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At least 235 records · Page 13Linked to original sources
PubMed2004

Hearing and performance during a 70-h exposure to noise simulating the space station environment.

INTRODUCTION: Elevated hearing thresholds have been documented in some astronauts after long-term spaceflights although noise levels were lower than those normally associated with noise-induced hearing loss in ground-based operations. The present study was conducted to determine whether prolonged exposure (70 h) to levels (72 dBA) recorded on the International Space Station (ISS) service module would impact diverse measures of auditory function, as well as cognition and memory, motivation, and cardiovascular function. METHOD: Five mixed gender subgroups of five normal-hearing subjects, aged 20-50 yr, were sequestered for 70 h in an environment that modeled conditions on the ISS. They were assigned to one of three background conditions: quiet (n = 5), continuous noise from the ISS service module (n = 10), or continuous noise during the day only (n = 10). Subjects were tested repeatedly within and across days as individuals or pair mates. RESULTS: There were no negative effects of the noise on any of the outcome measures. Introduction of a delay or noise in a communication channel used in the assessment of speech communicability significantly affected the time taken for joint problem solving by partners. DISCUSSION: The results of this study were not consistent with the observation of hearing loss measured after spaceflights. Nor were changes evident in cognition, motivation, or cardiovascular function. Factors which might account for the discrepancy are discussed.

Adult↗
PubMed2002

Real-time quantification of T(2)(*) changes using multiecho planar imaging and numerical methods.

Conventional approaches to quantify whole brain T(2)(*) maps use nonlinear regression with intensive computational requirements that therefore likely limit quantitative T(2)(*) mapping for real-time applications. To overcome these limitations an alternative method, NumART(2)(*) (NUMerical Algorithm for Real-time T(2)(*) mapping) that directly calculates T(2)(*) by a linear combination of images obtained at three or more different echo times was developed. NumART(2)(*), linear least-squares, and nonlinear regression techniques were applied to multiecho planar images of the human brain and to simulated data. Although NumART(2)(*) may overestimate T(2)(*), it yields comparable values to regression techniques in cortical and subcortical areas, with only moderate deviations for echo spacings between 18 and 40 ms. NumART(2)(*), like linear regression, requires 2% of the computational time needed for nonlinear regression and compares favorably with linear regression due to its higher precision. The use of NumART(2)(*) for continuous on-line T(2)(*) mapping in real time fMRI studies is shown.

Adult↗
PubMed1992

Merocyanine 540-sensitized photoinactivation of leukemia cells: effects of dose fractionation.

The differential sensitivity to merocyanine 540 (MC540)-sensitized photoirradiation of leukemia cells, selected solid tumor cells, and normal pluripotent hematopoietic stem cells has been successfully exploited for the extracorporeal purging of simulated autologous remission bone marrow grafts. In this communication, we compare the effects of fractionated vs continuous irradiation upon the MC540-sensitized photoinactivation of L1210 and K562 leukemia cells. Exposure to MC540 (15 micrograms/mL) and fractionated doses of white light inactivated fewer in vitro clonogenic cells than exposure to an equivalent dose of continuous irradiation, provided the irradiation doses were small (8.1-16.2 kJ/m2) and spaced 1-2 h apart. The dye-sensitized photoinactivation of leukemia cells was enhanced when cells were stored at 4 degrees C instead of 37 degrees C between irradiation periods, most likely in part because the cells were unable to repair sublethal photodynamic damages at the lower temperature. These data suggest that cells can recover from sublethal damage inflicted by the plasma membrane-active photosensitizer, MC540.

Animals↗
PubMed2005

Short- and long-range topological correlations in two-dimensional aggregation of dense colloidal suspensions.

