Showing papers by "Pierre-and-Marie-Curie University published in 1993"

Comparison of numerical models for computing underwater light fields

Abstract: Seven models for computing underwater radiances and irradiances by numerical solution of the radiative transfer equation are compared. The models are applied to the solution of several problems drawn from optical oceanography. The problems include highly absorbing and highly scattering waters, scattering by molecules and by particulates, stratified water, atmospheric effects, surface-wave effects, bottom effects, and Raman scattering. The models provide consistent output, with errors (resulting from Monte Carlo statistical fluctuations) in computed irradiances that are seldom larger, and are usually smaller, than the experimental errors made in measuring irradiances when using current oceanographic instrumentation. Computed radiances display somewhat larger errors.

Forward, backward and symmetric stochastic integration

Abstract: We define three types of non causal stochastic integrals: forward, backward and symmetric. Our approach consists in approximating the integrator. Two optics are considered: the first one is based on traditional usual stochastic calculus and the second one on Wiener distributions.

Domain Decomposition by the Mortar Element Method

Abstract: The paper reviews recent results concerning the mortar element method, which allows for coupling variational discretizations of different types on nonoverlapping subdomains. The basic ideas and proofs are recalled on a model problem, and new extensions are presented.

Initiation of subduction and the generation of slab melts in western and eastern Mindanao, Philippines

Abstract: Adakite, found in both the eastern and western parts of Mindanao Island, Philippines, is a rare rock type, characterized by low heavy rare earth elements and Y contents together with high Sr/Y ratios, and is considered to be the result of the melting of young subducted oceanic crust, which leaves an eclogite residue. Pliocene-Quaternary adakites from western Mindanao (Zamboanga Peninsula) are probably derived from the melting of the young Miocene Sulu Sea crust, which is currently subducting beneath Zamboanga. Associated Nb-enriched basalts are thought to come from mantle metasomatized through interaction with adakitic melts. In eastern Mindanao, Pliocene-Quaternary cones and plugs of typical adakitic composition mark the trace of the Philippine fault in Surigao and north Davao. The underlying Philippine Sea crust is of Eocene age and therefore cannot melt under normal subduction thermal conditions. Thermal models indicate that melting at the start of subduction can occur. Subduction of the Philippine Sea plate began 3 to 4 Ma beneath eastern Mindanao and probably accounts for the presence of adakites along the Philippine fault.

An intervertebral disk prosthesis.

Abstract: A disk prosthesis for replacing a damaged spinal disk has two metal half-envelopes which confine between them a compression cushion having a controlled differential compression. The structure employs a membrane or diaphragm surrounding the compression cushion in order to insure a seal between the cushion and the environment and also has an anti-expulsion system for shaping the compression cushion in order to limit the expansion of the cushion out of the volume defined by the two metal cups and the membrane.

Vertical distribution and temporal variation of dissolved organic carbon in the North-Western Mediterranean Sea

Abstract: During 1991 and 1992, detailed vertical profiles of dissolved organic carbon (DOC) content were obtained monthly, in the North-Western Mediterranean at the DYFAMED-1 site (43°25′N, 07°52′E). DOC was measured using a high temperature catalytic oxidation method. In the 150–2000 m layer, DOC distribution was quite uniform, with a content of about 0.6–0.7 mg C l−1. In surface waters, DOC concentration changed according to hydrological situation and primary production cycle. During February, waters were DOC-homogeneous through the whole water column; then surface concentrations increased until autumn with maxima of about 1.1–1.2 mg C l−1 between 0 and 50 m depths. Assuming that the DOC is continuously removed from surface waters by diffusive processes during the stratified period and that the DOC accumulated in surface waters until the end of autumn is removed by advection during the short period of winter deep water formation, we find that an amount of 18.4 g Cm−2y−1 is transferred to the deep layer. This value is more than twice the particulate organic carbon flux measured on sediment traps (4–7 g Cm−2 y−1), but agrees with published new production values.

Dynamics of orientation coding in area V1 of the awake primate.

