15,896 research outputs found
PrAGMATiC: a Probabilistic and Generative Model of Areas Tiling the Cortex
Much of the human cortex seems to be organized into topographic cortical
maps. Yet few quantitative methods exist for characterizing these maps. To
address this issue we developed a modeling framework that can reveal
group-level cortical maps based on neuroimaging data. PrAGMATiC, a
probabilistic and generative model of areas tiling the cortex, is a
hierarchical Bayesian generative model of cortical maps. This model assumes
that the cortical map in each individual subject is a sample from a single
underlying probability distribution. Learning the parameters of this
distribution reveals the properties of a cortical map that are common across a
group of subjects while avoiding the potentially lossy step of co-registering
each subject into a group anatomical space. In this report we give a
mathematical description of PrAGMATiC, describe approximations that make it
practical to use, show preliminary results from its application to a real
dataset, and describe a number of possible future extensions
Polarized Diffuse Emission at 2.3 GHz in a High Galactic Latitude Area
Polarized diffuse emission observations at 2.3 GHz in a high Galactic
latitude area are presented. The 2\degr X 2\degr field, centred in
(\alpha=5^h,\delta=-49\degr), is located in the region observed by the
BOOMERanG experiment. Our observations has been carried out with the Parkes
Radio telescope and represent the highest frequency detection done to date in
low emission areas. Because of a weaker Faraday rotation action, the high
frequency allows an estimate of the Galactic synchrotron contamination of the
Cosmic Microwave Background Polarization (CMBP) that is more reliable than that
done at 1.4 GHz. We find that the angular power spectra of the E- and B-modes
have slopes of \beta_E = -1.46 +/- 0.14 and \beta_B = -1.87 +/- 0.22,
indicating a flattening with respect to 1.4 GHz. Extrapolated up to 32 GHz, the
E-mode spectrum is about 3 orders of magnitude lower than that of the CMBP,
allowing a clean detection even at this frequency. The best improvement
concerns the B-mode, for which our single-dish observations provide the first
estimate of the contamination on angular scales close to the CMBP peak (about 2
degrees). We find that the CMBP B-mode should be stronger than synchrotron
contamination at 90 GHz for models with T/S > 0.01. This low level could move
down to 60-70 GHz the optimal window for CMBP measures.Comment: 5 pages, 6 figures, accepted for publication in MNRAS Letter
Weak Lensing Determination of the Mass in Galaxy Halos
We detect the weak gravitational lensing distortion of 450,000 background
galaxies (20<R<23) by 790 foreground galaxies (R<18) selected from the Las
Campanas Redshift Survey (LCRS). This is the first detection of weak lensing by
field galaxies of known redshift, and as such permits us to reconstruct the
shear profile of the typical field galaxy halo in absolute physical units
(modulo H_0), and to investigate the dependence of halo mass upon galaxy
luminosity. This is also the first galaxy-galaxy lensing study for which the
calibration errors are negligible. Within a projected radius of 200 \hkpc, the
shear profile is consistent with an isothermal profile with circular velocity
164+-20 km/s for an L* galaxy, consistent with typical disk rotation at this
luminosity. This halo mass normalization, combined with the halo profile
derived by Fischer et al (2000) from lensing analysis SDSS data, places a lower
limit of (2.7+-0.6) x 10^{12}h^{-1} solar masses on the mass of an L* galaxy
halo, in good agreement with satellite galaxy studies. Given the known
luminosity function of LCRS galaxies, and the assumption that for galaxies, we determine that the mass within 260\hkpc of normal
galaxies contributes to the density of the Universe (for
) or for . These lensing data suggest
that (95% CL), only marginally in agreement with the usual
Faber-Jackson or Tully-Fisher scaling. This is the most
complete direct inventory of the matter content of the Universe to date.Comment: 18 pages, incl. 3 figures. Submitted to ApJ 6/7/00, still no response
from the referee after four months
Entanglement requirements for implementing bipartite unitary operations
We prove, using a new method based on map-state duality, lower bounds on
entanglement resources needed to deterministically implement a bipartite
unitary using separable (SEP) operations, which include LOCC (local operations
and classical communication) as a particular case. It is known that the Schmidt
rank of an entangled pure state resource cannot be less than the Schmidt rank
of the unitary. We prove that if these ranks are equal the resource must be
uniformly (maximally) entangled: equal nonzero Schmidt coefficients. Higher
rank resources can have less entanglement: we have found numerical examples of
Schmidt rank 2 unitaries which can be deterministically implemented, by either
SEP or LOCC, using an entangled resource of two qutrits with less than one ebit
of entanglement.Comment: 7 pages Revte
Consistent Resolution of Some Relativistic Quantum Paradoxes
A relativistic version of the (consistent or decoherent) histories approach
to quantum theory is developed on the basis of earlier work by Hartle, and used
to discuss relativistic forms of the paradoxes of spherical wave packet
collapse, Bohm's formulation of Einstein-Podolsky-Rosen, and Hardy's paradox.
