6,721 research outputs found
Out of Equilibrium Solutions in the -Hamiltonian Mean Field model
Out of equilibrium magnetised solutions of the -Hamiltonian Mean Field
(-HMF) model are build using an ensemble of integrable uncoupled pendula.
Using these solutions we display an out-of equilibrium phase transition using a
specific reduced set of the magnetised solutions
Photometric variability in the old open cluster M 67. II. General Survey
We use differential CCD photometry to search for variability in BVI among 990
stars projected in and around the old open cluster M 67. In a previous paper we
reported results for 22 cluster members that are optical counterparts to X-ray
sources; this study focuses on the other stars in our observations. A variety
of sampling rates were employed, allowing variability on time scales ranging
from \sim 0.3 hours to \sim 20 days to be studied. Among the brightest sources
studied, detection of variability as small as sigma approx 10 mmag is achieved
(with > 3 sigma confidence); for the typical star observed, sensitivity to
variability at levels sigma approx 20 mmag is achieved. The study is unbiased
for stars with 12.5 < B < 18.5, 12.5 < V < 18.5, and 12 < I < 18 within a
radius of about 10 arcmin from the cluster centre. In addition, stars with 10 <
BVI < 12.5 were monitored in a few small regions in the cluster. We present
photometry for all 990 sources studied, and report the variability
characteristics of those stars found to be variable at a statistically
significant level. Among the variables, we highlight several sources that merit
future study, including stars located on the cluster binary sequence, stars on
the giant branch, blue stragglers, and a newly discovered W UMa system.Comment: 12 pages, including 6 figures and 5 tables. Tables 1 and 3 only
available in electronic version of paper. Accepted by A&
Gravitational diffraction radiation
We show that if the visible universe is a membrane embedded in a
higher-dimensional space, particles in uniform motion radiate gravitational
waves because of spacetime lumpiness. This phenomenon is analogous to the
electromagnetic diffraction radiation of a charge moving near to a metallic
grating. In the gravitational case, the role of the metallic grating is played
by the inhomogeneities of the extra-dimensional space, such as a hidden brane.
We derive a general formula for gravitational diffraction radiation and apply
it to a higher-dimensional scenario with flat compact extra dimensions.
Gravitational diffraction radiation may carry away a significant portion of the
particle's initial energy. This allows to set stringent limits on the scale of
brane perturbations. Physical effects of gravitational diffraction radiation
are briefly discussed.Comment: 5 pages, 2 figures, RevTeX4. v2: References added. Version to appear
in Phys. Rev.
A key role for stimulus-specific updating of the sensory cortices in the learning of stimulus-reward associations
Successful adaptive behavior requires the learning of associations between stimulus-specific choices and rewarding outcomes. Most research on the mechanisms underlying such processes has focused on subcortical reward-processing regions, in conjunction with frontal circuits. Given the extensive stimulus-specific coding in the sensory cortices, we hypothesized they would play a key role in the learning of stimulus-specific reward associations. We recorded electrical brain activity (EEG) during a learning-based, decision-making, gambling task where, on each trial, participants chose between a face and a house and then received feedback (gain or loss). Within each 20-trial set, either faces or houses were more likely to predict a gain. Results showed that early feedback processing (~200-1200ms) was independent of the choice made. In contrast, later feedback processing (~1400-1800ms) was stimulus-specific, reflected by decreased alpha power (reflecting increased cortical activity) over face-selective regions. For winning-versus-losing after a face choice, but not after a house choice. Finally, as the reward association was learned in a set, there was increasingly stronger attentional bias towards the more likely winning stimulus, reflected by increasing attentional-orienting-related brain activity and increasing likelihood of choosing that stimulus. These results delineate the processes underlying the updating of stimulus-reward associations during feedback-guided learning, which then guides future attentional allocation and decision making
Photometric variability in the open cluster M67 I. Cluster members detected in X-rays
We study photometric variability among the optical counterparts of X-ray
sources in the old open cluster M67. The two puzzling binaries below the giant
branch are both variables: for S1113 the photometric period is compatible with
the orbital period, S1063 either varies on a period longer than the orbital
period, or does not vary periodically. For the spectroscopic binaries S999,
S1070 and S1077 the photometric and orbital periods are similar. Another new
periodic variable is the main-sequence star S1112, not known to be a binary. An
increase of the photometric period in the WUMa system S1282 (AHCnc) is in
agreement with a previously reported trend. Six of the eight variables we
detected are binaries with orbital periods of 10 days or less and equal
photometric and orbital periods. This confirms the interpretation that their
X-ray emission arises in the coronae of tidally locked magnetically active
stars. No variability was found for the binaries with orbital periods longer
than 40 days; their X-ray emission remains to be explained.Comment: 11 pages, accepted for publication in A&
Improvement in visual search with practice: Mapping learning-related changes in neurocognitive stages of processing
© 2015 the authors. Practice can improve performance on visual search tasks; the neural mechanisms underlying such improvements, however, are not clear. Response time typically shortens with practice, but which components of the stimulus–response processing chain facilitate this behavioral change? Improved search performance could result from enhancements in various cognitive processing stages, including (1) sensory processing, (2) attentional allocation, (3) target discrimination, (4) motor-response preparation, and/or (5) response execution. We measured event-related potentials (ERPs) as human participants completed a five-day visual-search protocol in which they reported the orientation of a color popout target within an array of ellipses. We assessed changes in behavioral performance and in ERP components associated with various stages of processing. After practice, response time decreased in all participants (while accuracy remained consistent), and electrophysiological measures revealed modulation of several ERP components. First, amplitudes of the early sensory-evoked N1 component at 150 ms increased bilaterally, indicating enhanced visual sensory processing of the array. Second, the negative-polarity posterior–contralateral component (N2pc, 170–250 ms) was earlier and larger, demonstrating enhanced attentional orienting. Third, the amplitude of the sustained posterior contralateral negativity component (SPCN, 300–400 ms) decreased, indicating facilitated target discrimination. Finally, faster motor-response preparation and execution were observed after practice, as indicated by latency changes in both the stimulus-locked and response-locked lateralized readiness potentials (LRPs). These electrophysiological results delineate the functional plasticity in key mechanisms underlying visual search with high temporal resolution and illustrate how practice influences various cognitive and neural processing stages leading to enhanced behavioral performance
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