Biomedical subjects

F. T. Arecchi

Publications and source records attributed to F. T. Arecchi.

5 recordsLinked to original sources

PubMed1997

Adaptive strategies for recognition, noise filtering, control, synchronization and targeting of chaos.

Combining knowledge of the local variation rates with some information on the long time trends of a dynamical system, we introduce an adaptive recognition technique consisting in a sequence of variable resolution observation intervals at which the geometrical positions are sampled. The sampling times are chosen so that the sequence of observed points forms a regularized set, in the sense that the separation of adjacent points is almost uniform. We show how this adaptive technique is able to recognize the unstable periodic orbits embedded within a chaotic attractor and stabilize anyone of them even in the presence of noise, through small additive corrections to the dynamics. These techniques have been applied to the synchronization of three chaotic systems, assuring secure communication between a message sender and a message receiver; furthermore they provide robust solutions to the problems of targeting of chaos and of filtering the noise out of an experimental chaotic data set. Implementation of adaptive methods to chaotic Lorenz, three and four dimensional Roessler models and Mackey-Glass delayed system are reported.(c) 1997 American Institute of Physics.

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PubMed1994

Pattern formation and competition in photorefractive oscillators.

We introduce a general model of pattern formation in optical systems made of a cavity with an active medium as a photorefractive crystal fed by a pump. The model is based on the interplay of a diffractive equation for the optical field and a diffusive equation for the medium refractivity. The aim of the model is to describe a series of experiments which have shown mode competition (periodic or chaotic alternation) for low Fresnel numbers (F) and mode coexistence, leading to short range space correlations, for high F. For low F, a linear stability analysis provides the set of modes above threshold as a function of the transverse wave number. Due to the interplay of the optical and the diffusive interactions, different behaviors result depending on the thickness of the medium as compared to the optical absorption length and diffusion length. Including the leading nonlinearities compatible with the symmetry constraints, we introduce normal form equations which describe the time-dependent mode competition. In the case of a large number of modes (high F), nonlinear mode-mode interaction is equivalent to a self-induced noise. In this limit, the relevant feature to be compared with the experiment is the power spectrum.

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PubMed1994

Modeling excitable media by a one variable cellular automaton: Application to the cardiac case.

The dynamics of an assembly of cardiac cells is modeled by a simple cellular automaton that reduces to a single variable the two variable competition of the standard models of excitable media. Furthermore, a short superexcitability period is introduced, as suggested by the dynamics of the single cardiac miocyte. The model reproduces several pathological cardiac behaviors as, e.g., the fast transition from normal behavior to fibrillation, showing how this latter one can either occur over the whole spatial domain or can be confined within a limited region.

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PubMed1993

CO(2) laser with modulated losses: Theoretical models and experiments in the chaotic regime.

We compare two different theoretical models for a CO(2) laser, namely, the two- and four-level model, and show that the second one traces with much better accuracy the experimentally observed chaotic dynamics when the cavity losses are sinusoidally modulated. Even though the two-level model provides a qualitative explanation of the chaotic dynamics, only the four-level one assures a quantitative fitting. We also show that, at the onset of chaos, the chaotic dynamics is low dimensional and can be described in terms of a noninvertible one-dimensional map.

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PubMed1991

Rate processes in nonlinear optical dynamics with many attractors.

Kramers' 1940 paper and its successive elaborations have extensively explored the transition rate between two stable situations, that is, in the language of system dynamics, the transition between the basins of attraction of two stable fixed point attractors. In a nonequilibrium system some of the above conditions may be violated, either because one of the two fixed points is unstable, as in the case of transient phenomena, or because both fixed points are unstable, as in the case of heteroclinic chaos, or because the attractors are more complex than fixed points, as in a chaotic dynamics where two or more strange attractors coexist. Furthermore, there is recent experimental evidence of space-time complexity consisting in the alternate or simultaneous oscillation of many modes, each one with its own (possibly chaotic) dynamics. In all the above cases, coexistence of many alternative paths implies a choice, either due to noise or self-triggered by the same interacting degrees of freedom. A review of the above phenomena in the case of nonequilibrium optical systems is here presented, with the aim of stimulating theoretical investigation on these novel rate processes.

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