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Search Results: 1 - 10 of 8070 matches for " PHILIPPE; LEFèVRE "
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Ruin Time and Severity for a Lévy Subordinator Claim Process: A Simple Approach
Claude Lefèvre,Philippe Picard
Risks , 2013, DOI: 10.3390/risks1030192
Abstract: This paper is concerned with an insurance risk model whose claim process is described by a Lévy subordinator process. Lévy-type risk models have been the object of much research in recent years. Our purpose is to present, in the case of a subordinator, a simple and direct method for determining the finite time (and ultimate) ruin probabilities, the distribution of the ruin severity, the reserves prior to ruin, and the Laplace transform of the ruin time. Interestingly, the usual net profit condition will be essentially relaxed. Most results generalize those known for the compound Poisson claim process.
3D kinematics using dual quaternions: theory and applications in neuroscience
Guillaume Leclercq,Philippe Lefèvre,Gunnar Blohm
Frontiers in Behavioral Neuroscience , 2013, DOI: 10.3389/fnbeh.2013.00007
Abstract: In behavioral neuroscience, many experiments are developed in 1 or 2 spatial dimensions, but when scientists tackle problems in 3-dimensions (3D), they often face problems or new challenges. Results obtained for lower dimensions are not always extendable in 3D. In motor planning of eye, gaze or arm movements, or sensorimotor transformation problems, the 3D kinematics of external (stimuli) or internal (body parts) must often be considered: how to describe the 3D position and orientation of these objects and link them together? We describe how dual quaternions provide a convenient way to describe the 3D kinematics for position only (point transformation) or for combined position and orientation (through line transformation), easily modeling rotations, translations or screw motions or combinations of these. We also derive expressions for the velocities of points and lines as well as the transformation velocities. Then, we apply these tools to a motor planning task for manual tracking and to the modeling of forward and inverse kinematics of a seven-dof three-link arm to show the interest of dual quaternions as a tool to build models for these kinds of applications.
Individual Differences in Impulsivity Predict Anticipatory Eye Movements
Laetitia Cirilli, Philippe de Timary, Phillipe Lefèvre, Marcus Missal
PLOS ONE , 2011, DOI: 10.1371/journal.pone.0026699
Abstract: Impulsivity is the tendency to act without forethought. It is a personality trait commonly used in the diagnosis of many psychiatric diseases. In clinical practice, impulsivity is estimated using written questionnaires. However, answers to questions might be subject to personal biases and misinterpretations. In order to alleviate this problem, eye movements could be used to study differences in decision processes related to impulsivity. Therefore, we investigated correlations between impulsivity scores obtained with a questionnaire in healthy subjects and characteristics of their anticipatory eye movements in a simple smooth pursuit task. Healthy subjects were asked to answer the UPPS questionnaire (Urgency Premeditation Perseverance and Sensation seeking Impulsive Behavior scale), which distinguishes four independent dimensions of impulsivity: Urgency, lack of Premeditation, lack of Perseverance, and Sensation seeking. The same subjects took part in an oculomotor task that consisted of pursuing a target that moved in a predictable direction. This task reliably evoked anticipatory saccades and smooth eye movements. We found that eye movement characteristics such as latency and velocity were significantly correlated with UPPS scores. The specific correlations between distinct UPPS factors and oculomotor anticipation parameters support the validity of the UPPS construct and corroborate neurobiological explanations for impulsivity. We suggest that the oculomotor approach of impulsivity put forth in the present study could help bridge the gap between psychiatry and physiology.
The HelioClim Project: Surface Solar Irradiance Data for Climate Applications
Philippe Blanc,Beno?t Gschwind,Mireille Lefèvre,Lucien Wald
Remote Sensing , 2011, DOI: 10.3390/rs3020343
Abstract: Meteosat satellite images are processed to yield values of the incoming surface solar irradiance (SSI), one of the Essential Climate Variables. Two HelioClim databases, HC-1 and HC-3, were constructed covering Europe, Africa and the Atlantic Ocean, and contain daily and monthly means of SSI. The HC-1 database spans from 1985 to 2005; HC?3 began in 2004 and is updated daily. Their quality and limitations in retrieving monthly means of SSI have been studied by a comparison between eleven stations offering long time-series of measurements. A good agreement was observed for each site: bias was less than 10?W/m2 in absolute value (5% in relative value) for HC-3. HC-1 offers a similar quality, though it underestimates the SSI for latitudes greater than 45° and less than ?45°. Time-series running from 1985 to date can be created by concatenating the HC-1 and HC-3 values and could help in assessing SSI and its?changes.
