(a) Snapshots showing distribution of membrane potential in the 2D model 400, 500, 600, and 700 ms after delivery of S1 stimulus. S1 S2 interval was 200 ms, and S2 S3 interval 120 ms. The colour scheme shows resting tissue as black, and brighter colours show depolarised tissue. Dotted grey lines show boundary between R1 and R2 regions with different APD restitution, and grey arrows show direction of propagation. (b) Recordings of membrane potential from sites indicated by stars in (a). (c) APD restitution curves for R1 and R2, showing DIS1S2, and the APD produced in R1 and R2. See text for details.