Ophthalmic factors in dyslexia

Although the role of ophthalmic factors in dyslexia remains the subject of controversy, recent research has indicated that the correlates of dyslexia may include binocular dysfunction, unstable motor ocular dominance, a deficit of the transient visual subsystem, and an anomaly that can be treated with tinted lenses. These features, typically, have been studied in isolation and their inter-relationship has received little attention. The aim of the present research was to investigate ophthalmic factors in dyslexia, with a particular emphasis on the interaction between optometric variables. Further aims were to establish the most appropriate investigative techniques for optometric practice and to explore the relationship between optometric and psychometric variables. A pilot study was used to refine the experimental design for a subsequent detailed study of 39 children with a specific reading disability and 43 good readers, who were selected from 240 children. The groups were matched for age, sex, and performance IQ. The following factors emerged as correlates of dyslexia: slight impaired visual acuity; reduced vergence amplitudes; increased vergence instability; decreased accommodative amplitude; poor peformance at tests that were designed to assess the function of the transient visual system; and slightly slower performance at a non-verbal simulated reading visual search task. The `transient system deficit', as measured by reduced flicker sensitivity, was significantly associated with decreased accommodative and vergence amplitudes. This links the motor and sensory visual correlates of dyslexia. Although the binocular dysfunction was correlated with increased symptoms, the difference in the groups' simulated reading visual search task performance was largely attributable to psychometric variables. The results suggest tht optometric problems may be a contributory factor in dyslexia, but are unlikely to play a key causative role. Several optometric variables were confounded by psychometric parameters, and this interaction should be a priority for future investigation

Averaging the intensity of many-layered structures for accurate stacking-fault analysis using Rietveld refinement

Many technologically important synthetic and natural materials display stacking faults which lead to complex peak broadenings, asymmetries and shifts in their powder diffraction patterns. The patterns can be described using an enlarged unit cell (called a supercell) containing an explicit description of the layers. Since the supercell can contain hundreds of thousands of atoms with hundreds of thousands of hkl reflections, a Rietveld approach has been too computationally demanding for all but the simplest systems. This article describes the implementation of the speed-ups necessary to allow Rietveld refinement in the computer program TOPAS Version 6 (Bruker AXS, Karlsruhe, Germany). Techniques implemented include: a peaks buffer that allows hundreds of thousands of hkl-dependent peak shapes to be automatically approximated by a few hundred peaks; an averaging process for hundreds of large supercells with minimum impact on computational time; a smoothing technique that allows for the use of small supercells which approximate supercells ten to 20 times larger; and efficient algorithms for stacking sequence generation. The result is Rietveld refinement of supercells operating at speeds several thousand times faster than traditional Rietveld refinements. This allows quantitative and simultaneous analysis of structure and microstructure in complex stacking-faulted samples

Determination of step--edge barriers to interlayer transport from surface morphology during the initial stages of homoepitaxial growth

We use analytic formulae obtained from a simple model of crystal growth by molecular--beam epitaxy to determine step--edge barriers to interlayer transport. The method is based on information about the surface morphology at the onset of nucleation on top of first--layer islands in the submonolayer coverage regime of homoepitaxial growth. The formulae are tested using kinetic Monte Carlo simulations of a solid--on--solid model and applied to estimate step--edge barriers from scanning--tunneling microscopy data on initial stages of Fe(001), Pt(111), and Ag(111) homoepitaxy.Comment: 4 pages, a Postscript file, uuencoded and compressed. Physical Review B, Rapid Communications, in press

Visual stress, its treatment with spectral filters, and its relationship to visually induced motion sickness

We review the concept of visual stress and its relation to neurological disease. Visual stress can occur from the observation of images with unnatural spatial structure and an excess of contrast energy at spatial frequencies to which the visual system is generally most sensitive. Visual stress can often be reduced using spectral filters, provided the colour is selected with precision to suit each individual. The use of such filters and their effects on reading speed are reviewed. The filters have been shown to benefit patients with a variety of neurological conditions other than reading difficulty, all associated with an increased risk of seizures. © 2009 Elsevier Ltd

