  We show using simulations that when interstitial vortices are present in
superconductors with periodic pinning arrays, the transport in two
perpendicular directions can be anisotropic. The degree of the anisotropy
varies as a function of field due to the fact that the interstitial vortex
lattice has distinct orderings at different matching fields. The anisotropy is
most pronounced at the matching fields but persists at incommensurate fields,
and it is most prominent for triangular, honeycomb, and kagome pinning arrays.
Square pinning arrays can also show anisotropic transport at certain fields in
spite of the fact that the perpendicular directions of the square pinning array
are identical. We show that the anisotropy results from distinct vortex
dynamical states and that although the critical depinning force may be lower in
one direction, the vortex velocity above depinning may also be lower in the
same direction for ranges of external drives where both directions are
depinned. For honeycomb and kagome pinning arrays, the anisotropy can show
multiple reversals as a function of field. We argue that when the pinning sites
can be multiply occupied such that no interstitial vortices are present, the
anisotropy is strongly reduced or absent.
