Nuestros investigadores

Jean Bragard Monier

Líneas de investigación
Computational cardiology, Image analysis, Fluid mechanics
Índice H
14, (WoS, 25/08/2017)
17, (Google Scholar, 25/08/2017)

Publicaciones científicas más recientes (desde 2010)

Autores: Pérez, L. M.; Bragard, Jean René Alfred; Díaz, P . ; et al.
ISSN 0304-8853  Vol. 444  2017  págs. 432 - 438
In magnetic fluids the viscosity can depend on an external magnetic field. We theoretically investigate the influence of this magneto-viscous effect on the thermal convection thresholds for viscoelastic fer-rofluids, which are described by a linear Oldroyd model. Such a system is influenced by a static magnetic field not only via the Kelvin force, but also through the magneto-viscous effect. In particular, we find that these two contributions compete oppositely when the threshold for the oscillatory instability is considered. While the Kelvin force tends to decrease the critical Rayleigh number, the magneto-viscous effect increases it. The critical properties at the onset of the oscillatory instability are discussed as a function of the viscoelastic parameters, the external field strength, and the magneto-viscous coefficient. The transition between the stationary and the oscillatory instability is only slightly affected by the magneto-viscous effect. Examples for codimension-2 lines are given. (C) 2017 Elsevier B.V. All rights reserved.
Autores: Collet, A.; Bragard, Jean René Alfred; Dauby, P. C.;
Revista: CHAOS (EEUU)
ISSN 1054-1500  Vol. 27  Nº 9  2017  págs. 093924
This article characterizes the cardiac autonomous electrical activity induced by the mechanical deformations in the cardiac tissue through the mechano-electric feedback. A simplified and qualitative model is used to describe the system and we also account for temperature effects. The analysis emphasizes a very rich dynamics for the system, with periodic solutions, alternans, chaotic behaviors, etc. The possibility of self-sustained oscillations is analyzed in detail, particularly in terms of the values of important parameters such as the dimension of the system and the importance of the stretch-activated currents. It is also shown that high temperatures notably increase the parameter ranges for which self-sustained oscillations are observed and that several attractors can appear, depending on the location of the initial excitation of the system. Finally, the instability mechanisms by which the periodic solutions are destabilized have been studied by a Floquet analysis, which has revealed period-doubling phenomena and transient intermittencies. Published by AIP Publishing.
Autores: Bragard, Jean René Alfred; Mossay, P.;
ISSN 0960-0779  Vol. 83  2016  págs. 140 - 146
We study a spatial model of social interactions. Though the properties of the spatial equilibrium have been largely discussed in the existing literature, the stability of equilibrium remains an unaddressed issue. Our aim is to fill up this gap by introducing dynamics in the model and by determining the stability of equilibrium. First we derive a variational equation useful for the stability analysis. This allows to study the corresponding eigenvalue problem. While odd modes are shown to be always stable, there is a single even mode of which stability depends on the model parameters. Finally various numerical simulations illustrate our theoretical results.
Autores: Carmona, N. ; Elorza, Jorge; Recasens, J. ; et al.
ISSN 0020-0255  Vol. 310  2015  págs. 36 - 51
Fuzzy operators are an essential tool in many fields and the operation of composition is often needed. In general, composition is not a commutative operation. However, it is very useful to have operators for which the order of composition does not affect the result. In this paper, we analyze when permutability appears. That is, when the order of application of the operators does not change the outcome. We characterize permutability in the case of the composition of fuzzy consequence operators and the dual case of fuzzy interior operators. We prove that for these cases, permutability is completely connected to the preservation of the operator type. We also study the particular case of fuzzy operators induced by fuzzy relations through Zadeh¿s compositional rule and the inf-¿¿ composition. For this cases, we connect permutability of the fuzzy relations (using the sup-¿ composition) with permutability of the induced operators. Special attention is paid to the cases of operators induced by fuzzy preorders and similarities. Finally, we use these results to relate the operator induced by the transitive closure of the composition of two reflexive fuzzy relations with the closure of the operator this composition induces.
Autores: Pérez, L. M. ; Bragard, Jean René Alfred; Mancini, Héctor Luis; et al.
