Fig. 1
Action potential in each pixel is characterized by three parameters: phase-0 slope, early phase-4 slope and APD50. Lines 1 and 2 are least-squares fits of a straight line, representing phase-0 and phase-4 slopes, respectively. The horizontal dotted line indicates the x-axis (time-axis). Frame rate was 2.0Â ms in most cases, so that 100 frames corresponded to 200Â ms