Segregation from direction differences in dynamic random-dot stimuli.

Previous research has shown that a field of random dots in which each dot alternates between a slow and a fast speed, can give rise to the percept of two superimposed sheets of moving dots when the alternations are out of phase or asynchronous with each other [Vis. Res. 35 (1995) 1691]. Under those conditions, observers can discriminate changes in the slow speed independent of changes in the fast speed. The present study investigated whether such motion-based segregation could result when dots alternated between two different directions. Three observers viewed a variety of displays containing two directions of motion, one upward and one oblique, with the task of discriminating small trial-to-trial changes in the direction of the upward component. The oblique direction component also changed direction from trial-to-trial. The field of dots either alternated synchronously (all dots moved in the same direction and switched to the other direction simultaneously) or asynchronously. Results showed that when the dots alternated synchronously between the directions, observers' direction discrimination performance was generally poor. However, when dots switched directions asynchronously, direction discrimination was only slightly elevated in comparison to that produced by a field of dots all moving in a single direction. Additional experiments demonstrated that this performance was not due to judging the global direction of the random-dot display. Thus the visual system had to segregate the stimulus into its component directions before integrating to arrive at the motion signal to be discriminated. It is concluded that for displays comprising elements that alternate between different directions, local direction signals can be used by the human visual system to effectively segregate a display so long as both direction signals are present simultaneously.