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 th eir 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 fin dings on luminance-defined objects. Two identical rectangles (1.1 x 1.4 deg rees) composed of random dot patterns moved toward each other at a speed of 3.5 degrees /s. In experiment 1 we used backgrounds of a grey field, stati c random dots, or dynamic noise, and examined the effect of introducing a p ause in motion and a visual distractor. In experiment 2 we introduced visua l 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 m otion 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 distra ction 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 perce ption of bouncing does not lie in the modulation of object visibility.