Half-quadratic regularization for MRI image restoration

We consider the reconstruction of MRI images by minimizing regularized cost-functions. To accelerate the computation of the estimate, two forms of half-quadratic regularization, multiplicative and additive, are often used. In Nikolova and Ng (2002), we have compared both theoretically and experimentally the efficiency of these two forms using one-dimensional signals. The goal of this paper is to compare experimentally the efficiency of these two forms using MRI image reconstruction. We find that using the additive form is more computationally effective than using the multiplicative form.published_or_final_versio

Composition chimique et propriétés antibactériennes des huiles essentielles d’Ocimum basilicum et d’Hyptis suaveolens (L.) Poit récoltés dans la région de Dakar au Sénégal

Les huiles essentielles des feuilles d’Ocimum basilicumet d’Hyptis suaveolens, récoltées dans la région de Dakar au Sénégal ont été extraites par entraînement à la vapeur et analysées en CPG et CPG-SM. Les monoterpènes oxygénés dont l’estragol (38,78 %), le linalol (19,45 %) et le méthyl-eugénol (9,98 %) sont majoritaires dans l’huile essentielle d’O. basilicum. Ils sont suivis d’un sesquiterpène hydrocarboné: le bergamotène (8,48 %). Par contre, l’huile essentielle de H. suaveolensest essentiellement constituée de composés hydrocarbonés: le β-caryophyllène (16,63 %), le sabinène (11,30 %), le terpinolène (8,58 %), le limonène (8,45 %) et le bergamotène (5,26 %). Les propriétés antimicrobiennes des huiles essentielles de ces plantes ont été testées in vitro sur cinq souches bactériennes (Staphylococcus aureus, Bacillus sp.,Salmonella sp., Escherichia coli, Streptococcus sp.). Une activité inhibitrice des huiles sur les souches étudiées a été observée. Toutefois, celle d’O.basilicums’est révélée plus active, particulièrement contre Bacillus sp., Salmonella sp., et Escherichia coli.Mots-clés: Ocimum basilicum, Hyptis suaveolens, huiles essentielles, composition chimique, propriétés antimicrobiennes. Chemical composition and antimicrobial properties of the essential oils of Ocimum basilicum andHyptis suaveolensharvested from Dakar region in SenegalEssential oils of leaves from Ocimum basilicum and Hyptis suaveolens collected in the region of Dakar in Senegal have been extracted by steam distillation and analyzed by GC and GC-MS. The oxygenated monoterpenes which estragol (38.78%), linalool (19.45%) and methyl-eugenol (9.98%) constitute the major portion of the essential oils of O. basilicum followed by bergamotene (8.48%) which is a sesquiterpene hydrocarbon. Principal compounds of H.suaveolens essential oilsare: β-caryophyllene (16.63%), sabinene (11.30%), terpinolene (8.58%), limonene (8.45%) and bergamotene (5.26%).The antimicrobial properties of essential oils of these plants were tested in vitro against five bacterial strains (Staphylococcus aureus, Bacillussp, Salmonella sp., Escherichia coli, Streptococcus sp.). The inhibitory activity of the oils on the strains studied was observed. However, the essential oil of O. basilicum was more active, especially against Bacillus sp., Salmonella sp. and Escherichia coli.Keywords: Ocimum basilicum, Hyptis suaveolens, essential oils, chemical composition, antimicrobial properties

Introduction: Meeting in the Middle

Compromise is routinely evoked in everyday language and in scholarly debates across the social sciences. Yet, it has been subjected to relatively little systematic study. The introduction to this inter-disciplinary volume addresses the research gap in three steps. First, we offer three reasons for the study of compromise: its empirical omnipresence in politics, its theoretical potential to bridge the rationalist-constructivist divide, and its normative promise to recognize the plurality of society. Second, we introduce different approaches to the coherence, legitimacy and limits of compromise found in the existing explanatory and normative literatures. We discuss why these literatures need to speak to one another, and identify possible applications in empirical research. Third, we conceptualize compromise as one possible solution to a conflict. Distinct from both dissensus and consensus, all compromises share three characteristics: concessions, non-coercion and continued controversy. However, different types of compromise can be distinguished by how mutual, costly and painful concessions are; by whether all forms of coercion are absent; and by the degree to which the relevant parties’ grounds for conflict are transformed. We conclude by discussing the challenge and appeal of ‘politics as compromise’ in plural and complex societies

