Rigid 3-D percept from stochastic 1-D motion

We have earlier reported on a percept of structure-from-motion resulting from a random-dot cinematogram (RDC) in which each dot took an independent 2-D random walk. 1 Although such a stimulus is stochastic, we observed an apparently rigid organization of the dots into a rotating 3-D volume. We now provide a somewhat more dramatic example of structure-from-motion using a RDC in which each dot takes an independent 1-D random walk of variable step size. Instead of a distribution of directions as in the previous display, there is now a distribution of dot velocities along a single direction. Again, we obtain a percept of a rigidly rotating 3-D volume. Our results are difficult to reconcile with rigidity-based models of structure-from-motion. However, they can be interpreted in the context of cooperative interactions among velocity-selective mechanisms.