  In this paper, we introduce and systematically study, in terms of phase
response curves (PRC), the effect of a dual pulse excitation on the dynamics of
an autonomous oscillator. Specifically, we test the deviations from a linear
summation of phase advances from two small perturbations. We derive
analytically a correction term, which generally appears for oscillators whose
intrinsic dimensionality is greater than one. We demonstrate this effect in
case of the Stuart-Landau model, and also in various higher dimensional
neuronal model.
  The non-linear correction is found to be proportional to the square of the
perturbation. This deviation from the superposition principle needs to be taken
into account in studies of networks of pulse-coupled oscillators. Further, this
deviation could be used for verification of oscillator models via a dual pulse
excitation.
