Showing papers by "University of Rennes published in 2002"

Epileptic fast activity can be explained by a model of impaired GABAergic dendritic inhibition.

Abstract: This paper focuses on high-frequency (gamma band) EEG activity, the most characteristic electrophysiological pattern in focal seizures of human epilepsy. It starts with recent hypotheses about: (i) the behaviour of inhibitory interneurons in hippocampal or neocortical networks in the generation of gamma frequency oscillations; (ii) the nonuniform alteration of GABAergic inhibition in experimental epilepsy (reduced dendritic inhibition and increased somatic inhibition); and (iii) the possible depression of GABA A , f a s t circuit activity by GABA A , s l o w inhibitory postsynaptic currents. In particular, these hypotheses are introduced in a new computational macroscopic model of EEG activity that includes a physiologically relevant fast inhibitory feedback loop. Results show that strikingly realistic activity is produced by the model when compared to real EEG signals recorded with intracerebral electrodes. They show that, in the model, the transition from interictal to fast ictal activity is explained by the impairment of dendritic inhibition.

An introduction to o-minimal geometry

Abstract: Preface These notes have served as a basis for a course in Pisa in Spring 1999. A parallel course on the construction of o-minimal structures was given by A. Macintyre. The content of these notes owes a great deal to the excellent book by L. van den Dries [vD]. Some interesting topics contained in this book are not included here, such as the Vapnik-Chervonenkis property. Part of the material which does not come from [vD] is taken from the paper [Co1]. This includes the sections on the choice of good coordinates and the triangulation of functions in Chapter 4 and Chapter 5. The latter chapter contains the results on triviality in families of sets or functions which were the main aim of this course. The last chapter on smoothness was intended to establish property " DC k-all k " which played a crucial role in the course of Macintyre (it can be easily deduced from the results in [vDMi]). It is also the occasion to give a few results on tubular neighborhoods. I am pleased to thank Francesca Acquistapace, Fabrizio Broglia and all colleagues of the Dipartimento di Matematica for the invitation to give this course in Pisa and their friendly hospitality. Also many thanks to Antonio Ponchio for reading these notes and correcting mistakes.

Connected components of the moduli spaces of Abelian differentials with prescribed singularities

Abstract: Consider the moduli space of pairs (C,w) where C is a smooth compact complex curve of a given genus and w is a holomorphic 1-form on C with a given list of multiplicities of zeroes. We describe connected components of this space. This classification is important in the study of dynamics of interval exchange transformations and billiards in rational polygons, and in the study of geometry of translation surfaces.

Filtration-consistent nonlinear expectations and related g-expectations

Abstract: From a general definition of nonlinear expectations, viewed as operators preserving monotonicity and constants, we derive, under rather general assumptions, the notions of conditional nonlinear expectation and nonlinear martingale. We prove that any such nonlinear martingale can be represented as the solution of a backward stochastic equation, and in particular admits continuous paths. In other words, it is a g-martingale.

Legitimizing fluvial ecosystems as users of water: An overview

Abstract: We suggest that fluvial ecosystems are legitimate users of water and that there are basic ecological principles guiding the maintenance of long-term ecological vitality. This article articulates some fundamental relationships between physical and ecological processes, presents basic principles for maintaining the vitality of fluvial ecosystems, identifies several major scientific challenges and opportunities for effective implementation of the basic ecological principles, and acts as an introduction to three specific articles to follow on biodiversity, biogeochemistry, and riparian communities. All the objectives, by necessity, link climate, land, and fresh water. The basic principles proposed are: (1) the natural flow regime shapes the evolution of aquatic biota and ecological processes, (2) every river has a characteristic flow regime and an associated biotic community, and (3) aquatic ecosystems are topographically unique in occupying the lowest position in the landscape, thereby integrating catchment-scale processes. Scientific challenges for the immediate future relate to quantifying cumulative effects, linking multidisciplinary knowledge and models, and formulating effective monitoring and assessment procedures. Additionally, forecasting the ecological consequences of changing water regimes is a fundamental challenge for science, especially as environmental issues related to fresh waters escalate in the next two to three decades.

On the role of aurora-A in centrosome function.

