79,753 research outputs found
Speeding up Permutation Testing in Neuroimaging
Multiple hypothesis testing is a significant problem in nearly all
neuroimaging studies. In order to correct for this phenomena, we require a
reliable estimate of the Family-Wise Error Rate (FWER). The well known
Bonferroni correction method, while simple to implement, is quite conservative,
and can substantially under-power a study because it ignores dependencies
between test statistics. Permutation testing, on the other hand, is an exact,
non-parametric method of estimating the FWER for a given -threshold,
but for acceptably low thresholds the computational burden can be prohibitive.
In this paper, we show that permutation testing in fact amounts to populating
the columns of a very large matrix . By analyzing the spectrum of this
matrix, under certain conditions, we see that has a low-rank plus a
low-variance residual decomposition which makes it suitable for highly
sub--sampled --- on the order of --- matrix completion methods. Based
on this observation, we propose a novel permutation testing methodology which
offers a large speedup, without sacrificing the fidelity of the estimated FWER.
Our evaluations on four different neuroimaging datasets show that a
computational speedup factor of roughly can be achieved while
recovering the FWER distribution up to very high accuracy. Further, we show
that the estimated -threshold is also recovered faithfully, and is
stable.Comment: NIPS 1
Experimental analysis of the pressure–velocity correlations of external unsteady flow over rocket launchers
Based on simultaneous wall pressure and velocity measurements, the aerodynamic load of a launcher body model is investigated. Two different configurations are considered in order to study the influence of geometrical artifacts on the after body flow and consequently on the aerodynamic load. After a brief presentation of the experimental setup used to get the unsteady wall pressure and external velocity field, a global analysis of the integrated pressure along the nozzle is presented for both configurations. It is shown that the unsteady load induced on a configuration with attachment device involves characteristic frequencies which fits the mechanical response of the structure whereas no particular behavior is observed on the configuration without attachment device. Then, a Proper Orthogonal Decomposition is successively performed from the wall pressure field and from the external velocity field highlighting the relationship between the most energetic structures of the flow and the involved phenomenon. Finally, a pressure/velocity correlation of the POD modes is presented and the most energetic structures of the velocity field are linked to the unsteady load observed on the nozzle. It is then confirmed the structural influence of the attachment device and its contribution to the unsteady loads acting on the nozzle.ESA TRP: "Unsteady Subscale Force Measurements within a Launch Vehicle Base Buffeting Environment"
Measurement Matrix Design for Compressive Sensing Based MIMO Radar
In colocated multiple-input multiple-output (MIMO) radar using compressive
sensing (CS), a receive node compresses its received signal via a linear
transformation, referred to as measurement matrix. The samples are subsequently
forwarded to a fusion center, where an L1-optimization problem is formulated
and solved for target information. CS-based MIMO radar exploits the target
sparsity in the angle-Doppler-range space and thus achieves the high
localization performance of traditional MIMO radar but with many fewer
measurements. The measurement matrix is vital for CS recovery performance. This
paper considers the design of measurement matrices that achieve an optimality
criterion that depends on the coherence of the sensing matrix (CSM) and/or
signal-to-interference ratio (SIR). The first approach minimizes a performance
penalty that is a linear combination of CSM and the inverse SIR. The second one
imposes a structure on the measurement matrix and determines the parameters
involved so that the SIR is enhanced. Depending on the transmit waveforms, the
second approach can significantly improve SIR, while maintaining CSM comparable
to that of the Gaussian random measurement matrix (GRMM). Simulations indicate
that the proposed measurement matrices can improve detection accuracy as
compared to a GRMM
Improving Negative Sampling for Word Representation using Self-embedded Features
Although the word-popularity based negative sampler has shown superb
performance in the skip-gram model, the theoretical motivation behind
oversampling popular (non-observed) words as negative samples is still not well
understood. In this paper, we start from an investigation of the gradient
vanishing issue in the skipgram model without a proper negative sampler. By
performing an insightful analysis from the stochastic gradient descent (SGD)
learning perspective, we demonstrate that, both theoretically and intuitively,
negative samples with larger inner product scores are more informative than
those with lower scores for the SGD learner in terms of both convergence rate
and accuracy. Understanding this, we propose an alternative sampling algorithm
that dynamically selects informative negative samples during each SGD update.
More importantly, the proposed sampler accounts for multi-dimensional
self-embedded features during the sampling process, which essentially makes it
more effective than the original popularity-based (one-dimensional) sampler.
Empirical experiments further verify our observations, and show that our
fine-grained samplers gain significant improvement over the existing ones
without increasing computational complexity.Comment: Accepted in WSDM 201
The organization of collective group movements in wild Barbary macaques (Macaca sylvanus): social structure drives processes of group coordination in macaques
Social animals have to coordinate activities and collective movements to benefit from the advantages of group living.
Animals in large groups maintain cohesion by self-organization processes whereas in smaller groups consensus decisions
can be reached. Where consensus decisions are relevant leadership may emerge. Variation in the organization of collective
movements has been linked to variation in female social tolerance among macaque species ranging from despotic to
egalitarian. Here we investigated the processes underlying group movements in a wild macaque species characterized by a
degree of social tolerance intermediate to previously studied congeneric species. We focused on processes before, during
and after the departure of the first individual. To this end, we observed one group of wild Barbary macaques (Macaca
sylvanus) in the Middle Atlas, Morocco using all-occurrence behaviour sampling of 199 collective movements. We found that
initiators of a collective movement usually chose the direction in which more individuals displayed pre-departure behavior.
Dominant individuals contributed to group movements more than subordinates, especially juveniles, measured as
frequencies of successful initiations and pre-departure behaviour. Joining was determined by affiliative relationships and
the number of individuals that already joined the movement (mimetism). Thus, in our study group partially shared
consensus decisions mediated by selective mimetism seemed to be prevalent, overall supporting the suggestion that a
species’ social style affects the organization of group movements. As only the most tolerant species show equally shared
consensus decisions whereas in others the decision is partially shared with a bias to dominant individuals the type of
consensus decisions seems to follow a stepwise relation. Joining order may also follow a stepwise, however opposite,
relationship, because dominance only determined joining in highly despotic, but not in intermediate and tolerant species
Turbulent jet interaction with a long rise-time pressure signature
A sonic boom signature with a long rise time has the ability to reduce the sonic boom, but it does not necessarily minimize the sonic boom at the ground level because of the real atmospheric turbulence. In this study, an effect of the turbulence on a long rise-time pressure signature was experimentally investigated in a ballistic range facility. To compare the effects of the turbulence on the long and short rise-time pressure signatures, a cone-cylinder projectile that simultaneously produces these pressure signatures was designed. The pressure waves interacted with a turbulent field generated by a circular nozzle. The turbulence effects were evaluated using flow diagnostic techniques: high-speed schlieren photography, a point-diffraction interferometer, and a pressure measurement. In spite of the fact that the long and short rise-time pressure signatures simultaneously travel through the turbulent field, the turbulence effects do not give the same contribution to these overpressures. Regarding the long rise-time pressure signature, the overpressure fluctuation due to the turbulence interaction is almost uniform, and a standard deviation 1.5 times greater than that of the no-turbulence case is observed. By contrast, a short rise-time pressure signature which passed through the same turbulent field is strongly affected by the turbulence. A standard deviation increases by a factor of 14 because of the turbulence interaction. Additionally, there is a non-correlation between the overpressure fluctuations of the long and short rise-time pressure signatures. These results deduce that the length of the rise time is important to the turbulence effects such as the shock focusing/diffracting
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