Stochastic bifurcations in the nonlinear parallel Ising model.

We investigate the phase transitions of a nonlinear, parallel version of the Ising model, characterized by an antiferromagnetic linear coupling and ferromagnetic nonlinear one. This model arises in problems of opinion formation. The mean-field approximation shows chaotic oscillations, by changing the couplings or the connectivity.

Bifurcations in models of a society of reasonable contrarians and conformists.

We study models of a society composed of a mixture of conformist and reasonable contrarian agents that at any instant hold one of two opinions. Conformists tend to agree with the average opinion of their neighbors and reasonable contrarians tend to disagree, but revert to a conformist behavior in the presence of an overwhelming majority, in line with psychological experiments. The model is studied in the mean-field approximation and on small-world and scale-free networks.

Topological bifurcations in a model society of reasonable contrarians.

People are often divided into conformists and contrarians, the former tending to align to the majority opinion in their neighborhood and the latter tending to disagree with that majority. In practice, however, the contrarian tendency is rarely followed when there is an overwhelming majority with a given opinion, which denotes a social norm. Such reasonable contrarian behavior is often considered a mark of independent thought and can be a useful strategy in financial markets.

Control of cellular automata.

We study the problem of master-slave synchronization and control of totalistic cellular automata. The synchronization mechanism is that of setting a fraction of sites of the slave system equal to those of the master one (pinching synchronization). The synchronization observable is the distance between the two configurations.

Acoustic gaps in a chain of magnetic spheres.

Acoustic gaps are normally observed in granular inhomogeneous structures made of composite materials. The modulation of the elastic properties in such media creates the coherent effects of scattering and interference that ultimately lead to frequency intervals where sound propagation is forbidden. Contrastingly, we report here an experimental observation of acoustic gaps in homogeneous media; specifically, in granular chains.

Thermodynamic entropy and chaos in a discrete hydrodynamical system.

We show that the thermodynamic entropy density is proportional to the largest Lyapunov exponent (LLE) of a discrete hydrodynamical system, a deterministic two-dimensional lattice gas automaton. The definition of the LLE for cellular automata is based on the concept of Boolean derivatives and is formally equivalent to that of continuous dynamical systems. This relation is justified using a Markovian model.

Cultural standards, power and subversion in cross-cultural psychotherapy.

In a creolizing world, psychotherapy is challenged by the growing influence of cultural standards, systems of meaning and idioms of distress supported by new groups who play a major role in public health policies. While these elements were originally introduced in psychotherapy with ethnic minorities both to improve the patient-doctor relationship and to bring relevant material from patients' cultural background, they also became a way to empower nonwestern patients in western health care systems. When in the 1980s it became possible to readdress the issue of power in psychiatric practice, something had already changed in the social regulation that psychiatry should endorse.

Synchronization universality classes and stability of smooth coupled map lattices.

We study two problems related to spatially extended systems: the dynamical stability and the universality classes of the replica synchronization transition. We use a simple model of one-dimensional coupled map lattices and show that chaotic behavior implies that the synchronization transition belongs to the multiplicative noise universality class, while stable chaos implies that the synchronization transition belongs to the directed percolation universality class.

The Survivor's Paradox: Psychological Consequences of the Khmer Rouge Rhetoric of Extermination.

From 1975 to 1979 in Cambodia, during the four years of the Pol Pot regime, the rhetoric of extermination that perpetrators used to legitimize mass murder was a powerful instrument to deprive individuals of their humankind before killing them. Because every human community is founded on a metaphor claiming the inalterability of the social bond beyond the dead, the genocide project aims for its obliteration. By breaking the possible representation of a continuity and a social permanence between the dead and the living, the rhetoric of extermination claims, in a terrifying reality, that the only possible connection between the dead and the living concerns an identical physical condition: the living have already died or will pass away.

An anthropological hybrid: the pragmatic arrangement of universalism and culturalism in French mental health.

As in most European countries, the mental health of immigrants in France has recently been the subject of scientific scrutiny. Since the end of World War II voluntary special mental health services for migrants and refugees have been created in France and especially in Paris, but none has been based on epidemiological data. Generally, this lack of objective data gave rise to the assumption that many immigrants might not be getting the type of services they required.

Phase transitions of extended-range probabilistic cellular automata with two absorbing states.

We study phase transitions in a long-range one-dimensional cellular automaton with two symmetric absorbing states. It includes and extends several other models, like the Ising and Domany-Kinzel ones. It is characterized by competing ferromagnetic linear and antiferromagnetic nonlinear couplings.

Complex dynamics in simple systems with periodic parameter oscillations.

We study systems with periodically oscillating parameters that can give way to complex periodic or nonperiodic orbits. Performing the long time limit, we can define ergodic averages such as Lyapunov exponents, where a negative maximal Lyapunov exponent corresponds to a stable periodic orbit. By this, extremely complicated periodic orbits composed of contracting and expanding phases appear in a natural way.

The rebirth of PTSD: the rise of a new paradigm in psychiatry.

The new conception of psychological trauma that arose in the 1980s with the definition of PTSD in the DSM-III was a major change compared to the previous traumatic neurosis. While the clinical features were in some way similar, the political and sociological meanings of trauma were absolutely different. At that time, the invention of PTSD was much more the consequence of a broad mutation in mentality that introduced a new moral perspective in trauma studies than of a scientific discovery.

Sensitive dependence on initial conditions for cellular automata.

The property of sensitive dependence on intial conditions is the basis of a rigorous mathematical construction of local maximum Lyapunov exponents for cellular automata. The maximum Lyapunov exponent is given by the fastest average velocity of either the left or right propagating damage fronts. Deviations from the long term behavior of the finite time Lyapunov exponents due to generation of information are quantified and could be used for the characterization of the space time complexity of cellular automata.

Noise-induced fluctuations of period lengths of stable periodic orbits.

We discuss a class of one-dimensional maps, which possesses a globally attracting stable periodic orbit. Despite a strongly negative Lyapunov exponent, a small amount of noise can introduce fluctuations of the period length. It is shown that this is a reasonable model for the observed dynamics of a bubble formation experiment in a heated capillary embedded in boiling water.

Transport properties of the diluted Lorentz slab.

We study the behavior of a point particle incident on a slab of a randomly diluted triangular array of circular scatterers. Various scattering properties, such as the reflection and transmission probabilities and the scattering time are studied as a function of thickness and dilution. We show that a diffusion model satisfactorily describes the mentioned scattering properties.

Nature of phase transitions in a probabilistic cellular automaton with two absorbing states.

We present a probabilistic cellular automaton with two absorbing states, which can be considered a natural extension of the Domany-Kinzel model. Despite its simplicity, it shows a very rich phase diagram, with two second-order and one first-order transition lines that meet at a bicritical point. We study the phase transitions and the critical behavior of the model using mean field approximations, direct numerical simulations and field theory.

[Transcultural approach to non-francophone patients originating from Southeast Asia in the psychiatric service network of the XIIIth district of Paris].

Following the political upheaval that shook the Indochinese peninsula in the middle of the 1970s, many refugees originating from Southeast Asia re-established themselves in France. Since 1976, many of these refugees have formed a large Asian community in the XIIIth arrondissement of Paris. As a result, the arrondissement's psychiatric services, which cover the entire Asian neighborhood, have been confronted with the arrival of increasing numbers of non francophone patients from this ethnic minority.