Fixation patterns during recognition of personally familiar and unfamiliar faces
Goedele van Belle,Meike Ramon,Philippe Lefèvre,Bruno Rossion
Frontiers in Psychology , 2010, DOI: 10.3389/fpsyg.2010.00020
Abstract: Previous studies recording eye gaze during face perception have rendered somewhat inconclusive findings with respect to fixation differences between familiar and unfamiliar faces. This can be attributed to a number of factors that differ across studies: the type and extent of familiarity with the faces presented, the definition of areas of interest subject to analyses, as well as a lack of consideration for the time course of scan patterns. Here we sought to address these issues by recording fixations in a recognition task with personally familiar and unfamiliar faces. After a first common fixation on a central superior location of the face in between features, suggesting initial holistic encoding, and a subsequent left eye bias, local features were focused and explored more for familiar than unfamiliar faces. Although the number of fixations did not differ for un-/familiar faces, the locations of fixations began to differ before familiarity decisions were provided. This suggests that in the context of familiarity decisions without time constraints, differences in processing familiar and unfamiliar faces arise relatively early – immediately upon initiation of the first fixation to identity-specific information – and that the local features of familiar faces are processed more than those of unfamiliar faces.
Active Collisions in Altered Gravity Reveal Eye-Hand Coordination Strategies
Olivier White, Philippe Lefèvre, Alan M. Wing, R. Martyn Bracewell, Jean-Louis Thonnard
PLOS ONE , 2012, DOI: 10.1371/journal.pone.0044291
Abstract: Most object manipulation tasks involve a series of actions demarcated by mechanical contact events, and gaze is usually directed to the locations of these events as the task unfolds. Typically, gaze foveates the target 200 ms in advance of the contact. This strategy improves manual accuracy through visual feedback and the use of gaze-related signals to guide the hand/object. Many studies have investigated eye-hand coordination in experimental and natural tasks; most of them highlighted a strong link between eye movements and hand or object kinematics. In this experiment, we analyzed gaze strategies in a collision task but in a very challenging dynamical context. Participants performed collisions while they were exposed to alternating episodes of microgravity, hypergravity and normal gravity. First, by isolating the effects of inertia in microgravity, we found that peak hand acceleration marked the transition between two modes of grip force control. Participants exerted grip forces that paralleled load force profiles, and then increased grip up to a maximum shifted after the collision. Second, we found that the oculomotor strategy adapted visual feedback of the controlled object around the collision, as demonstrated by longer durations of fixation after collision in new gravitational environments. Finally, despite large variability of arm dynamics in altered gravity, we found that saccades were remarkably time-locked to the peak hand acceleration in all conditions. In conclusion, altered gravity allowed light to be shed on predictive mechanisms used by the central nervous system to coordinate gaze, hand and grip motor actions during a mixed task that involved transport of an object and high impact loads.
Estrategias de subsistencia de cazadores recolectores de Isla Dawson (Estrecho de Magallames) durante la segunda mitad del Holoceno: primeras aproximaciones
Magallania (Punta Arenas) , 2011, DOI: 10.4067/S0718-22442011000200011
Abstract: the development of an archaeological survey on the coast of dawson and wickham island, located in a strategic position between different path roads that connect the central portion of the magellan strait with the fueguian archipelago, allowed detecting 27 sites that correspond to former marine hunter-gatherer camps of the inhabitants of the zone. the accomplishment of excavations in six of these sites allowed recovering an interesting set of archaeological fauna remains. from the analysis of these assemblages, we describe the main observed characteristics and offer a first panorama referred to the subsistence of the groups that lived in this zone of the fueguian archipelago.
