  Phase response curves are important for analysis and modeling of oscillatory
dynamics in various applications, particularly in neuroscience. Standard
experimental technique for determining them requires isolation of the system
and application of a specifically designed input. However, isolation is not
always feasible and we are compelled to observe the system in its natural
environment under free-running conditions. To that end we propose an approach
relying only on passive observations of the system and its input. We illustrate
it with simulation results of an oscillator driven by a stochastic force.
