The detection of stimuli rotating in depth amid linear motion and rotation distractors.

In three experiments, observers watched displays consisting of two or more areas that contained unidirectionally moving pixels. In half of the displays, one area of pixels contained movement that corresponded to the projection of the front surface of a rotating cylinder. The total duration of the displays and the number of stimulus areas per display were varied. The subjects' task was to indicate whether or not a given display contained rotation. When the display time required to reach 75% accuracy was determined, it was found that the number of stimuli per display had no effect; nor did it interact with other variables. One control experiment eliminated "pixel crowding" at the edges of the rotating cylinders, with little effect on the results. Another control experiment found that the ability to discriminate rotating from linear motion declines with distance away from fixation. A fourth experiment showed that under conditions similar to the first three, subjects can make accurate shape discrimination, thereby suggesting that three-dimensional information contributed to the decisions made in the original experiments. On the basis of these results and previous data, it is suggested that in the present experiments structure was recovered from motion by the short-range process, ad that this recovery engages attention to a relatively constant extent, regardless of the number of stimuli contained in a display. Shape discrimination based on structure from motion may required a more effortful form of attention.