Stochastic resonance in a dipole

We show that the dipole, a system usually proposed to model relaxation phenomena, exhibits a maximum in the signal-to-noise ratio at a nonzero noise level, thus indicating the appearance of stochastic resonance. The phenomenon occurs in two different situations, i.e., when the minimum of the potential of the dipole remains fixed in time and when it switches periodically between two equilibrium points. We have also found that the signal-to-noise ratio has a maximum for a certain value of the amplitude of the oscillating field.

Late Cretaceous to Eocene geology of the South Central American forearc area (southern Costa Rica and western Panama) : initiation and evolution of an intra-oceanic convergent margin

Résumé : L'arc volcanique du sud de l'Amérique Centrale se situe sur la marge SW de la Plaque Caraïbe, au-dessus des plaques subduites de Cocos et Nazca. Il s'agit de l'un des arcs intra-océaniques les plus étudiés au monde, qui est généralement considéré comme s'étant développé à la fin du Crétacé le long d'un plateau océanique (le Plateau Caraïbe ou CLIP) et se trouvant actuellement dans un régime de subduction érosive. Au cours des dernières décennies, des efforts particuliers ont été faits pour comprendre les processus liés à la subduction sur la base d'études géophysiques et géochimiques. Au sud du Costa Rica et à l'ouest du Panama, des complexes d'accrétions et structures à la base de l'arc volcanique ont été exposés grâce à la subduction de rides asismiques et de failles transformantes. Des affleurements, situés jusqu'à seulement 15 km de la fosse, offrent une possibilité unique de mieux comprendre quelques uns des processus ayant lieu le long de la zone de subduction. Nous présentons de nouvelles contraintes sur l'origine de ces affleurements en alliant une étude de terrain poussée, de nouvelles données géochimiques, sédimentaires et paléontologiques, ainsi que des observations structurales effectuées en télédétection. Une nouvelle stratigraphie tectonique entre le Campanien et l'Éocène est définie pour la région d'avant-arc située entre la Péninsule d'Osa (Costa Rica) et la Péninsule d'Azuero (Panama). Nos résultats montrent que la partie externe de la marge est composée d'un arrangement complexe de roches ignées et de séquences sédimentaires de recouvrement qui comprennent principalement le socle de l'arc, des roches d'arc primitif, des fragments de monts sous-marins accrétés et des mélanges d'accrétion. Des preuves sont données pour le développement de l'arc volcanique du sud de l'Amérique Centrale sur un plateau océanique. Le début de la subduction le long de la marge SW de la Plaque Caraïbe a eu lieu au Campanien et a généré des roches d'arc primitif caractérisées par des affinités géochimiques particulières, globalement intermédiaires entre des affinités de plateau et d'arc insulaire. L'arc était mature au Maastrichtien et formait un isthme essentiellement continu entre l'Amérique du Nord et l'Amérique du Sud. Ceci a permis la migration de faunes terrestres entre les Amériques et pourrait avoir contribué à la crise fin Crétacé -Tertiaire en réduisant les courants océaniques subéquatoriaux entre le Pacifique et l'Atlantique. Plusieurs unités composées de fragments de monts sous-marins accrétés sont définies. La nature et l'arrangement structural de ces unités définissent de nouvelles contraintes sur les modes d'accrétion des monts sous-marins/îles océaniques et sur l'évolution de la marge depuis la formation de la zone de subduction. Entre la fin du Crétacé et l'Éocène moyen, la marge a enregistré plusieurs épisodes ponctuels d'accrétion de monts sous-marins alternant avec de la subduction érosive. A l'Éocène moyen, un événement tectonique régional pourrait avoir causé un fort couplage entre les plaques supérieure et inférieure, menant à des taux plus important d'accrétion de monts sous-marins. Durant cette période, la situation le long de la marge était très semblable à la situation actuelle et caractérisée par la présence de monts sous-marins subductants et l'absence d'accrétion de sédiments. L'enregistrement géologique montre qu'il n'est pas possible d'attribuer une nature érosive ou accrétionnaire à la marge dans le passé ou -par analogie- aujourd'hui, parce que (1) les processus d'accrétion et érosifs varient fortement spatialement et temporellement et (2) il est impossible d'évaluer la quantité exacte de matériel tectoniquement enlevé à la marge depuis le début de la subduction. Au sud du Costa Rica, certains fragments de monts sous-marins accrétés sont représentatifs d'une interaction entre une ride et un point chaud dans le Pacifique au Crétacé terminal/Paléocène. L'existence de ces fragments de monts sous-marins et la morphologie du fond de l'Océan Pacifique indiquent que la formation de la ride de Cocos-Nazca s'est formée au moins ~40 Ma avant l'âge proposé par les modèles tectoniques actuels. Au Panama, nous avons identifié une île océanique d'âge début Éocène qui a été accrétée à l'Éocène moyen. L'accrétion a eu lieu à très faible profondeur par détachement de l'île dans la fosse, et a mené à une exceptionnelle préservation des structures volcaniques. Des affleurement comprenant aussi bien des