The human visual system breaks an incoming image into separate elements, but also reassembles these elements into groups. Here, we examine the principle of common fate, according to which objects appear grouped when they display the same pattern of motion.
It seems like we can process several common fate groups at once. However, each set of moving dots in this example could also be grouped by their proximity, instead of by their motion.
When proximity cues cannot be used to form groups, it becomes apparent that our ability to group objects based on their motion is in fact quite limited! Here, it is difficult to report properties of more than one group at a time.
We argue that this surprising deficit reveals the mechanism by which common-fate grouping occurs: The primary mechanism for grouping by common fate is attentional selection of a direction of motion. This account makes a unique prediction, that the visual system must be limited to forming only a single common-fate group at a time. Attempts to find a particular common-fate group among other groups (or among nongroups) should be highly inefficient.
We have tested this prediction using two paradigms. First, participants searched for a vertically oriented group among horizontally oriented groups. Searches were inefficient: The vertically oriented group does not "pop out." This result suggests that the visual system may be limited to constructing only one common-fate group at a time.
Can observers efficiently determine the presence or absence of a common fate group? Here, participants searched for a group among non-groups. Again, searches were inefficient. Together, these two experiments support the conclusion that selection limits the ability of the visual system to form common-fate groups.