Displacement thresholds for coherent apparent motion in random dot-patterns

Two correlated random-dot patterns (A and B) were generated on a CRT screen and presented in rapid alternation; (B) was shifted horizontally by varying amounts in relation to (A) so that coherent apparent motion was seen. We found that larger shifts were tolerated if (i) the stimulus onset asynchrony (SOA) was longer; (ii) if the patterns were optically blurred; and (iii) if there were fewer dots on the screen. Hence apparent motion in random-dot patterns may involve a global pattern matching operation as in stereopsis. Two uncorrelated random-dot patterns were alternated to produce incoherent dynamic "noise". A low spatial frequency sine wave grating was then projected on this "noise" and moved in step with the alternating random-dot patterns. This resulted in "motion-capture"--i.e. all the dots now seemed to move synchronously with the moving grating. The effect could not be obtained with high spatial frequency gratings or with stationary dots. As a tentative solution to the "correspondence problem" it is suggested that low spatial frequencies are matched first and these matches impose constraints on subsequent high frequency matches--thus allowing the system to home in on a unique solution.