64,533 research outputs found
Omnidirectional Sensory and Motor Volumes in Electric Fish
Active sensing organisms, such as bats, dolphins, and weakly electric fish, generate a 3-D space for active sensation by emitting self-generated energy into the environment. For a weakly electric fish, we demonstrate that the electrosensory space for prey detection has an unusual, omnidirectional shape. We compare this sensory volume with the animal's motor volume—the volume swept out by the body over selected time intervals and over the time it takes to come to a stop from typical hunting velocities. We find that the motor volume has a similar omnidirectional shape, which can be attributed to the fish's backward-swimming capabilities and body dynamics. We assessed the electrosensory space for prey detection by analyzing simulated changes in spiking activity of primary electrosensory afferents during empirically measured and synthetic prey capture trials. The animal's motor volume was reconstructed from video recordings of body motion during prey capture behavior. Our results suggest that in weakly electric fish, there is a close connection between the shape of the sensory and motor volumes. We consider three general spatial relationships between 3-D sensory and motor volumes in active and passive-sensing animals, and we examine hypotheses about these relationships in the context of the volumes we quantify for weakly electric fish. We propose that the ratio of the sensory volume to the motor volume provides insight into behavioral control strategies across all animals
A Robust and Efficient Method for Solving Point Distance Problems by Homotopy
The goal of Point Distance Solving Problems is to find 2D or 3D placements of
points knowing distances between some pairs of points. The common guideline is
to solve them by a numerical iterative method (\emph{e.g.} Newton-Raphson
method). A sole solution is obtained whereas many exist. However the number of
solutions can be exponential and methods should provide solutions close to a
sketch drawn by the user.Geometric reasoning can help to simplify the
underlying system of equations by changing a few equations and triangularizing
it.This triangularization is a geometric construction of solutions, called
construction plan. We aim at finding several solutions close to the sketch on a
one-dimensional path defined by a global parameter-homotopy using a
construction plan. Some numerical instabilities may be encountered due to
specific geometric configurations. We address this problem by changing
on-the-fly the construction plan.Numerical results show that this hybrid method
is efficient and robust
Dynamics of the Narrow-Line Region in the Seyfert 2 Galaxy NGC 1068
We present dynamical models based on a study of high-resolution long-slit
spectra of the narrow-line region (NLR) in NGC 1068 obtained with the Space
Telescope Imaging Spectrograph (STIS) aboard The Hubble Space Telescope (HST).
The dynamical models consider the radiative force due to the active galactic
nucleus (AGN), gravitational forces from the supermassive black hole (SMBH),
nuclear stellar cluster, and galactic bulge, and a drag force due to the NLR
clouds interacting with a hot ambient medium. The derived velocity profile of
the NLR gas is compared to that obtained from our previous kinematic models of
the NLR using a simple biconical geometry for the outflowing NLR clouds. The
results show that the acceleration profile due to radiative line driving is too
steep to fit the data and that gravitational forces along cannot slow the
clouds down, but with drag forces included, the clouds can slow down to the
systemic velocity over the range 100--400 pc, as observed. However, we are not
able to match the gradual acceleration of the NLR clouds from ~0 to ~100 pc,
indicating the need for additional dynamical studies.Comment: Paper prepared by emulateapj version 10/09/06 and accepted for print
in Ap
Multiscale sampling model for motion integration
Biologically plausible strategies for visual scene integration across spatial and temporal domains continues to be a challenging topic. The fundamental question we address is whether classical problems in motion integration, such as the aperture problem, can be solved in a model that samples the visual scene at multiple spatial and temporal scales in parallel. We hypothesize that fast interareal connections that allow feedback of information between cortical layers are the key processes that disambiguate motion direction. We developed a neural model showing how the aperture problem can be solved using different spatial sampling scales between LGN, V1 layer 4, V1 layer 6, and area MT. Our results suggest that multiscale sampling, rather than feedback explicitly, is the key process that gives rise to end-stopped cells in V1 and enables area MT to solve the aperture problem without the need for calculating intersecting constraints or crafting intricate patterns of spatiotemporal receptive fields. Furthermore, the model explains why end-stopped cells no longer emerge in the absence of V1 layer 6 activity (Bolz & Gilbert, 1986), why V1 layer 4 cells are significantly more end-stopped