Vertical circularly polarized ELF magnetic fields and induced electric fields in culture media

Some properties of induced electric fields in cell culture media produced by vertical circularly polarized magnetic fields are examined. The described geometry is not advantageous for determining effects that may be attributable to induced electric fields or currents.     

Solidification of metallic alloys under magnetic fields

The present paper is aimed at showing how solidification of metallic alloys can be influenced by AC or DC magnetic fields through various types of effects. The application of AC magnetic fields leads to the generation of either fluid flows or vibration. It has been shown both numerically and experimentally that the electromagnetically-driven flows created by travelling or rotating magnetic fields promoted segregations and influenced their distribution. The flow may also promote the CET thanks to its effects on both the temperature and solute fields as well as the possible fragmentation mechanism. As far as DC magnetic fields are concerned, it was known that they usually exert a damping of the bulk fluid flows. However, it has been shown recently that for some alloys high intensity magnetic field interacts with the small thermoelectric current to create significant electromagnetic forces which are responsible for strong liquid metal flows both in the bulk and in the mushy zone. Orientation changes as well as possible modifications of thermodynamic properties were also observed.     

A simple network model simulates hippocampal place fields: II. Computing goal-directed trajectories and memory fields.

Place cells have been described as the computational elements of a neuronal cognitive mapping system that encodes and stores relationships among spatial stimuli (J. O'Keefe and L. Nadel, 1978). Furthermore, place cells seem to encode remembered locations because neural activity is maintained when the visual stimuli that influence place field location are vastly degraded, such as when cues are removed or the lights are turned off (O'Keefe and A. Speakman, 1987; G. J. Quirk et al, 1990). A feed-forward network model that mapped visual input onto a representation of location simulated some basic properties of hippocampal place fields, including resistance to disruption after partial cue memory (M. L. Shapiro and P. A. Hetherington, 1993). However, the simulated place fields required visual input for their activation. It is reported that a network that incorporates feedback (1) computed correct trajectories toward simulated goals and (2) simulated place fields that persist in the absence of visual input. The simulation suggests that feedback properties can provide a computational account of O'Keefe and Speakman's data. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

喷嘴出口与基板间距离对冷喷涂超音速流场的影响

采用计算流体力学（CFD）模拟研究了冷气动力喷涂技术中喷嘴出口与基板距离（SoD）对超音速流场（包括气体流场和粒子流流场）的影响,通过计算流场的分布特性,分析了流场中速度、温度、压力和密度的变化规律。模拟结果发现,由于基体前激波的存在,极大影响了气体和粒子流流场。气流形成连续交替的膨胀波、压缩波波系,在马赫盘处气体速度达到超声速,越过马赫盘以后速度下降很大。SoD为40 mm时,1μm粒子流流场达到最优;SoD为50 mm时,5μm和22μm粒子流流场达到最优,此时粒子流对基体的轰击速度最大。     

Head-scatter factors in rectangular photon fields

Head-scatter factors were measured for a range of field sizes of rectangular shape from a linear accelerator that provides 6 and 25 MV photon beams. For a given field length and width, exchange of the inner and outer collimator pairs produces differences in the head-scatter factor of up to about 2% and 3% in open and wedged fields, respectively. Dependence on elongation deviates by up to 2% from that given by the equivalent squares. Such effects can easily be taken into account by using head-scatter data that have been directly measured in rectangular fields or by applying empirical corrections to values for square or equivalent square fields.     

Equivalent squares of irregular photon fields

The tables of equivalent fields published by the British Journal of Radiology (BJR) are intended for calculation of depth-dose functions in rectangular photon fields. We have investigated the validity of the equivalent-field concept for fields of arbitrary shape over a range of photon energies, field sizes and depths. We show that the empirical scatter-radius function (Day function) used to generate the equivalent-field tables is a good approximation to the average over energy of normalized scatter-air ratios extracted from BJR beam data for depths up to 10 cm. However, this function tends to diverge from the data as depth increases. Accuracy can be improved by making the Day function depend on depth. Equivalent squares, determined by sector integration of the original or modified Day functions, are suitable for megavoltage photon-beam dose calculations at central-axis and off-axis points in irregular as well as rectangular fields.     

