2,046 research outputs found
Amyloid imaging in aging and dementia: testing the amyloid hypothesis in vivo.
Amyloid imaging represents a major advance in neuroscience, enabling the detection and quantification of pathologic protein aggregations in the brain. In this review we survey current amyloid imaging techniques, focusing on positron emission tomography (PET) with (11)carbon-labelled Pittsburgh Compound-B ((11)C-PIB), the most extensively studied and best validated tracer. PIB binds specifically to fibrillar beta-amyloid (Abeta) deposits, and is a sensitive marker for Abeta pathology in cognitively normal older individuals and patients with mild cognitive impairment (MCI) and Alzheimer's disease (AD). PIB-PET provides us with a powerful tool to examine in vivo the relationship between amyloid deposition, clinical symptoms, and structural and functional brain changes in the continuum between normal aging and AD. Amyloid imaging studies support a model in which amyloid deposition is an early event on the path to dementia, beginning insidiously in cognitively normal individuals, and accompanied by subtle cognitive decline and functional and structural brain changes suggestive of incipient AD. As patients progress to dementia, clinical decline and neurodegeneration accelerate and proceed independently of amyloid accumulation. In the future, amyloid imaging is likely to supplement clinical evaluation in selecting patients for anti-amyloid therapies, while MRI and FDG-PET may be more appropriate markers of clinical progression
Statistical Entropy of Schwarzschild Black Holes
The entropy of a seven dimensional Schwarzschild black hole of arbitrary
large radius is obtained by a mapping onto a near extremal self-dual
three-brane whose partition function can be evaluated. The three-brane arises
from duality after submitting a neutral blackbrane, from which the
Schwarzschild black hole can be obtained by compactification, to an infinite
boost in non compact eleven dimensional space-time and then to a Kaluza-Klein
compactification. This limit can be defined in precise terms and yields the
Bekenstein-Hawking value up to a factor of order one which can be set to be
exactly one with the extra assumption of keeping only transverse brane
excitations. The method can be generalized to five and four dimensional black
holes.Comment: 11 pages, LaTex, no figures, corrected typ
Self-Duality and New TQFTs for Forms
We discuss theories containing higher-order forms in various dimensions. We
explain how Chern--Simons-type theories of forms can be defined from TQFTs in
one less dimension. We also exhibit new TQFTs with interacting Yang--Mills
fields and higher--order forms. They are obtained by the dimensional reduction
of TQFTs whose gauge functions are free self-duality equations. Interactions
are due to the gauging of global internal symmetries after dimensional
reduction. We list possible symmetries and give a brief discussion on the
possible relation of such systems to interacting field theories.Comment: teX-fil
On the Nature of the Hagedorn Transition in NCOS Systems
We extend the study of the nature of the Hagedorn transition in NCOS systems
in various dimensions. The canonical analysis results in a microscopic
ionization picture of a bound state system in which the Hagedorn transition is
postponed till irrelevancy. A microcanonical analysis leads to a limiting
Hagedorn behaviour dominated by highly excited, long open strings. The study of
the full phase diagram of the NCOS system using the AdS/CFT correspondence
suggests that the microscopic ionization picture is the correct one. We discuss
some refinements of the ionization mechanism for NCOS systems, including
the formation of a temperature-dependent barrier for the process. Some possible
consequences of this behaviour, including a potential puzzle for , are
discussed. Phase diagrams of a regularized form of NCOS systems are introduced
and do accomodate a phase of long open strings which disappears in the strict
NCOS limit.Comment: 37 pages, 3 Postscript figure
Long time scales and eternal black holes
We discuss the various scales determining the temporal behaviour of
correlation functions in the presence of eternal black holes. We point out the
origins of the failure of the semiclassical gravity approximation to respect a
unitarity-based bound suggested by Maldacena. We find that the presence of a
subleading (in the large-N approximation involved) master field does restore
the compliance with one bound but additional configurations are needed to
explain the more detailed expected time dependence of the Poincare recurrences
and their magnitude.Comment: 10 pages, 6 figures. Presented at Johns Hopkins 2003 and Ahrenshoop
2003 workshop
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