dmax for stereopsis and motion in random dot displays

The upper displacement limit for motion was compared with the upper disparity limit for stereopsis using two-frame random dot kinematograms or briefly presented stereograms. dmax (the disparity/displacement at which subjects make 20% errors in a forced-choice paradigm) was found to be very similar for motion and stereo at all dot densities, and to fall with increasing dot density (0.006% or two dots to 50%) according to a power law (exponent -0.2). If dmax is limited by the spacing of false targets, this pattern of results suggests that the spatial primitives in the input to the correspondence process may be derived from multiple spatial scales. A model using MIRAGE centroids provides a good fit to the data.