Low dimensionality of supraspinally induced force fields

Recent experiments using electrical and N-methyl-D-aspartate microstimulation of the spinal cord gray matter and cutaneous stimulation of the hindlimb of spinalized frogs have provided evidence for a modular organization of the frog's spinal cord circuitry. A "module" is a functional unit in the spinal cord circuitry that generates a specific motor output by imposing a specific pattern of muscle activation. The output of a module can be characterized as a force field: the collection of the isometric forces generated at the ankle over different locations in the leg's workspace. Different modules can be combined independently so that their force fields linearly sum. The goal of this study was to ascertain whether the force fields generated by the activation of supraspinal structures could result from combinations of a small number of modules. We recorded a set of force fields generated by the electrical stimulation of the vestibular nerve in seven frogs, and we performed a principal component analysis to study the dimensionality of this set. We found that 94% of the total variation of the data is explained by the first five principal components, a result that indicates that the dimensionality of the set of fields evoked by vestibular stimulation is low. This result is compatible with the hypothesis that vestibular fields are generated by combinations of a small number of spinal modules.     

Doubly Spectral Stochastic Finite-Element Method for Linear Structural Dynamics

Uncertainties in complex dynamic systems play an important role in the prediction of a dynamic response in the mid- and high-frequency ranges. For distributed parameter systems, parametric uncertainties can be represented by random fields leading to stochastic partial differential equations. Over the past two decades, the spectral stochastic finite-element method has been developed to discretize the random fields and solve such problems. On the other hand, for deterministic distributed parameter linear dynamic systems, the spectral finite-element method has been developed to efficiently solve the problem in the frequency domain. In spite of the fact that both approaches use spectral decomposition (one for the random fields and the other for the dynamic displacement fields), very little overlap between them has been reported in literature. In this paper, these two spectral techniques are unified with the aim that the unified approach would outperform any of the spectral methods considered on their own. An exponential autocorrelation function for the random fields, a frequency-dependent stochastic element stiffness, and mass matrices are derived for the axial and bending vibration of rods. Closed-form exact expressions are derived by using the Karhunen-Loève expansion. Numerical examples are given to illustrate the unified spectral approach.     

A simple network model simulates hippocampal place fields: Parametric analyses and physiological predictions.

Hippocampal place cells may be the computational units of a neuronal cognitive mapping system. A network model trained to compute locations from distal cues simulated the defining properties of hippocampal place cells (i.e., place-specific activation). The model produced units with detailed properties of place cells, including multiple subfields, "silent" and "noisy" cells, fields that persisted after cue removal, and groups of simulated field that overlapped in multiple clusters. Quantitative variants of the model showed that different properties of the fields were influenced by the complexity of the visual input (the number of spatial cues), the available computational resources (the number of hidden units), and the output encoding used to represent location. The simulations provide a framework for testing relationships between place field properties, variations in spatial environments, and the integrity of the hippocampal system. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

A new technique for positioning tangential fields

PURPOSE: A technique that eliminates the use of a mechanical "breast-bridge" for positioning tangential fields for treatment of the intact breast or chest wall has been developed. METHODS AND MATERIALS: Treatment set-up parameters are determined using measuring capabilities (gantry angles and source-skin distances) available on a standard simulator unit. A programmable scientific calculator is used to determine field geometry from polar coordinates for various points on the patient's skin. The calculator program determines the field size, a depth and lateral shift from a skin reference point to the isocenter for the tangential fields, and the gantry angles. The program provides additional information which facilitates the simulation process: First, the coordinates of the isocenter for the tangential fields are expressed relative to couch coordinates for an initial arbitrary isocenter so that the "auto go to" capability available on some simulators can be used. Second, the coordinates of the medial and lateral entry points can be edited when the first set of tangents are not accepted. This part of the program allows quick and efficient adjustment of the fields to obtain adequate treatment volume coverage and a minimum of irradiated lung or heart. RESULTS: Simulation of more than 300 patients has shown the technique to be a practical and efficient method for positioning tangential fields for breast or chest wall irradiation. CONCLUSION: The technique described here takes full advantage of the capabilities of the new generation of computer controlled simulators, and offers an alternative to previous methods employing a mechanical "breast-bridge."     

