Eye movements were monitored to examine search efficiency and infer how color is mentally represented to guide search for multiple targets. Observers located a single color target very efficiently by fixating colors similar to the target. However, simultaneous search for 2 colors produced a dual-target cost. In addition, as the similarity between the 2 target colors decreased, search efficiency suffered, resulting in more fixations on colors dissimilar to both target colors, which we describe as a "split-target cost." The patterns of fixations provide evidence to the type of mental representations guiding search. When the 2 targets are dissimilar, they are apparently encoded as separate and discrete representations. The fixation patterns for more similar targets can be explained with either 2 discrete target representations or a single, unitary range containing the target colors as well as the colors between them in color space.

With the use of X-ray images, performance in the simultaneous search for two target categories was compared with performance in two independent searches, one for each category. In all cases, displays contained one target at most. Dual-target search, for both categories simultaneously, produced a cost in accuracy, although the magnitude of this dual-target cost was affected by the nature of the targets. When target feature sets shared values, accuracy in dual-target search was equivalent to that in the less accurate of the two single-target searches. However, when targets comprised different feature sets, accuracy in dual-target search was lower than in either single-target search. These results held after practice. In conclusion, dual-target search performance depends on the target representations required for search. When combined representations contain conflicting values within the most informative feature dimensions, then there is a cost in performance. When target representations share features, the search can be guided by the common values so that resources are not wasted on irrelevant distractors. The implication is that security screener performance might be improved by specializing in searching for threat categories that share features. (PsycINFO Database Record (c) 2009 APA, all rights reserved).

The cost of searching for two visual targets simultaneously was compared against two separate single-target searches using exposure time and accuracy measures within a staircase procedure. Dualtarget search for all stimuli (colour, shape and orientation) exhibited a loss of accuracy for one target. For orientation and shape, this dual-target cost in accuracy was extreme, with chance-level performance on one target.

Previous research has shown that during visual search tasks target prevalence (the proportion of trials in which a target appears) influences both the probability that a target will be detected, and the speed at which participants will quit searching and provide an 'absent' response. When prevalence is low (e.g., target presented on 2 % of trials), participants are less likely to detect the target than when prevalence is higher (e.g., 50 % of trials). In the present set of experiments, we examined perceptual failures to detect low prevalence targets in visual search. We used a relative prevalence search task in order to be able to present an overall 50 % target prevalence and thereby prevent the results being accounted for by early quitting behavior. Participants searched for two targets, one of which appeared on 45 % of trials and another that appeared on 5 % of trials, leaving overall target prevalence at 50 %. In the first experiment, participants searched for two dissimilar targets; in the second experiment, participants searched for two similar targets. Overall, the results supported the notion that a reduction in prevalence primarily influenced perceptual failures of identification, rather than of selection. Together, these experiments add to a growing body of research exploring how and why observers fail to detect low prevalence targets, especially in real-world tasks in which some targets are more likely to appear than others.

Searching for two targets simultaneously is often less efficient than conducting two separate searches. Eye movements were tracked to understand this dual-target cost. Findings are discussed in the context of security screening. In both single-target and dual-target search, displays contained one target at most. Stimuli were abstract shapes modelled after guns and other threat items. With these targets and distractors, colour information helped more in guiding search than shape information. When the two targets had different colours, distractors with colours different from either target were fixated more often in dual-target search than in single-target searches. Thus a dual-target cost arose from a reduction in colour selectivity, reflecting limitations in the ability to represent two target features simultaneously and use them to guide search. Because of these limitations, performance in security searches may improve if each image is searched by two screeners, each specializing in a different category of threat item.