  Excitable media are systems which are at rest in the absence of external
input but which respond to a sufficiently strong stimulus by sending a wave of
"excitation" across the medium. Examples include cardiac and cortical tissue,
and in each of these examples rotating spiral waves have been observed and
associated with abnormalities. How such waves are initiated is not well
understood. In this numerical study of a standard mathematical model of
excitable media, we obtain spirals and other oscillatory patterns by a method,
simple in design, which had previously been ruled out. We analyze the early
stages of this process and show that long term stable oscillatory behavior,
including spiral waves, can start with very simple initial conditions, such as
two small spots of excitation, and no subsequent input. Thus, there are no
refractory cells in the initial condition. For this model, even random spots of
stimulation result in periodic rotating patterns relatively often, leading us
to suggest that this could happen in living tissue.
