Reappraising the apparent costs of attending to two separate visual objects.

Support for object-based accounts of visual attention has been drawn from several different types of effect. One effect is found when observers try to restrict their attention to a particular region of a display. Other regions belonging to the same object are often selected as well, suggesting that attention spreads spatially over entire objects. Another effect is found when judging two visual attributes; performance is often less efficient when the attributes belong to separate objects rather than both belonging to a single object. This latter effect has been taken to imply that only one segmented object can be attended at a time. However, it may instead merely be a variant of the first effect. If, as we assume here, attention spreads to task-irrelevant regions of relevant objects, it will encompass a larger spatial region and more information when judging attributes of two objects rather than one. Here we compared judging one versus two objects, while manipulating whether the two objects occupied a wider extent than the single object condition (as in previous work), or not. Costs were found for judging two objects versus one only when together they occupied a wider spatial extent. We conclude that reported difficulties in attending two objects may be due to attention spreading across the entire spatial extent of objects when judging their parts, rather than a fixed inability to process more than object at a time.