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PubMed2006

Entangled trajectory dynamics in the Husimi representation.

We solve quantum dynamical equations of simple systems by propagating ensembles of interacting trajectories. A scheme is proposed which uses adaptive kernel density estimation for representing probability distribution functions and their derivatives. The formulation is carried on in the Husimi representation to ensure the positiveness of the distribution functions. By comparing to previous work, the effect of changing representations is studied as well as the advantage of using adaptive kernels for the estimation of probability distributions. We found significant improvement in the accuracy of the results.

Journal Article↗

PubMed2006

A detailed quantum mechanical and quasiclassical trajectory study on the dynamics of the H+ + H2 --> H2 + H+ exchange reaction.

The H+ + H2 exchange reaction has been studied theoretically by means of a different variety of methods as an exact time independent quantum mechanical, approximate quantum wave packet, statistical quantum, and quasiclassical trajectory approaches. Total and state-to-state reaction probabilities in terms of the collision energy for different values of the total angular momentum obtained with these methods are compared. The dynamics of the reaction is extensively studied at the collision energy of E(coll)=0.44 eV. Integral and differential cross sections and opacity functions at this collision energy have been calculated. In particular, the fairly good description of the exact quantum results provided by the statistical quantum method suggests that the dynamics of the process is governed by an insertion mechanism with the formation of a long-lived collision complex.

Journal Article↗

PubMed2006

Refinement trajectory and determination of eigenstates by a wavelet based adaptive method.

The detail structure of the wave function is analyzed at various refinement levels using the methods of wavelet analysis. The eigenvalue problem of a model system is solved in granular Hilbert spaces, and the trajectory of the eigenstates is traced in terms of the resolution. An adaptive method is developed for identifying the fine structure localization regions, where further refinement of the wave function is necessary.

Journal Article↗

PubMed2006

Bohmian mechanics with complex action: a new trajectory-based formulation of quantum mechanics.

In recent years there has been a resurgence of interest in Bohmian mechanics as a numerical tool because of its local dynamics, which suggest the possibility of significant computational advantages for the simulation of large quantum systems. However, closer inspection of the Bohmian formulation reveals that the nonlocality of quantum mechanics has not disappeared-it has simply been swept under the rug into the quantum force. In this paper we present a new formulation of Bohmian mechanics in which the quantum action, S, is taken to be complex. This leads to a single equation for complex S, and ultimately complex x and p but there is a reward for this complexification-a significantly higher degree of localization. The quantum force in the new approach vanishes for Gaussian wave packet dynamics, and its effect on barrier tunneling processes is orders of magnitude lower than that of the classical force. In fact, the current method is shown to be a rigorous extension of generalized Gaussian wave packet dynamics to give exact quantum mechanics. We demonstrate tunneling probabilities that are in virtually perfect agreement with the exact quantum mechanics down to 10(-7) calculated from strictly localized quantum trajectories that do not communicate with their neighbors. The new formulation may have significant implications for fundamental quantum mechanics, ranging from the interpretation of non-locality to measures of quantum complexity.

Journal Article↗

PubMed2006

Grouping and trajectory storage in multiple object tracking: impairments due to common item motions.

In our natural viewing, we notice that objects change their locations across space and time. However, there has been relatively little consideration of the role of motion information in the construction and maintenance of object representations. We investigated this question in the context of the multiple object tracking (MOT) paradigm, wherein observers must keep track of target objects as they move randomly amid featurally identical distractors. In three experiments, we observed impairments in tracking ability when the motions of the target and distractor items shared particular properties. Specifically, we observed impairments when the target and distractor items were in a chasing relationship or moved in a uniform direction. Surprisingly, tracking ability was impaired by these manipulations even when observers failed to notice them. Our results suggest that differentiable trajectory information is an important factor in successful performance of MOT tasks. More generally, these results suggest that various types of common motion can serve as cues to form more global object representations even in the absence of other grouping cues.

Cues↗

PubMed2000

Place recognition monitored by location-driven operant responding during passive transport of the rat over a circular trajectory.