parties basses et hautes de l'édifice volcanique on été étudiées, depuis la phase sous-marine bouclier jusqu'à la phase subaérienne post-bouclier. La stratigraphie nous a permis de différencier les laves de la phase sous-marine de celles de la phase subaérienne. La composition des laves indique une diminution progressive de l'intensité de la fusion partielle de la source et une diminution de la température des laves produites durant les derniers stades de l'activité volcanique. Nous interprétons ces changements comme étant liés à l'éloignement progressif de l'île océanique de la zone de fusion ou point chaud. Abstract The southern Central American volcanic front lies on the SW edge of the Caribbean Plate, inboard of the subducting Cocos and Nazca Plates. It is one of the most studied intra-oceanic convergent margins around the world, which is generally interpreted to have developed in the late Cretaceous along an oceanic plateau (the Caribbean Large Igneous Province or CLIP) and to be currently undergoing a regime of subduction erosion. In the last decades a particular effort has been made to understand subduction-related processes on the basis of geophysical and geochemical studies. In southern Costa Rica and western Panama accretionary complexes and structures at the base of the volcanic front have been exposed in response to subduction of aseismic ridges and transforms. Onland exposures are located as close as to 15 km from the trench and provide a unique opportunity to better understand some of the processes occurring along the subduction zone. We provide new constraints on the origins of these exposures by integrating a comprehensive field work, new geochemical, sedimentary and paleontological data, as well as structural observations based on remote imaging. A new Campanian to Eocene tectonostratigraphy is defined for the forearc area located between the Osa Peninsula (Costa Rica) and the Azuero Peninsula (Panama). Our results show that the outer margin is composed of a complicated arrangement of igneous complexes and overlapping sedimentary sequences that essentially comprise an arc basement, primitive island-arc rocks, accreted seamount fragments and accretionary mélanges. Evidences are provided for the development of the southern Central American arc on the top an oceanic plateau. The subduction initiation along the SW edge of the Caribbean Plate occurred in the Campanian and led to formation of primitive island-arc rocks characterized by unusual geochemical affinities broadly intermediate between plateau and arc affinities. The arc was mature in the Maastrichtian and was forming a predominantly continuous landbridge between the North and South Americas. This allowed migration of terrestrial fauna between the Americas and may have contributed to the Cretaceous-Tertiary crisis by limiting trans-equatorial oceanic currents between the Pacific and the Atlantic. Several units composed of accreted seamount fragments are defined. The nature of the units and their structural arrangement provide new constraints on the modes of accretion of seamounts/oceanic islands and on the evolution of the margin since subduction initiation. Between the late Cretaceous and the middle Eocene, the margin recorded several local episodes of seamount accretion alternating with tectonic erosion. In the middle Eocene a regional tectonic event may have triggered strong coupling between the overriding and subducting plates, leading to higher rates of seamount accretion. During this period the situation along the margin was very similar to the present and characterized by subducting seamounts and absence of sediment accretion. The geological record shows that it is not possible to ascribe an overall erosive or accretionary nature to the margin in the past and, by analogy, today, because (1) accretionary and erosive processes exhibit significant lateral and temporal variations and (2) it is impossible to estimate the exact amount of material tectonically eroded from the margin since subduction initiation. In southern Costa Rica, accreted seamount fragments point toward a plume-ridge interaction in the Pacific in the late Cretaceous/Paleocene. This occurrence of accreted seamount fragments and morphology of the Pacific Ocean floor is indicative of the formation of the Cocos-Nazca spreading system at least ~40 Ma prior to the age proposed in current tectonic models. In Panama, we identified a remarkably-well preserved early Eocene oceanic island that accreted in the middle Eocene. The accretion probably occurred at very shallow depth by detachment of the island in the trench and led to an exceptional preservation of the volcanic structures. Exposures of both deep and superficial parts of the volcanic edifice have been studied, from the submarine-shield to subaerial-postshield stages. The stratigraphy allowed us to distinguish lavas produced during the submarine and subaerial stages. The lava compositions likely define a progressive diminution of source melting and a decrease in the temperature of erupted melts in the latest stages of volcanic activity. We interpret these changes to primarily reflect the progressive migration of the oceanic island out of the melting region or hotspot.

