  The effect of a temporal modulation at three times the critical frequency on
a Hopf bifurcation is studied in the framework of amplitude equations. We
consider a complex Ginzburg-Landau equation with an extra quadratic term,
resulting from the strong coupling between the external field and the unstable
modes. We show that, by increasing the intensity of the forcing, one passes
from an oscillatory regime to an excitable one with three equivalent frequency
locked states. In the oscillatory regime, topological defects are one-armed
phase spirals, while in the excitable regime they correspond to three-armed
excitable amplitude spirals. Analytical results show that the transition
between these two regimes occurs at a critical value of the forcing intensity.
The transition between phase and amplitude spirals is confirmed by numerical
analysis and it might be observed in periodically forced reaction-diffusion
systems.
