Object-based attention with endogenous cuing and positional certainty.

Previous studies have concluded that object-based attention does not always arise if attention is cued endogenously (Macquistan, 1997) or if the target location is known with certainty (Shomstein & Yantis, 2002). In the Experiments reported here, we found object-based attention even when the locations of the two targets were known with certainty due to presentation of an endogenous cue. However, object-based attention can be prevented by limiting the exposure time of the object stimuli. These findings provide additional evidence against a search prioritization account of object-based attention. They lead to a fuller view of the role of object boundaries in directing attention. Object-based attention is frequently used spontaneously, even when the object boundaries are irrelevant to the task. However, object segmentation is not necessary for all tasks, because attention can be allocated before the stimulus has been segregated into objects.