Visual apparent motion in three-dimensional space

When two spatially separated stimuli are presented in succession, a strong impression of motion may be perceived. This phenomenon has been termed apparent motion. In cases in which more than one stimulus is presented in each frame of an apparent motion sequence, the correspondence problem arises. Correspondence between any two stimuli from consecutive frames results from the interpretation that these two stimuli represent one object. One cue for correspondence is proximity, whereby correspondence matches over shorter distances are preferred. Previous research suggests that this distance should be defined in three-dimensional space, implying that correspondence matching is a relatively high-level process. The current research, however, shows that only in relatively few situations an effect on the correspondence process resulting from a higher-level inference of the visual scene (specifically, depth) can be found. Moreover, this effect is relatively minor when compared to the effect of retinal disparities alone. By using cyclopean stimuli, it was found that, even when the visual system is forced to extract depth information from the display before it assigns correspondence matches, these matches are still based on retinal distances. The results as a whole suggest strongly that correspondence matching in visual apparent motion is a low-level process acting on the distances between motion tokens in the retinal projection, with little (if any) regard for the object-centered three-dimensional distances between motion tokens.