A method for the real-time rendering of formless dot field structure-from-motion stimuli

The perception of visual motion relies on different computations and different neural substrates than the perception of static form. It is therefore useful to have psychophysical stimuli that carry mostly or entirely motion information, conveying little or nothing about form in any single frame. Structure-from-motion stimuli can sometimes achieve this dissociation, with some examples in studies of biological motion using point-light walkers. It is, however, generally not trivial to provide motion information without also providing static form information. The problem becomes more computationally difficult when the structures and the motions in question are complex. Here we present a technique by which an animated three-dimensional scene can be rendered in real-time as a pattern of dots. Each dot follows the trajectory of the underlying object in the animation, but each static frame of the animation appears to be a uniform random field of dots. The resulting stimuli capture motion vectors across arbitrary complex scenes, while providing virtually no instantaneous information about the structure of that scene. We also present the results of a psychophysical experiment demonstrating the efficacy and the limitations of the technique. The ability to create such stimuli on the fly allows for interactive adjustment and control of the stimuli, real-time parametric variations of structure and motion, and the creation of large libraries of actions without the need to prerender a prohibitive number of movies. This technique provides a powerful tool for the dissociation of complex motion from static form.