Opponent motion interactions in the perception of transparent motion

Interactions in the perception of motion transparency were investigated using a signal-detection paradigm. The stimuli were the linear sum of two independent, moving, random-check "signal" textures and a third texture consisting of dynamic random "noise." Performance was measured as the ratio of squared signal and noise contrasts was varied (S2/N2). Motion detectability was poorest when the two signal textures moved in opposite directions (180 degrees), intermediate when they moved in the same direction (0 degrees), and best when the textures moved in directions separated by 90 degrees in the stimulus plane. This pattern of results held across substantial variations in velocity, field size, duration, and texture-element size. Motion identification was also impaired, relative to 0 degrees, in the 180 degrees but not in the 90 degrees condition. These results are consistent with the idea that performance in the opponent-motion condition is limited by inhibitory (or suppressive) interactions. These interactions, however, appear to be direction specific: little, if any, inhibition was observed for perpendicular motion.