  Inverse stochastic resonance comprises a nonlinear response of an oscillatory
system to noise where the frequency of noise-perturbed oscillations becomes
minimal at an intermediate noise level. We demonstrate two generic scenarios
for inverse stochastic resonance by considering a paradigmatic model of two
adaptively coupled stochastic active rotators whose local dynamics is close to
a bifurcation threshold. In the first scenario, shown for the two rotators in
the excitable regime, inverse stochastic resonance emerges due to a biased
switching between the oscillatory and the quasi-stationary metastable states
derived from the attractors of the noiseless system. In the second scenario,
illustrated for the rotators in the oscillatory regime, inverse stochastic
resonance arises due to a trapping effect associated with a noise-enhanced
stability of an unstable fixed point. The details of the mechanisms behind the
resonant effect are explained in terms of slow-fast analysis of the
corresponding noiseless systems.
