Attention in Visual Search: Distinguishing Four Causes of a Set-Size Effect

Visual search is a common task both in naturalistic settings and in the laboratory. Outside the labora tory, one might look for a car in a parking lot, a name in text, or a nav igation marker on the horizon. In the laboratory, search is simplified in several ways; commonly, the sub ject views a set of distinct objects and is asked to detect the presence of a particular object (the target) among a set of distractors. Two ex amples are shown in Figure 1. The top two panels illustrate the contrast increment task, in which the target is a disk of high luminance and the dis tractors are disks of lower lumi nance; the bottom two panels illus trate the line bisection task, in which the target is a rotated L and the dis tractors are rotated Ts. One of the most studied aspects of visual search is the effect on per formance of the number of objects, here referred to as the display set size. Display set sizes of 2 and 24 are illustrated in Figure 1. In the top panels, the target pops out?even for a large display set size. More pre cisely, display set size has little or no effect on search time or accuracy when the target is much brighter than the distractors. In contrast, in the bottom panels, finding the target requires scrutiny for the large set size. Display set size has a large ef feet on both search time and accu