Spatial memory of animals is usually tested in navigation tasks that do not allow recognition and recall processes to be separated from the mechanisms of goal-directed locomotion. In the present study, place recognition was examined in rats (n = 7) confined in an operant chamber mounted on the periphery of a slowly rotating disk (diameter 1 m, angular velocity 9 degrees /s). The animals were passively transported over a circular trajectory and were rewarded for bar pressing when they passed across a 60 degrees -wide segment of the path. This segment was recognizable with reference to room landmarks visible from the operant box. Responding defined in the coordinate system of the room increased when the chamber entered the 60 degrees -wide approach zone, culminated at the entrance into the reward sector, was decreased inside it by eating the available reward, and rapidly declined to zero at the exit from this zone. When reward was discontinued, the skewed response distribution changed into a symmetric one with a maximum in the center of the reward sector. With advancing extinction, the response peak in the reward sector decreased in most rats proportionally to the overall decline of bar pressing. The rewarded and nonrewarded response patterns indicate that passively transported rats can recognize their position in the environment with an accuracy comparable to that of actively navigating animals and that location-driven operant responding can serve as a useful tool in the analysis of the underlying neural mechanisms.

Animals↗

PubMed1993

Poincaré resonances and the limits of trajectory dynamics.

In previous papers we have shown that the elimination of the resonance divergences in large Poincare systems leads to complex irreducible spectral representations for the Liouville-von Neumann operator. Complex means that time symmetry is broken and irreducibility means that this representation is implementable only by statistical ensembles and not by trajectories. We consider in this paper classical potential scattering. Our theory applies to persistent scattering. Numerical simulations show quantitative agreement with our predictions.

Journal Article↗

PubMed1997

Trajectory of DNA in the RNA polymerase II transcription preinitiation complex.

By using site-specific protein-DNA photocrosslinking, we define the positions of TATA-binding protein, transcription factor IIB, transcription factor IIF, and subunits of RNA polymerase II (RNAPII) relative to promoter DNA within the human transcription preinitiation complex. The results indicate that the interface between the largest and second-largest subunits of RNAPII forms an extended, approximately 240 A channel that interacts with promoter DNA both upstream and downstream of the transcription start. By using electron microscopy, we show that RNAPII compacts promoter DNA by the equivalent of approximately 50 bp. Together with the published structure of RNAPII, the results indicate that RNAPII wraps DNA around its surface and suggest a specific model for the trajectory of the wrapped DNA.

Animals↗

PubMed2002

Electron transfer from the Rieske iron-sulfur protein (ISP) to cytochrome f in vitro. Is a guided trajectory of the ISP necessary for competent docking?

The time course of electron transfer in vitro between soluble domains of the Rieske iron-sulfur protein (ISP) and cytochrome f subunits of the cytochrome b(6)f complex of oxygenic photosynthesis was measured by stopped-flow mixing. The domains were derived from Chlamydomonas reinhardtii and expressed in Escherichia coli. The expressed 142-residue soluble ISP apoprotein was reconstituted with the [2Fe-2S] cluster. The second-order rate constant, k(2)((ISP-f)) = 1.5 x 10(6) m(-1) s(-1), for ISP to cytochrome f electron transfer was <10(-2) of the rate constant at low ionic strength, k(2)((f-PC))(> 200 x 10(6) m(-1) s(-1)), for the reduction of plastocyanin by cytochrome f, and approximately 1/30 of k(2)((f-PC)) at the ionic strength estimated for the thylakoid interior. In contrast to k(2)((f-PC)), k(2)((ISP-f)) was independent of pH and ionic strength, implying no significant role of electrostatic interactions. Effective pK values of 6.2 and 8.3, respectively, of oxidized and reduced ISP were derived from the pH dependence of the amplitude of cytochrome f reduction. The first-order rate constant, k(1)((ISP-f)), predicted from k(2)((ISP-f)) is approximately 10 and approximately 150 times smaller than the millisecond and microsecond phases of cytochrome f reduction observed in vivo. It is proposed that in the absence of electrostatic guidance, a productive docking geometry for fast electron transfer is imposed by the guided trajectory of the ISP extrinsic domain. The requirement of a specific electrically neutral docking configuration for ISP electron transfer is consistent with structure data for the related cytochrome bc(1) complex.

Cytochromes↗

PubMed1995

MR image classification algorithms for neurosurgical and stereotactic radiosurgical trajectory planning and volumetric analysis.

Neurosurgical and stereotactic radiosurgical trajectory planning involves a phase of image data acquisition followed by localization of tissues to be avoided or crossed during the procedure. Success of the procedure is often assessed during follow-up by performing volumetric analysis of tissues of interest. Both localization and volumetric analysis can be facilitated by classifying the input data into different tissue types. This study compares classification results based on four different input image data: (1) MR data obtained with one TR and TE parameter combination, (2) MR data acquired with eight combinations of TR and TE parameters, (3) input data compressed into four principal components, and (4) the input data (3) together with first- and second-order texture features. The algorithms (a) to reduce the dimensionality using principal components, (b) to generate texture features, (c) to determine feature usefulness in classification using the Wilk's lambda criterion, and (d) to classify the input set are described. Classification results are found to be poor for input data 1 and much improved (qualitatively and quantatively) for input data 2. Input data 3 produces classification results (quantitatively and qualitatively) similar to that of 2. Input data 4 is found to produce quantitative results similar to those of 2 and 3, but the qualitative results show enhanced classification of some tissues and distortion of others.