than V1 layer 6 cells (Pack, Livingstone, Duffy, & Born, 2003), and how it is possible to have a solution to the aperture problem in area MT with no solution in V1 in the presence of driving feedback. In summary, while much research in the field focuses on how a laminar architecture can give rise to complicated spatiotemporal receptive fields to solve problems in the motion domain, we show that one can reframe motion integration as an emergent property of multiscale sampling achieved concurrently within lamina and across multiple visual areas.This work was supported in part by CELEST, a National Science Foundation Science of Learning Center; NSF SBE-0354378 and OMA-0835976; ONR (N00014-11-1-0535); and AFOSR (FA9550-12-1-0436). (CELEST, a National Science Foundation Science of Learning Center; SBE-0354378 - NSF; OMA-0835976 - NSF; N00014-11-1-0535 - ONR; FA9550-12-1-0436 - AFOSR)Published versio
Quasar Tomography: Unification of Echo Mapping and Photoionisation Models
Reverberation mapping uses time-delayed variations in photoionised emission
lines to map the geometry and kinematics of emission-line gas in active
galactic nuclei. In previous work, the light travel time delay
tau=R(1+cos(theta))/c and Doppler shift v give a 2-d map Psi(tau,v) for each
emission line. Here we combine the velocity-delay information with
photoionisation physics in a maximum entropy fit to the full reverberating
spectrum F_lam(lam,t) to recover a 5-d map of the differential covering
fraction f(R,theta,n,N,v), with n and N the density and column density of the
gas clouds. We test the method for a variety of geometries (shells, rings,
disks, clouds, jets) by recovering a 3-d map f(R,theta,n) from reverberations
in 7 uv emission lines. The best test recovers a hollow shell geometry,
defining R to 0.15 dex, n to 0.3 dex, and ionisation parameter U ~ 1/(n R^2) to
0.1 dex. The results are sensitive to the adopted distance and luminosity,
suggesting that these parameters may be measurable as well.Comment: Accepted 4 Sep 2002 for publication in MNRA
Turbulence induced collisional velocities and density enhancements: large inertial range results from shell models
To understand the earliest stages of planet formation, it is crucial to be
able to predict the rate and the outcome of dust grains collisions, be it
sticking and growth, bouncing, or fragmentation. The outcome of such collisions
depends on the collision speed, so we need a solid understanding of the rate
and velocity distribution of turbulence-induced dust grain collisions. The rate
of the collisions depends both on the speed of the collisions and the degree of
clustering experienced by the dust grains, which is a known outcome of
turbulence. We evolve the motion of dust grains in simulated turbulence, an
approach that allows a large turbulent inertial range making it possible to
investigate the effect of turbulence on meso-scale grains (millimeter and
centimeter). We find three populations of dust grains: one highly clustered,
cold and collisionless; one warm; and the third "hot". Our results can be fit
by a simple formula, and predict both significantly slower typical collisional
velocities for a given turbulent strength than previously considered, and
modest effective clustering of the collisional populations, easing difficulties
associated with bouncing and fragmentation barriers to dust grain growth.
Nonetheless, the rate of high velocity collisions falls off merely
exponentially with relative velocity so some mid- or high-velocity collisions
will still occur, promising some fragmentation.Comment: 14 pages, 8 figures, 4 tables, Accepted, MNRA
In loco intellegentia: Human factors for the future European train driver
The European Rail Traffic Management System (ERTMS) represents a step change in technology for rail operations in Europe. It comprises track-to-train communications and intelligent on-board systems providing an unprecedented degree of support to the train driver. ERTMS is designed to improve safety, capacity and performance, as well as facilitating interoperability across the European rail network. In many ways, particularly from the human factors perspective, ERTMS has parallels with automation concepts in the aviation and automotive industries. Lessons learned from both these industries are that such a technology raises a number of human factors issues associated with train driving and operations. The interaction amongst intelligent agents throughout the system must be effectively coordinated to ensure that the strategic benefits of ERTMS are realised. This paper discusses the psychology behind some of these key issues, such as Mental Workload (MWL), interface design, user information requirements, transitions and migration and communications. Relevant experience in aviation and vehicle automation is drawn upon to give an overview of the human factors challenges facing the UK rail industry in implementing ERTMS technology. By anticipating and defining these challenges before the technology is implemented, it is hoped that a proactive and structured programme of research can be planned to meet them
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