Splitting attention over multiple objects

Abstract

We often need to deal with multiple objects at once, when we monitor for changes in a set of objects, compare the features or locations of multiple objects, or store the appearances of objects in memory. In some tasks, juggling multiple objects might require sequential processing, while in others it may be possible to draw information from multiple objects simultaneously. In a series of experiments using both static and moving objects, we explore the underlying mechanism and limits of selecting multiple objects. First, we show that simultaneous selection can occur. Participants were asked to mentally mark a set of locations in an array that would contain a search target, and task accuracy suggested that they could search exclusively through those locations in a later display. But when the search task was made more difficult by making targets more featurally similar to distractors, fewer locations could be marked. This result suggests that marking locations entails encoding information from those locations (Awh & Jonides, 2001), and that tougher searches require selecting fewer locations, which cannot be recovered. Second, we show that multiple locations are not encoded as a constellation that relies on shape memory. Participants searched through several marked locations, and performance was not impaired by adding a shape memory dual task. A control task showed that adding an identical dual task to a single shape memory task did impair performance, suggesting that marking does not rely on shape memory. A third set of experiments using multiple object tracking tasks suggests that once objects are selected, they can move and selection can be maintained with no additional cost. Finally, we argue that in both location marking and multiple object tracking tasks the key performance and capacity-limiting factor is the spacing among objects.