Stochastic resonance in an autonomous system with a nonuniform limit cycle

In a recent numerical study, Gang et al. [Phys. Rev. Lett. 71, 807 (1993)] presented the first example of stochastic resonance in an autonomous system. They considered a two-variable model in which a limit cycle is born as some parameter is varied. Their numerical experiments revealed various noise-induced effects, including a noise-induced shift in the frequency of the limit-cycle oscillations, and noise-induced oscillations in the absence of a deterministic limit cycle. We show that both of these effects are simple consequences of the nonuniformity of the motion along the limit cycle.