Polarized Helium to Image the Lung

The main findings of the european PHIL project (Polarised Helium to Image the Lung) are reported. State of the art optical pumping techniques for polarising ^3He gas are described. MRI methodological improvements allow dynamical ventilation images with a good resolution, ultimately limited by gas diffusion. Diffusion imaging appears as a robust method of lung diagnosis. A discussion of the potential advantage of low field MRI is presented. Selected PHIL results for emphysema are given, with the perspectives that this joint work opens up for the future of respiratory medicine.Comment: To be published in Proc. ICAP 2004 (19th Int. Conf. on Atomic Physics, Rio, July 26-30 2004

DMFSGD: A Decentralized Matrix Factorization Algorithm for Network Distance Prediction

The knowledge of end-to-end network distances is essential to many Internet applications. As active probing of all pairwise distances is infeasible in large-scale networks, a natural idea is to measure a few pairs and to predict the other ones without actually measuring them. This paper formulates the distance prediction problem as matrix completion where unknown entries of an incomplete matrix of pairwise distances are to be predicted. The problem is solvable because strong correlations among network distances exist and cause the constructed distance matrix to be low rank. The new formulation circumvents the well-known drawbacks of existing approaches based on Euclidean embedding. A new algorithm, so-called Decentralized Matrix Factorization by Stochastic Gradient Descent (DMFSGD), is proposed to solve the network distance prediction problem. By letting network nodes exchange messages with each other, the algorithm is fully decentralized and only requires each node to collect and to process local measurements, with neither explicit matrix constructions nor special nodes such as landmarks and central servers. In addition, we compared comprehensively matrix factorization and Euclidean embedding to demonstrate the suitability of the former on network distance prediction. We further studied the incorporation of a robust loss function and of non-negativity constraints. Extensive experiments on various publicly-available datasets of network delays show not only the scalability and the accuracy of our approach but also its usability in real Internet applications.Comment: submitted to IEEE/ACM Transactions on Networking on Nov. 201

