Figure 3.

(a) Space–time plot of two colliding wave trains. See Movie 3, which is published as supporting information on the PNAS web site. (b) Two colliding waves appear to pass through one another. Those cells in the incoming wave that are in their refractory period continue to pass through, whereas those in their sensitive period may reverse on collision with counter-moving cells. Therefore, the outgoing waves after a collision consist of individuals from both incoming waves. (c) Plot of the directional densities, n₊ (solid) and n₋ (dashed), in counterpropagating wave trains moving at velocities v and −v, respectively.