126 research outputs found
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 propriĂ©tĂ©s antibactĂ©riennes des huiles essentielles dâOcimum basilicum et dâHyptis suaveolens (L.) Poit rĂ©coltĂ©s dans la rĂ©gion de Dakar au SĂ©nĂ©gal
Les huiles essentielles des feuilles dâOcimum basilicumet dâHyptis suaveolens, rĂ©coltĂ©es dans la rĂ©gion de Dakar au SĂ©nĂ©gal ont Ă©tĂ© extraites par entraĂźnement Ă la vapeur et analysĂ©es en CPG et CPG-SM. Les monoterpĂšnes oxygĂ©nĂ©s dont lâestragol (38,78 %), le linalol (19,45 %) et le mĂ©thyl-eugĂ©nol (9,98 %) sont majoritaires dans lâhuile essentielle dâO. basilicum. Ils sont suivis dâun sesquiterpĂšne hydrocarbonĂ©: le bergamotĂšne (8,48 %). Par contre, lâhuile essentielle de H. suaveolensest essentiellement constituĂ©e de composĂ©s hydrocarbonĂ©s: le ÎČ-caryophyllĂšne (16,63 %), le sabinĂšne (11,30 %), le terpinolĂšne (8,58 %), le limonĂšne (8,45 %) et le bergamotĂšne (5,26 %). Les propriĂ©tĂ©s antimicrobiennes des huiles essentielles de ces plantes ont Ă©tĂ© testĂ©es in vitro sur cinq souches bactĂ©riennes (Staphylococcus aureus, Bacillus sp.,Salmonella sp., Escherichia coli, Streptococcus sp.). Une activitĂ© inhibitrice des huiles sur les souches Ă©tudiĂ©es a Ă©tĂ© observĂ©e. Toutefois, celle dâO.basilicumsâest rĂ©vĂ©lĂ©e plus active, particuliĂšrement contre Bacillus sp., Salmonella sp., et Escherichia coli.Mots-clĂ©s: Ocimum basilicum, Hyptis suaveolens, huiles essentielles, composition chimique, propriĂ©tĂ©s 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: ÎČ-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 partiesâ grounds for conflict are transformed. We conclude by discussing the challenge and appeal of âpolitics as compromiseâ 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
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