6,922 research outputs found
A Vector-Integration-to-Endpoint Model for Performance of Viapoint Movements
Viapoint (VP) movements are movements to a desired point that are constrained to pass through an intermediate point. Studies have shown that VP movements possess properties, such as smooth curvature around the VP, that are not explicable by treating VP movements as strict concatenations of simpler point-to-point (PTP) movements. Such properties have led some theorists to propose whole-trajectory optimization models, which imply that the entire trajectory is pre-computed before movement initiation. This paper reports new experiments conducted to systematically compare VP with PTP trajectories. Analyses revealed a statistically significant early directional deviation in VP movements but no associated curvature change. An explanation of this effect is offered by extending the Vector-Integration-To-Endpoint (VITE) model (Bullock and Grossberg, 1988), which postulates that voluntary movement trajectories emerge as internal gating signals control the integration of continuously computed vector commands based on the evolving, perceptible difference between desired and actual position variables. The model explains the observed trajectories of VP and PTP movements as emergent properties of a dynamical system that does not precompute entire trajectories before movement initiation. The new model includes a working memory and a stage sensitive to time-to-contact information. These cooperate to control serial performance. The structural and functional relationships proposed in the model are consistent with available data on forebrain physiology and anatomy.Office of Naval Research (N00014-92-J-1309, N00014-93-1-1364, N0014-95-1-0409
The Implications of Galaxy Formation Models for the TeV Observations of Current Detectors
This paper represents a step toward constraining galaxy formation models via
TeV gamm a ray observations. We use semi-analytic models of galaxy formation to
predict a spectral distribution for the intergalactic infrared photon field,
which in turn yields information about the absorption of TeV gamma rays from
extra-galactic sources. By making predictions for integral flux observations at
>200 GeV for several known EGRE T sources, we directly compare our models with
current observational upper limits obtained by Whipple. In addition, our
predictions may offer a guide to the observing programs for the current
population of TeV gamma ray observatories.Comment: 6 pages, 11 figures, to appear in the proceedings of the 6th TeV
Workshop at Snowbird, U
Cosmological Constraints from Primordial Black Holes
Primordial black holes may form in the early Universe, for example from the
collapse of large amplitude density perturbations predicted in some
inflationary models. Light black holes undergo Hawking evaporation, the energy
injection from which is constrained both at the epoch of nucleosynthesis and at
the present. The failure as yet to unambiguously detect primordial black holes
places important constraints. In this article, we are particularly concerned
with the dependence of these constraints on the model for the complete
cosmological history, from the time of formation to the present. Black holes
presently give the strongest constraint on the spectral index of density
perturbations, though this constraint does require to be constant over a
very wide range of scales.Comment: 8 pages LaTeX file, using elsart.sty, with three figures incorporated
using epsf. To appear, proceedings of DM98, Los Angeles (ed D Cline,
Elsevier
Hierarchical galaxy formation and substructure in the Galaxy's stellar halo
We develop an explicit model for the formation of the stellar halo from
tidally disrupted, accreted dwarf satellites in the cold dark matter (CDM)
framework, focusing on predictions testable with the Sloan Digital Sky Survey
(SDSS) and other wide-field surveys. Subhalo accretion and orbital evolution
are calculated using a semi-analytic approach within the Press-Schechter
formalism. Motivated by our previous work, we assume that low-mass subhalos (v
< 30 km/s) can form significant populations of stars only if they accreted a
substantial fraction of their mass before the epoch of reionization. With this
assumption, the model reproduces the observed velocity function of galactic
satellites in the Local Group, solving the ``dwarf satellite problem'' without
modifying the popular LCDM cosmology. The disrupted satellites yield a stellar
distribution with a total mass and radial density profile consistent with those
observed for the Milky Way stellar halo. Most significantly, the model predicts
the presence of many large-scale, coherent substructures in the outer halo.
