Perceptual organization in moving patterns.

Is human motion perception based on a local piecemeal analysis of the image or do global effects play an important role? We used metastable apparent motion displays (Fig. 2) to try to answer this question. Two spots were flashed simultaneously on diagonally opposite corners of a square and then switched off and replaced by two spots appearing on the remaining corners. One could either see vertical or horizontal oscillation of the spots and the display was bistable just as a Necker cube is. We found that if several such bistable figures were randomly scattered on the screen (Fig. 3b), and presented simultaneously, then one always saw the same motion-axis in all of them, suggesting the presence of global field-like effects for resolving ambiguity in apparent motion. Surprisingly, the appearance of these displays could not be influenced by voluntary effort unless the speed of alternation was very slow. (Less than 3 frames per second.) It may be that if the events in the module that computes apparent motion are too rapid then it cannot be coupled with the "will" mechanism, which may have a long time constant.