14,106 research outputs found
Can parametric statistical methods be trusted for fMRI based group studies?
The most widely used task fMRI analyses use parametric methods that depend on
a variety of assumptions. While individual aspects of these fMRI models have
been evaluated, they have not been evaluated in a comprehensive manner with
empirical data. In this work, a total of 2 million random task fMRI group
analyses have been performed using resting state fMRI data, to compute
empirical familywise error rates for the software packages SPM, FSL and AFNI,
as well as a standard non-parametric permutation method. While there is some
variation, for a nominal familywise error rate of 5% the parametric statistical
methods are shown to be conservative for voxel-wise inference and invalid for
cluster-wise inference; in particular, cluster size inference with a cluster
defining threshold of p = 0.01 generates familywise error rates up to 60%. We
conduct a number of follow up analyses and investigations that suggest the
cause of the invalid cluster inferences is spatial auto correlation functions
that do not follow the assumed Gaussian shape. By comparison, the
non-parametric permutation test, which is based on a small number of
assumptions, is found to produce valid results for voxel as well as cluster
wise inference. Using real task data, we compare the results between one
parametric method and the permutation test, and find stark differences in the
conclusions drawn between the two using cluster inference. These findings speak
to the need of validating the statistical methods being used in the
neuroimaging field
High-resolution simulations of planetesimal formation in turbulent protoplanetary discs
We present high-resolution computer simulations of dust dynamics and
planetesimal formation in turbulence generated by the magnetorotational
instability. We show that the turbulent viscosity associated with
magnetorotational turbulence in a non-stratified shearing box increases when
going from 256^3 to 512^3 grid points in the presence of a weak imposed
magnetic field, yielding a turbulent viscosity of at high
resolution. Particles representing approximately meter-sized boulders
concentrate in large-scale high-pressure regions in the simulation box. The
appearance of zonal flows and particle concentration in pressure bumps is
relatively similar at moderate (256^3) and high (512^3) resolution. In the
moderate-resolution simulation we activate particle self-gravity at a time when
there is little particle concentration, in contrast with previous simulations
where particle self-gravity was activated during a concentration event. We
observe that bound clumps form over the next ten orbits, with initial birth
masses of a few times the dwarf planet Ceres. At high resolution we activate
self-gravity during a particle concentration event, leading to a burst of
planetesimal formation, with clump masses ranging from a significant fraction
of to several times the mass of Ceres. We present a new domain decomposition
algorithm for particle-mesh schemes. Particles are spread evenly among the
processors and the local gas velocity field and assigned drag forces are
exchanged between a domain-decomposed mesh and discrete blocks of particles. We
obtain good load balancing on up to 4096 cores even in simulations where
particles sediment to the mid-plane and concentrate in pressure bumps.Comment: Accepted for publication in Astronomy & Astrophysics, with some
changes in response to referee repor
Prospects of long-time-series observations from Dome C for transit search
The detection of transiting extrasolar planets requires high-photometric
quality and long-duration photometric stellar time-series. In this paper, we
investigate the advantages provided by the Antarctic observing platform Dome C
for planet transit detections during its long winter period, which allows for
relatively long, uninterrupted time-series. Our calculations include limiting
effects due to the Sun and Moon, cloud coverage and the effect of reduced
photometric quality for high extinction of target fields. We compare the
potential for long time-series from Dome C with a single site in Chile, a
three-site low-latitude network as well as combinations of Dome C with Chile
and the network, respectively. Dome C is one of the prime astronomical sites on
Earth for obtaining uninterrupted long-duration observations in terms of
prospects for a high observational duty cycle. The duty cycle of a project can,
however, be significantly improved by integrating Dome C into a network of
sites.Comment: 10 pages, 9 figures, accepted by PAS
Traffic matrix estimation on a large IP backbone: a comparison on real data
This paper considers the problem of estimating the point-to-point
traffic matrix in an operational IP backbone. Contrary to previous studies, that have used a partial traffic matrix or demands estimated from aggregated Netflow traces, we use a unique data set of complete traffic matrices from a global IP network measured over five-minute intervals. This allows us to do an accurate data analysis on the time-scale of typical link-load measurements and enables us to make a balanced evaluation of different traffic matrix estimation techniques. We describe the data collection infrastructure, present spatial and temporal demand distributions, investigate the stability of fan-out factors, and analyze the mean-variance relationships between demands. We perform a critical evaluation of existing and novel methods for traffic matrix estimation, including recursive fanout estimation, worst-case bounds, regularized estimation techniques, and methods that rely on mean-variance relationships. We discuss the weaknesses and strengths of the various methods, and highlight differences in the results for the European and American subnetworks
An algorithm for lifting points in a tropical variety
The aim of this paper is to give a constructive proof of one of the basic
theorems of tropical geometry: given a point on a tropical variety (defined
using initial ideals), there exists a Puiseux-valued ``lift'' of this point in
the algebraic variety. This theorem is so fundamental because it justifies why
a tropical variety (defined combinatorially using initial ideals) carries
information about algebraic varieties: it is the image of an algebraic variety
over the Puiseux series under the valuation map. We have implemented the
``lifting algorithm'' using Singular and Gfan if the base field are the
rational numbers. As a byproduct we get an algorithm to compute the Puiseux
expansion of a space curve singularity in (K^{n+1},0).Comment: 33 page
Cluster Failure Revisited: Impact of First Level Design and Data Quality on Cluster False Positive Rates
Methodological research rarely generates a broad interest, yet our work on
the validity of cluster inference methods for functional magnetic resonance
imaging (fMRI) created intense discussion on both the minutia of our approach
and its implications for the discipline. In the present work, we take on
various critiques of our work and further explore the limitations of our
original work. We address issues about the particular event-related designs we
used, considering multiple event types and randomisation of events between
subjects. We consider the lack of validity found with one-sample permutation
(sign flipping) tests, investigating a number of approaches to improve the
false positive control of this widely used procedure. We found that the
combination of a two-sided test and cleaning the data using ICA FIX resulted in
nominal false positive rates for all datasets, meaning that data cleaning is
not only important for resting state fMRI, but also for task fMRI. Finally, we
discuss the implications of our work on the fMRI literature as a whole,
estimating that at least 10% of the fMRI studies have used the most problematic
cluster inference method (P = 0.01 cluster defining threshold), and how
individual studies can be interpreted in light of our findings. These
additional results underscore our original conclusions, on the importance of
data sharing and thorough evaluation of statistical methods on realistic null
data
- …