These substructures are remnants of individual, tidally disrupted dwarf
satellite galaxies. Substructure is more pronounced at large galactocentric
radii because of the smaller number density of tidal streams and the longer
orbital times. This model provides a natural explanation for the coherent
structures in the outer stellar halo found in the SDSS commissioning data, and
it predicts that many more such structures should be found as the survey covers
more of the sky. The detection (or non-detection) and characterization of such
structures could eventually test variants of the CDM scenario, especially those
that aim to solve the dwarf satellite problem by enhancing satellite
disruption.Comment: 12 pages, 8 figures, Submitted to Ap
An investigation of magnetic field effects on plume density and temperature profiles of an applied-field MPD thruster
Applied-field magnetoplasmadynamic (MPD) thruster performance is below levels required for primary propulsion missions. While MPD thruster performance has been found to increase with the magnitude of the applied-field strength, there is currently little understanding of the impact of applied-field shape on thruster performance. The results of a study in which a single applied-field thruster was operated using three solenoidal magnets with diameters of 12.7, 15.2, and 30.4-cm are presented. Thruster voltage and anode power deposition were measured for each applied field shape over a range of field strengths. Plume electron number density and temperature distributions were measured using a Langmuir probe in an effort to determine the effect of field shape on plume confinement by the diverging magnetic-field for each of the three magnetic field shapes. Results show that the dependence of the measured thruster characteristics on field shape were non-monotonic and that the field shape had a significant effect on the plume density and temperature profiles
Automated and semi-automated contact tracing: Protocol for a rapid review of available evidence and current challenges to inform the control of COVID-19
Abstract Introduction Traditional approaches to case-finding, case isolation, and contact tracing methods have so far proved insufficient on their own to prevent the development of local epidemics of COVID-19 in many high-income countries despite relatively advanced public health systems. As a result, many governments have resorted to widespread social distancing measures and mass quarantines (âlock-downsâ) to reduce transmission and to prevent healthcare systems from being overwhelmed. However, such measures impose heavy human and societal costs. Automated or semi-automated digital contact tracing, in conjunction with scaled-up community testing, has been proposed as a key part of exit strategies from lockdowns. However, the effectiveness of these approaches to contact tracing is unclear, and to be effective, trusted, and widely adopted such technology must overcome several challenges. Methods and analysis We will perform a rapid systematic review to assess the effectiveness of automated and semi-automated digital tools for contact tracing, and identify key considerations for successful implementation, to inform the control of COVID-19. We will search PubMed, EMBASE, EBSCO Medical COVID information portal, OVID Global Health, Cochrane Library, medRxiv, BioRxiv, and arXiv for peer-reviewed and pre-print papers on automated or semi-automated digital tools for contact tracing of COVID-19, another respiratory disease with pandemic potential (limited to SARS, MERS, or pandemic influenza), or Ebola, in human populations. Studies will be eligible if published in English between 1 January 2000 and 14 April 2020. We will synthesise study findings narratively and will consider meta-analysis if â„ 3 suitable studies with comparable interventions and outcomes are available. Ethics and dissemination Ethical approval is not required for this review. We plan to disseminate findings via pre-print, journal publication, through social media and web-based platforms and through direct stakeholder engagement
Alternatives to Raised Pavement Markers (RPMs)
Empirical evidence shows that raised pavement markers (RPMs) can become dislodged from the pavement when struck by snowplows, presenting a road hazard. Furthermore, a dislodged RPM opens an ingress point for water to reach the sub pavement, potentially reducing the design life of the pavement. To address this issue, Indiana conducted an evaluation of centerline rumble stripes (CLRSs) as an alternative to RPMs on rural, non-interstate, roadways.
Five pilot test sites consisting of 41.2 miles of roadway were constructed to evaluate CLRS performance in multiple regions of Indiana under various pavement conditions. CLRS were subsequently incorporated into INDOTâs 2013 Goals and approximately 238 project miles were programmed for construction. The project team participated in the development of a new specification for use in contract lettings after September 1, 2013.
This technical report summarizes that evaluation, briefly summarizes the project background and development of CLRS specifications, provides detailed photos of a fall 2013 deployment, and summarizes the postâconstruction close out meeting held on December 4, 2013
Painted Rumble Stripes: Alternatives to Raised Pavement Markers (RPMs)
This poster highlights research completed by the Joint Transportation Research Program (JTRP) led by Darcy Bullock, Lyles School of Civil Engineering, Purdue University, for JTRP Project SPR-3528, Alternatives to Raised Pavement Markers (RPMs). The poster includes photos taken at a construction site on State Road 25 located near Shadeland, Indiana. The top photo demonstrates how the center line corrected a driver when snow covered to avoid oncoming traffic. Photo was taken on US 231 near Country Road 800 in Tippecanoe County
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