This is a standard psychophysical stimulus used to study the properties of low-level motion detectors. A dense pattern of random black-white dots is displaced coherently in one direction or the opposite (eg. left or right), and the observer must report perceived direction. The use of dense random dots prevents observers identifying and tracking individual features. When the displacement is small (lefthand demonstration), the judgement is easy; when the displacement is large (righthand demonstration), the judgement becomes difficult. Which directions do you perceive in the two patterns? The lefthand pattern is jumping in steps of two picture elements, and the righthand pattern is jumping in steps of twelve picture elements. Braddick (1974) thought that this limited performance reflected the response properties of low-level detectors. For a re-assessment of this idea, see Cavanagh & Mather (1989).
Try blurring the patterns by de-focusing your eyes, or placing a sheet
of tracing paper over the screen. What do you see, and why? See Morgan (1992) for one explanation.