ISSN 1556-1801  Vol. 10  Nº 1 Supl. 1  2015  págs. 209 - 221
We report a systematic investigation of the magnetic anisotropy effects observed in the deterministic spin dynamics of a magnetic particle in the presence of a time-dependent magnetic field. The system is modeled by the Landau-Lifshitz-Gilbert equation and the magnetic field consists of two terms, a constant term and a term involving a harmonic time modulation. We consider a general quadratic anisotropic energy with three different preferential axes. The dynamical behavior of the system is represented in Lyapunov phase diagrams, and by calculating bifurcation diagrams, Poincare sections and Fourier spectra. We find an intricate distribution of shrimp-shaped regular island embedded in wide chaotic phases. Anisotropy effects are found to play a key role in defining the symmetries of regular and chaotic stability phases.
Autores: Barseghyan, M.; Baghramyan, H.; Laroze, D.; et al.
ISSN 1386-9477  Vol. 74  2015  págs. 421 - 425
The effects of a lateral electric field on intraband absorption in GaAs/GaAlAs two-dimensional coupled quantum dot-ring structure with an on-center hydrogenic donor impurity are investigated. The confining potential of the system consists of two parabolas with various confinement energies. The calculations are made using the exact diagonalization technique. A selection rule for intraband transitions was found for x-polarized incident. light. The absorption spectrum mainly exhibits a redshift with the increment of electric field strength. On the other hand, the absorption spectrum can exhibit either a blue- or redshift depending on the values of confinement energies of dot and ring. Additionally, electric field changes the energetic shift direction influenced by the variation of barrier thickness of the structure.
Autores: Bragard, Jean René Alfred; Laroze, D.; et al.
ISSN 1539-3755  Vol. 92  Nº 6  2015  págs. 2919
Defibrillation is the standard clinical treatment used to stop ventricular fibrillation. An electrical device delivers a controlled amount of electrical energy via a pair of electrodes in order to reestablish a normal heart rate. We propose a technique that is a combination of biphasic shocks applied with a four-electrode system rather than the standard two-electrode system. We use a numerical model of a one-dimensional ring of cardiac tissue in order to test and evaluate the benefit of this technique. We compare three different shock protocols, namely a monophasic and two types of biphasic shocks. The results obtained by using a four-electrode system are compared quantitatively with those obtained with the standard two-electrode system. We find that a huge reduction in defibrillation threshold is achieved with the four-electrode system. For the most efficient protocol (asymmetric biphasic), we obtain a reduction in excess of 80% in the energy required for a defibrillation success rate of 90%. The mechanisms of successful defibrillation are also analyzed. This reveals that the advantage of asymmetric biphasic shocks with four electrodes lies in the duration of the cathodal and anodal phase of the shock.
Autores: Elorza, Jorge; Fuentes-González, R.; Bragard, Jean René Alfred; et al.
ISSN 0165-0114  Vol. 218  2013  págs. 73 - 89
In a previous paper, Elorza and Burillo explored the coherence property in fuzzy consequence operators. In this paper we show that fuzzy closing operators of mathematical morphology are always coherent operators. We also show that the coherence property is the key to link the four following families: fuzzy closing morphological operators, fuzzy consequence operators, fuzzy preorders and fuzzy closure and co-closure systems. This will allow to translate important well-known properties from the field of approximate reasoning to the field of image processing.
Autores: Bragard, Jean René Alfred; Elorza, Jorge; et al.
Revista: CHAOS (EEUU)
ISSN 1054-1500  Vol. 23  2013  págs. 043119
In this article, we compare quantitatively the efficiency of three different protocols commonly used in commercial defibrillators. These are based on monophasic and both symmetric and asymmetric biphasic shocks. A numerical one¿dimensional model of cardiac tissue using the bidomain formulation is used in order to test the different protocols. In particular, we performed a total of 4.8 × 106 simulations by varying shock waveform, shock energy, initial conditions, and heterogeneity in internal electrical conductivity. Whenever the shock successfully removed the reentrant dynamics in the tissue, we classified the mechanism. The analysis of the numerical data shows that biphasic shocks are significantly more efficient (by about 25%) than the corresponding monophasic ones. We determine that the increase in efficiency of the biphasic shocks can be explained by the higher proportion of newly excited tissue through the mechanism of direct activation.
