  We study, both theoretically and experimentally, the occurrence of
topological defects in polariton superfluids in the optical parametric
oscillator (OPO) regime. We explain in terms of local supercurrents the
deterministic behaviour of both onset and dynamics of spontaneous
vortex-antivortex pairs generated by perturbing the system with a pulsed probe.
Using a generalised Gross-Pitaevskii equation, including photonic disorder,
pumping and decay, we elucidate the reason why topological defects form in
couples and can be detected by direct visualizations in multi-shot OPO
experiments.
