46,017 research outputs found
Verbal paired associates and the hippocampus: The role of scenes
It is widely agreed that patients with bilateral hippocampal damage are impaired at binding pairs of words together. Consequently, the verbal paired associates (VPA) task has become emblematic of hippocampal function. This VPA deficit is not well understood and is particularly difficult for hippocampal theories with a visuospatial bias to explain (e.g., cognitive map and scene construction theories). Resolving the tension among hippocampal theories concerning the VPA could be important for leveraging a fuller understanding of hippocampal function. Notably, VPA tasks typically use high imagery concrete words and so conflate imagery and binding. To determine why VPA engages the hippocampus, we devised an fMRI encoding task involving closely matched pairs of scene words, pairs of object words, and pairs of very low imagery abstract words. We found that the anterior hippocampus was engaged during processing of both scene and object word pairs in comparison to abstract word pairs, despite binding occurring in all conditions. This was also the case when just subsequently remembered stimuli were considered. Moreover, for object word pairs, fMRI activity patterns in anterior hippocampus were more similar to those for scene imagery than object imagery. This was especially evident in participants who were high imagery users and not in mid and low imagery users. Overall, our results show that hippocampal engagement during VPA, even when object word pairs are involved, seems to be evoked by scene imagery rather than binding. This may help to resolve the issue that visuospatial hippocampal theories have in accounting for verbal memory
A preliminary shield design for a SNAP-8 power system
A preliminary shield design for a nuclear power system utilizing a SNAP-8 reactor for space base application is presented. A representative space base configuration was selected to set the geometry constraints imposed on the design. The base utilizes two independent power packages each with a reactor operating at 600 kwt and each producing about 50 kwe. The crew compartment is located about 200 feet from each reactor and is large enough in extent to intercept a total shadow angle of 60 deg measured about the center line of each reactor
Magic wavelengths for optical cooling and trapping of lithium
Using first-principles calculations, we identify magic wavelengths for the
2s-2p and 2s-3p transitions in lithium. The ns and np atomic levels have the
same ac Stark shifts at the corresponding magic wavelength, which facilitates
state-insensitive optical cooling and trapping. Tune-out wavelengths for which
the ground-state frequency-dependent polarizability vanishes are also
calculated. Differences of these wavelengths between 6Li and 7Li are reported.
Our approach uses high-precision, relativistic all-order methods in which all
single, double, and partial triple excitations of the Dirac-Fock wave functions
are included to all orders of perturbation theory. Recommended values are
provided for a large number of Li electric-dipole matrix elements. Static
polarizabilities for the 2s, 2p, 3s, 3p, and 3d levels are compared with other
theory and experiment where available. Uncertainties of all recommended values
are estimated. The magic wavelengths for the uv 2s-3p transition are of
particular interest for the production of a quantum gas of lithium [Duarte et
al., Phys. Rev. A 84, 061406R (2011)].Comment: 7 pages, 5 figure
An Evidence Based Time-Frequency Search Method for Gravitational Waves from Pulsar Glitches
We review and expand on a Bayesian model selection technique for the
detection of gravitational waves from neutron star ring-downs associated with
pulsar glitches. The algorithm works with power spectral densities constructed
from overlapping time segments of gravitational wave data. Consequently, the
original approach was at risk of falsely identifying multiple signals where
only one signal was present in the data. We introduce an extension to the
algorithm which uses posterior information on the frequency content of detected
signals to cluster events together. The requirement that we have just one
detection per signal is now met with the additional bonus that the belief in
the presence of a signal is boosted by incorporating information from adjacent
time segments.Comment: 6 pages, 4 figures, submitted to AMALDI 7 proceeding
Lifshitz-like transition and enhancement of correlations in a rotating bosonic ring lattice
We study the effects of rotation on one-dimensional ultra-cold bosons
confined to a ring lattice. For commensurate systems, at a critical value of
the rotation frequency, an infinitesimal interatomic interaction energy opens a
gap in the excitation spectrum, fragments the ground state into a macroscopic
superposition of two states with different circulation and generates a sudden
change in the topology of the momentum distribution. These features are
reminiscent of the topological changes in the Fermi surface that occurs in the
Lifshitz transition in fermionic systems. The entangled nature of the ground
state induces a strong enhancement of quantum correlations and decreases the
threshold for the Mott insulator transition. In contrast to the commensurate
case, the incommensurate lattice is rather insensitive to rotation. Our studies
demonstrate the utility of noise correlations as a tool for identifying new
physics in strongly correlated systems.Comment: 5 pages, 4 figure
Phase transitions, entanglement and quantum noise interferometry in cold atoms
We show that entanglement monotones can characterize the pronounced
enhancement of entanglement at a quantum phase transition if they are sensitive
to long-range high order correlations. These monotones are found to develop a
sharp peak at the critical point and to exhibit universal scaling. We
demonstrate that similar features are shared by noise correlations and verify
that these experimentally accessible quantities indeed encode entanglement
information and probe separability.Comment: 4 pages 4 figure
Radio emission from the massive stars in the Galactic Super Star Cluster Westerlund 1
Current mass-loss rate estimates imply that main sequence winds are not
sufficient to strip away the H-rich envelope to yield Wolf-Rayet (WR) stars.
The rich transitional population of Westerlund 1 (Wd 1) provides an ideal
laboratory to observe mass-loss processes throughout the transitional phase of
stellar evolution. An analysis of deep radio continuum observations of Wd 1 is
presented. We detect 18 cluster members. The radio properties of the sample are
diverse, with thermal, non-thermal and composite thermal/non-thermal sources
present. Mass-loss rates are ~10^{-5} solar mass/year across all spectral
types, insufficient to form WRs during a massive star lifetime, and the stars
must undergo a period of enhanced mass loss. The sgB[e] star W9 may provide an
example, with a mass-loss rate an order of magnitude higher than the other
cluster members, and an extended nebula of density ~3 times the current wind.
This structure is reminiscent of luminous blue variables, and one with evidence
of two eras of high, possibly eruptive, mass loss. Three OB supergiants are
detected, implying unusually dense winds. They also may have composite spectra,
suggesting binarity. Spatially resolved nebulae are associated with three of
the four RSGs and three of the six YHGs in the cluster, which are due to
quiescent mass loss rather than outbursts. For some of the cool star winds, the
ionizing source may be a companion star though the cluster radiation density is
sufficiently high to provide the necessary ionizing radiation. Five WR stars
are detected with composite spectra, interpreted as arising in colliding-wind
binaries.Comment: 15 pages, 6 figures. Accepted for publication in Astronomy and
Astrophysic
Analysing I/O bottlenecks in LHC data analysis on grid storage resources
We describe recent I/O testing frameworks that we have developed and applied within the UK GridPP Collaboration, the ATLAS experiment and the DPM team, for a variety of distinct purposes. These include benchmarking vendor supplied storage products, discovering scaling limits of SRM solutions, tuning of storage systems for experiment data analysis, evaluating file access protocols, and exploring I/O read patterns of experiment software and their underlying event data models. With multiple grid sites now dealing with petabytes of data, such studies are becoming essential. We describe how the tests build, and improve, on previous work and contrast how the use-cases differ. We also detail the results obtained and the implications for storage hardware, middleware and experiment software
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