Regional coherence evaluation in mild cognitive impairment and Alzheimer's disease based on adaptively extracted magnetoencephalogram rhythms

This study assesses the connectivity alterations caused by Alzheimer's disease (AD) and mild cognitive impairment (MCI) in magnetoencephalogram (MEG) background activity. Moreover, a novel methodology to adaptively extract brain rhythms from the MEG is introduced. This methodology relies on the ability of empirical mode decomposition to isolate local signal oscillations and constrained blind source separation to extract the activity that jointly represents a subset of channels. Inter-regional MEG connectivity was analysed for 36 AD, 18 MCI and 26 control subjects in δ, θ, α and β bands over left and right central, anterior, lateral and posterior regions with magnitude squared coherence—c(f). For the sake of comparison, c(f) was calculated from the original MEG channels and from the adaptively extracted rhythms. The results indicated that AD and MCI cause slight alterations in the MEG connectivity. Computed from the extracted rhythms, c(f) distinguished AD and MCI subjects from controls with 69.4% and 77.3% accuracies, respectively, in a full leave-one-out cross-validation evaluation. These values were higher than those obtained without the proposed extraction methodology

Time-frequency detection of Gravitational Waves

We present a time-frequency method to detect gravitational wave signals in interferometric data. This robust method can detect signals from poorly modeled and unmodeled sources. We evaluate the method on simulated data containing noise and signal components. The noise component approximates initial LIGO interferometer noise. The signal components have the time and frequency characteristics postulated by Flanagan and Hughes for binary black hole coalescence. The signals correspond to binaries with total masses between $45 M_\odot$ to $70 M_\odot$ and with (optimal filter) signal-to-noise ratios of 7 to 12. The method is implementable in real time, and achieves a coincident false alarm rate for two detectors $\approx$ 1 per 475 years. At this false alarm rate, the single detector false dismissal rate for our signal model is as low as 5.3% at an SNR of 10. We expect to obtain similar or better detection rates with this method for any signal of similar power that satisfies certain adiabaticity criteria. Because optimal filtering requires knowledge of the signal waveform to high precision, we argue that this method is likely to detect signals that are undetectable by optimal filtering, which is at present the best developed detection method for transient sources of gravitational waves.Comment: 24 pages, 5 figures, uses REVTE

An excess power statistic for detection of burst sources of gravitational radiation

We examine the properties of an excess power method to detect gravitational waves in interferometric detector data. This method is designed to detect short-duration (< 0.5 s) burst signals of unknown waveform, such as those from supernovae or black hole mergers. If only the bursts' duration and frequency band are known, the method is an optimal detection strategy in both Bayesian and frequentist senses. It consists of summing the data power over the known time interval and frequency band of the burst. If the detector noise is stationary and Gaussian, this sum is distributed as a chi-squared (non-central chi-squared) deviate in the absence (presence) of a signal. One can use these distributions to compute frequentist detection thresholds for the measured power. We derive the method from Bayesian analyses and show how to compute Bayesian thresholds. More generically, when only upper and/or lower bounds on the bursts duration and frequency band are known, one must search for excess power in all concordant durations and bands. Two search schemes are presented and their computational efficiencies are compared. We find that given reasonable constraints on the effective duration and bandwidth of signals, the excess power search can be performed on a single workstation. Furthermore, the method can be almost as efficient as matched filtering when a large template bank is required. Finally, we derive generalizations of the method to a network of several interferometers under the assumption of Gaussian noise.Comment: 22 pages, 6 figure

Anisotropic diffusion equations for adaptive quadratic representations

Adaptive diffusion techniques for processing timefrequency representations were first proposed by Payot and Gonçalvès in 1998 as an application of the Perona and Malik adaptive diffusion. In this communication we consider both this technique and the anisotropic diffusion of Weickert, which allows to tune orientation and shape of smoothing kernels. We propose a new adaptive diffusion scheme where the strength and the orientation of the anisotropic kernel are locally tailored to the processed time-frequency representation. We provide a comparison with other signal-dependent techniques. Finally we define a diffusion tensor that can be used to process time-frequency representations of the affine class, ensuring the preservation of their covariance properties. 1

Hybrid Linear/Bilinear Time-Scale Analysis

using the instantaneous power spectrum, ” IEEE Trans. Acoust.

