Coarse-grained information dominates fine-grained information in judgments of time-to-contact from retinal flow

To investigate the relative importance of fine- and coarse-grained structure in the analysis of retinal flow, subjects made estimates of time-to-contact from random dot kinematograms depicting movement towards a flat, sparsely textured surface. Individual display elements moved smoothly away from each other while expanding smoothly in size. By artificially manipulating the rate at which the individual elements expanded we showed that this cue has only a small effect upon performance. When individual elements were replaced by small clusters of dots, expansion of the clusters had a similarly small effect upon performance. However, estimates of time-to-contact were possible when a single expanding cluster was presented in isolation. We conclude that both types of information are available to the subject but that estimates of time-to-contact are based primarily on coarse-grained changes in the position of image elements and that fine-grained changes in element size or position play only a minor role.