We have studied the average properties and the topological correlations of computer-simulated two-dimensional (2D) aggregating systems at different initial surface packing fractions. For this purpose, the centers of mass of the growing clusters have been used to build the Voronoi diagram, where each cell represents a single cluster. The number of sides (n) and the area (A) of the cells are related to the size of the clusters and the number of nearest neighbors, respectively. We have focused our paper in the study of the topological quantities derived from number of sides, n , and we leave for a future work the study of the dependence of these magnitudes on the area of the cells, A . In this work, we go beyond the adjacent cluster correlations and explore the organization of the whole system of clusters by dividing the space in concentric layers around each cluster: the shell structure. This method allows us to analyze the time behavior of the long-range intercluster correlations induced by the aggregation process. We observed that kinetic and topological properties are intimately connected. Particularly, we found a continuous ordering of the shell structure from the earlier stages of the aggregation process, where clusters positions approach a hexagonal distribution in the plane. For long aggregation times, when the dynamic scaling regime is achieved, the short- and long-range topological properties reached a final stationary state. This ordering is stronger for high particle densities. Comparison between simulation and theoretical data points out the fact that 2D colloidal aggregation in the absence of interactions (diffusion-limited cluster aggregation regimen) is only able to produce short-range cluster-cluster correlations. Moreover, we showed that the correlation between adjacent clusters verifies the Aboav-Weaire law, while all the topological properties for nonadjacent clusters are mainly determined by only two parameters: the second central moment of number-of-sides distribution mu(2) = sumP (n) (n-6)(2) and the screening factor a (defined through the Aboav-Weaire equation). We also found that the values of mu(2) and a calculated for two-dimensional aggregating system are related through a single universal common form a proportional to mu2(-0.89), which is independent of the particle concentration.

Journal Article↗
PubMed2004

Weak first-order superfluid-solid quantum phase transitions.

We study superfluid-solid zero-temperature transitions in two-dimensional lattice boson-spin models using worm-algorithm Monte Carlo simulations. We observe that such transitions are typically first order with the exception of special high-symmetry points which require fine-tuning in the Hamiltonian parameter space. We present evidence that the superfluid-checkerboard solid and superfluid-valence-bond solid transitions at half-integer filling factor are extremely weak first-order transitions and in small systems can be confused with continuous or high-symmetry points.

Journal Article↗
PubMed1999

Effects of head-slaved and peripheral displays on lane-keeping performance and spatial orientation.

To improve the efficiency of images presented in low-cost vehicle simulators, the virtual viewing direction (i.e., the direction in which the image is rendered) can be head-slaved, the display can be surrounded with a less detailed peripheral image, or both. Three simulator experiments were used to evaluate the effect of these techniques on lane-keeping performance and spatial orientation. In Experiment 1, vehicle references or a head-slaved display (HSD) provided feedback on the virtual viewing direction. Vehicle references improved lane-keeping performance somewhat with a standard 50 degrees h x 50 degrees v display. An HSD (50 degrees h x 50 degrees v) allowed better steering performance, but not to the levels obtained with a wide display (150 degrees h x 50 degrees v). Experiments 2a and 2b evaluated the effects of surrounding the HSD with a less detailed peripheral image and of moving the HSD discretely or continuously. With the peripheral image, lane-keeping performance (Experiment 2a) and spatial orientation (Experiment 2b) were similar to those with a wide display. In both experiments, performance with the discretely moving HSD was superior to that with the continuously moving HSD. The results show that low-cost driving simulators can be equipped with more efficient displays that are as effective as wide displays for lane-keeping and spatial orientation.

Adult↗
PubMed1998

Virtual percutaneous transluminal coronary angioplasty system for an educational support system.