Decoding the retina with the first wave of spikes

International audienceUnderstanding how the retina encodes visual information remains an open question. Using MEAs on salamander retinas, Gollisch & Meister (2008) showed that the relative latencies between some neuron pairs carry sufficient information to identify the phase of square-wave gratings (,. Using gratings of varying phase, spatial frequency, and contrast on mouse retinas, we extended this idea by systematically considering the relative order of all spike latencies, i.e. the shape of the first wave of spikes after stimulus onset. The discrimination task was to identify the phase among gratings of identical spatial frequency. We compared the performance (fraction correct predictions) of our approach under classical Bayesian and LDA decoders to spike count and response latency of each recorded neuron. Best results were obtained for the lowest spatial frequency. There, results showed that the spike count discrimination performance was higher than for latency under both the Bayesian (0,95+-0,02 and 0,75+-0,11 respectively) and LDA (0,95+-0,01 and 0,62+-0,03 respectively) decoders. The first wave of spikes decoder is (0,46+-0,06) less efficient than the spike count. Nevertheless, it accounts for 50% of the overall performance. Interestingly, these results tend to confirm the rank order coding hypothesis (Thorpe & Gautrais, 1998)

Accuracy and Conditioning of Surface-Source Based Near-Field to Far-Field Transformations

The conditioning and accuracy of various inverse surface-source formulations are investigated. First, the normal systems of equations are discussed. Second, different implementations of the zero-field condition are analyzed regarding their effect on solution accuracy, conditioning, and source ambiguity. The weighting of the Love-current side constraint is investigated in order to provide an accurate problem-independent methodology. The transformation results for simulated and measured near-field data show a comparable behavior regarding accuracy and conditioning for most of the formulations. Advantages of the Love-current solutions are found only in diagnostic capabilities. Regardless of this, the Love side constraint is a computationally costly way to influence the iterative solver threshold, which is more conveniently controlled with the appropriate type of normal equation. The solution behavior of the inverse surface-source formulations is mostly influenced by the choice of the reconstruction surface. A spherical Huygens surface leads to the best conditioning, whereas the most accurate solutions are found with a tight, possibly convex hull around the antenna under test.Comment: 15 pages, 13 figures, 4 tables, accepted for publication in IEEE Transactions on Antennas and Propagatio

Rank order coding: a retinal information decoding strategy revealed by large-scale multielectrode array retinal recordings

International audienceHow a population of retinal ganglion cells (RGCs) encodes the visual scene remains an open question. Going beyond individual RGC coding strategies, results in salamander suggest that the relative latencies of an RGC pair encodes spatial information. Thus a population code based on this concerted spiking could be a powerful mechanism to transmit visual information rapidly and efficiently. Here, we tested this hypothesis in mouse by recording simultaneous light-evoked responses from hundreds of RGCs, at pan-retinal level, using a new generation of large-scale, high density multielectrode array consisting of 4096 electrodes. Interestingly, we did not find any RGCs exhibiting a clear latency tuning to the stimuli, suggesting that in mouse, individual RGC pairs may not provide sufficient information. We show that a significant amount of information is encoded synergistically in the concerted spiking of large RGC populations. Thus, the RGC population response described with relative activities, or ranks, provides more relevant information than classical independent spike count- or latency- based codes. In particular, we report for the first time that when considering the relative activities across the whole population, the wave of first stimulus-evoked spikes (WFS) is an accurate indicator of stimulus content. We show that this coding strategy co-exists with classical neural codes, and that it is more efficient and faster. Overall, these novel observations suggest that already at the level of the retina, concerted spiking provides a reliable and fast strategy to rapidly transmit new visual scenes

On a coupled PDE model for image restoration

In this paper, we consider a new coupled PDE model for image restoration. Both the image and the edge variables are incorporated by coupling them into two different PDEs. It is shown that the initial-boundary value problem has global in time dissipative solutions (in a sense going back to P.-L. Lions), and several properties of these solutions are established. This is a rough draft, and the final version of the paper will contain a modelling part and numerical experiments

Bifurcation study of a neural field competition model with an application to perceptual switching in motion integration.

Perceptual multistability is a phenomenon in which alternate interpretations of a fixed stimulus are perceived intermittently. Although correlates between activity in specific cortical areas and perception have been found, the complex patterns of activity and the underlying mechanisms that gate multistable perception are little understood. Here, we present a neural field competition model in which competing states are represented in a continuous feature space. Bifurcation analysis is used to describe the different types of complex spatio-temporal dynamics produced by the model in terms of several parameters and for different inputs. The dynamics of the model was then compared to human perception investigated psychophysically during long presentations of an ambiguous, multistable motion pattern known as the barberpole illusion. In order to do this, the model is operated in a parameter range where known physiological response properties are reproduced whilst also working close to bifurcation. The model accounts for characteristic behaviour from the psychophysical experiments in terms of the type of switching observed and changes in the rate of switching with respect to contrast. In this way, the modelling study sheds light on the underlying mechanisms that drive perceptual switching in different contrast regimes. The general approach presented is applicable to a broad range of perceptual competition problems in which spatial interactions play a role