Algorithms↗

PubMed1992

Individual trajectories of cognitive decline in patients with dementia of the Alzheimer type.

The course of decline was studied in 16 patients with probable or definite dementia of the Alzheimer type (DAT) over 2.7 to 6.8 years from first to last evaluation. Overall severity of dementia was measured with the Wechsler Adult Intelligence Scale (WAIS), the Dementia Rating Scale (DRS), and the Mini-Mental State Examination (MMSE), at approximately annual intervals. An initial plateau phase, during which language and cognitive functions did not change for periods of 9 to 35 months, was observed in 5 patients who initially had an isolated memory impairment without significant impairment of nonmemory language or visuospatial function. Once nonmemory functions began to decline, the rate of decline was remarkably steady in most individual patients but varied markedly among patients. The initial rate of decline after the plateau phase, as measured with the WAIS and DRS, was a significant predictor of subsequent rate in individual patients (r = .66, p less than .01, and r = .67, p less than .01, for the WAIS and DRS, respectively). The MMSE was a less reliable measure of longitudinal change in dementia severity and did not predict future rates of decline (r = .29). These results demonstrate a biphasic trajectory of decline in patients with DAT. Stable interindividual differences in rate of decline may provide a basis for designing more sensitive studies of treatments intended to slow or halt the progress of DAT.

Adult↗

PubMed2001

On the tactical significance of game situations in anticipating ball trajectories in tennis.

It is hypothesized that live playing situations preceding an opponent's strokes in tennis have sufficient significance to provide expert players with anticipative cues to estimate accurately the spatiotemporal characteristics of oncoming ball trajectories. Seven participants (all expert tennis players) had to watch two players opposed in high, moderate, and low tactical significance situations terminated by a stroke delivered by one of the two players in the direction of the participants. The participants' vision was occluded 100 ms after the stroke, and they had to indicate the zone reached by the ball at the moment of its rebound. Results showed that the essential anticipative information is contained in the view of the opponent's stroke movements, whatever the tactical significance of the situation.

Adult↗

PubMed2004

Influence of spermine on DNA conformation in a molecular dynamics trajectory of d(CGCGAATTCGCG)(2): major groove binding by one spermine molecule delays the A-->B transition.

The effect of spermine on the A-DNA to B-DNA transition in d(CGCGAATTCGCG)(2) has been investigated by five A-start molecular dynamics simulations, using the Cornell et al. potential. In the absence of spermine an A-->B transition is initiated immediately and the DNA becomes equidistant from the A- and B-forms at 200ps. In three DNA-spermine simulations, when a spermine is located across the major groove of A-DNA in one of three different initial locations, the time taken to reach equidistance from the A- and B-forms is delayed until 800, 950 or 1000ps. In each case the A-form appears to be temporarily stabilized by spermine's electrostatic interactions with phosphates on both sides of the major groove. The onset of the A-->B transition can be correlated with the spermine losing contact with phosphates on one side of the groove and with A-like --> B-like sugar pucker transitions in the vicinity of the spermine bridge. However in the fifth trajectory, in which the spermine initially threads from the major groove via the backbone into the minor groove, the B-->A transition occurs rapidly once again and the DNA is equidistant between the A- and B-forms within 300ps. This indicates that the mere presence of spermine is insufficient to delay the transition and that major groove binding stabilizes A-DNA.

Computer Simulation↗

PubMed1998

AIDS-related illness trajectories in Mexico: findings from a qualitative study in two marginalized communities.

This paper describes findings from a recent study examining how people affected directly and indirectly by the HIV/AIDS epidemic cope with HIV-related illness in Mexico. One-hundred-and-thirteen in-depth interviews were conducted with key informants in two contrasting communities: Ciudad Netzahualcóyotl (an economically marginalized community) and the gay community in Mexico City (a sexually marginalized community). This paper describes the AIDS-related wellness/illness careers or trajectories followed by individuals in both communities, and identifies critical points for material and emotional intervention. This career comprises four stages: (1) life before infection; (2) life surrounding the discovery of seropositivity; (3) living as an HIV-positive person; and (4) facing death. Comparisons are drawn between the processes of adjustment and coping found in both communities. In Ciudad Netzahualcóyotl, wellness/illness careers are closely linked to prevailing poverty and oppression, as well as the sense of urgency in which local people live their lives. In the case of the gay community, wellness/illness careers are associated with the intolerance and social repression faced by homosexual men. The paper concludes by suggesting possible interventions to improve the lives of people with HIV/AIDS in Mexico today.

