Metal-Insulator Transition Revisited for Cold Atoms in Non-Abelian Gauge Potentials

We discuss the possibility of realizing metal-insulator transitions with ultracold atoms in two-dimensional optical lattices in the presence of artificial gauge potentials. Such transitions have been extensively studied for magnetic fields corresponding to Abelian gauges; they occur when the magnetic flux penetrating the lattice plaquette is an irrational multiple of the magnetic flux quantum. Here we present the first study of these transitions for non-Abelian U(2) gauge fields, which can be realized with atoms with two pairs of degenerate internal states. In contrast to the Abelian case, the spectrum and localization transition in the non-Abelian case is strongly influenced by atomic momenta. In addition to determining the localization boundary, the momentum fragments the spectrum and the minimum energy viewed as a function of momentum exhibits a step structure. Other key characteristics of the non-Abelian case include the absence of localization for certain states and satellite fringes around the Bragg peaks in the momentum distribution and an interesting possibility that the transition can be tuned by the atomic momenta.Comment: 4 pages, 4 figures, see http://physics.gmu.edu/~isatija/recentpub.htm for high resolution figure

The visual representation of texture

This research is concerned with texture: a source of visual information, that has motivated a huge amount of psychophysical and computational research. This thesis questions how useful the accepted view of texture perception is. From a theoretical point of view, work to date has largely avoided two critical aspects of a computational theory of texture perception. Firstly, what is texture? Secondly, what is an appropriate representation for texture? This thesis argues that a task dependent definition of texture is necessary, and proposes a multi-local, statistical scheme for representing texture orientation. Human performance on a series of psychophysical orientation discrimination tasks are compared to specific predictions from the scheme. The first set of experiments investigate observers' ability to directly derive statistical estimates from texture. An analogy is reported between the way texture statistics are derived, and the visual processing of spatio-luminance features. The second set of experiments are concerned with the way texture elements are extracted from images (an example of the generic grouping problem in vision). The use of highly constrained experimental tasks, typically texture orientation discriminations, allows for the formulation of simple statistical criteria for setting critical parameters of the model (such as the spatial scale of analysis). It is shown that schemes based on isotropic filtering and symbolic matching do not suffice for performing this grouping, but that the scheme proposed, base on oriented mechanisms, does. Taken together these results suggest a view of visual texture processing, not as a disparate collection of processes, but as a general strategy for deriving statistical representations of images common to a range of visual tasks

Authors' response to commentaries: Do image descriptions underlie word recognition in reading?

This article is a response to commentaries on our original article 'The utility of image descriptions in the initial stages of vision: A case study of printed text' (Watt & Dakin, 2010)

The role of crowding in contextual influences on contour integration

Dakin and Baruch (2009) investigated how context influences contour integration, specifically reporting that near-perpendicular surrounding-elements reduced the exposure-duration observers required to localize and determine the shape of contours (compared to performance with randomly oriented surrounds) while near-parallel surrounds increased this time. Here, we ask if this effect might be a manifestation of visual crowding (the disruptive influence of "visual clutter" on object recognition). We first report that the effect generalizes to simple contour-localization (without explicit shape-discrimination) and influences tolerance to orientation jitter in the same way it affects threshold exposure-duration. We next directly examined the role of crowding by quantifying observers' local uncertainty (about the orientation of the elements that comprised our contours), showing that this largely accounts for the effects of context on global contour integration. These findings support the idea that context influences contour integration at a predominantly local stage of processing and that the local effects of crowding eventually influence downstream stages in the cortical processing of visual form

Theories of developmental dyslexia: Insights from a multiple case study of dyslexic adults

A multiple case study was conducted in order to assess three leading theories of developmental dyslexia: the phonological, the magnocellular (auditory and visual) and the cerebellar theories. Sixteen dyslexic and 16 control university students were administered a full battery of psychometric, phonological, auditory, visual and cerebellar tests. Individual data reveal that all 16 dyslexics suffer from a phonological deficit, 10 from an auditory deficit, 4 from a motor deficit, and 2 from a visual magnocellular deficit. Results suggest that a phonological deficit can appear in the absence of any other sensory or motor disorder, and is sufficient to cause a literacy impairment, as demonstrated by 5 of the dyslexics. Auditory disorders, when present, aggravate the phonological deficit, hence the literacy impairment. However, auditory deficits cannot be characterised simply as rapid auditory processing problems, as would be predicted by the magnocellular theory. Nor are they restricted to speech. Contrary to the cerebellar theory, we find little support for the notion that motor impairments, when found, have a cerebellar origin, or reflect an automaticity deficit. Overall, the present data support the phonological theory of dyslexia, while acknowledging the presence of additional sensory and motor disorders in certain individuals

