Visual search with negative slopes: the statistical power of numerosity guides attention

Four experiments were performed to examine the hypothesis that abstract, nonspatial, statistical representations of object numerosity can be used for attentional guidance in a feature search task. Participants searched for an odd-colored target among distractors of one, two, or three other colors. An enduring advantage of large over small sets (i.e., negative slopes of search functions) was found, and this advantage grew with the number of colored subsets among distractors. The results of Experiments 1 and 2 showed that the negative slopes cannot be ascribed to the spatial grouping between distractors but can be partially explained by the spatial density of the visual sets. Hence, it appears that observers relied on numerosity of subsets to guide attention. Experiments 3a and 3b tested the processes within and between color subsets of distractors more precisely. It was found that the visual system collects numerosity statistics that can be used for guidance within each subset independently. However, each subset representation should be serially selected by attention. As attention shifts from one subset to another, the "statistical power" effects from every single subset are accumulated to provide a more pronounced negative slope.