Estimating traveltimes and groundwater flow patterns using 3D time-lapse crosshole ERT imaging of electrical resistivity fluctuations induced by infiltrating river water

The infiltration of river water into aquifers is of high relevance to drinking-water production and is a key driver of biogeochemical processes in the hyporheic and riparian zone, but the distribution and quantification of the infiltrating water are difficult to determine using conventional hydrological methods (e.g., borehole logging and tracer tests). By time-lapse inverting crosshole ERT (electrical resistivity tomography) monitoring data, we imaged groundwater flow patterns driven by river water infiltrating a perialpine gravel aquifer in northeastern Switzerland. This was possible because the electrical resistivity of the infiltrating water changed during rainfall-runoff events. Our time-lapse resistivity models indicated rather complex flow patterns as a result of spatially heterogeneous bank filtration and aquifer heterogeneity. The upper part of the aquifer was most affected by the river infiltrate, and the highest groundwater velocities and possible preferential flow occurred at shallow to intermediate depths. Time series of the reconstructed resistivity models matched groundwater electrical resistivity data recorded on borehole loggers in the upper and middle parts of the aquifer, whereas the resistivity models displayed smaller variations and delayed responses with respect to the logging data. in the lower part. This study demonstrated that crosshole ERT monitoring of natural electrical resistivity variations of river infiltrate could be used to image and quantify 3D bank filtration and aquifer dynamics at a high spatial resolution.

L'autre du même : emprunts et répétitions dans le "Roman de Perceforest"

Issu de la fin du Moyen Âge, le Roman de Perceforest est le plus long texte que le Moyen Âge nous ait laissé. Il entend décrire la vie des ancêtres préchrétiens d'Arthur et de ses chevaliers en les faisant descendre d'Alexandre le Grand. Au fil de son récit, l'auteur met en place une véritable poétique de la reprise, tant externe qu'interne. Il multiplie les références à des textes préexistants issus de différentes matières et va même jusqu'à en intégrer des morceaux entiers. Il reprend par ailleurs des parties de sa propre oeuvre, n'hésitant pas à reproduire des schémas, voire des séquences narratives précises. Cette esthétique répétitive a plusieurs conséquences, tant sur l'appartenance générique du texte, sur sa construction, que sur sa réception par le lecteur. Elle est au coeur de notre étude. Nous nous intéressons plus spécifiquement à un phénomène particulier de reprise qui montre l'intégration d'une séquence préexistante dans un contexte distinct de son apparition d'origine et que nous qualifions d'emprunt. Notre travail s'organise autour d'un examen successif des différents types d'emprunts qui apparaissent au sein du texte, tant intertextuels qu'intratextuels. À terme, c'est la cohérence et l'individualité du Perceforest, ainsi que la conception de l'écriture qui anime son auteur que nous mettons en avant, tout en proposant des jalons pour une théorie générale de l'emprunt. - The Roman de Perceforest, the longest known text from the Middle Ages, aims to describe the life of Arthur's pre-christian ancestors and knights, presenting them as descendants of Alexander the Great. Along the storytelling, a genuine poetics of external and internal repetition takes place: the Perceforest s authors multiply the references to previous texts belonging to several materials, integrating sometimes entire parts of other texts. Furthermore the narrative reproduces its own patterns and particular sequences in different places of the text. Throughout this research, we consider the influence of such repetition aesthetics on the literary genre definition, on its construction as well as on the reader's reception. In this dissertation, we explore a specific category of repetitions where pre-existing sequences are embedded in a narrative context that differs from the original context of occurrence, which we called emprunt (s). Reviewing the species of inter- and intra-textual recurrences occurring within the text, we reveal some overlooked aspects of the consistency, the specificity of the Perceforest and its author's idea of writing, striving to groundwork on general theory of «emprunts» that shall thereby be laid.

