K. Agladze, J. P. Keener, S. C. Muller, and A. V. Panfilov, Geometry creates spirals, Science 264,1746-1748 (1994).

Abstract:

We use experiments with Belousov-Zhabotinsky (BZ) reagent and numerical simulations to show that under high frequency stimulus, rotating spiral waves can be initiated in a homogeneous excitable medium in the vicinity of domain boundaries or inexcitable barriers with sharp corners.