Autores: Penaranda, A.; Cantalapiedra, I. R; Bragard, Jean René Alfred; et al.
ISSN 1742-4682  Vol. 9  2012 
This paper analyzes a new semiphysiological ionic model, used recently to study reexitations and reentry in cardiac tissue [I.R. Cantalapiedra et al, PRE 82 011907 (2010)]. The aim of the model is to reproduce action potencial morphologies and restitution curves obtained, either from experimental data, or from more complex electrophysiological models. The model divides all ion currents into four groups according to their function, thus resulting into fast-slow and inward-outward currents. We show that this simplified model is flexible enough as to accurately capture the electrical properties of cardiac myocytes, having the advantage of being less computational demanding than detailed electrophysiological models. Under some conditions, it has been shown to be amenable to mathematical analysis. The model reproduces the action potential (AP) change with stimulation rate observed both experimentally and in realistic models of healthy human and guinea pig myocytes (TNNP and LRd models, respectively). When simulated in a cable it also gives the right dependence of the conduction velocity (CV) with stimulation rate. Besides reproducing correctly these restitution properties, it also gives a good fit for the morphology of the AP, including the notch typical of phase 1. Finally, we perform simulations in a realistic geometric model of the rabbit's ventricles, finding a good qualitative agreement in AP propagation and the ECG. Thus, this simplified model represents an alternative to more complex models when studying instabilities in wave propagation.
Autores: Laroze, D.; Bragard, Jean René Alfred; Suárez, O.J.; et al.
ISSN 0018-9464  Vol. 47  Nº 10  2011  págs. 3032-3035
Autores: Laroze, D.; Pérez, L.M.; Bragard, Jean René Alfred; et al.
Revista: Magnetohydrodynamics
ISSN 0024-998X  Vol. 47  Nº 2  2011  págs. 159 - 165
We report both theoretical and numerical results on convection for a magnetic fluid under rotation in a viscoelastic carrier liquid. The viscoelastic properties are given by the Oldroyd model. We focus on the stationary convection for idealized boundary conditions. Close to the first bifurcation, the coefficients of the corresponding amplitude equation are determined. The effect of the Kelvin force and of the rotation on instability thresholds for a diluted suspension are also emphasized.
Autores: Pérez, L.M.; Bragard, Jean René Alfred; Laroze, D.; et al.
ISSN 0304-8853  Vol. 323  Nº 6  2011  págs. 691 - 698
Autores: Bragard, Jean René Alfred; Pleiner, H.; Suárez, O. J.; et al.
ISSN 1539-3755  Vol. 84  Nº 3  2011  págs. 037202
We study the deterministic spin dynamics of an anisotropic magnetic particle in the presence of a magnetic field with a constant longitudinal and a time-dependent transverse component using the Landau-Lifshitz-Gilbert equation. We characterize the dynamical behavior of the system through calculation of the Lyapunov exponents, Poincare sections, bifurcation diagrams, and Fourier power spectra. In particular we explore the positivity of the largest Lyapunov exponent as a function of the magnitude and frequency of the applied magnetic field and its direction with respect to the main anisotropy axis of the magnetic particle. We find that the system presents multiple transitions between regular and chaotic behaviors. We show that the dynamical phases display a very complicated structure of intricately intermingled chaotic and regular phases.
Autores: Carneiro, J.; Solé, R. V.; et al.
ISSN 1752-0509  Vol. 5  Nº 1  2011  págs. 114
Autores: Cantalapiedra, I. R.; Peñaranda, A.; Echebarria, B.; et al.