Characterization of the neuronal microtubule regulator SCG10 in transfected cells and " in vivo "

SUMMARY The ability of neuronal processes to find their way along complex paths and to establish appropriate connections depends on continual rearrangements of the cytoskeletal components. The regulation of microtubules plays an important role for morphological changes underlying nevrite outgrowth, axonal elongation, and growth cone steering. SCG10 (superior cervical ganglion clone 10) is a neuronal growthassociated protein developmentally regulated and highly enriched in the neuronal growth cones. SCG10 presents a microtubule destabilizing activity that could participate to the regulation of microtubule dynamics and thus explain microtubule behaviors in the growth cone during axonal elongation and turning. It is here suggested that a tight control of the opposite effects on microtubules of SCG10 and the stabilizing microtubule-associated protein MAP1B allows a fine tuning of cytoskeletal rearrangement and may provide the required microtubule dynamic instability to promote axonal growth. Moreover, antibodyblockade of SCG10 function, that leads to growth cone pauses similar as those triggered by the guidance molecule EphB, and the modulation of SCG10 activity by the Rho GTPase Rnd1 suggest a potential role for SCG10 in the signal transduction pathways of extracellular guidance cues. The identification of the active zone protein Bassoon as a potential interaction partner for the SCG10-related protein NPC2, using atomic force microscopy as well as COS-7 and neuronal cell cultures, also gives new insights for a role of this protein family into the processes of synapse genesis or plasticity. Finally, SCG10 mutant mice generated by gene targeting and expressing a soluble form of the protein have been characterized during early postnatal development and in the adulthood. Due to the deletion of its membrane binding domain, SCG10 specific subcellular targeting to growth cones is compromised and results in impairments of motor and coordination development. Further histological analysis in the sciatic nerve reveal that these symptoms are associated with neurodegenerative signs. RESUME Une navigation correcte des prolongements cellulaires neuronaux leur permettant de former des connections appropriées repose sur de continuels réarrangements des constituants de leur cytosquelette. La régulation des microtubules joue notamment un rôle important dans les changements morphologiques qui accompagnent la croissance axonale et les réorientations du cône de croissance. SCG10 (superior cervical ganglion clone 10) est une protéine étroitement associée à la croissance neuronale, hautement régulée durant le développement et abondante au niveau du cône de croissance. SCG10 présente une activité déstabilisatrice sur les microtubules qui pourrait permettre une régulation des paramètres dynamiques propres aux microtubules et ainsi expliquer leur comportement durant la navigation du cône de croissance. Il est ici proposé qu'un contrôle précis des effets opposés de SCG10 et d'une autre protéine stabilisante associée aux microtubules (MAP1 B) permette un réglage fin des réarrangements du cytosquelette et puisse ainsi produire l'instabilité dynamique nécessaire à la croissance anale. Par ailleurs, le blocage de la fonction de SCG10 par un anticorps spécifique, conduisant à des pauses du cônes de croissance similaires à celles provoquées par la molécule de guidage EphB, ainsi que la modulation de l'activité de SCG10 par la Rho GTPase Rnd1 suggèrent une potentielle implication de SCG10 dans les voies de transduction des signaux provenant de molécules de guidage extracellulaires. L'identification d'une interaction de la protéine synaptique Bassoon avec la protéine NPC2 apparentée à SCG10, au moyen de la microscopie à force atomique et dans des cultures de cellules neuronales et COS-7, ouvre des perspectives concernant ces protéines dans la formation et la plasticité synaptiques. Finalement, des souris mutantes pour SCG10 produites par ciblage de gène et exprimant une forme soluble de la protéine ont été caractérisées durant la phase précoce du développement et à l'âge adulte. La délétion du domaine permettant l'ancrage de SCG10 aux membranes compromet sa sub-localisation au niveau du cône de croissance et résulte en l'apparition de troubles moteurs et de la coordination. Des analyses histologiques complémentaires au niveau du nerf sciatique montrent que ces symptômes sont associés avec des signes neurodégénératifs.