Acquired Immunodeficiency Syndrome↗

PubMed2004

Living with a chronic deteriorating disease: the trajectory with muscular dystrophy over ten years.

PURPOSE: The aim of the study was to elucidate experiences of living with muscular dystrophy in terms of consequences for activity over 10 years. METHODS: The study population was identified in a prevalence study in a county of Sweden. Forty-six persons of this cohort with MD were interviewed. A qualitative research approach was chosen. The World Health Organization's International Classification of Functioning, Disability and Health (ICF) was used for categorization. RESULTS: Nearly all the subjects experienced a deterioration of physical capacity. Most obvious were the restrictions on mobility and increased fatigue and feebleness. The persons described psychosocial consequences of the muscular dystrophy as well as stigma when the disability had become more obvious. In spite of reported distress several persons experienced better psychological adaptation over time. The image of the future was often dark but several focus on today and avoid thoughts about the future. ICF showed some limitation with regard to temporal aspects, emotions and the subjective perspective. CONCLUSIONS: The knowledge of the trajectory with MD is important in order to offer the best possible treatment and support. Repeated assessment by ICF can serve as a valuable source of such knowledge, and a development of the classification would increase its usefulness in future analysis of functioning and disability.

Accidental Falls↗

PubMedDate not supplied

Comparing ontogenetic trajectories using growth process data.

Ontogenetic trajectories are commonly quantified by characterizing changes in the sizes and shapes of organisms over the course of development. This formulation of ontogenetic transformations can be misleading in that it ignores critical aspects of the biological processes responsible for constructing morphology. Hypothetical examples are used to illustrate some of the shortcomings of methods that rely exclusively on size and shape data for ontogenetic analyses. By characterizing growth as a vector field, and representing growth vectors as complex numbers, one can simultaneously analyze size, shape, and growth processes. The utility of such an approach is demonstrated in a study of shape and growth process variation in turtle shells.

Analysis of Variance↗

PubMed2005

Concurrent use of alcohol and cigarettes from adolescence to young adulthood: an examination of developmental trajectories and outcomes.

Concurrent use of alcohol and tobacco is common among adolescents, yet little is known about the developmental patterns of concurrent use, or the consequences associated with such patterns during young adulthood. Using data collected at six time points during 1985-1995 as part of an evaluation of a school-based substance abuse prevention program in California and Oregon, this study used latent growth mixture modeling to identify five distinct developmental trajectories of concurrent use of alcohol and tobacco from ages 13-23 in a cohort of 5873 individuals and compared these distinct groups with respect to demographic characteristics and young adult outcomes (at age 23 and age 29). Results suggest that while it is common during adolescence to drink but not smoke, it is very unusual to smoke and not drink. Compared to young people who smoked and drank consistently throughout their teens and early twenties, those who drank consistently but smoked only occasionally or dramatically decreased their smoking over time had lower rates of deviant behavior and predatory violence at age 23 and were less likely to have a history of arrest and substance use problems by age 29. This close examination of concurrent use of alcohol and cigarette use from ages 13-23 further accentuates the importance of curbing smoking behavior among adolescents before it becomes habitual.

Adolescent↗

PubMed2001

Angular weighted hybrid cone-beam CT reconstruction for circular trajectories.

Hybrid reconstruction techniques have been introduced for the volume reconstruction of axially truncated cone-beam computed tomography projection data acquired along a circular source-detector trajectory. The introduction of weighted half-scan techniques into this framework is described in this paper. Due to the cone-beam geometry it is not possible to perform the weighting on the projections as is typically done in conventional single-line computed tomography. Hence, in this paper we present an efficient way to incorporate angular weighting functions, depending on the object point position, into the framework of hybrid cone-beam reconstruction. Four different angular weighting functions are introduced and discussed with respect to their cone-beam artefact behaviour and their influence on the signal-to-noise ratio. As a result, the most effective angular weighting function for hybrid circular cone-beam reconstruction is determined by means of a simulation study based on mathematical phantoms and clinical data sets. This distance-weighted angular weighting scheme yields the best results in terms of high image quality, low computational complexity and signal-to-noise variations in the reconstruction volume.

Computer Simulation↗