With the increase of ischemic diseases of the circulatory system in Japan, the necessity for cardiovascular intervention continues to increase. However, because intervention operations have been developed only recently, the education system for the corresponding specialists in our country has not been established yet. In this study, an intervention educational support system using a virtual reality (VR) technique was developed. Of course, intervention includes percutaneous transluminal coronary angioplasty (PTCA). PTCA is an operation enlarging the stenosis of the coronary artery with a balloon through a guide catheter. The technique that lets the guide wire for the PTCA balloon cross the stenosis is one of the most difficult. A simulation system for the manipulation of a torque device for the PTCA guide wire was developed with this technology for VR. In virtual space, reconstitution of the 3-dimensional coronary artery with atherosclerosis was performed, and the virtual PTCA system was produced experimentally. An interesting system was produced experimentally as a system for the training of doctors studying to become interventional specialists. After the system was combined with an Expert system for treating ischemic heart diseases, its usefulness was steadily increased. With the development of more sophisticated VR methodology in the future, a PTCA training system without using a patient will be embodied.

Angioplasty, Balloon, Coronary↗
PubMed2000

Modelling exposure opportunities: estimating relative risk for motor neurone disease in Finland.

This paper addresses the issues surrounding an individual's exposure to potential environmental risk factors, which can be implicated in the aetiology of a disease. We hope to further elucidate the 'lag' or latency period between the initial exposure to potential pathogens and the physical emergence of the disease, with specific reference to the rare neurological condition, motor neurone disease (MND), using a dataset obtained from the Finnish Death Certificate registry, for MND deaths between the period 1985-1995. A space-time approach is adopted, whereby patterns in both time and space are considered. No prior assumptions about the aetiology of MND are adopted. By using methods for the analysis of point processes, which preserve the continuous nature of the data, we resolve some of the problems of analysis that are often based on arbitrary areal units, such as postcode boundaries, or political boundaries. We use kernel estimation to model space-time patterns. Raised relative risk is assessed by adopting appropriate adjustments for the underlying population at risk, with the use of controls. Significance of the results is assessed using Monte Carlo simulation, and comparisons are made with results obtained from Openshaw's geographical analysis machine (GAM). Our results demonstrate the utility of kernel estimation as a visualisation tool. Small areas of elevated risk are identified, which need to be more closely examined before any firm conclusions can be drawn. We highlight a number of issues concerning the inadequacies of the data, and possibly of the techniques themselves.

Finland↗
PubMed1997

Self-sustained pH oscillations in a compartmentalized enzyme reactor system.

This work represents our continued effort toward fulfilling the need to discover a model system for experimental investigations of temporal oscillations in an enzyme-membrane system. In this paper, the regions in the parameter space where self-sustained pH oscillations can be induced for a compartmentalized enzyme reactor system, which consists of a well-stirred reactor, a reservoir and a membrane containing no enzyme, were determined via numerical simulation with two proteolytic enzymes: papain (EC 3.4.22.2) and alpha-chymotrypsin (EC 3.4.21.1). The sizes of the regions were qualitatively compared with those associated with enzymic membrane system. As a result, we found that the possibility of experimentally observing self-sustained oscillations in the compartmentalized papain reactor system, as well as in the papain-membrane system, is high. However, self-sustained pH oscillations are less likely in the compartmentalized alpha-chymotrypsin reactor system than in the alpha-chymotrypsin-membrane system.

Journal Article↗
PubMed2001

Evolution of cooperation without reciprocity.

A long-standing problem in biological and social sciences is to understand the conditions required for the emergence and maintenance of cooperation in evolving populations. For many situations, kin selection is an adequate explanation, although kin-recognition may still be a problem. Explanations of cooperation between non-kin include continuing interactions that provide a shadow of the future (that is, the expectation of an ongoing relationship) that can sustain reciprocity, possibly supported by mechanisms to bias interactions such as embedding the agents in a two-dimensional space or other context-preserving networks. Another explanation, indirect reciprocity, applies when benevolence to one agent increases the chance of receiving help from others. Here we use computer simulations to show that cooperation can arise when agents donate to others who are sufficiently similar to themselves in some arbitrary characteristic. Such a characteristic, or 'tag', can be a marking, display, or other observable trait. Tag-based donation can lead to the emergence of cooperation among agents who have only rudimentary ability to detect environmental signals and, unlike models of direct or indirect reciprocity, no memory of past encounters is required.