Abstract: To investigate the importance of feedback loops in visual information processing, we have analyzed the dynamic aspects of neuronal responses to oriented gratings in cortical area V1 of the awake primate. If recurrent feedback is important in generating orientation selectivity, the initial part of the neuronal response should be relatively poorly selective, and full orientation selectivity should only appear after a delay. Thus, by examining the dynamics of the neuronal responses it should be possible to assess the importance of feedback processes in the development of orientation selectivity. The results were base on a sample of 259 cells recorded in two monkeys, of which 89% were visually responsive. Of these, approximately two-thirds were orientation selective. Response latency varied considerably between neurons, ranging from a minimum of 41 ms to over 150 ms, although most had latencies of 50–70 ms. Orientation tuning (defined as the bandwidth at half-height) ranged from 16 deg to over 90 deg, with a mean value of around 55 deg. By examining the selectivity of these different neurons by 10-ms time slices, starting at the onset of the neuronal response, we found that the orientation selectivity of virtually every neuron was fully developed at the very start of the neuronal response. Indeed, many neurons showed a marked tendency to respond at somewhat longer latencies to stimuli that were nonoptimally oriented, with the result that orientation selectivity was highest at the very start of the neuronal response. Furthermore, there was no evidence that the neurons with the shortest onset latencies were less selective. Such evidence is inconsistent with the hypothesis that recurrent intracortical feedback plays an important role in the generation of orientation selectivity. Instead, we suggest that orientation selectivity is primarily generated using feedforward mechanisms, including feedforward inhibition. Such a strategy has the advantage of allowing orientation to be computed rapidly, and avoids the initially poorly selective neuronal responses that characterize processing involving recurrent loops.

Quantized motion of cold cesium atoms in two- and three-dimensional optical potentials.

Abstract: Quantization of atomic motion is observed in two- and three-dimensional cesium optical molasses by stimulated Raman spectroscopy. A new geometry is used with the minimum number of laser beams, so that the topography of the optical potential is not sensitive to phase drifts of the molasses beams. The deepest potential wells are located on a hexagonal lattice in two dimensions and on a body-centered-cubic lattice in three dimensions. They correspond to purely circularly polarized light, allowing a Lamb-Dicke narrowing of the transitions between vibrational levels to occur.

Theoretical analysis of the structures of titanium dioxide crystals

Abstract: This paper presents results of pseudopotential Hartree-Fock calculations on the titanium dioxide crystals. The optimized structures are in good agreement with experimental data. Both anatase and rutile structures have a large ionic character. The distortions from the undistorted crystals made of regular octahedra are discussed and qualitative trends for the distortions are justified by semiempirical calculations (extended H\"uckel theory). When correlation is taken into account, a large part of the binding energy is recovered and the rutile structure is found to be more stable than the anatase one.

Myelin/oligodendrocyte glycoprotein is a member of a subset of the immunoglobulin superfamily encoded within the major histocompatibility complex

Abstract: Myelin/oligodendrocyte glycoprotein (MOG) is found on the surface of myelinating oligodendrocytes and external lamellae of myelin sheaths in the central nervous system, and it is a target antigen in experimental autoimmune encephalomyelitis and multiple sclerosis. We have isolated bovine, mouse, and rat MOG cDNA clones and shown that the developmental pattern of MOG expression in the rat central nervous system coincides with the late stages of myelination. The amino-terminal, extracellular domain of MOG has characteristics of an immunoglobulin variable domain and is 46% and 41% identical with the amino terminus of bovine butyrophilin (expressed in the lactating mammary gland) and B-G antigens of the chicken major histocompatibility complex (MHC), respectively; these proteins thus form a subset of the immunoglobulin superfamily. The homology between MOG and B-G extends beyond their structure and genetic mapping to their ability to induce strong antibody responses and has implications for the role of MOG in pathological, autoimmune conditions. We colocalized the MOG and BT genes to the human MHC on chromosome 6p21.3-p22. The mouse MOG gene was mapped to the homologous band C of chromosome 17, within the M region of the mouse MHC.

