Different localization of motion onset with pointing and relative judgements

When observers are asked to judge the first position of a moving object, displacements of the judged onset in the direction of and in the direction opposite to the motion have been reported. These errors have been referred to as the Fröhlich illusion and the onset repulsion effect, respectively. To resolve the apparent contradiction between these results, a number of experimental parameters were investigated. Displacement in the direction opposite to motion was only observed when observers pointed to the onset of a slowly moving target. At higher velocities, no displacement with pointing was observed. In contrast, relative judgements of motion onset were accurate at slow velocities, but displaced in the direction of motion at fast velocities. Whether the target moved on a linear or circular trajectory did not alter the results. In one experiment, a dissociation between perceptual and memory-based judgements was found. Overall, the experimental task determined whether displacement in the direction of or in the direction opposite to motion occurred.