Response of the mutual inhibition bistable switch module. X∗(solid line) is shown when the module is subjected to (A,B) gradient signals and (C,D) localized signal inputs. (A,C) When the value of the signal in the entire domain is either above or below the critical value S2 (see text), the response is qualitatively similar to the signal. (B,D) A spatial switching effect is observed when the signal crosses the critical value in some part of the domain. (E) The above switching effects are seen even if some of the elements are weakly diffusible. Furthermore, even if the signal is entirely in the bistable regime, a spatial switching effect can be seen in the profile of X∗(with different initial conditions, see text). (F) A bifurcation diagram for this module is shown. The module has two stable steady states and one unstable steady state (between the two ’LP’ markers) in a signal range (S1 to S2, see text).