Hierarchical Motion Organization in Random-Dot Configurations

Motion fields differ in the degree to which they can serve as hierarchical frames of reference. To assess the strength of hierarchical motion organisation, we measured the ability of human observers to extract the motion of a target dot relative to a background of moving dots. The task was to discriminate circular from elliptical relative motions for the target dot (2AFC). Observers performed above chance for translational and divergence backgrounds, but were at chance level for rotational backgrounds. This finding, however, was based on random configurations of dots, where locations and trajectories were unrelated. When spatial constraints were introduced, observers used a different strategy and performed above chance also for rotational backgrounds. Both the effect of motion transformations and the effect of spatial information can be understood if we assume that the task is to find the most likely mechanical description of an object in a three-dimensional environment.