based on the stimulus reconstruction method, is assessed as a function of firing rate and stimulus contrast, in the context of bandlimited Gaussian stimuli. Our main conclusion is that the quality of encoding increases strongly with firing rate, but also depends on the actual combination of biophysical parameters that determine this rate.

Animals↗

PubMed1992

A three-dimensional statistical analysis for CBF activation studies in human brain.

Many studies of brain function with positron emission tomography (PET) involve the interpretation of a subtracted PET image, usually the difference between two images under baseline and stimulation conditions. The purpose of these studies is to see which areas of the brain are activated by the stimulation condition. In many cognitive studies, the activation is so slight that the experiment must be repeated on several subjects and the subtracted images are averaged to improve the signal-to-noise ratio. The averaged image is then standardized to have unit variance and then searched for local maxima. The main problem facing investigators is which of these local maxima are statistically significant. We describe a simple method for determining an approximate p value for the global maximum based on the theory of Gaussian random fields. The p value is proportional to the volume searched divided by the product of the full widths at half-maximum of the image reconstruction process or number of resolution elements. Rather than working with local maxima, our method focuses on the Euler characteristic of the set of voxels with a value larger than a given threshold. The Euler characteristic depends only on the topology of the regions of high activation, irrespective of their shape. For large threshold values this is approximately the same as the number of isolated regions of activation above the threshold. We can thus not only determine if any activation has taken place, but we can also estimate how many isolated regions of activation are present.

Brain↗

PubMed1999

A neural model of motion processing and visual navigation by cortical area MST.

Cells in the dorsal medial superior temporal cortex (MSTd) process optic flow generated by self-motion during visually guided navigation. A neural model shows how interactions between well-known neural mechanisms (log polar cortical magnification, Gaussian motion-sensitive receptive fields, spatial pooling of motion-sensitive signals and subtractive extraretinal eye movement signals) lead to emergent properties that quantitatively simulate neurophysiological data about MSTd cell properties and psychophysical data about human navigation. Model cells match MSTd neuron responses to optic flow stimuli placed in different parts of the visual field, including position invariance, tuning curves, preferred spiral directions, direction reversals, average response curves and preferred locations for stimulus motion centers. The model shows how the preferred motion direction of the most active MSTd cells can explain human judgments of self-motion direction (heading), without using complex heading templates. The model explains when extraretinal eye movement signals are needed for accurate heading perception, and when retinal input is sufficient, and how heading judgments depend on scene layouts and rotation rates.

Algorithms↗

PubMed2002

Exact occupation time distribution in a non-Markovian sequence and its relation to spin glass models.

We compute exactly the distribution of the occupation time in a discrete non-Markovian toy sequence that appears in various physical contexts such as the diffusion processes and Ising spin glass chains. The non-Markovian property makes the results nontrivial even for this toy sequence. The distribution is shown to have non-Gaussian tails characterized by a nontrivial large deviation function that is computed explicitly. An exact mapping of this sequence to an Ising spin glass chain via a gauge transformation raises an interesting question for a generic finite sized spin glass model; at a given temperature, what is the distribution (over disorder) of the thermally averaged number of spins that are aligned to their local fields? We show that this distribution remains nontrivial even at infinite temperature and can be computed explicitly in few cases such as in the Sherrington-Kirkpatrick model with Gaussian disorder.

Journal Article↗

PubMed2005

Quantum key distillation from Gaussian states by Gaussian operations.

We study the secrecy properties of Gaussian states under Gaussian operations. Although such operations are useless for quantum distillation, we prove that it is possible to distill a secret key secure against any attack from sufficiently entangled Gaussian states with nonpositive partial transposition. Moreover, all such states allow for key distillation, when Eve is assumed to perform finite-size coherent attacks before the reconciliation process.

Journal Article↗

PubMed2006

Application of eigenmode in the adaptive optics system based on a micromachined membrane deformable mirror.

The construction process and characteristics of a deformable mirror eigenmode are introduced. The eigenmode of a 37-element micromachined membrane deformable mirror (MMDM) from OKO, Ltd. is analyzed. The Gaussian-Seidel low-order aberrations are fitted with eigenmodes as basic functions. An experimental adaptive optics (AO) system is constructed with the MMDM as the wavefront corrector, a deformable mirror eigenmode as the wavefront control algorithm, and a Shack-Hartmann wavefront sensor as the wavefront detector. The experimental results demonstrate that the deformable mirror eigenmode can act as the wavefront control algorithm for the AO system based on the MMDM.

Journal Article↗

PubMed1989

The QUEST system for quantitative analysis of two-dimensional gels.

