Exploring the perceptual causes of search set-size effects in complex scenes

We explored search set-size effects in the context of scenes, where objects are difficult to delineate and define. Observers searched for a tank target in landscape scenes populated by boulder, shrub, and tree distractors, which varied in their color and size similarity to the target. Scenes contained either 75 or 125 distractors. In two experiments we found that adding boulder or shrub distractors that were similar to the target in color and in size produced a consistent set-size effect, but adding distractors that were similar to the target only in size did not. Tree distractors, which were larger than the target, also produced a set-size effect regardless of whether they had a target-similar color (experiment 1) or not (experiment 2). In a third experiment we varied target-tree color similarity and tree color heterogeneity and found no change in search efficiency. We interpret these data as evidence for two independent sources of set-size effects in scenes, one subject to target--distractor similarity relationships (shrubs/boulders) and the other reflecting overall visual clutter (trees), with these latter clutter effects definable by edge content.