14,010 research outputs found
An MRI-Derived Definition of MCI-to-AD Conversion for Long-Term, Automati c Prognosis of MCI Patients
Alzheimer's disease (AD) and mild cognitive impairment (MCI), continue to be
widely studied. While there is no consensus on whether MCIs actually "convert"
to AD, the more important question is not whether MCIs convert, but what is the
best such definition. We focus on automatic prognostication, nominally using
only a baseline image brain scan, of whether an MCI individual will convert to
AD within a multi-year period following the initial clinical visit. This is in
fact not a traditional supervised learning problem since, in ADNI, there are no
definitive labeled examples of MCI conversion. Prior works have defined MCI
subclasses based on whether or not clinical/cognitive scores such as CDR
significantly change from baseline. There are concerns with these definitions,
however, since e.g. most MCIs (and ADs) do not change from a baseline CDR=0.5,
even while physiological changes may be occurring. These works ignore rich
phenotypical information in an MCI patient's brain scan and labeled AD and
Control examples, in defining conversion. We propose an innovative conversion
definition, wherein an MCI patient is declared to be a converter if any of the
patient's brain scans (at follow-up visits) are classified "AD" by an
(accurately-designed) Control-AD classifier. This novel definition bootstraps
the design of a second classifier, specifically trained to predict whether or
not MCIs will convert. This second classifier thus predicts whether an
AD-Control classifier will predict that a patient has AD. Our results
demonstrate this new definition leads not only to much higher prognostic
accuracy than by-CDR conversion, but also to subpopulations much more
consistent with known AD brain region biomarkers. We also identify key
prognostic region biomarkers, essential for accurately discriminating the
converter and nonconverter groups
On the black-box complexity of Sperner's Lemma
We present several results on the complexity of various forms of Sperner's
Lemma in the black-box model of computing. We give a deterministic algorithm
for Sperner problems over pseudo-manifolds of arbitrary dimension. The query
complexity of our algorithm is linear in the separation number of the skeleton
graph of the manifold and the size of its boundary. As a corollary we get an
deterministic query algorithm for the black-box version of the
problem {\bf 2D-SPERNER}, a well studied member of Papadimitriou's complexity
class PPAD. This upper bound matches the deterministic lower
bound of Crescenzi and Silvestri. The tightness of this bound was not known
before. In another result we prove for the same problem an
lower bound for its probabilistic, and an
lower bound for its quantum query complexity, showing
that all these measures are polynomially related.Comment: 16 pages with 1 figur
Statistical Network Analysis for Functional MRI: Summary Networks and Group Comparisons
Comparing weighted networks in neuroscience is hard, because the topological
properties of a given network are necessarily dependent on the number of edges
of that network. This problem arises in the analysis of both weighted and
unweighted networks. The term density is often used in this context, in order
to refer to the mean edge weight of a weighted network, or to the number of
edges in an unweighted one. Comparing families of networks is therefore
statistically difficult because differences in topology are necessarily
associated with differences in density. In this review paper, we consider this
problem from two different perspectives, which include (i) the construction of
summary networks, such as how to compute and visualize the mean network from a
sample of network-valued data points; and (ii) how to test for topological
differences, when two families of networks also exhibit significant differences
in density. In the first instance, we show that the issue of summarizing a
family of networks can be conducted by adopting a mass-univariate approach,
which produces a statistical parametric network (SPN). In the second part of
this review, we then highlight the inherent problems associated with the
comparison of topological functions of families of networks that differ in
density. In particular, we show that a wide range of topological summaries,
such as global efficiency and network modularity are highly sensitive to
differences in density. Moreover, these problems are not restricted to
unweighted metrics, as we demonstrate that the same issues remain present when
considering the weighted versions of these metrics. We conclude by encouraging
caution, when reporting such statistical comparisons, and by emphasizing the
importance of constructing summary networks.Comment: 16 pages, 5 figure
Accurate classification of 29 objects detected in the 39 months Palermo Swift/BAT hard X-ray catalogue
Through an optical campaign performed at 4 telescopes located in the northern
and the southern hemispheres, plus archival data from two on-line sky surveys,
we have obtained optical spectroscopy for 29 counterparts of unclassified or
poorly studied hard X-ray emitting objects detected with Swift/BAT and listed
in the 39 months Palermo catalogue. All these objects have also observations
taken with Swift/XRT or XMM-EPIC which not only allow us to pinpoint their
optical counterpart, but also to study their X-ray spectral properties (column
density, power law photon index and F2-10 keV flux). We find that 28 sources in
our sample are AGN; 7 are classified as type 1 while 21 are of type 2; the
remaining object is a galactic cataclysmic variable. Among our type 1 AGN, we
find 5 objects of intermediate Seyfert type (1.2-1.9) and one Narrow Line
Seyfert 1 galaxy; for 4 out of 7 sources, we have been able to estimate the
central black hole mass. Three of the type 2 AGN of our sample display optical
features typical of the LINER class and one is a likely Compton thick AGN. All
galaxies classified in this work are relatively nearby objects since their
redshifts lie in the range 0.008-0.075; the only galactic object found lies at
an estimated distance of 90 pc. We have also investigated the optical versus
X-ray emission ratio of the galaxies of our sample to test the AGN unified
model. For them, we have also compared the X-ray absorption (due to gas) with
the optical reddening (due to dust): we find that for most of our sources,
specifically those of type 1.9-2.0 the former is higher than the latter
confirming early results by Maiolino et al. (2001); this is possibly due to the
properties of dust in the circumnuclear obscuring torus of the AGN.Comment: 15 pages, 4 figures, 8 tables, accepted for publication on Astronomy
and Astrophysic
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