The effect of artifacts on dependence measurement in fMRI

The study of effective connectivity by means of neuroimaging depends on the measurement of similarity between activity patterns at different locations in the brain, without necessarily presupposing a particular model for this dependence. When these interactions are measured using functional magnetic resonance imaging (fMRI) techniques, however, imaging and physiological artifacts create patterns of dependence that may be unrelated to cortical activity. We demonstrate some of these effects through the measurement of short-range dependencies present in fMRI scans of the primary visual cortex (V1) in the anaesthetized macaque monkey. High-field (4.7 T) fMRI scans were conducted to measure responses based on the blood oxygen level-dependent contrast mechanism, during periods of no sensory stimulation and of visual stimulation with rotating polar-transformed checkerboard gratings. Dependence between the haemodynamic activity at different spatial locations (i.e., different voxels) was measured using correlation, mutual information and functional covariance. Particular attention was paid to understanding the sources of spurious dependence that may be observed during such investigations. Two main effects were detected: (a) short-range correlations introduced by the process of image reconstruction and (b) perturbations in the haemodynamic response caused by breathing. The image reconstruction artifacts were shown to create an artificially high short-range dependence in the readout direction of the scan, and the breathing artifacts caused enhanced short-range dependence in both the readout and phase-encode directions. Additional dependence in the phase-encode direction due to image-ghosting is also possible but will not be discussed in this report, as it can be alleviated by fine adjustment of preemphasis (elimination of eddy currents). A technique is described for removing breathing artifacts, and the effect of breathing on the apparent dependence between voxels is illustrated. The correlation of haemodynamic activity with the stimulus was found to be affected by breathing, although this effect can be neutralised by averaging the haemodynamic responses over many repetitions of the stimulus. Nonetheless, patterns of dependent activity between voxels may be lost in this averaging process, which makes the removal of breathing artifacts necessary if statistical dependence and the study of effective connectivity is the primary aim of an investigation.     

A finite element formulation for the analysis of interlaminar shear stresses in layered composite plates

Fiber reinforced composite plates are often highly stressed structures. For an efficient design of such plates interlaminar stresses have to be evaluated as precisely as possible. In this paper we propose a finite element formulation for the analysis of transverse shear stresses in layered composite plates. For this purpose the equilibrium equations are exploited with appropriate shape functions in weak form which leads to a local discrete boundary value problem. Two different models to compute the derivatives of the strains are considered. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Using a Moving Mesh PDE for Cell Centres to Adapt a Finite Volume Grid

The paper is concerned with a grid adaptation approach based on a moving mesh partial differential equation. A method is proposed for discretizing this equation with a cell-centred finite volume method so that solution-dependent relocation of a fixed number of grid points without changing their topology becomes available as an attractive add-on for many finite volume solvers. Several interpolation strategies to determine appropriate cell corners from moved cell centre points are discussed and compared to each other. For a turbulent hill flow, numerical results are presented for two-dimensional adaptation based on an equidistribution of the gradient of the streamwise velocity and the production of turbulent kinetic energy.     

Statistical Analysis of the First Passage Path Ensemble of Jump Processes

The transition mechanism of jump processes between two different subsets in state space reveals important dynamical information of the processes and therefore has attracted considerable attention in the past years. In this paper, we study the first passage path ensemble of both discrete-time and continuous-time jump processes on a finite state space. The main approach is to divide each first passage path into nonreactive and reactive segments and to study them separately. The analysis can be applied to jump processes which are non-ergodic, as well as continuous-time jump processes where the waiting time distributions are non-exponential. In the particular case that the jump processes are both Markovian and ergodic, our analysis elucidates the relations between the study of the first passage paths and the study of the transition paths in transition path theory. We provide algorithms to numerically compute statistics of the first passage path ensemble. The computational complexity of these algorithms scales with the complexity of solving a linear system, for which efficient methods are available. Several examples demonstrate the wide applicability of the derived results across research areas.     

Ion beam sputter deposition of TiO<Subscript>2</Subscript> films using oxygen ions

TiO₂ thin films were grown by ion beam sputter deposition (IBSD) using oxygen ions, with the ion energy and geometrical parameters (ion incidence angle, polar emission angle, and scattering angle) being varied systematically. Metallic Ti and ceramic TiO₂ served as target materials. The thin films were characterized concerning thickness, growth rate, surface topography, structural properties, mass density, and optical properties. It was found that the scattering geometry has the main impact on the film properties. Target material, ion energy, and ion incidence angle have only a marginal influence. Former studies on reactive IBSD of TiO₂ using Ar and Xe ions reported equivalent patterns. Nevertheless, the respective ion species distinctively affects the film properties. For instance, mass density and the refractive index of the TiO₂ thin films are remarkably lower for sputtering with oxygen ions than for sputtering with Ar or Xe ions. The variations in the thin film properties are tentatively attributed to the angular and the energy distribution of the film-forming particles, especially, to those of the backscattered primary particles.     