Abstract: Mammalian aurora-A belongs to a multigenic family of mitotic serine/threonine kinases comprising two other members: aurora-B and aurora-C. In this review we will focus on aurora-A that starts to localize to centrosomes only in S phase as soon as centrioles have been duplicated, the protein is then degraded in early G1. Works in various organisms have revealed that the kinase is involved in centrosome separation, duplication and maturation as well as in bipolar spindle assembly and stability. Aurora kinases are found in all organisms in which their function has been conserved throughout evolution, namely the control of chromosome segregation. In human, aurora-A has focused a lot of attention, since its overexpression has been found to be correlated with the grade of various solid tumours. Ectopic kinase overexpression in any culture cell line leads to polyploidy and centrosome amplification. However, overexpression of aurora-A in particular cell lines such as NIH3T3 is sufficient to induce growth on soft agar. Those transformed cells form tumours when implanted in immunodeficient mice, indicating that the kinase is an oncogene.

Archean granite-greenstone tectonics at Kolar (South India): Interplay of diapirism and bulk inhomogeneous contraction during juvenile magmatic accretion

Abstract: [1] The structural study of the Kolar greenstone belt and surrounding granite-gneiss terrains combined with U-Pb dating reveals that the middle and lower crustal tectonoplutonic pattern of the eastern Dharwar craton developed during a major magmatic accretion event between 2550 and 2530 Ma The granite-greenstone pattern resulted from the interference of gravity-driven sagging of the greenstones (ie, diapirism), E-W bulk inhomogeneous shortening combined with horizontal N-S stretching, and syntectonic juvenile pluton emplacement Bulk inhomogeneous contraction is accommodated by the synchronous development of a pervasive, N-S trending vertical foliation, shallow stretching lineation, and conjugate strike-slip shear zone pattern within and outside the greenstone belt, resulting in regional horizontal pure shear deformation The plutons around the greenstone belt record regional contraction by developing one set of strike-slip C-S fabrics of the shear zone pattern The development of the granite-greenstone pattern was coeval and compatible with deformation during juvenile magmatic accretion, melting, and granulite metamorphism in the lower crust The Kolar example points to a specific crustal rheology that allowed sagduction of the greenstones and regional distributed bulk inhomogeneous strain, due to mechanical homogeneity and low viscosity provided by large-scale melting during the accretion event This example further suggests specific boundary conditions to the craton that allowed E-W inhomogeneous shortening to be accommodated by N-S stretching and spreading of the crust without significant tectonic thickening Such tectonoplutonic pattern is specific to the Archean and may develop as a consequence of mantle plume activity in intracontinental settings

Genotype-phenotype relationships in Berardinelli-Seip congenital lipodystrophy

Abstract: Generalised lipodystrophy of the Berardinelli-Seip type (BSCL) is a rare autosomal recessive human disorder with severe adverse metabolic consequences. A gene on chromosome 9 (BSCL1) has recently been identified, predominantly in African-American families. More recently, mutations in a previously undescribed gene of unknown function (BSCL2) on chromosome 11, termed seipin, have been found to be responsible for this disorder in a number of European and Middle Eastern families. We have studied the genotype/phenotype relationships in 70 affected subjects from 44 apparently unrelated pedigrees of diverse ethnic origin. In all subjects, hepatic dysfunction, hyperlipidaemia, diabetes mellitus, and hypertrophic cardiomyopathy were significant contributors to morbidity with no clear differences in their prevalence between subjects with BSCL1 or BSCL2 and those with evidence against cosegregation with either chromosome 9 or 11 (designated BSCLX). BSCL2 appears to be a more severe disorder than BSCL1 with a higher incidence of premature death and a lower prevalence of partial and/or delayed onset of lipodystrophy. Notably, subjects with BSCL2 had a significantly higher prevalence of intellectual impairment than those with BSCL1 or BSCLX (p<0.0001, OR 17.0, CI 3.6 to 79.0). The higher prevalence of intellectual impairment and the increased risk of premature death in BSCL2 compared to BSCL1 emphasise the importance of molecular diagnosis of this syndrome and have clear implications for genetic counselling.