The strategies and methods used by the QUEST system for two-dimensional gel analysis are described, and the performance of the system is evaluated. Radiolabeled proteins, resolved on two-dimensional gels and detected using calibrated exposures to film, are quantified in units of disintegrations per minute or as a fraction of the total protein radioactivity applied to the gel. Spot quantitation and resolution of overlapping spots is performed by two-dimensional gaussian fitting. Pattern matching is carried out for groups of gels called matchsets, and within each matchset every gel is matched to every other gel. During the matching process, spots are automatically added to each pattern at positions where unmatched spots were detected in other patterns. This results in enhanced accuracy for both spot detection and for matching. The spot fitting procedure is repeated after matching. Tests show that up to 97% of spots in each pattern can be matched and that fewer than 1% of the spots are matched inconsistently. Approximately 2000 proteins are detected from typical gels. Of these 1600 are high quality spots. Tests to measure the coefficient of variation of spot quantitation versus spot quality show that the average coefficient of variation for high quality spots is 21%. The intensities of the detected proteins range from 4 to 20,000 ppm of total protein synthesis. The QUEST analysis system has been used to build a quantitative database for the proteins of normal and transformed REF52 cells, as presented in the accompanying reports (Garrels, J., and Franza, B. R., Jr. (1989) J. Biol. Chem. 264, 5283-5298, 5299-5312).

Algorithms↗

PubMed1984

[Hirsutism].

The symptom of hirsutism, i.e. excessive sexual hair of the male pattern for the individual woman, must be strictly differentiated from hypertrichosis, which is not androgendependent and occurs in both sexes, and also from the disease of virilization. The suffering derives from a divergence from the feminine ideal rather than from reality, and most forms of solitary hirsutism are probably to be assigned to the outer area of a Gaussian curve of sexual hair growth in normal women, by analogy to baldness in men. Although slightly elevated androgen levels are by no means rare, receptorial and postreceptorial processes in the hair follicle cells are at least of equal importance; both are modulated by a variety of partial factors. The salient point in workup is to exclude virilization and other primary diseases: this is usually possible on clinical grounds and by limited hormone measurements. Longterm therapy of solitary hirsutism is chiefly cosmetic, if there is no simultaneous wish for contraception. If this is desired, and if the hirsutism is mild, an initial low-dose cyproterone acetate-estrogen combination may be (appropriate. A higher dosage of cyproterone acetate Hammerstein 's regimen) over many years should be confined to severe forms.

Adult↗

PubMed1998

A statistical paradigm for neural spike train decoding applied to position prediction from ensemble firing patterns of rat hippocampal place cells.

The problem of predicting the position of a freely foraging rat based on the ensemble firing patterns of place cells recorded from the CA1 region of its hippocampus is used to develop a two-stage statistical paradigm for neural spike train decoding. In the first, or encoding stage, place cell spiking activity is modeled as an inhomogeneous Poisson process whose instantaneous rate is a function of the animal's position in space and phase of its theta rhythm. The animal's path is modeled as a Gaussian random walk. In the second, or decoding stage, a Bayesian statistical paradigm is used to derive a nonlinear recursive causal filter algorithm for predicting the position of the animal from the place cell ensemble firing patterns. The algebra of the decoding algorithm defines an explicit map of the discrete spike trains into the position prediction. The confidence regions for the position predictions quantify spike train information in terms of the most probable locations of the animal given the ensemble firing pattern. Under our inhomogeneous Poisson model position was a three to five times stronger modulator of the place cell spiking activity than theta phase in an open circular environment. For animal 1 (2) the median decoding error based on 34 (33) place cells recorded during 10 min of foraging was 8.0 (7.7) cm. Our statistical paradigm provides a reliable approach for quantifying the spatial information in the ensemble place cell firing patterns and defines a generally applicable framework for studying information encoding in neural systems.

Action Potentials↗

PubMed1985

Mortality and aging in a heterogeneous population: a stochastic process model with observed and unobserved variables.

Various multivariate stochastic process models have been developed to represent human physiological aging and mortality. These efforts are extended by considering the effects of observed and unobserved state variables on the age trajectory of physiological parameters. This is done by deriving the Kolmogorov-Fokker-Planck equations describing the distribution of the unobserved state variables conditional on the history of the observed state variables. Given some assumptions, it is proved that the distribution is Gaussian. Strategies for estimating the parameters of the distribution are suggested based on an extension of the theory of Kalman filters to include systematic mortality selection. Various empirical applications of the model to studies of human aging and mortality as well as to other types of "failure" processes in heterogeneous populations are discussed.

Aging↗