Intraattribute and interattribute motion induction.

The phenomenon of motion induction occurs, for example, when a bar that is presented next to a spot, which itself was presented slightly earlier, is not correctly perceived to appear everywhere simultaneously, but seems to grow out of the spot. The spot is said to prime one end of the bar. Experiments have been designed to throw more light on the local and global aspects of this phenomenon, in particular to establish whether this illusory motion percept can be observed when the spot and the bar stimuli are defined with respect to the background by one of a variety of attributes, such as luminance, color, stereodepth (crossed and uncrossed), texture, and motion (start and stop). It was found that all attribute combinations supported motion induction readily, but that the strength of the perceived motion (as measured by magnitude estimation) varied and depended more on the attribute defining the bar than on the attribute of the spot. Luminance and color gave the most vivid effects, whereas motion and depth showed the least vivid effects. The influence of the amount of luminance and color contrast on the strength of the effect was also determined and it was found that these variables affected motion induction most at very low contrast levels close to detection threshold. It is concluded that the illusory motion in this effect depends only slightly on the particular visual attribute channel that carries the stimulus information. This is consistent with the contention that it is a high-level, attention-related effect, phenomenologically similar to polarized gamma movement.