Spiral competition in three-component excitable media

Excitable wave patterns consisting of large coherent spirals are observed during the aggregation of Dictyostelium amoebae. These emerge from an initially disordered state which arises via random cell firing. In this work, we show that this phenomenon can be understood as being due to a specific spiral competition instability which occurs in certain three-component excitable medium models. This instability can be understood as symmetry breaking of a spiral pair leading to one spiral suppressing and expelling another.