Effects of standoff distance on the supersonic flow fields in cold gas dynamic spraying

In this study,the effects of standoff distance(SoD)on the supersonic flow fields(including gas and particle flow fields)during cold gas dynamic spraying(CGDS)are investigated by means of computational fluid dynamics (CFD).The variation of velocity,temperature,pressure and density with different SoD is elucidated through the analysis of the distribution properties of the flow fields.It is found that the shock waves in front of the substrate remarkably influence the gas and particle flow fields.The wave system of expansion waves and pressure waves come into being continuously.The velocity of gas reaches the supersonic speed at the position of the Mach disc,while it decreases sharply when the gas flow crosses the Mach disc.The optimal SoDs are 40 mm for 1 μm particles and 50 mm for both 5μm and 22 μm particles.     

The effect of low-frequency electromagnetic fields on the neuroendocrine system

This paper presents literature data about effects of low-intensity variable electromagnetic fields on the neuroendocrine system of experimental animals. We mostly paid attention to electromagnetic fields frequently found in our environment, in technological processes, even in our everyday life. This study shows that the regulatory systems (nervous and endocrine) are extremely sensitive to effects of electromagnetic fields. In regard to structures of the central nervous system hypothalamus shows particularly high sensitivity whereas we can consider a hypothesis that effects of this physical factor may be expected in other systems too. It has been emphasized that the effects of electromagnetic fields on regulatory mechanisms may be connected with primary disturbances on the cellular and subcellular (mitochondrial) level.     

Anisotropic elastic fields of twist boundaries

Relations for linear anisotropic elastic fields of parallel arrays of dislocations are developed. These fields are used to compute the displacement fields of twist boundaries composed of a square grid of screw dislocations. For gold, the results are shown to agree to first order with fields deduced from X-ray diffraction studies and from atomistic simulations for twist boundaries in gold.     

Algorithm for Generating Samples of Homogeneous Gaussian Fields

A new algorithm is developed for generating samples of stationary Gaussian random fields. The algorithm is based on a model derived from the spectral representation theorem for weakly stationary random fields. The model consists of a superposition of a random number of waves with random amplitude and frequency, can match the second moment properties of any target random field, and becomes Gaussian as the intensity of two independent Poisson processes, defining the number of waves and their frequencies, increases indefinitely. In contrast to the current Monte Carlo simulation algorithms, the proposed algorithm: (1) does not produce periodic samples; and (2) does not require the discretization of the frequency domain. The proposed Monte Carlo algorithm is applied to generate samples of a stationary Gaussian field defined on 2.     

Large receptive fields for optic flow detection in humans

We used a psychophysical summation technique to study the properties of detectors tuned to radial, circular and translational motion, and to determine the spatial extent of their receptive fields. Signal-to-noise motion thresholds were measured for patterns curtailed spatially in various ways. Sensitivity for radial, circular and translational motion increased with stimulus area at a rate predicted by an ideal integrator. When sectors of noise were added to the stimulus, sensitivity decreased at a rate consistent with an ideal integrator. Summation was tested for large annular stimuli, and shown to hold up to 70 degrees in some cases, suggesting very large receptive fields for this type of motion (consistent with the physiology of neurones in the dorsal region of the medial superior temporal area (MSTd)). This is a far greater area than observed for summation of contrast sensitivity to gratings (Anderson SJ and Burr DC, Vis Res 1987;29:621-635, and to this type of stimuli (Morrone MC, Burr DC and Vaina LM, Nature 1995;376:507-509, consistent with the suggestion that the two techniques examine different levels of motion analysis.     

The effect of strong longitudinal magnetic fields on dose deposition from electron and photon beams

