Perceived direction of moving two-dimensional patterns depends on duration, contrast and eccentricity.

Type II two-dimensional motion is produced by superimposing two one-dimensional drifting cosine gratings with velocity vectors lying on the same side of the intersection-of-constraints (IOC) resultant. When type II patterns were constructed with components having the same spatial frequency and contrast, perceived direction was found to be biased toward the vector sum direction at short durations and approached the direction predicted by IOC only after some time lag. This time lag was contrast dependent. At 5% contrast, the perceived direction after 1 sec of presentation remained biased by more than 20 degrees. Direction perception was also measured at 15 degrees eccentricity. At this eccentricity the perceived direction of type II patterns was grossly biased away from the IOC prediction in the direction of the component vectors by an average of 25 degrees.