Can mental rotation begin before perception finishes?

We conducted six experiments to determine if mental rotation can begin before perception finishes, as allowed by continuous flow models but not discrete state models of information processing. The results of Experiments 1-3 showed that the effect of shape discriminability on RT was underadditive with the effect of stimulus orientation, suggesting that mental rotation began before shape discrimination had finished and that the two processes overlapped in time. The results of experiments 4-6 indicated that mental rotation can overlap with color discriminations as well. In both sets of experiments, however, the amount of underadditivity tended to be much less than predicted by models allowing interference-free overlap. This suggests that mental rotation can overlap with perceptual analysis, contrary to fully discrete models, but that little rotation is carried out during this overlap due to interference between simultaneous discrimination and rotation processes.