The Bookend Effect

The principle of “common fate” tells us that objects that move together, group together. But what happens in dynamic illusions where physical paths are shared, but illusory paths differ? Here a horizontal row of targets physically moves up and down as a group over a zigzag set of vertical columns each filled with a static, oblique grating (the furrow illusion). Each target moves in its own column and when the columns all have the same orientation, the targets drift obliquely together, following the furrows of the static gratings. However, if the background orientations for the outer two “bookend” targets (say, right tilt) differ from those of the inner “book” targets (say, left tilt), all items still appear to drift initially to the right as a group, following the illusory direction set by the bookends and ignoring the opposing illusory motion of the inner books. The effect does not rely on blocking access to the features of the inner books – as might be expected from crowding – as they can have easily discriminable shape and/or colour, and their relative motion within the frame of the bookends may still be recoverable. What is missing is any sense of their opposing illusory path. We suggest that the spatial proximity of the items together with their common vertical motion provides the visual system with enough evidence to form a single “grouped” object representation. As objects tend not to move in two directions at once, the more salient bookends dominate our perception of where the group is going.