Direction repulsion goes global

When viewing two superimposed, translating sets of dots moving in different directions, one overestimates direction difference. This phenomenon of direction repulsion is thought to be driven by inhibitory interactions between directionally tuned motion detectors [1 and 2]. However, there is disagreement on where this occurs - at early stages of motion processing [1 and 3], when local motions are extracted; or at the later, global motion-processing stage following "pooling" of these local measures [4, 5 and 6]. These two stages of motion processing have been identified as occurring in area V1 and the human homolog of macaque MT/V5, respectively [7 and 8]. We designed experiments in which local and global predictions of repulsion are pitted against one another. Our stimuli contained a target set of dots, moving at a uniform speed, superimposed on a "mixed-speed" distractor set. Because the perceived speed of a mixed-speed stimulus is equal to the dots' average speed [9], a global-processing account of direction repulsion predicts that repulsion magnitude induced by a mixed-speed distractor will be indistinguishable from that induced by a single-speed distractor moving at the same mean speed. This is exactly what we found. These results provide compelling evidence that global-motion interactions play a major role in driving direction repulsion.