The Influence of Soil Crusts on Heat and Water Storage, Chapter 5

There is a continuous exchange of water, air, and heat between the soil and atmosphere. This has a profound influence on weather, plant growth, and ground water storage. Increased advective energy transport and air turbulence are often consequences of heat exchanged between the soil and the atmosphere. The release of heat from a firmly packed soil which makes crop plants less apt to be frozen than those growing on a loose, recently cultivated soil is another example. The interchange of water between soil and atmosphere also profoundly affects our environment. If water reaches the soil surface faster than it can be absorbed, runoff and flooding occur. Water absorption by the soil is a basic requirement for ground water recharge and, consequently, necessary for the continued flow of all springs and wells. Taken from this veiwpoint, the exchange of heat and water through soil crusts is of quite general interest

Jamming coverage in competitive random sequential adsorption of binary mixture

We propose a generalized car parking problem where cars of two different sizes are sequentially parked on a line with a given probability $q$. The free parameter $q$ interpolates between the classical car parking problem of only one car size and the competitive random sequential adsorption (CRSA) of a binary mixture. We give an exact solution to the CRSA rate equations and find that the final coverage, the jamming limit, of the line is always larger for a binary mixture than for the uni-sized case. The analytical results are in good agreement with our direct numerical simulations of the problem.Comment: 4 pages 2-column RevTeX, Four figures, (there was an error in the previous version. We replaced it (including figures) with corrected and improved version that lead to new results and conclusions

Equilibrium Properties of A Monomer-Monomer Catalytic Reaction on A One-Dimensional Chain

We study the equilibrium properties of a lattice-gas model of an $A + B \to 0$ catalytic reaction on a one-dimensional chain in contact with a reservoir for the particles. The particles of species $A$ and $B$ are in thermal contact with their vapor phases acting as reservoirs, i.e., they may adsorb onto empty lattice sites and may desorb from the lattice. If adsorbed $A$ and $B$ particles appear at neighboring lattice sites they instantaneously react and both desorb. For this model of a catalytic reaction in the adsorption-controlled limit, we derive analytically the expression of the pressure and present exact results for the mean densities of particles and for the compressibilities of the adsorbate as function of the chemical potentials of the two species.Comment: 19 pages, 5 figures, submitted to Phys. Rev.

Weak spin interactions in Mott insulating La2O2Fe2OSe2

Identifying and characterizing the parent phases of iron-based superconductors is an important step towards understanding the mechanism for their high-temperature superconductivity. We present an investigation into the magnetic interactions in the Mott insulator La2O2Fe2OSe2. This iron oxyselenide adopts a 2-k magnetic structure with low levels of magnetic frustration. This magnetic ground state is found to be dominated by next-nearest-neighbor interactions J2 and J2′ and the magnetocrystalline anisotropy of the Fe2+ site, leading to 2D-Ising-like spin S=2 fluctuations. In contrast to calculations, the values are small and confine the spin excitations below ∼25 meV. This is further corroborated by sum rules of neutron scattering. This indicates that superconductivity in related materials may derive from a weakly coupled and unfrustrated magnetic structure

Asymptotic Capture-Number and Island-Size Distributions for One-Dimensional Irreversible Submonolayer Growth

Using a set of evolution equations [J.G. Amar {\it et al}, Phys. Rev. Lett. {\bf 86}, 3092 (2001)] for the average gap-size between islands, we calculate analytically the asymptotic scaled capture-number distribution (CND) for one-dimensional irreversible submonolayer growth of point islands. The predicted asymptotic CND is in reasonably good agreement with kinetic Monte-Carlo (KMC) results and leads to a \textit{non-divergent asymptotic} scaled island-size distribution (ISD). We then show that a slight modification of our analytical form leads to an analytic expression for the asymptotic CND and a resulting asymptotic ISD which are in excellent agreement with KMC simulations. We also show that in the asymptotic limit the self-averaging property of the capture zones holds exactly while the asymptotic scaled gap distribution is equal to the scaled CND.Comment: 4 pages, 1 figure, submitted to Phys. Rev.