Effect of noise on the tuning properties of excitable systems

We analyze the effect of additive dynamical noise on simple phase-locking p atterns in the Fitzhugh-Nagumo (FHN) two-dimensional system in the excitabl e regime. In the absence of noise, the response amplitude for this system d isplays a classical resonance as a function of driving frequency. This tran slates into V-shaped tuning curves, which represent the amplitude threshold for one firing per cycle as a function of forcing frequency. We show that noise opens up these tuning curves at mid-to-low frequencies. We explain th is numerical result analytically using a heuristic form for the firing rate that incorporates the frequency dependence of the subthreshold voltage res ponse. We also present stochastic phase-locking curves in the noise intensi ty-forcing period subspace of parameter space. The relevance of our finding s for the tuning of electroreceptors of weakly electric fish and their enco ding of amplitude modulations of high frequency carriers are briefly discus sed. Our study shows that, in certain contexts, it is essential to take int o account the frequency sensitivity of neural responses and their modificat ion by sources of noise. (C) 2000 Elsevier Science Ltd. All rights reserved .