Long-Term Memory Motion-Compensated Prediction

Long-term memory motion-compensated prediction extends the spatial displacement vector utilized in block-based hybrid video coding by a variable time delay permitting the use of more frames than the previously decoded one for motion compensated prediction. The long-term memory covers several seconds of decoded frames at the encoder and decoder. The use of multiple frames for motion compensation in most cases provides significantly improved prediction gain. The variable time delay has to be transmitted as side information requiring an additional bit rate which may be prohibitive when the size of the long-term memory becomes too large. Therefore, we control the bit rate of the motion information by employing rate-constrained motion estimation. Simulation results are obtained by integrating long-term memory prediction into an H.263 codec. Reconstruction PSNR improvements up to 2 dB for the Foreman sequence and 1.5 dB for the Mother–Daughter sequence are demonstrated in comparison to the TMN-2.0 H.263 coder. The PSNR improvements correspond to bit-rate savings up to 34 and 30%, respectively. Mathematical inequalities are used to speed up motion estimation while achieving full prediction gain

Auto-organisation spatio-temporelle : analyse et extension de l'algorithme TOM

Rapport de DEA présentant un travail visant à étendre l'algorithme "Time-organized Map" de Wiemer au cas des cartes 2D

Mécanisme d'auto-organisation corticale : un modèle basé sur la règle de plasticité synaptique BCM

ISBN : 978-2-9532965-0-1L'auto-organisation est un principe de traitement et de représentation de l'information important dans le cortex cérébral. Kohonen a popularisé ce principe dans son célèbre modèle de carte auto-organisatrice. Cependant la règle d'apprentissage associée à ce modéle et portant sur les connexions corticales montantes souffre d'un certain nombre de faiblesses qui limitent sa plausibilité biologique et son domaine d'utilisation. Nous avons repris la règle d'apprentissage BCM (Bienenstock-Cooper-Munro) étudiée théoriquement en profondeur pour ses propriétés de stabilité et pour sa compatibilité avec les connaissances actuelles sur l'apprentissage cortical et nous l'avons adaptée pour une utilisation numérique intensive dans de larges cartes corticales soumises à des flux d'information continus. Nous reportons dans ce papier les résultats d'une première série d'expérimentations qui montrent que ce modèle peut étre utilisé pour retrouver ces principes d'auto-organisation tels qu'ils peuvent émerger de l'apprentissage des connexions montantes. Nous pensons également que cette même règle pourrait être utilisée, de façon conjointe, pour l'apprentissage des connexions corticales latérales, comme nous commencons à le mettre en ?uvre dans une seconde série d'expérimentations

Effects of a Modulatory Feedback upon the BCM learning rule

International audienceBy applying a modulatory feedback upon the BCM learning rule, we show that it is possible to introduce a bias in the learning process, thus promoting a certain kind of selectivity on the feedforward path

Toward a robust 2D spatio-temporal self-organization

International audienceSeveral models have been proposed for spatio-temporal self-organization, among which the TOM model by Wiemer is particularly promising. In this paper, we propose to adapt and extend this model to 2D maps to make it more generic and biologically plausible and more adapted to realistic applications, illustrated here by an application to speech analysis

Cortically-inspired Computational Models for Multimodality

International audienceIn the field of computational neuroscience, we develop distributed models of the cortex to account for perceptual and sensorimotor capabilities. Adopting a mesoscopic level of modeling with dynamic neural fields representing topologically organized populations of cortical columns, we propose various learning rules, competition mechanisms and interconnection schemes. Under the right conditions, these allow the emergence of spatially coherent bumps of activity yielding attentional properties and high robustness to noise. We focus in this poster on the distributed, continuous and unsupervised learning of multi-sensory representations and sensorimotor behaviors. Learning should lead to the emergence of coherent bumps of activity in cortical maps when correlated stimuli are presented, with the possibility to recall potentially missing modalities. This raises the issues of the necessity and nature of generalization in a multimodal context. Research projects on this topic range from the introduction of constrained self-organization with associative maps, adaptive learning rule parameters for multimodal selectivity, to local contingencies in high dimensional sensorimotor spaces

