We carried out experiments to study the effect of local disturbances on
the vortex dynamics in the light-sensitive Belousov - Zhabotinsky (BZ)
reaction. It was found that irradiation with visible light of the core
region of a rotating spiral wave (vortex) in an oscillatory medium
results in a bifurcation which dramatically changes the shape of the
vortex. The bifurcation is seen as an unfolding increase in the vortex
wavelength although only a negligible change of the vortex core size is
observed.  Similar dynamics is expected to occur in oscillatory media of
different nature.