Dynamical theory of proton tunneling transfer rates in solution: general formulation

Abstract: A general dynamical theory is presented for the rate constant of weak coupling, nonadiabatic proton-tunneling reactions in solution The theory incorporates the critical role of the solvent and the vibration of the separation of the heavy particles between which the proton transfers, including their dynamics The formulation which allows the computation of the quantum rate constant k via classical molecular dynamics simulation techniques is presented, as are a number of approximate analytic results for k in a variety of different important regimes The frequent appearance of (nearly) classical Arrhenius behavior for k — even though the intrinsic reactive event is quantum proton tunneling — is discussed, together with the solvent and vibrational contributions to the apparent activation energy In certain weak solvation limits, however, non-Arrhenius behavior for k is found and is related to vibrational Franck-Condon features in the reaction

Markovian Bridges: Construction, Palm Interpretation, and Splicing

Abstract: By a Markovian bridge we mean a process obtained by conditioning a Markov process X to start in some state x at time 0 and arrive at some state z at time t. Once the definition is made precise, we call this process the (x, t, z)-bridge derived from X. Important examples are provided by Brownian and Bessel bridges, which have been extensively studied and find numerous applications. See for example [PY1,SW,Sa,H,EL,AP,BP].It is part of Markovian folklore that the right way to define bridges in some generality is by a suitable Doob h -transform of the space-time process. This method was used by Getoor and Sharpe [GS4] for excursion bridges, and by Salminen [Sa] for one-dimensional diffusions, but the idea of using h-transforms to construct bridges seems to be much older. Our first object in this paper is to make this definition of bridges precise in a suitable degree of generality, with the aim of dispelling all doubts about the existence of clearly defined bridges for nice Markov processes. This we undertake in Section 2. In Section 3 we establish a conditioning formula involving bridges and continuous additive functionals of the Markov process. This formula can be found in [RY, Ex. (1.16) of Ch. X, p.378] under rather stringent continuity conditions. One of our goals here is to prove the formula in its “natural” setting. We apply the conditioning formula in Section 4 to show how Markovian bridges are involved in a family of Palm distributions associated with continuous additive functionals of the Markov process. This generalizes an approach to bridges suggested in a particular case by Kallenberg [K1], and connects this approach to the more conventional definition of bridges adopted here.

The Molting Gland of Crustaceans: Localization, Activity, and Endocrine Control (a Review)

Abstract: Y-organs are paired cephalic endocrine organs of higher Crustacea (Malacostraca). In lower groups (e.g., Entomostraca), they are absent. They were demonstrated as molting glands by Echalier in 1954. They originate from epidermis, and they may either remain attached to epidermis (e.g., in crayfishes) or become fully independent organs (e.g., in crabs); this is taken as an example of the formation of an endocrine gland from part of the target tissue of neurohormones (Buckmann, 1984). Their anatomical features show very large variations among species; these are extensively discussed here. Their ultrastructural characteristics were recently the subject of two excellent reviews (Birkenbeil, 1990; Spaziani, 1990) and are not fully described here. Y-organs secrete three different ecdysteroids, identified as ecdysone (E), 25-deoxyecdysone (25dE), and 3-dehydroecdysone (3DE). Usually these organs produce either E + 25dE or E + 3DE. The significance of these variations is unclear at the moment. These ecdysteroids are derived from dietary cholesterol, and our knowledge of the biosynthetic pathway is far from complete. Apart from the first step (conversion of cholesterol to 7-dehydrocholesterol) and the three last steps (hydroxylations at positions 25, 22, and 2), the reactions are still unknown, as is also the case in insects. The regulation of Y-organ activity is rather unusual, as it seems to be mainly exerted by an inhibitory neuropeptide secreted from the X-organ-sinus gland complex, the so-called molt-inhibiting hormone or MIH. MIH, recently isolated and sequenced in two species, is a member of a new 7-9 kDa peptide family that appears unique to crustaceans. The inhibitory mechanism of action of MIH on Y-organs has been extensively investigated, but it still remains controversial concerning the transduction mechanism involved, and the regulated steps are unknown. More recently, it has been shown that xanthurenic acid (XA), a derivative of tryptophan isolated from eyestalks, displays also an inhibitory effect on ecdysteroid biosynthesis that would be caused by a direct inhibition of cytochrome P-450 monooxygenase(s). The elucidation of the regulatory mechanisms of Y-organ activity by MIH (and other factors) should contribute to a better understanding of fundamental aspects of the regulation of steroidogenesis in general.

