Exploring set size effects in scenes: Identifying the objects of search

Traditional search paradigms utilize simple displays, allowing a precise determination of set size. However, objects in realistic scenes are largely uncountable, and typically visually and semantically complex. Can traditional conceptions of set size be applied to search in realistic scenes? Observers searched quasirealistic scenes for a tank target hidden among tree distractors varying in number and density. Search efficiency improved as trees were added to the display, a reverse set size effect. Eye movement analyses revealed that observers fixated individual trees when the set size was small, and the open regions between trees when the set size was large. Rather than a set size consisting of objectively countable objects, we interpret these data as evidence for a restricted functional set size consisting of idiosyncratically defined objects of search. Observers exploit low-level perceptual grouping processes and high-level semantic scene constraints to dynamically create objects that are appropriate to a given search task.