This demonstrates a standard psychophysical stimulus used to study the properties of low-level motion processes. A dense pattern of random black-and-white dots is displaced coherently in one direction, and the observer must report perceived direction. The use of dense random dot patterns prevents observers from identifying and tracking individual features in order to infer direction. When the displacement is small (lefthand array of dots), the judgement is easy; when the displacement is large (righthand array), 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. The upper spatial limit for detecting motion (Dmax) was initially thought to be fixed, but it is now accepted to vary with stimulus conditions. The value of Dmax is jointly determined by the statistical properties of the pattern (potential false matches at a given displacement) and the receptive field properties of motion-detecting cells in the visual system.
Try blurring the patterns by de-focusing your eyes, or placing a sheet of tracing paper over the screen. What do you see? Is the change in perceived motion a statistical effect or a 'neural' effect?
See:
Braddick, O. J. (1974) A short-range process in apparent motion. Vision Research, 14, 519-527.
Cavanagh, P., & Mather, G. (1989) Motion: the long and short of it. Spatial Vision, 4, 103-129. PDF
Mather, G., & Tunley, H. (1995) Temporal filtering enhances direction discrimination in random dot patterns. Vision Research, 35, 2105-2116. PDF
Morgan, M. J. (1992) Spatial filtering precedes motion detection. Nature, 355, 344-346.