Streaming and bouncing: Observations on motion defined objects

When two identical objects move in opposite directions on the same path and at the same speed, they can appear, after crossing over, to continue in their original directions (streaming), or to reverse direction (bouncing). In order to be able to manipulate visibility by adding noise, we used objects defined by contrast, flicker, or motion, and thereby extended previous findings on luminance-defined objects. Two identical rectangles (1.1 x 1.4°) composed of random dot patterns moved toward each other at a speed of 3.5°/s. In experiment 1 we used backgrounds of a grey field, static random dots, or dynamic noise, and examined the effect of introducing a pause in motion and a visual distractor. In experiment 2 we introduced visual noise at four levels. For all three types of motion display, we found an increase in the proportion of the bouncing percept when either a pause in motion or an attentional distractor was introduced. Experiment 2 showed that neither of these effects depends on the visibility of the moving objects. An increase in the bouncing percept, due to a pause in motion or the distraction of attention, can be observed for all types of object definition, and is not affected by decreasing the visibility of the motion-defined objects. This finding suggests that the role of attention in determining the perception of bouncing does not lie in the modulation of object visibility.