Genes, environment and sex : perspectives from ectothermic vertebrates

RESUMELes modèles classiques sur l'évolution des chromosomes sexuels supposent que des gènes sexe- antagonistes s'accumulent sur les chromosomes sexuels, entraînant ainsi l'apparition d'une région non- recombinante, qui se répand progressivement en favorisant l'accumulation de mutations délétères. En accord avec cette théorie, les chromosomes sexuels que l'on observe aujourd'hui chez les mammifères et les oiseaux sont considérablement différenciés. En revanche, chez la plupart des vertébrés ectothermes, les chromosomes sexuels sont indifférenciés et il existe une impressionnante diversité de mécanismes de détermination du sexe. Au cours de cette thèse, j'ai étudié l'évolution des chromosomes sexuels chez les vertébrés ectothermes, en outre pour mieux comprendre ce contraste avec les vertébrés endothermes. L'hypothèse « high-turnover » postule que les chromosomes sexuels sont remplacés régulièrement à partir d'autosomes afin d'éviter leur dégénérescence. L'hypothèse « fountain-of-youth » propose que la recombinaison entre le chromosome X et le chromosome Y au sein de femelles XY empêche la dégénérescence. Les résultats de ma thèse, basés sur des études théoriques et empiriques, suggèrent que les deux processus peuvent être entraînés par l'environnement et ainsi jouent un rôle important dans l'évolution des chromosomes sexuels chez les vertébrés ectothermes.SUMMARYClassical models of sex-chromosome evolution assume that sexually antagonistic genes accumulate on sex chromosomes leading to a non-recombining region, which progressively expands and favors the accumulation of deleterious mutations. Concordant with this theory, sex chromosomes in extant mammals and birds are considerably differentiated. In most ectothermic vertebrates, such as frogs, however, sex chromosomes are undifferentiated and a striking diversity of sex determination systems is observed. This thesis was aimed to investigate this apparent contrast of sex chromosome evolution between endothermic and ectothermic vertebrates. The "high-turnover" hypothesis holds that sex chromosomes arose regularly from autosomes preventing decay. The "fountain-of-youth" hypothesis posits that sex chromosomes undergo episodic X-Y recombination in sex-reversed XY females, thereby purging ("rejuvenating") the Y chromosome. We suggest that both processes likely played an important role in sex chromosome evolution of ectothermic vertebrates. The literature largely views sex determination as a dichotomous process: individual sex is assumed to be determined either by genetic (genotypic sex determination, GSD) or by environmental factors (environmental sex determination, ESD), most often temperature (temperature sex determination, TSD). We endorsed an alternative view, which sees GSD and TSD as the ends of a continuum. The conservatism of molecular processes among different systems of sex determination strongly supports the continuum view. We proposed to define sex as a threshold trait underlain by a liability factor, and reaction norms allowing modeling interactions between genotypic and temperature effects. We showed that temperature changes (due to e.g., climatic changes or range expansions) are expected to provoke turnovers in sex-determination mechanisms maintaining homomorphic sex chromosomes. The balanced lethal system of crested newts might be the result of such a sex determination turnover, originating from two variants of ancient Y-chromosomes. Observations from a group of tree frogs, on the other hand, supported the 'fountain of youth' hypothesis. We then showed that low rates of sex- reversals in species with GSD might actually be adaptive considering joint effects of deleterious mutation purging and sexually antagonistic selection. Ongoing climatic changes are expected to threaten species with TSD by biasing population sex ratios. In contrast, species with GSD are implicitly assumed immune against such changes, because genetic systems are thought to necessarily produce even sex ratios. We showed that this assumption may be wrong and that sex-ratio biases by climatic changes may represent a previously unrecognized extinction threat for some GSD species.

