The bookend effect

The principle of "common fate" tells us that objects that move together, group together. But what happens when physical paths are shared and illusory paths differ? Here, horizontally aligned targets move up and down over columns containing static, oblique gratings. If all columns share the same orientation, the target group appears to drift in that direction, as per the furrow illusion. However, when the two outer "bookend" targets have a different background orientation from the inner "book" targets, group motion is entirely captured by the bookends. This occurs despite the inner books having discriminable features and recoverable relative motion. What is missing is any sense of their opposing illusory path. We suggest that spatial proximity and common vertical motion help create a single grouped object representation. Since rigid objects tend not to move in two directions at once, the salient bookends dominate our perception of where the group is going.