Open Treatment of Acute Calcaneal Fractures

Abstract: Displaced intraarticular fractures of the calcaneus require operative intervention to restore the anatomy of the bone, which in turn is the requirement for recovery of subtalar joint mobility. Surgery through a lateral incision, without opening the sheath of the peroneal tendons, and the use of lag screws and a "Y-plate," must restore not only the respective positions of the "three poles" of the calcaneus but also the respective orientations of the articular surfaces. Stable internal fixation allows early active and passive mobilization of the subtalar and ankle joints. If this is not achieved, the functional outcome will be poor.

Multifractals and extreme rainfall events

Abstract: Based on a multifractal structure hypothesis for temporal rainfall processes, a general formula relating maximum possible point rainfall accumulations is derived as a function of the duration and sample size. This formula appears to be in agreement with empirical observations. Such a result may reconcile some opposite points of view regarding extreme rainfall events, and suggests new ways of exploiting the scaling properties of rain processes.

Anterior inferior cerebellar artery territory infarcts. Mechanisms and clinical features.

Abstract: • Arterial lesions, mechanisms, territory, and clinical features of anterior inferior cerebellar artery (AICA) territory infarcts are only based on necropsy cases. To our knowledge, no large clinical series has been reported. We selected nine consecutive patients with AICA territory infarction confirmed by magnetic resonance imaging and angiography. Atherosclerosis was the only cause and all patients were hypertensive. Patients with pure AICA territory infarcts (n=4) were diabetic and likely had basilar branch occlusion due to basilar artery plaques that extended into the AICA or microatheroma that blocked the AICA origin. These patients had no or had only recently had (1 day) prodromata. Patients with AICA plus infarct (n=5) had basilar artery occlusion at the AICA and reconstitution of the distal basilar artery by collaterals through hemispheric anastomoses from the posterior inferior cerebellar arteries and posterior communicating arteries. All these patients except one had prodromata. In seven of nine patients, cranial nerve involvement indicated a lateral pontine lesion in the territory supplied by the AICA. Only two patients had the complete AICA syndrome, and none of the patients had isolated vertigo. The outcome was good in seven of nine patients. Isolated unilateral AICA infarcts should be regarded as most likely due to small artery atherosclerotic disease in diabetic patients. More widespread infarctions that include the AICA territory are due to basilar artery occlusive disease.

Arching effect model for particle size segregation.

Abstract: Starting from 2D experimental observations and using topological arguments, we search for the set of stable positions when a large disk is raised step by step among a 2D array of small monodisperse disks. We show and confirm experimentally that larger disks should move upward continuously via the arching effect whereas smaller disks may only climb up intermittently. We find the critical diameter ratio for a continuous ascent in 2D and 3D. Our model sheds a new light on the decisive role played by vaults in particle size segregation.

On Decomposition Methods for Tandem Queueing Networks with Blocking

Abstract: Queueing networks with blocking are useful for modeling and analyzing discrete event systems, especially manufacturing systems. Most analysis methods for queueing networks with blocking are approximation methods that involve a decomposition of the network into a set of subsystems. This paper presents some insight into these decomposition methods as well as new results. Attention is mainly restricted to the case of tandem queueing networks with exponential service times and blocking-after-service. This type of blocking is especially encountered in manufacturing systems. The first aim of this paper is to improve the understanding and present a unified view of the decomposition methods. We show that decomposition methods can be classified according to three main approaches. One of these approaches is of special interest because it offers a symmetrical view of the decomposition. The second aim of the paper is to provide properties pertaining to these decomposition methods in the case of exponential characterizations of subsystems. We prove the existence and uniqueness of the solution. Moreover, we prove the convergence of the computational algorithm associated with the symmetrical approach.