First-passage and escape problems in the Feller process

The Feller process is an one-dimensional diffusion process with linear drift and state-dependent diffusion coefficient vanishing at the origin. The process is positive definite and it is this property along with its linear character that have made Feller process a convenient candidate for the modeling of a number of phenomena ranging from single-neuron firing to volatility of financial assets. While general properties of the process have long been well known, less known are properties related to level crossing such as the first-passage and the escape problems. In this work we thoroughly address these questions.

Entropic stochastic resonance

We present a novel scheme for the appearance of stochastic resonance when the dynamics of a Brownian particle takes place in a confined medium. The presence of uneven boundaries, giving rise to an entropic contribution to the potential, may upon application of a periodic driving force result in an increase of the spectral amplification at an optimum value of the ambient noise level. The entropic stochastic resonance, characteristic of small-scale systems, may constitute a useful mechanism for the manipulation and control of single molecules and nanodevices.

Effects of trimethyltin (TMT) on glial and neuronal cells in aggregate cultures: dependence on the developmental stage.

Long-term effects of trimethyltin (TMT) applied at concentrations below the cytotoxic level were examined in three-dimensional cell cultures of fetal rat telencephalon using biochemical, immunochemical and morphological criteria. It was found that in immature cultures low concentrations of TMT (10(-8) M) specifically induced a gliotic response in astrocytes, with increased immunoreactivity for glial fibrillary acidic protein, and a greater number of astrocytic processes. Significant changes in oligodendrocytic and neuronal parameters were found only at 10(-6) M of TMT. In differentiated cultures, distinct changes in cell type-specific parameters occurred at 10(-6) M of TMT (the lowest effective concentration). In addition, different patterns of responses were found for astrocytes and oligodendrocytes, as compared to immature cultures. These results suggest that among neural cells, astroblasts are most sensitive to TMT, and that the glial responses to this neurotoxicant are development-dependent.

Atomic force microscopy imaging of living cells

Over the last two decades, Atomic Force Microscopy (AFM) has emerged as the tool of choice to image living organisms in a near-physiological environment. Whereas fluorescence microscopy techniques allow labeling and tracking of components inside cells and the observation of dynamic processes, AFM is mainly a surface technique that can be operated on a wide range of substrates including biological samples. AFM enables extraction of topographical, mechanical and chemical information from these samples.

Mutations in B3GALT6, which encodes a glycosaminoglycan linker region enzyme, cause a spectrum of skeletal and connective tissue disorders.

Proteoglycans (PGs) are a major component of the extracellular matrix in many tissues and function as structural and regulatory molecules. PGs are composed of core proteins and glycosaminoglycan (GAG) side chains. The biosynthesis of GAGs starts with the linker region that consists of four sugar residues and is followed by repeating disaccharide units. By exome sequencing, we found that B3GALT6 encoding an enzyme involved in the biosynthesis of the GAG linker region is responsible for a severe skeletal dysplasia, spondyloepimetaphyseal dysplasia with joint laxity type 1 (SEMD-JL1). B3GALT6 loss-of-function mutations were found in individuals with SEMD-JL1 from seven families. In a subsequent candidate gene study based on the phenotypic similarity, we found that B3GALT6 is also responsible for a connective tissue disease, Ehlers-Danlos syndrome (progeroid form). Recessive loss-of-function mutations in B3GALT6 result in a spectrum of disorders affecting a broad range of skeletal and connective tissues characterized by lax skin, muscle hypotonia, joint dislocation, and spinal deformity. The pleiotropic phenotypes of the disorders indicate that B3GALT6 plays a critical role in a wide range of biological processes in various tissues, including skin, bone, cartilage, tendon, and ligament.