Three different glacier surges at a spot: what satellites observe and what not

In the Karakoram, dozens of glacier surges occurred in the past 2 decades, making the region a global hotspot. Detailed analyses of dense time series from optical and radar satellite images revealed a wide range of surge behaviour in this region: from slow advances longer than a decade at low flow velocities to short, pulse-like advances over 1 or 2 years with high velocities. In this study, we present an analysis of three currently surging glaciers in the central Karakoram: North and South Chongtar Glaciers and an unnamed glacier referred to as NN9. All three glaciers flow towards the same small region but differ strongly in surge behaviour. A full suite of satellites (e.g. Landsat, Sentinel-1 and 2, Planet, TerraSAR-X, ICESat-2) and digital elevation models (DEMs) from different sources (e.g. Shuttle Radar Topography Mission, SRTM; Satellite Pour l’Observation de la Terre, SPOT; High Mountain Asia DEM, HMA DEM) are used to (a) obtain comprehensive information about the evolution of the surges from 2000 to 2021 and (b) to compare and evaluate capabilities and limitations of the different satellite sensors for monitoring surges of relatively small glaciers in steep terrain. A strongly contrasting evolution of advance rates and flow velocities is found, though the elevation change pattern is more similar. For example, South Chongtar Glacier had short-lived advance rates above 10 km yr−1, velocities up to 30 m d−1, and surface elevations increasing by 170 m. In contrast, the neighbouring and 3-times-smaller North Chongtar Glacier had a slow and near-linear increase in advance rates (up to 500 m yr−1), flow velocities below 1 m d−1 and elevation increases up to 100 m. The even smaller glacier NN9 changed from a slow advance to a full surge within a year, reaching advance rates higher than 1 km yr−1. It seems that, despite a similar climatic setting, different surge mechanisms are at play, and a transition from one mechanism to another can occur during a single surge. The sensor inter-comparison revealed a high agreement across sensors for deriving flow velocities, but limitations are found on small and narrow glaciers in steep terrain, in particular for Sentinel-1. All investigated DEMs have the required accuracy to clearly show the volume changes during the surges, and elevations from ICESat-2 ATL03 data fit neatly to the other DEMs. We conclude that the available satellite data allow for a comprehensive observation of glacier surges from space when combining different sensors to determine the temporal evolution of length, elevation and velocity changes

Dynamic vulnerability revealed in the collapse of an Arctic tidewater glacier

Abstract Glacier flow instabilities can rapidly increase sea level through enhanced ice discharge. Surge-type glacier accelerations often occur with a decadal to centennial cyclicity suggesting internal mechanisms responsible. Recently, many surging tidewater glaciers around the Arctic Barents Sea region question whether external forces such as climate can trigger dynamic instabilities. Here, we identify a mechanism in which climate change can instigate surges of Arctic tidewater glaciers. Using satellite and seismic remote sensing observations combined with three-dimensional thermo-mechanical modeling of the January 2009 collapse of the Nathorst Glacier System (NGS) in Svalbard, we show that an underlying condition for instability was basal freezing and associated friction increase under the glacier tongue. In contrast, continued basal sliding further upstream increased driving stresses until eventual and sudden till failure under the tongue. The instability propagated rapidly up-glacier, mobilizing the entire 450 km2 glacier basin over a few days as the till entered an unstable friction regime. Enhanced mass loss during and after the collapse (5–7 fold compared to pre-collapse mass losses) combined with regionally rising equilibrium line altitudes strongly limit mass replenishment of the glacier, suggesting irreversible consequences. Climate plays a paradoxical role as cold glacier thinning and retreat promote basal freezing which increases friction at the tongue by stabilizing an efficient basal drainage system. However, with some of the most intense atmospheric warming on Earth occurring in the Arctic, increased melt water can reduce till strength under tidewater glacier tongues to orchestrate a temporal clustering of surges at decadal timescales, such as those observed in Svalbard at the end of the Little Ice Age. Consequently, basal terminus freezing promotes a dynamic vulnerability to climate change that may be present in many Arctic tidewater glaciers

High-level transgene expression by homologous recombination-mediated gene transfer

Gene transfer and expression in eukaryotes is often limited by a number of stably maintained gene copies and by epigenetic silencing effects. Silencing may be limited by the use of epigenetic regulatory sequences such as matrix attachment regions (MAR). Here, we show that successive transfections of MAR-containing vectors allow a synergistic increase of transgene expression. This finding is partly explained by an increased entry into the cell nuclei and genomic integration of the DNA, an effect that requires both the MAR element and iterative transfections. Fluorescence in situ hybridization analysis often showed single integration events, indicating that DNAs introduced in successive transfections could recombine. High expression was also linked to the cell division cycle, so that nuclear transport of the DNA occurs when homologous recombination is most active. Use of cells deficient in either non-homologous end-joining or homologous recombination suggested that efficient integration and expression may require homologous recombination-based genomic integration of MAR-containing plasmids and the lack of epigenetic silencing events associated with tandem gene copies. We conclude that MAR elements may promote homologous recombination, and that cells and vectors can be engineered to take advantage of this property to mediate highly efficient gene transfer and expression