Noise induced oscillations in excitable media

A simple model for the local temporal dynamics of such systems is the FitzHugh-Nagumo model (3.59)-(3.60), written here as  

The effect of mixing in a spatially extended excitable medium with additive noise has been studied by Zhou et al. (2003). The 

The frequency of the oscillations increases with the noise amplitude (Fig. 8.6). This is simply a consequence of the shorter waiting time needed for the development of a new excitation center. When the noise level is increased further, after the first coherent excitation the return to the homogeneous steady state does not happen uniformly in space. Due to the local fluctuations the coherence of the system is lost and the mean field is almost constant, while the spatial structure consists of a continuously changing mixture of randomly distributed excited filaments that do not develop a global excitation. When the stirring rate is increased, the three different regimes are shifted towards higher noise intensities. 

Thus the stirred excitable media under random perturbations can produce regular coherent oscillations whose frequency is controlled by the intensity of mixing. Note that this is in contrast with the 

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