476 research outputs found
An upper bound from helioseismology on the stochastic background of gravitational waves
The universe is expected to be permeated by a stochastic background of
gravitational radiation of astrophysical and cosmological origin. This
background is capable of exciting oscillations in solar-like stars. Here we
show that solar-like oscillators can be employed as giant hydrodynamical
detectors for such a background in the muHz to mHz frequency range, which has
remained essentially unexplored until today. We demonstrate this approach by
using high-precision radial velocity data for the Sun to constrain the
normalized energy density of the stochastic gravitational-wave background
around 0.11 mHz. These results open up the possibility for asteroseismic
missions like CoRoT and Kepler to probe fundamental physics.Comment: 6 pages, 2 figures. Updated to match published versio
BOLD fMRI Correlation Reflects Frequency-Specific Neuronal Correlation
SummaryThe brain-wide correlation of hemodynamic signals as measured with BOLD fMRI is widely studied as a proxy for integrative brain processes [1â3]. However, the relationship between hemodynamic correlation structure and neuronal correlation structure [4â6] remains elusive. We investigated this relation using BOLD fMRI and spatially co-registered, source-localized MEG in resting humans. We found that across the entire cortex BOLD correlation reflected the co-variation of frequency-specific neuronal activity. Resolving the relation between electrophysiological and hemodynamic correlation structures locally in cortico-cortical connection space, we found that this relation was subject specific and even persisted on the centimeter scale. At first sight, this relation was strongest in the alpha to beta frequency range (8â32Â Hz). However, correcting for differences in signal-to-noise ratios across electrophysiological frequencies, we found that the relation extended over a broad frequency range from 2 to 128Â Hz. Moreover, we found that the frequency with the tightest link to BOLD correlation varied across cortico-cortical space. For every cortico-cortical connection, we show which specific correlated oscillations were most related to BOLD correlations. Our work provides direct evidence for the neuronal origin of BOLD correlation structure. Moreover, our work suggests that, across the brain, BOLD correlation reflects correlation of different types of neuronal network processes and that frequency-specific electrophysiological correlation provides information about large-scale neuronal interactions complementary to BOLD fMRI
Integration Measure and Spectral Flow in the Critical N=2 String
I present the moduli space of the (2+2)-dimensional critical closed fermionic
string with two world-sheet supersymmetries. The integration of fermionic and
Maxwell moduli in the presence of punctures yields the string measure for
n-point amplitudes at arbitrary genus and instanton number. Generalized
picture-changing and spectral-flow operators emerge, connecting different
instanton sectors. Tree and loop amplitudes are computed.Comment: Talk at 29th Ahrenshoop Symposium in Buckow 1995, 6 pages,
espcrc2.st
Gso Projection, BRST Cohomology and Picture-Changing in N=2 String Theory
We investigate in detail the critical fermionic string with and
without a global twist. An analysis of BRST cohomology shows that
twisted sectors contain massless `spacetime' fermions which are {\it
non-local\/} with respect to the standard massless boson. However, two distinct
GSO projections exist, one (untwisted) retaining merely the usual boson and its
spectral-flow partner, the other (twisted) yielding two fermions and one boson,
on the massless level. The corresponding chiral BRST-invariant vertex operators
are constructed in certain pictures, and their fusion and picture-changing are
investigated, including the construction of inverse picture-changing operators.
The `spacetime supersymmetry' generators are {\it null\/} operators,
since the twisted massless states fail to interact. The untwisted three- and
four-point functions are recalculated at tree-level.Comment: 42 pages, LaTeX, no figures, 115 kb, ITP-UH-05/94, DESY 94-104
(extensive revision and expansion of earlier version
Gradual progression from sensory to task-related processing in cerebral cortex
Somewhere along the cortical hierarchy, behaviorally relevant information is distilled from raw sensory inputs. We examined how this transformation progresses along multiple levels of the hierarchy by comparing neural representations in visual, temporal, parietal, and frontal cortices in monkeys categorizing across three visual domains (shape, motion direction, and color). Representations in visual areas middle temporal (MT) and V4 were tightly linked to external sensory inputs. In contrast, lateral prefrontal cortex (PFC) largely represented the abstracted behavioral relevance of stimuli (task rule, motion category, and color category). Intermediate-level areas, including posterior inferotemporal (PIT), lateral intraparietal (LIP), and frontal eye fields (FEF), exhibited mixed representations. While the distribution of sensory information across areas aligned well with classical functional divisions (MT carried stronger motion information, and V4 and PIT carried stronger color and shape information), categorical abstraction did not, suggesting these areas may participate in different networks for stimulus-driven and cognitive functions. Paralleling these representational differences, the dimensionality of neural population activity decreased progressively from sensory to intermediate to frontal cortex. This shows how raw sensory representations are transformed into behaviorally relevant abstractions and suggests that the dimensionality of neural activity in higher cortical regions may be specific to their current task. Keywords: categorization; cognition; prefrontal cortex; posterior parietal cortex; dimensionalityNational Institute of Mental Health (U.S.) (Grant 5R37MH087027
An integrated architecture for shallow and deep processing
We present an architecture for the integration of shallow and deep NLP components which is aimed at flexible combination of different language technologies for a range of practical current and future applications. In particular, we describe the integration of a high-level HPSG parsing system with different high-performance shallow components, ranging from named entity recognition to chunk parsing and shallow clause recognition. The NLP components enrich a representation of natural language text with layers of new XML meta-information using a single shared data structure, called the text chart. We describe details of the integration methods, and show how information extraction and language checking applications for realworld German text benefit from a deep grammatical analysis
A Novel Long-Term ex vivo Model for Studying Vascular Calcification Pathogenesis: The Rat Isolated-Perfused Aorta
The investigation of vascular calcification and its underlying cellular and molecular pathways is of great interest in current research efforts. Therefore, suitable assays are needed to allow examination of the complex calcification process under controlled conditions. The current study describes a new ex vivo model of isolated-perfused rat aortic tissue with subsequent quantification and vessel staining to analyze the calcium content of the aortic wall. A rat aorta was perfused ex vivo with control and calcification media for 14 days, respectively. The calcification medium was luminally perfused and induced a significant increase in calcium deposition within the media of the vessel wall detected alongside the elastic laminae. Perfusion with control medium induced no calcification. In addition, the mRNA expression of the osteogenic marker bone morphogenetic protein 2 (BMP-2) increased in aortic tissue after perfusion, while SM22α as smooth muscle marker decreased. This newly developed ex vivo model of isolated-perfused rat aorta is suitable for vascular calcification studies testing inducers and inhibitors of vessel calcification and studying signaling pathways within calcification progression
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