This is similar to the case of the FHN model on a ring, however this model has a restitution with slope greater than one. Therefore, at high frequencies (as shown in the monophasic restitution applet) there will be APD oscillations. Click the Start button and see a pulse which oscillates. By increasing the ring size, the period is longer and the oscillations will disappear. By decreasing the ring size the oscillations will start again until conduction block is reached for a ring size of approximately 190 grid points.
For a description of the model see for example: PRL Vol 81, p481 July 1998 by Fenton and Karma. and Chaos Vol8, p20 1998.