1,387 research outputs found
Nilpotent Approximations of Sub-Riemannian Distances for Fast Perceptual Grouping of Blood Vessels in 2D and 3D
We propose an efficient approach for the grouping of local orientations
(points on vessels) via nilpotent approximations of sub-Riemannian distances in
the 2D and 3D roto-translation groups and . In our distance
approximations we consider homogeneous norms on nilpotent groups that locally
approximate , and which are obtained via the exponential and logarithmic
map on . In a qualitative validation we show that the norms provide
accurate approximations of the true sub-Riemannian distances, and we discuss
their relations to the fundamental solution of the sub-Laplacian on .
The quantitative experiments further confirm the accuracy of the
approximations. Quantitative results are obtained by evaluating perceptual
grouping performance of retinal blood vessels in 2D images and curves in
challenging 3D synthetic volumes. The results show that 1) sub-Riemannian
geometry is essential in achieving top performance and 2) that grouping via the
fast analytic approximations performs almost equally, or better, than
data-adaptive fast marching approaches on and .Comment: 18 pages, 9 figures, 3 tables, in review at JMI
A physical approach to modelling large-scale galactic magnetic fields
A convenient representation of the structure of the large-scale galactic
magnetic field is required for the interpretation of polarization data in the
sub-mm and radio ranges, in both the Milky Way and external galaxies. We
develop a simple and flexible approach to construct parametrised models of the
large-scale magnetic field of the Milky Way and other disc galaxies, based on
physically justifiable models of magnetic field structure. The resulting models
are designed to be optimised against available observational data.
Representations for the large-scale magnetic fields in the flared disc and
spherical halo of a disc galaxy were obtained in the form of series expansions
whose coefficients can be calculated from observable or theoretically known
galactic properties. The functional basis for the expansions is derived as
eigenfunctions of the mean-field dynamo equation or of the vectorial magnetic
diffusion equation. The solutions presented are axially symmetric but the
approach can be extended straightforwardly to non-axisymmetric cases. The
magnetic fields are solenoidal by construction, can be helical, and are
parametrised in terms of observable properties of the host object, such as the
rotation curve and the shape of the gaseous disc. The magnetic field in the
disc can have a prescribed number of field reversals at any specified radii.
Both the disc and halo magnetic fields can separately have either dipolar or
quadrupolar symmetry. The model is implemented as a publicly available software
package GalMag which allows, in particular, the computation of the synchrotron
emission and Faraday rotation produced by the model's magnetic field. The model
can be used in interpretations of observations of magnetic fields in the Milky
Way and other spiral galaxies, in particular as a prior in Bayesian analyses.
(Abridged.)Comment: 20 pages, 14 figures. Accepted for publication in A&
Implicit and Explicit Attitudes Toward Spiders: Sensitivity to Treatment and Predictive Value for Generalization of Treatment Effects
This study tested whether high spider fearful individuals’ implicit and explicit attitudes toward spiders are sensitive to exposure treatment, and whether post-treatment implicit and/or explicit attitudes are related to the generalization of treatment effects. Self-reported explicit and implicit attitudes (indexed with a pictorial Extrinsic Affective Simon Task) were assessed in high spider fearful, treatment-seeking individuals (n = 60) before and after a one-session exposure in vivo treatment and at 2-month follow-up. A group of non-fearful participants (n = 30) completed the same assessments once. Results show that implicit attitudes did not change following treatment over and above test–retest effects. In contrast, explicit attitudes did change favorably following treatment, but negative explicit attitudes at post-treatment were associated with less pronounced overt approach behavior at follow-up. These findings support the idea that residual negative explicit attitudes interfere with the generalization of treatment effects
Airborne Measurements of Gravity Wave Breaking at the Tropopause
2000 FLORIDA AVE NW, WASHINGTON, DC,
2000
Comment on “Effects of Arsenite during Fetal Development on Energy Metabolism and Susceptibility to Diet-Induced Fatty Liver Diseases in Male Mice” and “Mechanisms Underlying Latent Disease Risk Associated with Early-Life Arsenic Exposure: Current Trends and Scientific Gaps”
Peer reviewedPublisher PD
Imaging Electron Wave Functions of Quantized Energy Levels in Carbon Nanotubes
Carbon nanotubes provide a unique system to study one-dimensional
quantization phenomena. Scanning tunneling microscopy is used to observe the
electronic wave functions that correspond to quantized energy levels in short
metallic carbon nanotubes. Discrete electron waves are apparent from periodic
oscillations in the differential conductance as a function of the position
along the tube axis, with a period that differs from that of the atomic
lattice. Wave functions can be observed for several electron states at adjacent
discrete energies. The measured wavelengths are in good agreement with the
calculated Fermi wavelength for armchair nanotubes.Comment: 11 pages, 4 figures in seperate PDF fil
Anatomy of Cirrus Clouds: Results from the Emerald Airborne Campaigns
2000 FLORIDA AVE NW, WASHINGTON, USA, DC,
2000
Synchronization Dynamics in the Presence of Coupling Delays and Phase Shifts
In systems of coupled oscillators, the effects of complex signaling can be captured by time delays and phase shifts. Here, we show how time delays and phase shifts lead to different oscillator dynamics and how synchronization rates can be regulated by substituting time delays by phase shifts at a constant collective frequency. For spatially extended systems with time delays, we show that the fastest synchronization can occur for intermediate wavelengths, giving rise to novel synchronization scenarios.This work was supported by spanish Ministry of Economy and Competitiveness (MINECO) through Grant PHYSDEV (No. FIS2012-32349) and from CSIC
through the Junta para la Ampliación de Estudios program
(JAEDOC014, 2010 call) cofunded by the European Social FundPublicad
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