The use of strong, uniform, longitudinal magnetic fields for external electron and photon beam irradiation is considered. Using the EGS4 Monte Carlo code modified to account for the presence of magnetic fields, dramatic narrowing of penumbra for photon and electron irradiations is demonstrated. In the vicinity of heterogeneities, "hot" and "cold" spots due to multiple scattering in electron beams are reduced substantially. However, in the presence of strong magnetic fields, the effect of inhomogeneities can be observed far from the location of the inhomogeneity due to reduced "washout" caused by lateral multiple scattering. The enhanced "Bragg peak," proposed or calculated by other authors, is not observed on the central axis of broad beams, owing to lateral equilibrium. It is proven that for broad parallel beams, the central axis depth-dose curve is independent of the strength of the external longitudinal magnetic field, as long as it is uniform. However, strong longitudinal magnetic fields can induce enhancements by redirection of the electron fields coming from point sources. Strong uniform longitudinal magnetic fields provide a way of controlling the spreading of electron beams due to multiple scattering, making the electron beams more "geometrical" in character, simplifying dose-deposition patterns, possibly allowing electron beams to be used in new ways for radiotherapy. Photon therapy also benefits from strong uniform longitudinal magnetic fields since the penumbra or other lateral disequilibrium effects associated with lateral electron transport can be eliminated.     

Cyclic near-tip fields for fatigue cracks along metal-metal and metal-ceramic interfaces

Finite element analyses were conducted with the objective of determining the cyclic near-tip fields ahead of a stationary tensile fatigue crack lying along the interface between two dissimilar solids. The model systems analyzed are (i) a metal-metal bimaterial whose components have the same elastic properties but different plastic deformation characteristics, and (ii) a metal-ceramic bimaterial. In both cases, monotonic loading to the peak tensile load results in a predominantly mode I field ahead of the crack. However, unloading in one or both components of the bimaterial with prior plastic deformation generates mixed-mode conditions at the crack tip. The mode mixity persists during a significant portion of the next loading phase and is gradually removed upon reloading to the peak stress. The cyclic plastic zone size directly ahead of the fatigue crack is approximately one-fifth the size of the monotonic plastic zone. The residual compressive stresses within the cyclic plastic zone are computed. The implications of reversed yielding and mixed-mode near-tip fields to constant-amplitude and variable-amplitude fatigue fracture are discussed.     

Biological responses to electromagnetic fields

Electrification in developed countries has progressively increased the mean level of extremely low-frequency electromagnetic fields (ELF-EMFs) to which populations are exposed; these humanmade fields are substantially above the naturally occurring ambient electric and magnetic fields of approximately 10(-4) Vm(-1) and approximately 10(-13) T, respectively. Several epidemiological studies have concluded that ELF-EMFs may be linked to an increased risk of cancer, particularly childhood leukemia. These observations have been reinforced by cellular studies reporting EMF-induced effects on biological systems, most notably on the activity of components of the pathways that regulate cell proliferation. However, the limited number of attempts to directly replicate these experimental findings have been almost uniformly unsuccessful, and no EMF-induced biological response has yet been replicated in independent laboratories. Many of the most well-defined effects have come from gene expression studies; several attempts have been made recently to repeat these key findings. This review analyses these studies and summarizes other reports of major cellular responses to EMFs and the published attempts at replication. The opening sections discuss quantitative aspects of exposure to EMFs and the incidence of cancers that have been correlated with such fields. The concluding section considers the problems that confront research in this area and suggests feasible strategies.     

Behavioral spectral sensitivities of different retinal areas in pigeons.

The spectral sensitivity of the red and the yellow retinal fields of head-fixed pigeons was separately measured for wavelengths between 340 and 640 nm by a behavioral perimetric technique. Within this spectral range the mean spectral sensitivity of both fields was found to be maximal at 584 nm and minimal at the lower ultraviolet wavelengths. Differences in sensitivity were found, however, at shorter wavelengths, with the yellow field being more sensitive than the red at wavelengths below 500 nm and especially in the ultraviolet spectral range. These sensitivity differences are discussed in relation to other functional differences between the pigeon's retinal fields. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Biologic effects of low-level electromagnetic fields: current issues and controversies

There is uncertainty and controversy about the extent to which low level electromagnetic fields may cause deleterious effects, but even experts who are skeptical about many supposed hazards are willing to agree that electromagnetic fields even weaker than those in the MR environment can have effects under certain conditions. In order that readers can familiarize themselves enough with the subject to make an informed independent assessment, discuss it knowledgeably in public, and have the means with which to evaluate new developments and avoid experimental pitfalls if planning their own research in the area, they are provided with some of the most recent finding of in vitro and in vivo research from outside the MR literature as well as some of the results and controversies coming from recent epidemiological studies.