La notion d'incommensurabilité chez Thomas Kuhn

Cette thèse porte sur la notion d'incommensurabilité que Thomas Kuhn a introduite dans son oeuvre la plus connue, la structure des révolutions scientifiques, pour caractériser la relation entre paradigmes successifs. Selon Kuhn, dans la SRS, le passage d'un paradigme à un autre implique une rupture qui se manifeste sur trois plans, le plan méthodologique, le plan sémantique et le plan ontologique qu'il exprime par l'idée que les scientifiques postrévolutionnaires travaillent dans un monde différent. De ce point de vue, l'incommensurabilité est une relation complexe entre paradigmes successifs, que Kuhn ne définit pas de façon systématique, mais qui semble impliquer une rupture radicale entre traditions successives. Kuhn s'appuie sur la métaphore visuelle du renversement de gestalt pour rendre compte du passage révolutionnaire d'un paradigme à un autre. Si les sociologues ont bien accueilli les thèses de Kuhn, il n'en a pas été de même des philosophes des sciences qui lui ont reproché de proposer une vision à la fois irrationnelle, relativiste et idéaliste du développement des sciences. Puisque Kuhn a toujours prétendu que ces critiques reposaient en grande partie sur l'incompréhension, il a entrepris de clarifier ses positions pour répondre aux objections. Il entreprit, en particulier, de préciser la notion d'incommensurabilité qui est au coeur de son argumentation. Ce travail retrace, dans un premier temps, l'évolution de la pensée de Kuhn relativement à cette notion et aux conséquences de l'incommensurabilité sur sa conception de la science et du progrès scientifique. Pour répondre aux objections des philosophes, il amorce son tournant linguistique qui lui permet d'interpréter l'incommensurabilité en termes de rupture de la communication entre scientifiques qui défendent des paradigmes différents. La rupture de communication repose, de ce point de vue, sur l'impossibilité de traduire le langage théorique propre à une communauté scientifique dans celui du groupe révolutionnaire. Il substitue donc une approche sémantique à la métaphore visuelle de la SRS. Ensuite pour répondre aux objections de Putnam et, surtout de Davidson, selon qui, la notion d'incommensurabilité, entendue comme intraduisibilité entre langages théoriques est incohérente, Kuhn introduit la distinction entre traduction et interprétation. Il introduit, aussi, la notion d'incommensurabilité locale, qui ne touche que quelques termes interreliés. Cette approche le conduit à la solution taxinomique, selon laquelle les langages commensurables partagent une même structure lexicale. De ce point de vue, chaque lexique détermine un ensemble de mondes possibles qui ne sont descriptibles, et donc accessibles à l'expérience, que dans le cadre de ce lexique particulier. Deux lexiques successifs, constituent des mondes différents. Le chapitre six, expose le point de vue de Feyerabend sur l'incommensurabilité et ses conséquences, principalement pour mettre en perspective les idées de Kuhn. Les chapitres suivants abordent les thèses de Kuhn d'un point de vue critique. Le chapitre sept montre, à l'encontre des critiques de Putnam et de Davidson, que la notion d'incommensurabilité locale, complétée par la distinction entre interprétation et traduction, n'est pas incohérente. Si l'idée que deux langages puissent être radicalement intraduisibles semble impensable, il n'en va pas de même si deux langages ne diffèrent structuralement que pour un nombre restreint de termes théoriques centraux. Finalement, le chapitre VIII examine la thèse controversée selon laquelle des lexiques différents déterminent des mondes phénoménaux différents. Ce chapitre critique la forme de constructivisme néo-kantien que défend Kuhn en s'appuyant sur le fait que cette thèse repose sur une théorie descriptiviste de la référence, alors que le recours à une théorie causale de la référence permettrait d'accepter l'idée que les paradigmes successifs puissent être incommensurables sans devoir renoncer à une interprétation réaliste de la science. ______________________________________________________________________________ MOTS-CLÉS DE L’AUTEUR : Incommensurabilité, Paradigme, Épistémologie, Histoire des sciences, Sémantique, Théorie descriptiviste de la référence, Théorie causale de la référence

Climate and disturbance regime effects on aspen (Populus tremuloidesMichx.) stand structure and composition along an east–west transect in Canada's boreal forest

Stand structure and composition play a key role in maintaining the ecological integrity of the boreal forest. However, future changes in climate and disturbance regime could affect these forest attributes. Using provincial forest inventory datasets, we analysed stands dominated by aspen (=75% of the plot total basal area) distributed along a wide longitudinal gradient of environmental conditions across Canada. Stands were classified into three diameter structure types (inverted J, intermediate and advanced). There was no major difference in the distribution pattern of structural types of aspen-dominated stands between the western and eastern Canadian boreal mixedwood forests, despite a marked contrast in climatic conditions and fire regime. These results suggest that the predominance of juvenile structures in the western aspen forests is mainly related to the frequent recurrence of fires, while within eastern aspen forests, the longer fire cycle was not the controlling factor of stand structure. Anthropogenic activities would have strongly shaped the structure of aspen forests in eastern Canada. White spruce in the west and balsam fir in the east are among the main shade-tolerant conifer companion species associated with these stands. Although stand structure and composition were highly related to stand age and site productivity, regional climate and human activities, through their influence on disturbance regime, might have impacted these forest attributes

Conifer Recruitment in Trembling Aspen (Populus Tremuloides Michx.) Stands along an East-West Gradient in the Boreal Mixedwoods of Canada

