How does attention select and track spatially extended objects?: New effects of attentional concentration and amplification

Abstract

Much recent research has demonstrated that attention can be allocated to discrete objects in addition to spatial locations, but relatively little research has explored the allocation of attention within individual uniform objects. While it may be that attention spreads uniformly through relatively small objects, real-world situations (e.g. driving) often involve attending to spatially extended objects, often under conditions of motion and high processing load. Here we explore how attention is used to select and track spatially extended objects in a multiple object tracking (MOT) task. Instead of the punctate objects used in most previous MOT studies, observers had to track of a number of long moving overlapped line segments in a field of identical distractors. At the same time, observers had to respond to sporadic probes, and their probe detection performance is used as a measure of the distribution of attention across the lines. In four experiments we discovered two novel phenomena: First, attention seems to be concentrated at the centers of the lines during tracking, despite their uniformity: probe detection was much more accurate at the centers of the lines than near their endpoints. Second, this ‘center advantage’ grew as the lines became longer: not only did observers get relatively worse near the endpoints, but they became better at the lines' centers — as if attention became more concentrated as the objects became more extended. Both of these effects were unusually large and robust. Additional results suggest that these effects reflect automatic visual processing rather than higher-level strategies. Beyond demonstrating that objects can serve as units of attention, these results begin to show *how* attention is actively allocated to extended objects over time in complex dynamic displays.