FIGURE 4:

Diffusion can push the system across a threshold. (A, B) Diffusive spreading of a high-u state in the absence of reaction for two different diffusion coefficients. The values of u as a function of position and time are depicted both as curves and in a simplified red-green-blue heat map representation. Increasing the diffusion coefficient allows some regions within the low-u region to attain moderately high (green) levels of u more quickly, but for a shorter duration. (C) Phase plot for one point in space that is initially in the low-u region. Diffusion moves the value of u across the threshold into the green region, but in the absence of reaction, eventually returns it to the blue region. If the FHN reactions are fast enough, they can capture the suprathreshold trajectory and convert this point in space to a high-u (red) state. (D, E) Diffusion plus reaction allows self-regenerating trigger waves to propagate outward (D) unless the diffusion coefficient is too high (E).