Correlation of process parameters and properties of TiO<Subscript>2</Subscript> films grown by ion beam sputter deposition from a ceramic target

The correlation between process parameters and properties of TiO₂ films grown by ion beam sputter deposition from a ceramic target was investigated. TiO₂ films were grown under systematic variation of ion beam parameters (ion species, ion energy) and geometrical parameters (ion incidence angle, polar emission angle) and characterized with respect to film thickness, growth rate, structural properties, surface topography, composition, optical properties, and mass density. Systematic variations of film properties with the scattering geometry, namely the scattering angle, have been revealed. There are also considerable differences in film properties when changing the process gas from Ar to Xe. Similar systematics were reported for TiO₂ films grown by reactive ion beam sputter deposition from a metal target [C. Bundesmann et al., Appl. Surf. Sci. 421, 331 (2017)]. However, there are some deviations from the previously reported data, for instance, in growth rate, mass density and optical properties.     

Decompositions of injection patterns for nodal flow allocation in renewable electricity networks

The large-scale integration of fluctuating renewable power generation represents a challenge to the technical and economical design of a sustainable future electricity system. In this context, the increasing significance of long-range power transmission calls for innovative methods to understand the emerging complex flow patterns and to integrate price signals about the respective infrastructure needs into the energy market design. We introduce a decomposition method of injection patterns. Contrary to standard flow tracing approaches, it provides nodal allocations of link flows and costs in electricity networks by decomposing the network injection pattern into market-inspired elementary import/export building blocks. We apply the new approach to a simplified data-driven model of a European electricity grid with a high share of renewable wind and solar power generation.     

A Collaboration-based Approach to CFD Model Validation and Uncertainty Quantification (VUQ) Using Data from a Laminar Helium Plume

An effective approach to the model VUQ process by means of direct collaboration between computationalist and experimental data analyst is proposed. An analysis of data from a laminar helium plume experiment provides a demonstration of the proposed collaboration process. Consistency analysis serves a central role in the collaboration. It takes the data and uncertainties from both analyst and computationalist and provides an objective and quantifiable measure of agreement between the two. Despite the simplicity of the laminar helium system and the computational model, certain phenomena brought to light in the collaboration process make it difficult to find quantitative agreement in the data. These phenomena include the unsteady behavior of air flow in an open room, and the presence of helium permeation to the region near the plume. Important sources of error in the simulation include uncertainty in the room temperature (295.15 to 305.15 K), uncertainty in the helium inlet velocity (0.1215 \(\frac {m}{s}\) to 0.1415 \(\frac {m}{s}\)), and uncertainty in local helium permeation (0 % to 3 % by mass.) The collaboration process allows for a better understanding of the phenomena affecting the plume and the relative sensitivies of the system to these phenomena.     

Supply networks: Instabilities without overload

Supply and transport networks support much of our technical infrastructure as well as many biological processes. Their reliable function is thus essential for all aspects of life. Transport processes involving quantities beyond the pure loads exhibit alternative collective dynamical options compared to processes exclusively characterized by loads. Here we analyze the stability and bifurcations in oscillator models describing electric power grids and demonstrate that these networks exhibit instabilities without overloads. This phenomenon may well emerge also in other sufficiently complex supply or transport networks, including biological transport processes.     

Multinuclear NMR Study of Structure and Mobility in Cyclic Model Lithium Conducting Systems

The transport of the lithium ions is the basis of lithium ion conductivity of currently used electrolytes. Understanding how the transport of lithium ions within the matrix is influenced by the interactions with solvating moieties is needed to improve their performance. Along these lines well-defined model compounds based on cyclotriphosphazene (CTP) and hexaphenylbenzene (HPB) cores, bearing side groups of ethylene carbonate, a common solvent for lithium salts used as electrolytes in Li-ion batteries (Thielen et al. Chem. Mater, 23, 2120, 2011) and blended with different amounts of Lithium bis(trifluoromethanesulfonyl) imide (LiTFSI) have been studied by multinuclear nuclear magnetic resonance (NMR) spectroscopy. The local dynamics of the matrix was probed by ¹H and ³¹P NMR, while the local dynamics of the Li⁺ cations was unraveled by ⁷Li and ¹³C NMR. Transport of both ions was studied by pulsed-field gradient (PFG) NMR. Based on the different temperature dependences of the dynamics the bulk ion transport is not attributed to local dynamics, but to translational diffusion best characterized by PFG NMR. Although the glass transition temperatures of the blends are low, their conductivities are only in the range of typical polymer electrolytes. The results of NMR spectroscopy are in accord with the conjecture that the coordination between the cyclic carbonate functionality and the Li⁺-ion is too tight to allow for fast ion dynamics.