It is argued that wave function collapse is not needed for introducing
probabilities into relativistic quantum mechanics, and in any case should never
be thought of as a physical process. Alternative approaches to stochastic time
dependence can be used to construct a physical picture of the measurement
process that is less misleading than collapse models. In particular, one can
employ a coarse-grained but fully quantum mechanical description in which
particles move along trajectories, with behavior under Lorentz transformations
the same as in classical relativistic physics, and detectors are triggered by
particles reaching them along such trajectories. States entangled between
spacelike separate regions are also legitimate quantum descriptions, and can be
consistently handled by the formalism presented here. The paradoxes in question
arise because of using modes of reasoning which, while correct for classical
physics, are inconsistent with the mathematical structure of quantum theory,
and are resolved (or tamed) by using a proper quantum analysis. In particular,
there is no need to invoke, nor any evidence for, mysterious long-range
superluminal influences, and thus no incompatibility, at least from this
source, between relativity theory and quantum mechanics.Comment: Latex 42 pages, 7 figures in text using PSTrick
P,T-Violating Nuclear Matrix Elements in the One-Meson Exchange Approximation
Expressions for the P,T-violating NN potentials are derived for ,
and exchange. The nuclear matrix elements for and
exchange are shown to be greatly suppressed, so that, under the assumption of
comparable coupling constants, exchange would dominate by two orders of
magnitude. The ratio of P,T-violating to P-violating matrix elements is found
to remain approximately constant across the nuclear mass table, thus
establishing the proportionality between time-reversal-violation and
parity-violation matrix elements. The calculated values of this ratio suggest a
need to obtain an accuracy of order for the ratio of the
PT-violating to P-violating asymmetries in neutron transmission experiments in
order to improve on the present limits on the isovector pion coupling constant.Comment: 17 pages, LaTeX, no figure
Health professionals, their medical interventions and uncertainty : a study focusing on women at midlife
Health professionals face a tension between focusing on the individual and attending to health issues for the population as a whole. This tension is intrinsic to medicine and gives rise to medical uncertainty, which here is explored through accounts of three medical interventions focused on women at midlife: breast screening, hormone replacement therapy and bone densitometry. The accounts come from interviews with UK health professionals using these medical interventions in their daily work. Drawing on the analysis of Fox [(2002). Health and Healing: The public/private divide (pp. 236â253). London: Routledge] we distinguish three aspects of medical uncertainty and explore each one of them in relation to one of the interventions. First is uncertainty about the balance between the individual and distributive ethic of medicine, explored in relation to breast screening. Second is the dilemma faced by health professionals when using medicial evidence generated through studies of populations and applying this to individuals. We explore this dilemma for hormone replacement therapy. Thirdly there is uncertainty because of the lack of a conceptual framework for understanding how new micro knowledge, such as human genetic information, can be combined with knowledge of other biological and social dimensions of health. The accounts from the bone denistometry clinic indicate the beginnings of an understanding of the need for such a framework, which would acknowledge complexity, recognising that factors from many different levels of analysis, from heredity through to social factors, interact with each other and influence the individual and their health. However, our analysis suggests biomedicine continues to be dominated by an individualised, context free, concept of health and health risk with individuals alone responsible for their own health and for the health of the population. This may continue to dominate how we perceive responsibilities for health until we establish a conceptual framework that recognises the complex interaction of many factors at macro and micro level affecting health
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