Hardness, Toughness, and Scratchability of Germanium–Selenium Chalcogenide Glasses

Abstract: Ge-Se chalcogenide glasses are characterized by relatively low hardness (0.39-2.35 GPa) and low fracture toughness (0.1-0.28 MPa.m 1/2 ). Actually, the hardness of chalcogen-rich glasses is low enough so that the brittleness parameter, B = H/K c , is lower than that of silicate glasses. Whereas hardness and Young's modulus increase with increasing germanium contents, fracture toughness follows a trend similar to that of the density and exhibits a maximum for the Ge 20 Se 80 composition, which corresponds to the rigidity percolation threshold. Optical microscopy and atomic force microscopy observations suggest that the indentation deformation proceeds by a localized shear deformation phenomenon. Glasses in the chalcogen-rich region behave viscoelastically at room temperature. As a consequence, an increase of the loading time results in a decrease of hardness and toughness.

Seasonal dynamics of denitrification along topohydrosequences in three different riparian wetlands.

Abstract: We investigated the seasonal patterns of denitrification rates and potentials in soil profiles along the topohydrosequence formed at the upland-wetland interface in three riparian wetlands with different vegetation cover (i.e., forest, understory vegetation, and grass). Denitrification was measured using the acetylene inhibition method on soil cores and slurries, which provided a means of comparing the relative activity of this process in different locations. We evaluated, on a seasonal basis, the respective importance of the vegetative cover and the hydromorphic gradient as factors limiting denitrification. Regardless of the season, vegetation type, or lateral position along each topohydrosequence in the riparian wetlands, strong significant gradients of both in situ and potential denitrification rates were measured within a soil profile. Results confirm that the upper organic soil horizon is the most active, when in contact with the ground water. In deeper soil horizons, denitrification activity was low (from 0.004 to 0.5 mg N kg -1 dry soil d -1 ), but contributed significantly to the reduction of ground water NO 3 load along the riparian ground water flowpath (from 9.32 to 0.98 mg NO 3 -N L -1 ). Along the soil topohydrosequence, the denitrifying community of the upper soil horizons did not vary significantly on a seasonal basis despite the large seasonal ground water fluctuations. Along each topohydrosequence, the denitrification-limiting factor gradually shifted from anaerobiosis to NO 3 supply. In situ denitrification rates in the forested, understory vegetation and grass sites were not significantly different. This result emphasizes the importance of the topography of the valley rather than the vegetation cover in controlling denitrification activity in riparian wetlands.

Compaction dynamics of a granular medium under vertical tapping

Abstract: We report new experimental results on granular compaction under consecutive vertical taps. The evolution of the mean volume fraction and of the mean potential energy of a granular packing presents a slow densification until a final steady state, and is reminiscent of usual relaxation in glasses via a stretched exponential law. The intensity of the taps seems to rule the characteristic time of the relaxation according to an Arrhenius's type relation. Finally, the analysis of the vertical volume fraction profile reveals an almost homogeneous densification in the packing.

Evolution of soil surface roughness and flowpath connectivity in overland flow experiments

Abstract: During rainfall events, surface roughness affects runoff generation by providing water surface storage in the depressions and altering the flow direction on the surface. The process of runoff initiation, or triggering, involves the gradual filling of individual depressions and the connection of those overflowing depressions toward the outflow boundary. Although studies have been conducted to relate surface roughness to total depression storage, little work has been done in quantifying the roughness effects on runoff initiation. This study examines the role of surface roughness on overland flow triggering in interrill areas. Laboratory experiments were conducted on a 2.4-m×2.4-m soil box exposed to a sequence of four rainfall events with treatments including two levels of initial roughness and two slope gradients. Surface microtopography was digitised by a laser scanner before and after each rainfall event. Soil roughness was analysed by the variogram method and its role in runoff triggering was evaluated using a numerical model that gradually fills depressions with a conditioned-walker method. Experimental variograms showed a gradual lowering of semivariances in a homothetic way after each additional rainfall, indicating that all roughness scales are affected similarly. All variograms showed a distinct topographic correlation length that can be related to depressional storage capacity. The linear relationship between these two variables also has a threshold roughness term below which the storage capacity tends to zero. Analyses of the runoff triggering showed that a small modification of micro-topographic structure had a major effect on runoff initiation. Even if the storage capacity is an important parameter of the runoff characteristics, large differences are observed between the initial stages of each experiment and final stages. We attributed these differences to the creation of preferential connections between topography depressions due to the material redistribution. Since the variogram analysis may not be sensitive toward the development of preferential flow path in a local scale, additional methodologies, such as the conditioned-walker analysis used in this study, need to be incorporated in quantifying the role of surface microtopography on the dynamics of runoff generation.