Estrategias de subsistencia de cazadores recolectores de Isla Dawson (Estrecho de Magallames) durante la segunda mitad del Holoceno: primeras aproximaciones Subsistence strategies of hunter gatherers from Dawsson Island (Magellan's Strait) During the second half of the Holocene: first aproximations
Magallania , 2011,
Abstract: El desarrollo de una prospección arqueológica en la costa de isla Dawson e isla Wickham, ubicadas en una posición estratégica entre las diferentes vías de comunicación que conectan la porción central del estrecho de Magallanes con el archipiélago fueguino, registró 27 yacimientos que corresponden a antiguos campamentos de cazadores-recolectores marinos que habitaron la zona. La realización de excavaciones de sondeo en seis de estos sitios permitió recuperar un interesante conjunto de restos arqueofaunísticos. A partir del análisis de éstos conjuntos, describimos las principales características observadas y ofrecemos un primer panorama referido a la subsistencia de los grupos que habitaron esta zona del archipiélago fueguino. The development of an archaeological survey on the coast of Dawson and Wickham island, located in a strategic position between different path roads that connect the central portion of the Magellan strait with the fueguian archipelago, allowed detecting 27 sites that correspond to former marine hunter-gatherer camps of the inhabitants of the zone. The accomplishment of excavations in six of these sites allowed recovering an interesting set of archaeological fauna remains. From the analysis of these assemblages, we describe the main observed characteristics and offer a first panorama referred to the subsistence of the groups that lived in this zone of the fueguian archipelago.
Anomalous non-linear response of glassy liquids: general arguments and a Mode-Coupling approach
Marco Tarzia,Giulio Biroli,Jean-Philippe Bouchaud,Alexandre Lefèvre
Physics , 2008,
Abstract: We study theoretically the non-linear response properties of glass formers. We establish several general results which, together with the assumption of Time-Temperature Superposition, lead to a relation between the non-linear response and the derivative of the linear response with respect to temperature. Using results from Mode-Coupling Theory (MCT) and scaling arguments valid close to the glass transition, we obtain the frequency and temperature dependence of the non-linear response in the $\alpha$ and $\beta$-regimes. Our results demonstrate that supercooled liquids are characterized by responses to external perturbations that become increasingly non-linear as the glass transition is approached. These results are extended to the case of inhomogeneous perturbing fields.
Cholesterol Depletion Disorganizes Oocyte Membrane Rafts Altering Mouse Fertilization
Jorgelina Buschiazzo, Come Ialy-Radio, Jana Auer, Jean-Philippe Wolf, Catherine Serres, Brigitte Lefèvre, Ahmed Ziyyat
PLOS ONE , 2013, DOI: 10.1371/journal.pone.0062919
Abstract: Drastic membrane reorganization occurs when mammalian sperm binds to and fuses with the oocyte membrane. Two oocyte protein families are essential for fertilization, tetraspanins and glycosylphosphatidylinositol-anchored proteins. The firsts are associated to tetraspanin-enriched microdomains and the seconds to lipid rafts. Here we report membrane raft involvement in mouse fertilization assessed by cholesterol modulation using methyl-β-cyclodextrin. Cholesterol removal induced: (1) a decrease of the fertilization rate and index; and (2) a delay in the extrusion of the second polar body. Cholesterol repletion recovered the fertilization ability of cholesterol-depleted oocytes, indicating reversibility of these effects. In vivo time-lapse analyses using fluorescent cholesterol permitted to identify the time-point at which the probe is mainly located at the plasma membrane enabling the estimation of the extent of the cholesterol depletion. We confirmed that the mouse oocyte is rich in rafts according to the presence of the raft marker lipid, ganglioside GM1 on the membrane of living oocytes and we identified the coexistence of two types of microdomains, planar rafts and caveolae-like structures, by terms of two differential rafts markers, flotillin-2 and caveolin-1, respectively. Moreover, this is the first report that shows characteristic caveolae-like invaginations in the mouse oocyte identified by electron microscopy. Raft disruption by cholesterol depletion disturbed the subcellular localization of the signal molecule c-Src and the inhibition of Src kinase proteins prevented second polar body extrusion, consistent with a role of Src-related kinases in fertilization via signaling complexes. Our data highlight the functional importance of intact membrane rafts for mouse fertilization and its dependence on cholesterol.
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