  Rotating spiral waves are a form of self-organization observed in spatially
extended systems of physical, chemical, and biological nature. In the presence
of a small perturbation, the spiral wave's centre of rotation and fiducial
phase may change over time, i.e. the spiral wave drifts. In linear
approximation, the velocity of the drift is proportional to the convolution of
the perturbation with the spiral's Response Functions (RFs), which are the
eigenfunctions of the adjoint linearized operator corresponding to the critical
eigenvalues $\lambda = 0, \pm i\omega$. Here we demonstrate that the response
functions give quantitatively accurate prediction of the drift velocities due
to a variety of perturbations: a time dependent, periodic perturbation
(inducing resonant drift); a rotational symmetry breaking perturbation
(inducing electrophoretic drift); and a translational symmetry breaking
perturbation (inhomogeneity induced drift) including drift due to a gradient,
step-wise and localised inhomogeneity. We predict the drift velocities using
the response functions in FitzHugh-Nagumo (FHN) and Barkley models, and compare
them with the velocities obtained in direct numerical simulations. In all cases
good quantitative agreement is demonstrated.
