347 research outputs found
Consistency Conditions for Brane Worlds in Arbitrary Dimensions
We consider ``brane world sum rules'' for compactifications involving an
arbitrary number of spacetime dimensions. One of the most striking results
derived from such consistency conditions is the necessity for negative tension
branes to appear in five--dimensional scenarios. We show how this result is
easily evaded for brane world models with more than five dimensions. As an
example, we consider a novel realization of the Randall--Sundrum scenario in
six dimensions involving only positive tension branes.Comment: 18 pages, LaTex, refs. adde
Internal wave propagation normal to a geostrophic current
It is shown that (i) internal gravity waves propagating along a horizontal density gradient in a uniformly rotating ocean can exist for a range of slightly smaller frequencies than is possible in the absence of such a gradient, (ii) such waves exhibit a phase difference from top to bottom, and (iii) they exchange energy with the geostrophic current, which balances the density-induced pressure gradient...
The Relationship between Different Assays for Detection and Quantification of Amyloid Beta 42 in Human Cerebrospinal Fluid
Alzheimer's disease (AD), which is characterized by a degeneration of neurons and their synapses, is one of the most common forms of dementia. CSF levels of amyloid β42 (Aβ42) have been recognized as a strong candidate to serve as an AD biomarker. There are a number of commercial assays that are routinely employed for measuring Aβ42; however, these assays give diverse ranges for the absolute levels of CSF Aβ42. In order to employ CSF Aβ42 as a biomarker across multiple laboratories, studies need to be performed to understand the relationship between the different platforms. We have analyzed CSF samples from both diseased and nondiseased subjects with two different widely used assay platforms. The results showed that different values for the levels of CSF Aβ42 were reported, depending on the assay used. Nonetheless, both assays clearly demonstrated statistically significant differences in the levels of Aβ42 in CSF from AD relative to age-matched controls (AMC). This paper provides essential data for establishing the relationship between these assays and provides an important step towards the validation of Aβ42 as a biomarker for AD
Superstars and Giant Gravitons in M-theory
Following hep-th/0109127, we show that a certain class of BPS naked
singularities (superstars) found in compactifications of M-theory can be
interpreted as being composed of giant gravitons. More specifically, we study
superstars which are asymptotically AdS_7 x S^4 and AdS_4 x S^7 and show that
these field configurations can be interpreted as being sourced by continuous
distributions of spherical M2- and M5-branes, respectively, which carry
internal momenta and have expanded on the spherical component of the
space-time.Comment: 13 page
Motivational State and Reward Content Determine Choice Behavior under Risk in Mice
Risk is a ubiquitous feature of the environment for most organisms, who must often choose between a small and certain reward and a larger but less certain reward. To study choice behavior under risk in a genetically well characterized species, we trained mice (C57BL/6) on a discrete trial, concurrent-choice task in which they must choose between two levers. Pressing one lever (safe choice) is always followed by a small reward. Pressing the other lever (risky choice) is followed by a larger reward, but only on some of the trials. The overall payoff is the same on both levers. When mice were not food deprived, they were indifferent to risk, choosing both levers with equal probability regardless of the level of risk. In contrast, following food or water deprivation, mice earning 10% sucrose solution were risk-averse, though the addition of alcohol to the sucrose solution dose-dependently reduced risk aversion, even before the mice became intoxicated. Our results falsify the budget rule in optimal foraging theory often used to explain behavior under risk. Instead, they suggest that the overall demand or desired amount for a particular reward determines risk preference. Changes in motivational state or reward identity affect risk preference by changing demand. Any manipulation that increases the demand for a reward also increases risk aversion, by selectively increasing the frequency of safe choices without affecting frequency of risky choices
Preclinical Evaluation of Spatial Frequency Domain-Enabled Wide-Field Quantitative Imaging for Enhanced Glioma Resection
5-Aminolevelunic acid-induced protoporphyrin IX (PpIX) fluorescence-guided resection (FGR) enables maximum safe resection of glioma by providing real-time tumor contrast. However, the subjective visual assessment and the variable intrinsic optical attenuation of tissue limit this technique to reliably delineating only high-grade tumors that display strong fluorescence. We have previously shown, using a fiber-optic probe, that quantitative assessment using noninvasive point spectroscopic measurements of the absolute PpIX concentration in tissue further improves the accuracy of FGR, extending it to surgically curable low-grade glioma. More recently, we have shown that implementing spatial frequency domain imaging with a fluorescent-light transport model enables recovery of two-dimensional images of [PpIX], alleviating the need for time-consuming point sampling of the brain surface. We present first results of this technique modified for in vivo imaging on an RG2 rat brain tumor model. Despite the moderate errors in retrieving the absorption and reduced scattering coefficients in the subdiffusive regime of 14% and 19%, respectively, the recovered [PpIX] maps agree within 10% of the point [PpIX] values measured by the fiber-optic probe, validating its potential as an extension or an alternative to point sampling during glioma resection
Tachyon Effective Actions In Open String Theory
We argue that the Dirac-Born-Infeld (DBI) action coupled to a tachyon, that
is known to reproduce some aspects of open string dynamics, can be obtained
from open string theory in a certain limit, which generalizes the limit leading
to the usual DBI action. This helps clarify which aspects of the full open
string theory are captured by this action.Comment: harvmac, 19 pages; v2 two references added; v3 additional comments
and a referenc
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