Random Forests: some methodological insights

This paper examines from an experimental perspective random forests, the increasingly used statistical method for classification and regression problems introduced by Leo Breiman in 2001. It first aims at confirming, known but sparse, advice for using random forests and at proposing some complementary remarks for both standard problems as well as high dimensional ones for which the number of variables hugely exceeds the sample size. But the main contribution of this paper is twofold: to provide some insights about the behavior of the variable importance index based on random forests and in addition, to propose to investigate two classical issues of variable selection. The first one is to find important variables for interpretation and the second one is more restrictive and try to design a good prediction model. The strategy involves a ranking of explanatory variables using the random forests score of importance and a stepwise ascending variable introduction strategy

How to promote informal learning in the workplace? The need for incremental design methods

Informal Learning in the Workplace (ILW) is ensured by the everyday work activities in which workers are engaged. It accounts for over 75 per cent of learning in the workplace. Enterprise Social Media (ESM) are increasingly used as informal learning environments. According to the results of an implementation we have conducted in real context, we show that ESM are appropriate to promote ILW. Nevertheless, social aspects must be reconsidered to address users' needs regarding content and access, quality information indicators, moderation and control

Effects of the vasoactive intestinal peptide (VIP) and related peptides on glioblastoma cell growth in vitro

The growth rate of numerous cancer cell lines is regulated in part by actions of neuropeptides of the vasoactive intestinal peptide (VIP) family, which also includes pituitary adenylate cyclase-activating peptide (PACAP), glucagon, and peptide histidine/isoleucine (PHI). The aim of this work was to investigate the effect of these peptides on the growth of the rat glioblastoma cell line C6 in vitro. We also sought to determine which binding sites were correlated with the effects observed. Proliferation studies performed by means of a CyQuant trade mark assay showed that VIP and PACAP strongly stimulated C6 cell proliferation at most of the concentrations tested, whereas PHI increased cell proliferation only when associated with VIP. Two growth hormone-releasing factor (GRF) derivatives and the VIP antagonist hybrid peptide neurotensin-VIP were able to inhibit VIP-induced cell growth stimulation, even at very low concentrations. Binding experiments carried out on intact cultured C6 cells, using 125I-labeled VIP and PACAP as tracers, revealed that the effects of the peptides on cell growth were correlated with the expression on C6 cells of polyvalent high-affinity VIP-PACAP binding sites and of a second subtype corresponding to very high-affinity VIP-selective binding species. The latter subtype, which interacted poorly with PACAP with a 10,000-fold lower affinity than VIP, might mediate the antagonist effects of neurotensin- VIP and of both GRF derivatives on VIP-induced cell growth stimulation

Some effects of color incongruity in a visual Ad

While the effects of an ad's colour on attitudes towards it have already been investigated, far less work has been done on the influence of incongruous colours. This paper examines the effects of an ad using an incongruous colour on several variables of the advertising persuasion process. Analyses of variance have been conducted for an experimental study involving 407 respondents. The results show that a colour that is incongruous with the ad or with the product advertised affects the elements of surprise, perceived provocation, attitudes towards the ad and the positive beliefs about it.Incongruity, Surprise, Provocation, Attitude towards an advertisement, Beliefs about an advertisement.

Characterization of Posidonia Oceanica Seagrass Aerenchyma through Whole Slide Imaging: A Pilot Study

Characterizing the tissue morphology and anatomy of seagrasses is essential to predicting their acoustic behavior. In this pilot study, we use histology techniques and whole slide imaging (WSI) to describe the composition and topology of the aerenchyma of an entire leaf blade in an automatic way combining the advantages of X-ray microtomography and optical microscopy. Paraffin blocks are prepared in such a way that microtome slices contain an arbitrarily large number of cross sections distributed along the full length of a blade. The sample organization in the paraffin block coupled with whole slide image analysis allows high throughput data extraction and an exhaustive characterization along the whole blade length. The core of the work are image processing algorithms that can identify cells and air lacunae (or void) from fiber strand, epidermis, mesophyll and vascular system. A set of specific features is developed to adequately describe the convexity of cells and voids where standard descriptors fail. The features scrutinize the local curvature of the object borders to allow an accurate discrimination between void and cell through machine learning. The algorithm allows to reconstruct the cells and cell membrane features that are relevant to tissue density, compressibility and rigidity. Size distribution of the different cell types and gas spaces, total biomass and total void volume fraction are then extracted from the high resolution slices to provide a complete characterization of the tissue along the leave from its base to the apex