ISSN 1539-3755  Vol. 82  Nº 1  2010  págs. 11907
The involvement of the pedunculopontine nucleus (PPN) and the adjacent cuneiform nucleus (CuN), known as the mesencephalic locomotor area, in the pathophysiology of parkinsonian symptoms is receiving increasing attention. Taking into account the role of dopamine (DA) in motor control and its degeneration in Parkinson's disease, this neurotransmitter could induce dysfunction in the PPN and CuN through a direct dopaminergic innervation of these brainstem structures. This study provides the first demonstration that the PPN and CuN are innervated by dopamine transporter-bearing fibres in normal monkeys, which points to a novel dopaminergic system that targets the lower brainstem. Intoxication with MPTP induced a significant loss of dopamine transporter-positive fibres in the PPN and CuN of young (3-5 years old) acutely or chronically intoxicated monkeys compared with control animals. The more severe DA depletion found after chronic intoxication may explain, at least in part, deficits that appear late in the evolution of Parkinson's disease. A drastic loss of DA fibres was also observed in aged acutely intoxicated monkeys (about 30 years old) suggesting that age- and disease-related loss of dopaminergic fibres might be responsible for symptoms, such as gait disorders, that are more severe in elderly parkinsonian patients.
Autores: Hawks, C. ; Elorza, Jorge; Echebarria, B. ; et al.
Libro:  Computing in Cardiology 2015
Vol. 42  2015  págs. 437 - 440
Constant conductances are often assumed when model- ing cardiac tissue. However experimental evidences have shown that gap junctions (GJ) actually connect adjacent cardiac myocytes. These GJ are complex proteins of the connexin family (Cx40; Cx43; Cx45 are the most common in human). These GJ modify the conductances between cardiac cell through a dynamical process. The aim of this study is to develop a bidomain model of the cardiac tis- sue where the dynamics of the connexins is also included. In particular we will compare the differences associated with the use of a monodomain versus bidomain formula- tion in inducing intra-cellular conductance variations. We have found that the monodomain formulation gives con- ductance variations a factor four to five larger with respect to the bidomain formulation.
Autores: Carmona, N.; Elorza, Jorge; Recasens, J.; et al.
2014  págs. 45 - 49
Composition of fuzzy operators often appears and it is natural to ask when the order of compo- sition does not change the result. In previous pa- pers, we characterized permutability in the case of fuzzy consequence operators and fuzzy inte- rior operators. We also showed the connection between the permutability of the fuzzy relations and the permutability of their induced fuzzy op- erators. In this work we present some examples of permutability and non permutability of fuzzy operators and fuzzy relations in order to illustrate these results.
Autores: Cantalapiedra, I. R.; Elorza, Jorge; et al.
Libro:  Computing in Cardiology 2014
Vol. 41  2014  págs. 505 - 508
In the absence of a better solution, ventricular fibrillation is treated by applying one or several large electrical shocks to the patient. The question of how to lower the energy required for a successful shock is still a current issue in both fundamental research and clinical practice. In the study presented here we will compare defibrillation applied through a four electrode device with the standard procedure using two electrodes. The method is tested through intensive numerical simulations. Here we have used a one dimensional geometry. At the level of the cardiac tissue, the bidomain and the modified Beeler-Reuter models were used. Three different shock waveforms are tested: monophasic and two types of biphasic shocks. The results are compared with those obtained with standard two electrode device. A significant reduction in defibrillation thresholds is achieved for all the three tested waveforms when we use a four electrode device.
Autores: Carmona, N.; Elorza, Jorge; Recasens, J.; et al.
Libro:  Advances in artificial intelligence
Vol. 8109  2013  págs. 62 - 69
In this paper we study the permutability of the composition of fuzzy consequence operators (fuzzy closings) and fuzzy interior operators (fuzzy openings). We establish several characterizations and we show the relation of permutability with the fuzzy closure and fuzzy interior of a fuzzy operator. We also study the connection between permutability and the preservation of the operator type through the composition. More precisely, when the composition of two openings is an opening and the composition of two closings is a closing.
Autores: Carmona, N.; Rua-Seoane, J. C.; Elorza, Jorge; et al.
Libro:  Computing in Cardiology Conference 2013
Vol. 40  2013  págs. 1031 - 1034
The aim of this work was to find evidence of aging of ECG indicators in a population of young adults (52 subjects of age 19 ± 0.6 in 2002). For this purpose, an application which analyzes ECG signals was designed and implemented. The data consist in four different series for each subject of the studied population. Two ECG signals of 5 minute duration (one at rest and one after a mild effort) were taken in 2002 and the exact same procedure was repeated for each subject five years later in 2007. All these electrocardiographic signals were examined by computational techniques in order to extract the RR and QT intervals and the energy content of the T-wave. Next, we have compared all the data using first return maps and measured the data dispersion by evaluating the covariance ellipses. Relations between each of the four data series were studied. The analysis also included the gender and the categorical aspect ¿practicing sport¿.
