The effects of aging on orientation discrimination

AbstractThe current experiments measured orientation discrimination thresholds in younger (mean age≈23 years) and older (mean age≈66 years) subjects. In Experiment 1, the contrast needed to discriminate Gabor patterns (0.75, 1.5, and 3c/deg) that differed in orientation by 12deg was measured for different levels of external noise. At all three spatial frequencies, discrimination thresholds were significantly higher in older than younger subjects when external noise was low, but not when external noise was high. In Experiment 2, discrimination thresholds were measured as a function of stimulus contrast by varying orientation while contrast was fixed. The resulting threshold-vs-contrast curves had very similar shapes in the two age groups, although the curve obtained from older subjects was shifted to slightly higher contrasts. At contrasts greater than 0.05, thresholds in both older and younger subjects were approximately constant at 0.5deg. The results from Experiments 1 and 2 suggest that age differences in orientation discrimination are due solely to differences in equivalent input noise. Using the same methods as Experiment 1, Experiment 3 measured thresholds in 6 younger observers as a function of external noise and retinal illuminance. Although reducing retinal illumination increased equivalent input noise, the effect was much smaller than the age difference found in Experiment 1. Therefore, it is unlikely that differences in orientation discrimination were due solely to differences in retinal illumination. Our findings are consistent with recent physiological experiments that have found elevated spontaneous activity and reduced orientation tuning on visual cortical neurons in senescent cats (Hua, T., Li, X., He, L., Zhou, Y., Wang, Y., Leventhal, A. G. (206). Functional degradation of visual cortical cells in old cats. Neurobiology Aging, 27(1), 155–162) and monkeys (Yu, S., Wang, Y., Li, X., Zhou, Y. & Leventhal, A. G. (2006). Functional degradation of visual cortex in senescent rhesus monkeys. Neuroscience, 140(3), 1023–1029; Leventhal, A. G., Wang, Y., Pu, M., Zhou, Y. & Ma. Y. (2003). GABA and its agonists improved visual cortical function in senescent monkeys. Science,300 (5620), 812–815)

Influence of vegetation on the ITCZ and South Asian monsoon in HadCM3

The role of extra-tropical vegetation on the large-scale tropical circulation is examined in the version 3 Hadley Centre Climate Model (HadCM3). Alternative representations of present day vegetation from observations and a dynamic vegetation model were used as the land-cover component for a number of HadCM3 experiments under a nominal present day climate state, and are shown to induce perturbations to the simulated global dynamics. This results in a shift in the location of the Inter Tropical Convergence Zone (ITCZ) and changes in the South Asian monsoon circulation. This has a significant impact on the Indian land precipitation compared to the standard configuration of HadCM3. This large-scale forcing is consistent with documented mechanisms relating to temperature and snow perturbations in the Northern Hemisphere extra-tropics. This analysis demonstrates that uncertainties in the representation of present day vegetation cover can result in significant perturbations to the simulated climate. The role of the Northern Hemisphere extra-tropics is further demonstrated with a fourth representation of vegetation cover produced by imposing simulated changes in Northern Hemisphere extra-tropical vegetation from the end of the 21st century on the present day climate. This experiment shows that through similar processes extra-tropical vegetation changes in the future contribute to a strengthening of the South Asian monsoon in this model, with a particular influence on the monsoon onset. These findings provide renewed motivation to give careful consideration to the role of global scale vegetation feedbacks when looking at climate change and its impact on the tropics and South Asian monsoon in the latest generation of Earth System models

Interaction effects and quantum phase transitions in topological insulators

We study strong correlation effects in topological insulators via the Lanczos algorithm, which we utilize to calculate the exact many-particle ground-state wave function and its topological properties. We analyze the simple, noninteracting Haldane model on a honeycomb lattice with known topological properties and demonstrate that these properties are already evident in small clusters. Next, we consider interacting fermions by introducing repulsive nearest-neighbor interactions. A first-order quantum phase transition was discovered at finite interaction strength between the topological band insulator and a topologically trivial Mott insulating phase by use of the fidelity metric and the charge-density-wave structure factor. We construct the phase diagram at $T = 0$ as a function of the interaction strength and the complex phase for the next-nearest-neighbor hoppings. Finally, we consider the Haldane model with interacting hard-core bosons, where no evidence for a topological phase is observed. An important general conclusion of our work is that despite the intrinsic nonlocality of topological phases their key topological properties manifest themselves already in small systems and therefore can be studied numerically via exact diagonalization and observed experimentally, e.g., with trapped ions and cold atoms in optical lattices.Comment: 13 pages, 12 figures. Published versio