Retrotransposons and Genomic Stability in Populations of the Young Allopolyploid Species Spartina anglica C.E. Hubbard (Poaceae)

Abstract: Spartina x townsendii arose during the end of the 19th century in England by hybridization between the indigenous Spartina maritima and the introduced Spartina alterniflora, native to the eastern seaboard of North America. Duplication of the hybrid genome gave rise to Spartina anglica, a vigorous allopolyploid involved in natural and artificial invasions on several continents. This system allows investigation of the early evolutionary changes that accompany stabilization of new allopolyploid species. Because allopolyploidy may be a genomic shock, eliciting retroelement insertional activity, we examined whether retrotransposons present in the parental species have been activated in the genome of S. anglica. For this purpose we used inter-retrotransposon amplified polymorphism (IRAP) and retrotransposons-microsatellite amplified polymorphism (REMAP) markers, which are multilocus PCR-based methods detecting retrotransposon integration events in the genome. IRAP and REMAP allowed the screening of insertional polymorphisms in populations of S. anglica. The populations are composed mainly of one major multilocus genotype, identical to the first-generation hybrid S. x townsendii. Few new integration sites were encountered in the young allopolyploid genome. We also found strict additivity of the parental subgenomes in the allopolyploid. Both these findings indicate that the genome of S. anglica has not undergone extensive changes since its formation. This contrasts with previous results from the literature, which report rapid structural changes in experimentally resynthesized allopolyploids.

Basic principles and ecological consequences of changing water regimes on nitrogen cycling in fluvial systems.

Abstract: Understanding the environmental consequences of changing water regimes is a daunting challenge for both resource managers and ecologists. Balancing human demands for fresh water with the needs of the environment for water in appropriate amounts and at the appropriate times are shaping the ways by which this natural resource will be used in the future. Based on past decisions that have rendered many freshwater resources unsuitable for use, we argue that river systems have a fundamental need for appropriate amounts and timing of water to maintain their biophysical integrity. Biophysical integrity is fundamental for the formulation of future sustainable management strategies. This article addresses three basic ecological principles driving the biogeochemical cycle of nitrogen in river systems. These are (1) how the mode of nitrogen delivery affects river ecosystem functioning, (2) how increasing contact between water and soil or sediment increases nitrogen retention and processing, and (3) the role of floods and droughts as important natural events that strongly influence pathways of nitrogen cycling in fluvial systems. New challenges related to the cumulative impact of water regime change, the scale of appraisal of these impacts, and the determination of the impacts due to natural and human changes are discussed. It is suggested that cost of long-term and long-distance cumulative impacts of hydrological changes should be evaluated against short-term economic benefits to determine the real environmental costs.

A Machine Learning Approach to Automatic Production of Compiler Heuristics

Abstract: Achieving high performance on modern processors heavily relies on the compiler optimizations to exploit the microprocessor architecture. The efficiency of optimization directly depends on the compiler heuristics. These heuristics must be target-specific and each new processor generation requires heuristics reengineering.In this paper, we address the automatic generation of optimization heuristics for a target processor by machine learning. We evaluate the potential of this method on an always legal and simple transformation: loop unrolling. Though simple to implement, this transformation may have strong effects on program execution (good or bad). However deciding to perform the transformation or not is difficult since many interacting parameters must be taken into account. So we propose a machine learning approach.We try to answer the following questions: is it possible to devise a learning process that captures the relevant parameters involved in loop unrolling performance? Does the Machine Learning Based Heuristics achieve better performance than existing ones?

