Aleksic found that the hardest primitive element to make was the wire. To tackle this problem, he considered the elementary cellular automaton as an excitable medium, the cells with value 1 being excited cells. The evolution of the automaton could be seen as the propagation of the excitation. He found two sets of interesting rules. The elements of the set P are the rules for which the excitation disappears in time.
Rules belonging to the set A can propagate excitation in all directions with a fixed speed.
He also found that the propagation of excitation can be simply controlled in the class of automata where the rule for an inert medium belongs to the set P, and the rule for the active medium belongs to the set A. For this class, the excitation is sustained or propagated only in the areas where the value of the environment is 1. A pattern of 1's in the environment guides the propagation of excitation, and this is the phenomenon that can be used for making wires. Excitation that propagates under control can carry signals, and the propagation of excitation and how two signals interact can be controlled by a particular structure of the environment.