  We investigate, both experimentally and theoretically, the bifurcation to
alternans in heart tissue. Previously, this phenomenon has been modeled either
as a smooth or as border-collision period-doubling bifurcation. Using a new
experimental technique, we find a hybrid behavior: very close to the
bifurcation point the dynamics are smooth-like, whereas further away they are
border-collision-like. This behavior is captured by a new type of model, called
an unfolded border-collision bifurcation.