A statistical scaling model for fracture network geometry, with validation on a multiscale mapping of a joint network (Hornelen Basin, Norway)

Abstract: [1] Fracture patterns are characterized by a complex geometry which involves a large length distribution and nonhomogeneous density distributions. Here we address the issue of the modeling of this complex geometry over scales through a first-order model providing the characterization of the number of fractures of a given length and at a given scale. We propose that the simplest model is a power law in both space and fracture length, with three main parameters: the exponent a of a power law length distribution, the fractal dimension D which fixes the scale dependence of the number of fractures, and the fracture density α. We verify the applicability of this model on seven fracture patterns mapped from the metric scale up to almost the kilometric scale in the Hornelen basin. The model efficiently describes fracture network properties at all scales with a single set of parameters. For the Hornelen fracture networks we found a simple relationship between basic exponents, a = D + 1, implying that the fracture network is self-similar. Through this analysis, we present different methodological developments for deriving the basic exponents a and D and for verifying the consistency of the model. Overall, the main methodological development is about the normalization of the measurements made from different scales of observation. Finally, we discuss both the limitations and the uses of such a model for analyzing the hydraulic and mechanical properties of fracture networks.

Hierarchical Estimation and Segmentation of Dense Motion Fields

Abstract: In this paper we present a comprehensive energy-based framework for the estimation and the segmentation of the apparent motion in image sequences. The robust cost functions and the associated hierarchical minimization techniques that we propose mix efficiently non-parametric (dense) representations, local interacting parametric representations, and global non-interacting parametric representations related to a partition into regions. Experimental comparisons, both on synthetic and real images, demonstrate the merit of the approach on different types of photometric and kinematic contents ranging from moving rigid objects to moving fluids.

Sublethal effects of exposure to chemical compounds: a cause for the decline in Atlantic eels?

Abstract: Because of their unusual life cycle, American and European eels (Anguilla rostrata Lesueur and A. anguilla L.), are particularly exposed to pollutant effects. Because silver eels fast when they leave the freshwater system, the transoceanic migration forces them to constitute energy reserves in the form of muscle lipids, that are needed for successful spawning. Using species biological data, toxicological and ecotoxicological information, hypotheses are given to assess the contribution of pollution from freshwater sources to the recorded decline in the American and European eels fisheries since the 1980s. This paper first describes the lipid storage problems and the relative migratory capacities. Then several studies on the accumulation of xenobiotics in various anatomical compartments, on the biological half-lives of these compounds, and on their sublethal toxicity, are reviewed. During migration, lipid mobilization returns persistent lipophilic pollutants back into circulation, these being concentrated particularly in gonads at the crucial time of gametogenesis. Extrapolation of toxicological analysis (individual physiology) to the population level (spawning success) suggests that the quality of future spawners leaving freshwaters is one of the prime factors for the conservation of this threatened species.

Geochemical and isotopic studies of the sedimentary and granitic rocks of the Altai orogen of northwest China and their tectonic implications

Abstract: The Altai orogen (northwest China) represents the southwestern margin of the Central Asian Orogenic Belt. Geochemical and Nd-Sr isotope analyses were carried out on the Palaeozoic sedimentary and granitic rocks in order to trace their sources and to evaluate the pattern of continental growth of the orogen Nd isotopic data for both the granites and sediments suggest a significant proportion of middle Proterozoic crust beneath the Altai orogen. However, addition of juvenile material (arc/back-arc oceanic crust) during Palaeozoic times is also significant. Trace elements and isotopic data of sediments suggest their sources were immature. They represent mixtures between a Palaeozoic juvenile component and an evolved continental crust. The early Palaeozoic sediments show e N d (T) = -3.4 to -5.0, T D M = 1.5-1.8 Ga, and I S r = 0.710-0.712. They represent a passive margin setting, with a predominance of evolved crustal material in the source. The Devonian sequences, however, might have been deposited in a back-arc basin setting, produced by subduction of the Junggar oceanic crust along the Irtysh fault. A significant addition of arc material into the sedimentary basin is responsible for the highly variable e N d values (-6 to 0) and I S r (0.711-0.706). The Carboniferous rocks were also deposited in a back-arc basin setting but with predominantly arc material in the source as suggested by an abrupt increase in e N d (T) (+6 to +3) and decrease in I S r (0.7045-0.7051). Voluminous syn-orogenic granitoids have e N d (T) = +2.1 to -4.3, I S r = 0.705-0.714 and T D M = 0.7-1.6 Ga. They were not derived by melting of local metasedimentary rocks as suggested by previous workers, but by melting of a more juvenile source at depth. Post-orogenic granites have higher e N d (T) (-+4.4) than the syn-orogenic granitoids, indicating their derivation from a deeper crustal level where juvenile crust may predominate.

Validation of medical image processing in image-guided therapy

Abstract: Do we need an abstract ?.