Fig. 1.

Bifurcation diagrams of the canonical model (1) together with ‘frequency–current’ (f–I) plots: (Type I) $c=0.005$; SNIC bifurcation near $I=I_{\mathrm{bif}}=0$ where the frequency approaches zero; (Type II) $c=4$; supercritical singular Andronov–Hopf bifurcation near $I=I_{\mathrm{bif}}=0$; the subsequent canard explosion is clearly visible; note the small frequency band for the relaxation oscillation branch