Sándor Kádár

Noise-supported travelling waves in sub-excitable media

The detection of weak signals of nonlinear dynamical systems in noisy environments may improve with increasing noise, reaching an optimal level before the signal is overwhelmed by the noise. This phenomenon, known as stochastic resonance,, has been characterized in electronic, laser, magnetoelastic, physical and chemical systems. Studies of stochastic resonance and noise effects in biological, and excitable dynamical systems have attracted particular interest, because of the possibility of noise-supported signal transmission in neuronal tissue and other excitable biological media. Here we report the positive influence of noise on wave propagation in a photosensitive Belousov–Zhabotinsky reaction. The chemical medium, which is sub-excitable and unable to support sustained wave propagation, is illuminated with light that is spatially partitioned into an array of cells in which the intensity is randomly varied. Wave propagation is enhanced with increasing noise amplitude, and sustained propagation is achieved at an optimal level. Above this level, only fragmented waves are observed.

Transient turing structures in a gradient-free closed system

Transient, symmetry-breaking, spatial patterns were obtained in a closed, gradient-free, aqueous medium containing chlorine dioxide, iodine, malonic acid, and starch at 4� to 5�C. The conditions under which these Turing-type structures appear can be accurately predicted from a simple mathematical model of the system. The patterns, which consist of spots, stripes, or both spots and stripes, require about 25 minutes to form and remain stationary for 10 to 30 minutes.

Noise sustained waves in subexcitable media: From chemical waves to brain waves

We discuss a novel type of spatiotemporal pattern that can be observed in subexcitable media when coupled to a thermal environment. These patterns have been recently observed in several different types of systems: a subexcitable photosensitive Belousov-Zhabotinsky reaction, hippocampal slices of rat brains, and astrocyte syncytium. In this paper, we introduce the basic concepts of subexcitable media, describe recent experimental observations in chemistry and neurophysiology, and put these observation into context with computer simulations. (c) 1998 American Institute of Physics.

New systems for pattern formation studies

Abstract Recent years have witnessed remarkable developments in the study of spatial pattern formation in reaction-diffusion systems. One of the most notable achievements has been the discovery of Turing patterns in the chlorite-iodide-malonic acid (CIMA) system. We have developed a mechanism for the chemistry of that system and from that mechanism have derived: (a) an understanding of how the Turing patterns arise; (b) a simple two-variable model, amenable to analytic study, that reproduces both the homogeneous behavior of the system and the Turing patterns; and (c) a general approach to the design of new systems that will show Turing patterns. We present experimental results of Turing patterns in the chlorine dioxide-iodine-malonic acid system, which our mechanistic analysis suggests lies at the heart of the CIMA Turing patterns. We also discuss, using the example of traveling waves in the Belousov-Zhabotinsky reaction, another experimental configuration, a sol-gel glass impregnated with key reagents, that shows great promise for the study of pattern formation and wave behavior.

Formation and evolution of scroll waves in photosensitive excitable media.

Experimental and computational studies of the formation and evolution of scroll waves in three-dimensional excitable media are presented. Scroll waves are initiated in the photosensitive Belousov-Zhabotinsky reaction by perturbing traveling waves transverse to their direction of propagation. Scroll rings are generated by perturbing circular waves, which expand or contract depending on the strength of an imposed excitability gradient and its direction relative to the rotational direction of the scroll wave. (c) 1998 American Institute of Physics.