Stress cellulaire et remodelage de la matrice extracellulaire

Les tissus sont composés de cellules et d’un support protéique, la matrice extracellulaire (MEC). Cellules et MEC interagissent physiquement et fonctionnellement par l’intermédiaire des intégrines. Ce travail est consacré à l’influence du stress cellulaire sur les interactions cellules-MEC, en particulier dans le contexte du stress UV et du stress mécanique. Dans un premier travail, nous montrons pour les UVA activent dans les kératinocytes normaux la synthèse d’une sérine-protéase, le granzyme B responsable du clivage de la fibronectine au niveau des sites RGD, responsable du détachement de la cellule et sa mort par anoïkis. Ce phénomène devrait faciliter l’élimination des cellules photo-exposées. Dans un second travail, nous montrons que les UVA facilitent la migration monocellulaire des kératinocytes à travers un milieu collagénique non seulement en activant la synthèse de MMP1 mais aussi en provoquant la rupture du contact cellule-cellule via la dissociation des complexes E-cadhérine/Béta-caténine. Ces deux mécanismes sont coordonnés par la phosphorylation en Y653 de la Béta-caténine. Dans un troisième travail, nous montrons que le stress mécanique en rapport avec la culture en 3D des cellules lymphomateuses a pour effet de déréguler la synthèse de nombreuses composantes de la MEC et l’expression de certaines intégrines avec pour conséquence une régulation positive de la survie et négative de la prolifération. Sur la base de ces observations et de leur confrontation avec la littérature, nous proposons un modèle selon lequel le stress cellulaire a pour effet d’induire un remodelage intense de la MEC et d’influencer la biologie des intégrines avec des conséquences fonctionnelles majeures mais dépendantes du modèle cellulaire et des conditions environnementales, telles que la mort par détachement, la stimulation des capacités invasives ou la structuration spatiale des tumeurs.Tissues contain not only cells but also a complex network of pericellular proteins, the extracellular matrix (ECM). Cells and ECM interact physically and functionally through integrins. In this work, we will study the modifications on cells/MEC interactions induced by diverse cellular stress conditions, including UV and mechanical stress. In a first work, we showed that, in normal kératinocytes, UVA irradiation resulted in increased synthesis and excretion of granzyme B. This serine-protease, by cleaving fibronectin on RGD sites, is responsible for cell detachment and subsequent death by anoïkis. This phenomenon should facilitate the elimination of photo-exposed cells. In a second work, we showed that UVA facilitated the migration of isolated keratinocytes through collagen not only by stimulating MMP1-mediated collagen degradation but also by inducing the loss of cell-to-cell contact via the dissociation of E-cadherine/béta-catenin complex. These two mechanisms are coordinated by béta-catenin phosphorylation on Y653 residue. In the third work, we showed that mechanical stress, in touch with 3D culture of lymphomatous cells, induced up-regulation of ECM components synthesis and integrins expression. These two modifications resulted in increased cell survival and reduced cell proliferation. Based on these observations and on the literature, we proposed a model in which cellular stress induced an intense ECM remodeling, in association with significant changes in integrin biology. These modifications may have important consequences in terms of cell survival through anoikis, invasive capacities through ECM degradation, and spatial tumor organization

Meshless modelling of dynamic behaviour of glasses under intense shock loadings: application to matter ejection during high velocity impacts on thin brittle targets

The purpose of this study is to present a new material model adapted to SPH modelling of dynamic behaviour of glasses under shock loadings. This model has the ability to reproduce fragmentation and densification of glasses under compression as well as brittle tensile failure. It has been implemented in Ls-Dyna software and coupled with a SPH code. By comparison with CEA-CESTA experimental data the model has been validated for fused silica and Pyrex glass for stress level up to 35GPa. For Laser MegaJoule applications, the present material model was applied to 3D high velocity impacts on thin brittle targets with good agreement in term of damages and matter ejection with experimental data obtained using CESTA’s double stage light gas gun