Superposition catastrophe and form-motion binding.

Numerous studies indicate that perceiving global object motion results from the integration of local component motions across space and time. Less attention has been paid to the issue of motion selection, necessary to avoid spurious associations of component motions belonging to different objects and to solve the so-called "superposition catastrophe problem" (F. Rosenblatt, 1961). We address this issue using outlines of geometrical shapes moving behind apertures that concealed their vertices such that recovering their global motion requires the selection and integration of some, but not all, component motions. Depending on which local motions are selected for motion integration, these stimuli yield the perception of either expansion/contraction, of global translation, or of segments moving independently. We show that the selection process depends on local and global stimulus parameters, including the local direction of figure's line-endings or the spatial configuration of component motions. In contrast, motion selection depends less on the width-i.e., spatial frequency content-or polarity of the edges. Finally, synchronous temporal modulation of component motions in the gamma range has little effect on motion selection. These results indicate that selecting component motions for motion integration is primarily determined by form constraints. As a consequence, current models assuming that mutually consistent component motions are bounded in a velocity space-lacking spatial organization should be revised to account for the present data. Alternately, interactions between visual areas selectively processing form and motion could be introduced in order to account for the perceptual binding of moving objects.