In this display, a grating jumps back and forth between two positions separated by a quarter of its spatial period. At each position, a different field of random black dots is superimposed on the grating. Although the two sets of dots are completely uncorrelated, they are 'captured' by the apparent motion of the grating, and appear to move along with it.
This stimulus was first described by Ramachandran and Cavanagh (1987). Recent models of motion integration (eg. Kim and Wilson, 1993) attribute the capture effect to co-operative interactions between the unambiguous signals generated by the grating and the ambiguous signals from the uncorrelated dots. Random pairings of dots as the two fields alternate create incoherent motion signals in all directions. The lateral signals from the grating may serve to 'boost' the random lateral signals from the dots, at the expense of all the other random signals.