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