Are there control processes, and (if so) can they be studied?

Generally, so-called control processes are thought to be necessary when we must perform one out of several competing actions. Some examples include performance of a less well-practiced action instead of a well-practiced one (prepotency); learning a new action (novelty); and rapidly switching from one action to another (task-switching). While it certainly is difficult to perform the desired action in these circumstances, it is less clear that a separate set of processes (e.g., control processes) are necessary to explain the observed behavior. Another way to approach the study of control processes is to investigate physiological dependent measures (e.g., electrophysiological or neuroimaging measures). Although these offer another avenue of inquiry into control processes, they have yet to furnish unambiguous evidence that control processes exist. While this might suggest that there are no control processes, it is also possible that our methods are insufficiently sensitive to measure control processes. We have investigated this latter possibility using tasks that are neuroanatomically distinct, though within the same modality (vision). This approach did not yield evidence for a separable set of control processes. However, recent works using a task-switching paradigm in which subjects switch between a visual and an auditory task suggest that switching both task and modality may be importantly different than switching task within a given modality. This may represent a way forward in the study of control processes.

Statistical methods for river runoff prediction

Methods used to analyze one type of nonstationary stochastic processes?the periodically correlated process?are considered. Two methods of one-step-forward prediction of periodically correlated time series are examined. One-step-forward predictions made in accordance with an autoregression model and a model of an artificial neural network with one latent neuron layer and with an adaptation mechanism of network parameters in a moving time window were compared in terms of efficiency. The comparison showed that, in the case of prediction for one time step for time series of mean monthly water discharge, the simpler autoregression model is more efficient.

Contribution of neural networks to Alzheimer disease's progression.

The neuropathology of Alzheimer disease is characterized by senile plaques, neurofibrillary tangles and cell death. These hallmarks develop according to the differential vulnerability of brain networks, senile plaques accumulating preferentially in the associative cortical areas and neurofibrillary tangles in the entorhinal cortex and the hippocampus. We suggest that the main aetiological hypotheses such as the beta-amyloid cascade hypothesis or its variant, the synaptic beta-amyloid hypothesis, will have to consider neural networks not just as targets of degenerative processes but also as contributors of the disease's progression and of its phenotype. Three domains of research are highlighted in this review. First, the cerebral reserve and the redundancy of the network's elements are related to brain vulnerability. Indeed, an enriched environment appears to increase the cerebral reserve as well as the threshold of disease's onset. Second, disease's progression and memory performance cannot be explained by synaptic or neuronal loss only, but also by the presence of compensatory mechanisms, such as synaptic scaling, at the microcircuit level. Third, some phenotypes of Alzheimer disease, such as hallucinations, appear to be related to progressive dysfunction of neural networks as a result, for instance, of a decreased signal to noise ratio, involving a diminished activity of the cholinergic system. Overall, converging results from studies of biological as well as artificial neural networks lead to the conclusion that changes in neural networks contribute strongly to Alzheimer disease's progression.

Towards Machine Consciousness: Grounding Abstract Models as pi-Processes

We present a two-level model of concurrent communicating systems (CCS) to serve as a basis formachine consciousness. A language implementing threads within logic programming is ¯rstintroduced. This high-level framework allows for the de¯nition of abstract processes that can beexecuted on a virtual machine. We then look for a possible grounding of these processes into thebrain. Towards this end, we map abstract de¯nitions (including logical expressions representingcompiled knowledge) into a variant of the pi-calculus. We illustrate this approach through aseries of examples extending from a purely reactive behavior to patterns of consciousness.