An earth pole-sitter using hybrid propulsion

In this paper we investigate optimal pole-sitter orbits using hybrid solar sail and solar electric propulsion (SEP). A pole-sitter is a spacecraft that is constantly above one of the Earth's poles, by means of a continuous thrust. Optimal orbits, that minimize propellant mass consumption, are found both through a shape-based approach, and solving an optimal control problem, using a direct method based on pseudo-spectral techniques. Both the pure SEP case and the hybrid case are investigated and compared. It is found that the hybrid spacecraft allows consistent savings on propellant mass fraction. Finally, is it shown that for sufficiently long missions (more than 8 years), a hybrid spacecraft, based on mid-term technology, enables a consistent reduction in the launch mass for a given payload, with respect to a pure SEP spacecraft

The spin-half Heisenberg antiferromagnet on two Archimedian lattices: From the bounce lattice to the maple-leaf lattice and beyond

We investigate the ground state of the two-dimensional Heisenberg antiferromagnet on two Archimedean lattices, namely, the maple-leaf and bounce lattices as well as a generalized $J$-$J'$ model interpolating between both systems by varying $J'/J$ from $J'/J=0$ (bounce limit) to $J'/J=1$ (maple-leaf limit) and beyond. We use the coupled cluster method to high orders of approximation and also exact diagonalization of finite-sized lattices to discuss the ground-state magnetic long-range order based on data for the ground-state energy, the magnetic order parameter, the spin-spin correlation functions as well as the pitch angle between neighboring spins. Our results indicate that the "pure" bounce ($J'/J=0$) and maple-leaf ($J'/J=1$) Heisenberg antiferromagnets are magnetically ordered, however, with a sublattice magnetization drastically reduced by frustration and quantum fluctuations. We found that magnetic long-range order is present in a wide parameter range $0 \le J'/J \lesssim J'_c/J$ and that the magnetic order parameter varies only weakly with $J'/J$. At $J'_c \approx 1.45 J$ a direct first-order transition to a quantum orthogonal-dimer singlet ground state without magnetic long-range order takes place. The orthogonal-dimer state is the exact ground state in this large-$J'$ regime, and so our model has similarities to the Shastry-Sutherland model. Finally, we use the exact diagonalization to investigate the magnetization curve. We a find a 1/3 magnetization plateau for $J'/J \gtrsim 1.07$ and another one at 2/3 of saturation emerging only at large $J'/J \gtrsim 3$.Comment: 9 pages, 10 figure

Correcting a bias in a climate model with an augmented emulator

This is the final version. Available from Copernicus Publications via the DOI in this record. A key challenge in developing flagship climate model configurations is the process of setting uncertain input parameters at values that lead to credible climate simulations. Setting these parameters traditionally relies heavily on insights from those involved in parameterisation of the underlying climate processes. Given the many degrees of freedom and computational expense involved in evaluating such a selection, this can be imperfect leaving open questions about whether any subsequent simulated biases result from mis-set parameters or wider structural model errors (such as missing or partially parameterised processes). Here, we present a complementary approach to identifying plausible climate model parameters, with a method of bias correcting subcomponents of a climate model using a Gaussian process emulator that allows credible values of model input parameters to be found even in the presence of a significant model bias. A previous study (McNeall et al., 2016) found that a climate model had to be run using land surface input parameter values from very different, almost non-overlapping, parts of parameter space to satisfactorily simulate the Amazon and other forests respectively. As the forest fraction of modelled non-Amazon forests was broadly correct at the default parameter settings and the Amazon too low, that study suggested that the problem most likely lay in the model's treatment of non-plant processes in the Amazon region. This might be due to modelling errors such as missing deep rooting in the Amazon in the land surface component of the climate model, to a warm-dry bias in the Amazon climate of the model or a combination of both. In this study, we bias correct the climate of the Amazon in the climate model from McNeall et al. (2016) using an "augmented" Gaussian process emulator, where temperature and precipitation, variables usually regarded as model outputs, are treated as model inputs alongside land surface input parameters. A sensitivity analysis finds that the forest fraction is nearly as sensitive to climate variables as it is to changes in its land surface parameter values. Bias correcting the climate in the Amazon region using the emulator corrects the forest fraction to tolerable levels in the Amazon at many candidates for land surface input parameter values, including the default ones, and increases the valid input space shared with the other forests. We need not invoke a structural model error in the land surface model, beyond having too dry and hot a climate in the Amazon region. The augmented emulator allows bias correction of an ensemble of climate model runs and reduces the risk of choosing poor parameter values because of an error in a subcomponent of the model. We discuss the potential of the augmented emulator to act as a translational layer between model subcomponents, simplifying the process of model tuning when there are compensating errors and helping model developers discover and prioritise model errors to target.Alan Turing Institut