Altruism↗
PubMed2003

Rényi entropies characterizing the shape and the extension of the phase space representation of quantum wave functions in disordered systems.

We discuss some properties of the generalized entropies, called Rényi entropies, and their application to the case of continuous distributions. In particular, it is shown that these measures of complexity can be divergent; however, their differences are free from these divergences, thus enabling them to be good candidates for the description of the extension and the shape of continuous distributions. We apply this formalism to the projection of wave functions onto the coherent state basis, i.e., to the Husimi representation. We also show how the localization properties of the Husimi distribution on average can be reconstructed from its marginal distributions that are calculated in position and momentum space in the case when the phase space has no structure, i.e., no classical limit can be defined. Numerical simulations on a one-dimensional disordered system corroborate our expectations.

Journal Article↗
PubMed1984

Neonatal cerebral blood flow velocity I. An in vitro validation of the pulsed Doppler technique.

A combined pulsed and continuous Doppler instrument was used to assess flow velocities in an in vitro model designed to simulate small deeply lying arterial vessels. Diameters of the model vessels, depth under the transducers and the pulsatile flow patterns were chosen to simulate the corresponding conditions that can be expected when cerebral blood flow velocities are measured with this technique through the anterior fontanel in newborn infants. Computer analysis of the space average velocity signal from the pulsed Doppler mode, showed that the area under this curve, which corresponds to distance, was closely correlated to true flow over a wide range of flows in vessels of different diameters. This variable is suggested to be the closest estimate of true flow, when the diameter of the vessel under study cannot be accurately measured.

Blood Flow Velocity↗
PubMed2000

Changing objects lead briefly flashed ones.

Continuous, predictable events and spontaneous events may coincide in the visual environment. For a continuously moving object, the brain compensates for delays in transmission between a retinal event and neural responses in higher visual areas. Here we show that it similarly compensated for other smoothly changing features. A disk was flashed briefly during the presentation of another disk of continuously changing color, and observers compared the colors of the disks at the moment of flash. We also tested luminance, spatial frequency and pattern entropy; for all features, the continuously changing item led the flashed item in feature space. Thus the visual system's ability to compensate for delays in information about a continuously changing stimulus may extend to all features. We propose a model based on backward masking and priming to explain the phenomenon.

Adaptation, Physiological↗
PubMed1991

Evolution of bioconvective patterns in variable gravity.

Measurements are reported of the evolution of bioconvective patterns in shallow, dense cultures of microorganisms subjected to varying gravity. Various statistical properties of this random, quasi-two-dimensional structure have been found: Aboav's law is obeyed, the average vertex angles follow predictions for regular polygons, and the area of a pattern varies linearly with its number of sides. As gravity varies between 1 g and 1.8g (g = 9.8 m s-1), these statistical properties continue to hold despite a tripling of the number of polygons and a reduced average polygon dimension by a third. This work compares with experiments on soap foams, Langmuir monolayer foams, metal grains, and simulations.

Animals↗
PubMed1999

Simulation of spike-burst generation and Ca(2+) oscillation in pancreatic beta-cells.

Based on the experimental evidence that Na(+)-Ca(2+) exchange participates in the regulation of intracellular Ca(2+) concentration in pancreatic beta-cells, we construct a mathematical model for the cyclic spike-bursts and oscillations of intracellular Ca(2+) concentration. In our model, an increase in ATP concentration by the stimulation of glucose metabolism leads to the closure of ATP-sensitive K(+) channels (K(ATP) channels) and gradual depolarization to the threshold of voltage-gated Ca(2+) channels. Spikes are generated by the alternate activation of voltage-gated Ca(2+) and K(+) channels, causing Ca(2+) entry. The accumulated Ca(2+) ions are extruded by Na(+)-Ca(2+) exchange and Ca(2+) active transport. An increase in Na(+) influx through Na(+)-Ca(2+) exchangers results in a rise in intracellular Na(+) concentration and the activation of Na(+)-K(+) active transport. The consumption of ATP during the process of Ca(2+) extrusion leads to the opening of K(ATP) channels and repolarization. The present model could reproduce the main experimental features of the spike-burst activity and Ca(2+) oscillations following changes in the extracellular glucose concentration. As the rate of ATP production increases, the spike-burst pattern changes from bursts with long silent phases to continuous spiking. Changes in the pattern of electrical activity produced by the alteration of extracellular Na(+) and K(+) concentrations and the addition of ouabain could be reproduced in the present model.

