  Transformation between dense and sparse spirals is studied numerically based
on a bistable FitzHugh-Nagumo model. It is found that the dense spiral can
transform into two types of sparse spirals via a subcritical bifurcation:
Positive Phase Sparse Spiral (PPSS) and Negative Phase Sparse Spiral (NPSS).
The choice of the two types of sparse spirals after the transformation is
affected remarkably by the boundary effect if a small domain size is applied.
Moreover, the boundary effect gives rise to novel meandering of sparse spiral
with only outward petals.
