Fig. 3.

Neural model (1): nullclines under variation of I which leads to the (singular limit) definition of $I_{\mathrm{bif}}$: (Type I) $I=I_{\mathrm{bif}}$ at a saddle-node bifurcation; (Type II) $I=I_{\mathrm{bif}}$ at (singular) Andronov–Hopf bifurcation. In the transition from type II to type I, a cusp bifurcation appears as can be predicted by looking from the bottom pictures in this figure