Autores: Carmona, N.; Elorza, Jorge; Recasens, J.; et al.
Libro:  Modeling decisions for artificial intelligence
Vol. 8234  2013  págs. 237 - 247
In this paper we study the permutability of the composition of fuzzy consequence operators when they are induced by fuzzy relations using the usual Zadeh's compositional rule. In particular, we study the case of fuzzy indistinguishability operators and fuzzy preorders. We study the connection between the permutability of the fuzzy relations and the permutability of their induced fuzzy operators.
Autores: Carmona, N.; Elorza, Jorge; Recasens, J.; et al.
Libro:  Aggregation functions in theory and in practise
Vol. 228  2013  págs. 295 - 306
In this paper we generate fuzzy relations and fuzzy operators using different kind of generators and we study the relationship between them. Firstly, we introduce a new fuzzy preorder induced by a fuzzy operator. We generalize this preorder to a fuzzy relation generated by two fuzzy operators and we analyze its properties. Secondly, we introduce and explore two ways of inducing a fuzzy operator, one from a fuzzy operator and a fuzzy relation and the other one from two fuzzy operators. The first one is an extension of the well-known fuzzy operator induced by a fuzzy relation through Zadeh¿s compositional rule. Finally, we aggregate these operators using the quasi-arithmetic mean associated to a continuous Archimedean t-norm. The aim is to compare the operator induced by the quasi-arithmetic mean of the generators with the quasi-arithmetic mean of the generated operators.
Autores: Bragard, Jean René Alfred; Elorza, Jorge; Cherry, E. M.; et al.
Libro:  Computing in Cardiology Conference 2013
Vol. 40  2013  págs. 851 - 854
We develop and validate a three-dimensional model of ventricular fibrillation for use in studying defibrillation. Specifically, we use a rabbit ventricular geometry as a realistic model system for evaluating the efficacy of defibrillatory shocks. Data obtained from the simulations are analyzed in terms of a dose-response curve. In agreement with clinically relevant values, we find that an electric field strength of about 6.6 V/cm indicates a 50% probability of successful defibrillation for a 12-ms monophasic shock in our model. Our validated model will be useful for optimizing defibrillation protocols and for performing detailed studies of defibrillation mechanisms.
Autores: Elorza, Jorge; Fuentes-González, R.; Bragard, Jean René Alfred; et al.
Libro:  Actas del XVI Congreso Español sobre Tecnologías y Lógica Fuzzy
2012  págs. 54-59
Paer we show a comprehensive summary on the relationship between fuzzy morphological operators, fuzzy consequence operators and fuzzy preorders and we motivate the study of morphological operators by using binary relations as structuring elements in a more general context, i.e., the context of fuzzy re- lational systems for obtaining relevant infor- mation.
Autores: Elorza, Jorge; Fuentes-González, R.; Bragard, Jean René Alfred; et al.
Libro:  Actas del XV Congreso Español sobre Tecnologías y Lógica Fuzzy
2010  págs. 133 - 138
In a previous paper [10] we explored the no- tion of coherent fuzzy consequence operator. It is well-known that the operator induced by a fuzzy preorder through Zadeh's compo- sitional rule is always a coherent fuzzy con- sequence operator. It is also known that the relation induced by a fuzzy consequence operator is a fuzzy preorder if such opera- tor is coherent [7]. Fuzzy closing operators of mathematical morphology can be consid- ered as fuzzy consequence operators. In [12] we showed that they are coherent operators. The aim of this paper is to analyze the rela- tions between both classes of operators and the class of all fuzzy preorders in order to translate well know properties from Approxi- mate Reasoning to the one of Image Process- ing.



Cáculo numérico (F.Ciencias). 
Universidad de Navarra - Facultad de Ciencias.

Ecuaciones diferenciales. 
Universidad de Navarra - Facultad de Ciencias.