We are grateful to Profs. P. Coullet and A. Winfree and Drs. P. Ashwin, D. Barkley and E. Nikolaev for fruitful and encouraging discussions, and to I. Biktasheva for help in processing the results of numerical experiments. Special thanks to Prof. E. Shnol who has read the manuscript and suggested several corrections and improvements. This work was supported in part by grants from The Wellcome Trust (045192), EPSRC ANM (448838) and Russian Fund for Basic Research (96-01-00592).