De la simulation de séries climatiques en zone sahélienne à la simulation hydrologique : le cas de la reconstitution des niveaux du lac Tchad sur la période 1851-2011

National audienceThe reduction in the surface area of Lake Chad from 25,000 to 4,000 km2 between 1972 and 1987, in connection with the great drought of the 1980s in the Sahel, had a major impact on the way of life of the riparian populations, although very low lake levels had already been observed several times during the 20th century. The assessment of the hydrological response of the Lake Chad watershed to climate variability is therefore a key issue in addressing the vulnerability of Sahelian populations. In this study, the levels of Lake Chad are reconstructed between 1851 and 2011 from climatic data (precipitation (P) and temperature (T)), by a lake model whose main forcing is the Chari-Logone flow (draining 85% of the inflows to the lake), itself simulated by a rain-flow model. The lake model and rain-flow model were previously adjusted over the period 1955-2011. The climatic series used (P, T) are obtained by the analogue method. This reconstitution method consists in particular of making an analogy between atmospheric situations of the simulation period (here 1851-2011) and the observation period (here 1955-2011), in order to reconstruct the series of P and T over the whole simulation period by resampling the observed series. The lake levels simulated in this way are compared with observations over the period 1955-2011 and historical records of lake levels prior to 1955. This reconstitution of lake levels is compared with another simulation obtained this time from precipitation and temperatures simulated by global climate models of CMIP5 experiments. Confronting the methods of reconstitution of hydrology in the Sahelian zone seems relevant as the rainfall projections are divergent in this region.La réduction de 25 000 à 4 000 km2 de la surface du lac Tchad entre 1972 et 1987, en lien avec la grande sécheresse des années 80 au Sahel, a eu des répercutions majeures sur le mode de vie des populations riveraines, bien que des niveaux très bas du lac aient déjà été observés à plusieurs reprises au cours du 20e siècle. L’évaluation de la réponse hydrologique du bassin versant du lac Tchad à la variabilité climatique est donc un enjeu essentiel pour faire face à la vulnérabilité des populations sahéliennes. Dans cette étude, les niveaux du lac Tchad sont reconstruits entre 1851 et 2011 à partir de données climatiques (précipitation (P) et température (T)), par un modèle de lac dont le forçage principal est le débit du Chari-Logone (fleuve drainant 85% des apports au lac), lui-même simulé par un modèle pluie-débit. Le modèle de lac et le modèle pluie-débit ont été préalablement calés sur la période 1955-2011. Les séries climatiques utilisées (P, T) sont obtenues par la méthode des analogues. Cette méthode de reconstitution consiste notamment à faire une analogie entre des situations atmosphériques de la période de simulation (ici 1851-2011) et de la période d’observation (ici 1955-2011), afin de reconstruire les séries de P et T sur toute la période de simulation par ré-échantillonnage des séries observées. Les niveaux du lac ainsi simulés sont comparés aux observations sur la période 1955-2011 ainsi qu’aux archives historiques qui témoignent des niveaux de lac avant 1955. À cette reconstitution des niveaux du lac est comparée une autre simulation obtenue cette fois à partir des précipitations et des températures simulées par des modèles climatiques globaux des expériences CMIP5. Confronter les méthodes de reconstitution de l’hydrologie en zone sahélienne semble pertinent tant les projections de précipitations sont divergentes dans cette région

What is the pore pressure in a saturated shale layer?

International audienceElectrostatic interactions, associated with negatively charged surfaces of clay minerals, produce a so-called "disjoining pressure" when diffuse layers overlap, i.e., at low porosity. Disjoining pressure is the pressure difference between the water in the clay pore space and that in a bulk solution at the same depth. Another widely used concept in clay-rocks is the "swelling pressure." It corresponds in fact to the macroscopic average of the disjoining pressure. This study proposes to determine the value of the swelling pressure of a natural material by a simple volume-averaging approach of the disjoining pressure, calculated for each clay mineral present in the material. The swelling pressure, which is dependent on the salinity of the pore fluid, is introduced into a hydrochemomechanical coupling, yielding a more general pressure diffusion equation. The results are compared to swelling pressure measurements for natural shale samples. The implications of this swelling pressure for water pressure measurements in natural formations are also discussed