330 research outputs found
Cortical Surface Area Differentiates Familial High Risk Individuals Who Go on to Develop Schizophrenia
BACKGROUND: Schizophrenia is associated with structural brain abnormalities that may be present before disease
onset. It remains unclear whether these represent general vulnerability indicators or are associated with the clinical state itself.
METHODS: To investigate this, structural brain scans were acquired at two time points (mean scan interval
1.87 years) in a cohort of individuals at high familial risk of schizophrenia (n 5 142) and control subjects (n 5 36).
Cortical reconstructions were generated using FreeSurfer. The high-risk cohort was subdivided into individuals that
remained well during the study, individuals that had transient psychotic symptoms, and individuals that subsequently
became ill. Baseline measures and longitudinal change in global estimates of thickness and surface area and lobar
values were compared, focusing on overall differences between high-risk individuals and control subjects and then
on group differences within the high-risk cohort.
RESULTS: Longitudinally, control subjects showed a significantly greater reduction in cortical surface area
compared with the high-risk group. Within the high-risk group, differences in surface area at baseline predicted
clinical course, with individuals that subsequently became ill having significantly larger surface area than individuals
that remained well during the study. For thickness, longitudinal reductions were most prominent in the frontal,
cingulate, and occipital lobes in all high-risk individuals compared with control subjects.
CONCLUSIONS: Our results suggest that larger surface areas at baseline may be associated with mechanisms that
go above and beyond a general familial disposition. A relative preservation over time of surface area, coupled with a
thinning of the cortex compared with control subjects, may serve as vulnerability markers of schizophrenia
On the fourth-order accurate compact ADI scheme for solving the unsteady Nonlinear Coupled Burgers' Equations
The two-dimensional unsteady coupled Burgers' equations with moderate to
severe gradients, are solved numerically using higher-order accurate finite
difference schemes; namely the fourth-order accurate compact ADI scheme, and
the fourth-order accurate Du Fort Frankel scheme. The question of numerical
stability and convergence are presented. Comparisons are made between the
present schemes in terms of accuracy and computational efficiency for solving
problems with severe internal and boundary gradients. The present study shows
that the fourth-order compact ADI scheme is stable and efficient
Large negative velocity gradients in Burgers turbulence
We consider 1D Burgers equation driven by large-scale white-in-time random
force. The tails of the velocity gradients probability distribution function
(PDF) are analyzed by saddle-point approximation in the path integral
describing the velocity statistics. The structure of the saddle-point
(instanton), that is velocity field configuration realizing the maximum of
probability, is studied numerically in details. The numerical results allow us
to find analytical solution for the long-time part of the instanton. Its
careful analysis confirms the result of [Phys. Rev. Lett. 78 (8) 1452 (1997)
[chao-dyn/9609005]] based on short-time estimations that the left tail of PDF
has the form ln P(u_x) \propto -|u_x|^(3/2).Comment: 10 pages, RevTeX, 10 figure
Anisotropic London Penetration Depth and Superfluid Density in Single Crystals of Iron-based Pnictide Superconductors
In- and out-of-plane magnetic penetration depths were measured in three
iron-based pnictide superconducting systems. All studied samples of both 122
systems show a robust power-law behavior, , with the
sample-dependent exponent n=2-2.5, which is indicative of unconventional
pairing. This scenario could be possible either through scattering in a state or due to nodes in the superconducting gap. In the Nd-1111 system, the
interpretation of data may be obscured by the magnetism of rare-earth ions. The
overall anisotropy of the pnictide superconductors is small. The 1111 system is
about two times more anisotropic than the 122 system. Our data and analysis
suggest that the iron-based pnictides are complex superconductors in which a
multiband three-dimensional electronic structure and strong magnetic
fluctuations play important roles.Comment: submitted to a special issue of Physica C on superconducting
pnictide
Implementation of an Optimal First-Order Method for Strongly Convex Total Variation Regularization
We present a practical implementation of an optimal first-order method, due
to Nesterov, for large-scale total variation regularization in tomographic
reconstruction, image deblurring, etc. The algorithm applies to -strongly
convex objective functions with -Lipschitz continuous gradient. In the
framework of Nesterov both and are assumed known -- an assumption
that is seldom satisfied in practice. We propose to incorporate mechanisms to
estimate locally sufficient and during the iterations. The mechanisms
also allow for the application to non-strongly convex functions. We discuss the
iteration complexity of several first-order methods, including the proposed
algorithm, and we use a 3D tomography problem to compare the performance of
these methods. The results show that for ill-conditioned problems solved to
high accuracy, the proposed method significantly outperforms state-of-the-art
first-order methods, as also suggested by theoretical results.Comment: 23 pages, 4 figure
Recent Advances in Understanding Particle Acceleration Processes in Solar Flares
We review basic theoretical concepts in particle acceleration, with
particular emphasis on processes likely to occur in regions of magnetic
reconnection. Several new developments are discussed, including detailed
studies of reconnection in three-dimensional magnetic field configurations
(e.g., current sheets, collapsing traps, separatrix regions) and stochastic
acceleration in a turbulent environment. Fluid, test-particle, and
particle-in-cell approaches are used and results compared. While these studies
show considerable promise in accounting for the various observational
manifestations of solar flares, they are limited by a number of factors, mostly
relating to available computational power. Not the least of these issues is the
need to explicitly incorporate the electrodynamic feedback of the accelerated
particles themselves on the environment in which they are accelerated. A brief
prognosis for future advancement is offered.Comment: This is a chapter in a monograph on the physics of solar flares,
inspired by RHESSI observations. The individual articles are to appear in
Space Science Reviews (2011
Towards an understanding of unique and shared pathways in the psychopathophysiology of AD/HD
Most attention deficit hyperactivity disorder (ADHD) research has compared cases with unaffected controls. This has led to many associations, but uncertainties about their specificity to ADHD in contrast with other disorders. We present a selective review of research, comparing ADHD with other disorders in neuropsychological, neurobiological and genetic correlates. So far, a specific pathophysiologicalpathway has not been identified. ADHD is probably not specifically associated with executive function deficits. It is possible, but not yet established, that ADHD symptoms may be more specifically associated with motivational abnormalities, motor organization and time perception. Recent findings indicating common genetic liabilities of ADHD and other conditions raise questions about diagnostic boundaries. In future research, the delineation of the pathophysiological mechanisms of ADHD needs to match cognitive, imaging and genetic techniques to the challenge of defining more homogenous clinical groups; multi-site collaborative projects are needed. © Blackwell Publishing Ltd
A weakly stable algorithm for general Toeplitz systems
We show that a fast algorithm for the QR factorization of a Toeplitz or
Hankel matrix A is weakly stable in the sense that R^T.R is close to A^T.A.
Thus, when the algorithm is used to solve the semi-normal equations R^T.Rx =
A^Tb, we obtain a weakly stable method for the solution of a nonsingular
Toeplitz or Hankel linear system Ax = b. The algorithm also applies to the
solution of the full-rank Toeplitz or Hankel least squares problem.Comment: 17 pages. An old Technical Report with postscript added. For further
details, see http://wwwmaths.anu.edu.au/~brent/pub/pub143.htm
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