Action Potentials↗
PubMed2005

The FLUKA code: new developments and application to 1 GeV/n iron beams.

The modeling of ion transport and interactions in matter is a subject of growing interest, driven by the continuous increase of possible application fields. These include hadron therapy, dosimetry, and space missions, but there are also several issues involving fundamental research, accelerator physics, and cosmic ray physics, where a reliable description of heavy ion induced cascades is important. In the present work, the capabilities of the FLUKA code for ion beams will be briefly recalled and some recent developments presented. Applications of the code to the simulation of therapeutic carbon, nitrogen and oxygen ion beams, and of iron beams, which are of direct interest for space mission related experiments, will be also presented together with interesting consideration relative to the evaluation of dosimetric quantities. Both applications involve ion beams in the AGeV range.

Carbon↗
PubMed1993

Influence of diffusion and gravity on the adhesion of a two-component mixture of hard spheres on a flat surface.

The adhesion of hard spheres, modeling particles of biological interest (proteins, bacteria, cells), on flat surfaces is investigated by means of Monte Carlo simulations. These computations include the Brownian diffusion of the particles in the bulk fluid, as well as the systematic displacement due to the gravitational field. The size of the particles influences directly both diffusion coefficient and net weight, with the consequence that the coverage at the jamming limit depends on this parameter. Results obtained in a former paper based on a lattice model are confirmed by the present continuous space model. In order to gain a better understanding of the adsorption competition of two types of particles, the proposed model is applied to the case of binary mixtures of spheres. For polydispersed suspensions, various parameters determine the final coverage, as well as the distribution of the small and large particles on the surface: the radii of the particles and the respective proportions of them in the infinitely large reservoir from which they are randomly selected. In this way, it is shown that the chronology of the adhesion of the small and large particles strongly influences the final number of each type of spheres fixed on the surface. Qualitatively, the present results resemble those obtained with disks placed by means of a classical random sequential adsorption mechanisms. Quantitatively, however, the number densities and coverage values determined in this way are significantly different due to the inclusion of the gravity and of the diffusion in the model.

Adhesiveness↗
PubMed1998

Efficient scatter modelling for incorporation in maximum likelihood reconstruction.

Definition of a simplified model of scatter which can be incorporated in maximum likelihood reconstruction for single-photon emission tomography (SPET) continues to be appealing; however, implementation must be efficient for it to be clinically applicable. In this paper an efficient algorithm for scatter estimation is described in which the spatial scatter distribution is implemented as a spatially invariant convolution for points of constant depth in tissue. The scatter estimate is weighted by a space-dependent build-up factor based on the measured attenuation in tissue. Monte Carlo simulation of a realistic thorax phantom was used to validate this approach. Further efficiency was introduced by estimating scatter once after a small number of iterations using the ordered subsets expectation maximisation (OSEM) reconstruction algorithm. The scatter estimate was incorporated as a constant term in subsequent iterations rather than modifying the scatter estimate each iteration. Monte Carlo simulation was used to demonstrate that the scatter estimate does not change significantly provided at least two iterations OSEM reconstruction, subset size 8, is used. Complete scatter-corrected reconstruction of 64 projections of 40¿128 pixels was achieved in 38 min using a Sun Sparc20 computer.

Algorithms↗