Pion Photoproduction Amplitude Relations in the 1/N_c Expansion

We derive expressions for pion photoproduction amplitudes in the 1/N_c expansion of QCD, and obtain linear relations directly from this expansion that relate electromagnetic multipole amplitudes at all energies. The leading-order relations in 1/N_c compare favorably with available data, while the next-to-leading order relations seem to provide only a small improvement. However, when resonance parameters are compared directly, the agreement at O(1/N_c) or O(1/N_c^2) is impressive.Comment: 19 pages, ReVTeX, 50 eps files combine into 5 compound figure

Pion-Nucleon Scattering Relations at Next-to-Leading Order in 1/N_c

We obtain relations between partial-wave amplitudes for pi-N-->pi-N and pi-N-->pi-Delta directly from large N_c QCD. While linear relations among certain amplitudes holding at leading order (LO) in 1/N_c were derived in the context of chiral soliton models two decades ago, the present work employs a fully model-independent framework based on consistency with the large N_c expansion. At LO we reproduce the soliton model results; however, this method allows for systematic corrections. At next-to-leading order (NLO), most relations require additional unknown functions beyond those appearing at leading order (LO) and thus have little additional predictive power. However, three NLO relations for the pi-N-->pi-Delta reaction are independent of unknown functions and make predictions accurate at this order. The amplitudes relevant to two of these relations were previously extracted from experiment. These relations describe experiment dramatically better than their LO counterparts.Comment: 8 pages, 2 figures; references adde

The development of an automated sentence generator for the assessment of reading speed

Reading speed is an important outcome measure for many studies in neuroscience and psychology. Conventional reading speed tests have a limited corpus of sentences and usually require observers to read sentences aloud. Here we describe an automated sentence generator which can create over 100,000 unique sentences, scored using a true/false response. We propose that an estimate of the minimum exposure time required for observers to categorise the truth of such sentences is a good alternative to reading speed measures that guarantees comprehension of the printed material. Removing one word from the sentence reduces performance to chance, indicating minimal redundancy. Reading speed assessed using rapid serial visual presentation (RSVP) of these sentences is not statistically different from using MNREAD sentences. The automated sentence generator would be useful for measuring reading speed with button-press response (such as within MRI scanners) and for studies requiring many repeated measures of reading speed

HADRONIC INTERACTIONS IN LARGE N_c QCD: STUDIES OF EXCITED BARYON DECAYS AND SCATTERING RELATIONS

Decays and scattering events are two of the principal ways to learn about particle physics. Decays, in which a particle spontaneously disintegrates and we examine the debris, are quantified by a decay width. The decay of a resonance state provides information about the structure of the state and the interaction between its components. In particular, we can learn about the dynamics of quarks and gluons by studying the decay of hadrons. Scattering, in which particles are directed towards each other and interact, are quantified by partial-wave amplitudes. These amplitudes give us information about the interaction between the scattered particles. In principle, all of hadronic physics follows from quantum chromodynamics (QCD), which describes the interactions of quarks and gluons. However, the techniques of perturbation theory are not applicable to QCD at low energy because the strong coupling constant (the natural choice for the expansion parameter) is large at the energy scale of hadronic physics. A powerful model-independent method is the 1/N_c expansion in which the number of quark color degrees of freedom (N_c) is treated as a large number. This thesis presents the application of the 1/N_c expansion to the calculation of physical observables for excited baryons, pion-nucleon scattering, and pion photoproduction. The framework of the contracted SU(4) group that emerges in large N_c QCD is applied to the study of excited baryon decays. The N_c power scaling of the excited baryon's decay width depends on the symmetry of its spin-flavor wavefunction. The scaling with N_c for different symmetries is discussed in the context of a quark-shell model that permits mixing of different symmetry types. The subtle issues concerning the legitimacy of applying the contracted SU(4) group theory to excited baryons are discussed. The contracted SU(4) spin-flavor symmetry severely restricts the angular momentum and isospin dependence of partial-wave amplitudes. The consequences of this restriction on pion-nucleon scattering and pion photoproduction are discussed. In particular, model-independent linear relations among different hadronic scattering amplitudes holding to leading order in 1/N_c are obtained and compared with experimental data. The group-theoretic structure of large N_c QCD allows for a systematic expansion of scattering amplitudes in powers of 1/N_c which leads to model-independent relations holding to next-to-leading order in 1/N_c. These relations are derived and shown to compare more favorably with experiments to the extent expected for the 1/N_c expansion