Effects of display heterogeneity on visual search efficiency are well documented (Duncan & Humphreys, 1989). Even when searching a clearly distinguishable feature singleton, attentional salience falls down with heterogeneity of distractors (e.g., Santhi & Reeves, 2004). It is presumed that the visual system is able to preattentively separate heterogeneous features to homogenous subsets and attend each subset serially to find a singleton. The issue we addressed in our study was as follows: How does the visual system process heterogeneous sets that can’t be clearly distinguished? Theoretically, it should conjoin all heterogeneous items under the same subset representation and a singleton, therefore, would become more salient despite large heterogeneity. In our visual search task observers searched for an odd-sized target (either small, or large) among 13, 25, or 37 differently sized items. There were two homogenous conditions: (1) all distractors were of medium or (2) opposite size (e.g., large distractors with small targets and vice versa). Above, two heterogenous conditions were tested. In one such conditions all distractors were of (3) medium and opposite sizes (the difference between medium and each opposite size were clearly distinguishable ). Finally, in condition (4) four transition sizes filled the gap between medium and opposite distractors providing six heterogeneous sizes. We found in the result near parallel pattern of search performance in all positive conditions. The fastest detection was predictably found for homogenous displays with opposite sizes. The slowest detection was found for two distinct sizes of distractors. The intermediate efficiency was found for both medium homogenous and heterogeneous sets with transition sizes. RTs were substantially the same in these two conditions. This suggests that the visual system does fail to separate such transitional sets to subsets and treat them as a unitary perceptual entity opposing to a singleton (despite large heterogeneity and wide range of differences).
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.