  When human participants repeatedly encounter a velocity-dependent force field
that distorts their movement trajectories, they adapt their motor behavior to
recover straight trajectories. Computational models suggest that adaptation to
a force field occurs at the action selection level through changes in the
mapping between goals and actions. The quantitative prediction from these
models indicates that early perturbed trajectories before adaptation and late
unperturbed trajectories after adaptation should have opposite curvature, i.e.
one being a mirror image of the other. We tested these predictions in a human
adaptation experiment and we found that the expected mirror organization was
either absent or much weaker than predicted by the models. These results are
incompatible with adaptation occurring at the action selection level but
compatible with adaptation occurring at the goal selection level, as if
adaptation corresponds to aiming toward spatially remapped targets.
