Coherent global motion percepts from stochastic local motions (abstract only)

Abstract

A percept of global, coherent motion results when many different localized motion vectors are combined. We studied the percept with dynamic random dot kinematograms in which each element took an independent, random walk of constant step size. Directions of displacement from frame to frame were chosen from a uniform distribution. The tendency to see coherent, global flow along the mean of the uniform distribution varied with the range of the distribution.Psychometric functions were obtained with kinematograms having various step sizes (0.1 to 1.4 degrees) and element densities (0.2 to 1.6 dots per square degree). Results fall into two categories, depending on whether the step size is larger or smaller than 1.0 degree. For step sizes greater than 1.0 degree changes in dot density altered the psychometric function. No change was found if the step size was less than one degree. These changes in the psychometric function with step size and density are consistent with Ullman's "minimal map theory" of motion correspondence.For the smaller step sizes, the constancy of the results over a large range of dot densities suggests that spurious directions of displacement due to the interference of random walks for different dots are not important. That is, only the directions of local motion determined by the predefined distribution of directions significantly contribute to the percept. We also found that although temporal summation occurred in a nonlinear manner over frames, it depended only on the set of directions present from frame to frame, not on the spatial relationships between local motion vectors over time. Taken together, these two results suggest that directions of the individual steps are independently detected and that these responses are pooled over time and space to generate the perception of coherent motion.