View-based approaches to spatial representation in human vision

In an immersive virtual environment, observers fail to notice the expansion of a room around them and consequently make gross errors when comparing the size of objects. This result is difficult to explain if the visual system continuously generates a 3-D model of the scene based on known baseline information from interocular separation or proprioception as the observer walks. An alternative is that observers use view-based methods to guide their actions and to represent the spatial layout of the scene. In this case, they may have an expectation of the images they will receive but be insensitive to the rate at which images arrive as they walk. We describe the way in which the eye movement strategy of animals simplifies motion processing if their goal is to move towards a desired image and discuss dorsal and ventral stream processing of moving images in that context. Although many questions about view-based approaches to scene representation remain unanswered, the solutions are likely to be highly relevant to understanding biological 3-D vision

Transition stages of Rayleigh–Taylor instability between miscible fluids

Direct numerical simulations (DNS) are presented of three-dimensional, Rayleigh–Taylor instability (RTI) between two incompressible, miscible fluids, with a 3:1 density ratio. Periodic boundary conditions are imposed in the horizontal directions of a rectangular domain, with no-slip top and bottom walls. Solutions are obtained for the Navier–Stokes equations, augmented by a species transport-diffusion equation, with various initial perturbations. The DNS achieved outer-scale Reynolds numbers, based on mixing-zone height and its rate of growth, in excess of 3000. Initial growth is diffusive and independent of the initial perturbations. The onset of nonlinear growth is not predicted by available linear-stability theory. Following the diffusive-growth stage, growth rates are found to depend on the initial perturbations, up to the end of the simulations. Mixing is found to be even more sensitive to initial conditions than growth rates. Taylor microscales and Reynolds numbers are anisotropic throughout the simulations. Improved collapse of many statistics is achieved if the height of the mixing zone, rather than time, is used as the scaling or progress variable. Mixing has dynamical consequences for this flow, since it is driven by the action of the imposed acceleration field on local density differences

Nuclear mutations affecting mitochondrial structure and function in Chlamydomonas

Wild type cells of the green alga Chlamydomonas reinhardtii can grow in the in the dark by taking up and respiring exogenously supplied acetate. Obligate photoautotrophic (dark dier, dk) mutants of this alga have been selected which grow at near wild type rates in the light, but rapidly die when transferred to darkness because of defects in mitochondrial structure and function. In crosses of the dk mutants to wild type, the majority of the mutants are inherited in a mendelian fashion, although two have been isolated which are inherited in a clearly nonmendelian fashion. Nine mendelian dk mutants have been analyzed in detail, and belong to eight different complementation groups representing eight gene loci. These mutants have been tentatively grouped into three classes on the basis of the pleiotropic nature of their phenotypic defects. Mutants in Class I have gross alterations in the ultrastructure of their mitochondrial inner membranes together with deficiencies in cytochrome oxidase and antimycin/rotenone-sensitive NADH-cytochrome c reductase activities. Mutants in Class II have a variety of less severe alterations in mitochondrial ultrastructure and deficiencies in cytochrome oxidase activity. Mutants in Class III have normal or near normal mitochondrial ultrastructure and reduced cytochrome oxidase activity. Eight of the nine mutants show corresponding reductions in cyanide-sensitive respiration

Predicting the stability of atom-like and molecule-like unit-charge Coulomb three-particle systems

Non-relativistic quantum chemical calculations of the particle mass, m ± 2 , corresponding to the dissociation threshold in a range of Coulomb three-particle systems of the form {m ± 1 m ± 2 m ∓ 3 } , are performed variationally using a series solution method with a Laguerre-based wavefunction. These masses are used to calculate an accurate stability boundary, i.e., the line that separates the stability domain from the instability domains, in a reciprocal mass fraction ternary diagram. This result is compared to a lower bound to the stability domain derived from symmetric systems and reveals the importance of the asymmetric (mass-symmetry breaking) terms in the Hamiltonian at dissociation. A functional fit to the stability boundary data provides a simple analytical expression for calculating the minimum mass of a third particle required for stable binding to a two-particle system, i.e., for predicting the bound state stability of any unit-charge three-particle system

Nonabelian dark matter: models and constraints

Numerous experimental anomalies hint at the existence of a dark matter (DM) multiplet chi_i with small mass splittings. We survey the simplest such models which arise from DM in the low representations of a new SU(2) gauge symmetry, whose gauge bosons have a small mass mu < 1 GeV. We identify preferred parameters M_chi ~ 1 TeV, mu ~ 100 MeV, alpha_g ~ 0.04 and the chi chi -> 4e annihilation channel, for explaining PAMELA, Fermi, and INTEGRAL/SPI lepton excesses, while remaining consistent with constraints from relic density, diffuse gamma rays and the CMB. This consistency is strengthened if DM annihilations occur mainly in subhalos, while excitations (relevant to the excited DM proposal to explain the 511 keV excess) occur in the galactic center (GC), due to higher velocity dispersions in the GC, induced by baryons. We derive new constraints and predictions which are generic to these models. Notably, decays of excited DM states chi' -> chi gamma arise at one loop and could provide a new signal for INTEGRAL/SPI; big bang nucleosynthesis (BBN) constraints on the density of dark SU(2) gauge bosons imply a lower bound on the mixing parameter epsilon between the SU(2) gauge bosons and photon. These considerations rule out the possibility of the gauge bosons that decay into e^+e^- being long-lived. We study in detail models of doublet, triplet and quintuplet DM, showing that both normal and inverted mass hierarchies can occur, with mass splittings that can be parametrically smaller, e.g., O(100) keV, than the generic MeV scale of splittings. A systematic treatment of Z_2 symmetry which insures the stability of the intermediate DM state is given for cases with inverted mass hierarchy, of interest for boosting the 511 keV signal from the excited dark matter mechanism.Comment: 28 pages, 17 figures; v2. added brief comment, reference

Systemic Risk and the Refinancing Ratchet Effect

The confluence of three trends in the U.S. residential housing market-rising home prices, declining interest rates, and near-frictionless refinancing opportunities-led to vastly increased systemic risk in the financial system. Individually, each of these trends is benign, but when they occur simultaneously, as they did over the past decade, they impose an unintentional synchronization of homeowner leverage. This synchronization, coupled with the indivisibility of residential real estate that prevents homeowners from deleveraging when property values decline and homeowner equity deteriorates, conspire to create a "ratchet" effect in which homeowner leverage is maintained during good times without the ability to decrease leverage during bad times. If refinancing-facilitated homeowner-equity extraction is sufficiently widespread-as it was during the years leading up to the peak of the U.S. residential real-estate market-the inadvertent coordination of leverage during a market rise implies higher correlation of defaults during a market drop. To measure the systemic impact of this ratchet effect, we simulate the U.S. housing market with and without equity extractions, and estimate the losses absorbed by mortgage lenders by valuing the embedded put-option in non-recourse mortgages. Our simulations generate loss estimates of $1.5 trillion from June 2006 to December 2008 under historical market conditions, compared to simulated losses of$280 billion in the absence of equity extractions.Risk; Financial Crisis; Household Finance; Real Estate; Subprime