Simultaneous recording of action potentials at variable depth. (A) Stained cell placed at a 20° angle with the scanning line (broken line). The extent of the line comprised inside the cell is marked in A with a horizontal segment. The cell was paced at 50 Hz with field pulses of 0.2 ms. (B) Normalized SEER ratio image R(x,t). (C) Average R(t) over segments of the scanning line marked by color-coded brackets in B. In red trace is the ratio record in the central region (green), scaled to match peaks with the record from the surface (black). Note the lag between the nadirs of the two signals, interpreted in the text as reflecting a propagation speed of 30.1 m/s. ID: 082112, average of series 03–09.