As before, pedestrians move from source to sink but now have a choice to make. Because there is no a priori knowledge of the floorplan they must use memory in the form of a decision tree. If one attempt doesn't work another is tried after backtracking. The pedestrians are not following an invisible path but are discovering and avoiding walls while also deciding which route appears more likely to get them to their destination.

Notice, too, that the decision tree is not used in a deterministic way. Without going into the details here, you can see that often pedestrians move to the right but sometimes down. On rare occasions they will even "unexpectedly" back track.