Ongoing climate change is likely to result in shifts in successional dynamics in boreal mixedwood stands. Using data from provincial forest inventory databases, we examined the occurrence and abundance of the regeneration of various coniferous species (white spruce, black spruce and balsam fir) along an east-west Canadian gradient in aspen-dominated stands. The interpretation of the results was based on environmental conditions, including climate, natural fire regime and human impacts. We found that conifer regeneration was present in aspen stands along the entire gradient, despite differences in climatic conditions and fire regimes between the west (warmer and drier, with large recurrent fires) and east (more humid with relatively long fire cycles). However, abundance and distribution varied from one conifer species to the next. The abundance of white spruce decreased towards the eastern end of the longitudinal gradient, while balsam fir and black spruce abundance decreased towards the west. Although abundance decreased, balsam fir and black spruce regeneration was still present in western Canada. This study shows that it is difficult to interpret the effects of climate change on conifer recruitment without accounting for the superimposed effects of human activities

Decentralized Prediction of End-to-End Network Performance Classes

In large-scale networks, full-mesh active probing of end-to-end performance metrics is infeasible. Measuring a small set of pairs and predicting the others is more scalable. Under this framework, we formulate the prediction problem as matrix completion, whereby unknown entries of an incomplete matrix of pairwise measurements are to be predicted. This problem can be solved by matrix factorization because performance matrices have a low rank, thanks to the correlations among measurements. Moreover, its resolution can be fully decentralized without actually building matrices nor relying on special landmarks or central servers. In this paper we demonstrate that this approach is also applicable when the performance values are not measured exactly, but are only known to belong to one among some predefined performance classes, such as "good" and "bad". Such classification-based formulation not only fulfills the requirements of many Internet applications but also reduces the measurement cost and enables a unified treatment of various performance metrics. We propose a decentralized approach based on Stochastic Gradient Descent to solve this class-based matrix completion problem. Experiments on various datasets, relative to two kinds of metrics, show the accuracy of the approach, its robustness against erroneous measurements and its usability on peer selection.Peer reviewe

Landscape-scale disturbances and changes in bird communities of boreal mixed-wood forests

Bird community response to both landscape-scale and local (forest types) changes in forest cover was studied in three boreal mixed-wood forest landscapes modified by different types of disturbances: (1) a pre-industrial landscape where human settlement, agriculture, and logging activities date back to the early 1930s, (2) an industrial timber managed forest, and (3) a forest dominated by natural disturbances. Birds were sampled at 459 sampling stations distributed among the three landscapes. Local habitat and landscape characteristics of the context surrounding each sampling station (500-m and 1-km radius) were also computed. Bird communities were influenced by landscape-scale changes in forest cover. The higher proportion of early-successional habitats in both human-disturbed landscapes resulted in significantly higher abundance of early-successional bird species and generalists. The mean number of mature forest bird species was significantly lower in the industrial and pre-industrial landscapes than in the natural landscape. Landscape-scale conversion of mature forests from mixed-wood to deciduous cover in human-disturbed landscapes was the main cause of changes in mature forest bird communities. In these landscapes, the abundance of species associated with mixed and coniferous forest cover was lower, whereas species that preferred a deciduous cover were more abundant. Variation in bird community composition determined by the landscape context was as important as local habitat conditions, suggesting that predictions on the regional impact of forest management on songbirds with models solely based on local scale factors could be misleading. Patterns of bird species composition were related to several landscape composition variables (proportions of forest types), but not to configuration variables (e.g., interior habitat, amount of edge). Overall, our results indicated that the large-scale conversion of the southern portion of the boreal forest from a mixed to a deciduous cover may be one of the most important threats to the integrity of bird communities in these forest mosaics. Negative effects of changes in bird communities could be attenuated if current forestry practices are modified toward maintaining forest types (deciduous, mixed-wood, and coniferous) at levels similar to those observed under natural disturbances

Transcriptional landscape estimation from tiling array data using a model of signal shift and drift

Motivation: High-density oligonucleotide tiling array technology holds the promise of a better description of the complexity and the dynamics of transcriptional landscapes. In organisms such as bacteria and yeasts, transcription can be measured on a genome-wide scale with a resolution >25 bp. The statistical models currently used to handle these data remain however very simple, the most popular being the piecewise constant Gaussian model with a fixed number of breakpoints