Common-fate grouping as feature selection.

The visual system groups elements within the visual field that are physically separated yet similar to each other. Although grouping processes have been intensely studied for a century, the mechanisms of grouping remain elusive. We propose that a primary mechanism for grouping by common fate is attentional selection of a direction of motion. A unique prediction follows from this account: that the visual system must be limited to forming only a single common-fate group at a time, and that attempts to find a particular common-fate group among other groups, or among nongroups, should therefore be highly inefficient. We show that this is true in searches for vertically oriented groups of moving dots among horizontally oriented groups (Experiment 1) and in searches for motion-linked groups among nonlinked objects (Experiment 2). Feature selection may limit the visual system to the construction of only one common-fate group at a time, and thus the experience of simultaneous grouping may be an illusion.