Search for two categories of target produces fewer fixations to target-color items.

Searching simultaneously for metal threats (guns and knives) and improvised explosive devices (IEDs) in X-ray images is less effective than 2 independent single-target searches, 1 for metal threats and 1 for IEDs. The goals of this study were to (a) replicate this dual-target cost for categorical targets and to determine whether the cost remains when X-ray images overlap, (b) determine the role of attentional guidance in this dual-target cost by measuring eye movements, and (c) determine the effect of practice on guidance. Untrained participants conducted 5,376 trials of visual search of X-ray images, each specializing in single-target search for metal threats, single-target search for IEDs, or dual-target search for both. In dual-target search, only 1 target (metal threat or IED) at most appeared on any 1 trial. Eye movements, response time, and accuracy were compared across single-target and dual-target searches. Results showed a dual-target cost in response time, accuracy, and guidance, with fewer fixations to target-color objects and disproportionately more to non-target-color objects, compared with single-target search. Such reduction in guidance explains why targets are missed in dual-target search, which was particularly noticeable when objects overlapped. After extensive practice, accuracy, response time, and guidance remained better in single-target search than in dual-target search. The results indicate that, when 2 different target representations are required for search, both representations cannot be maintained as accurately as in separate single-target searches. They suggest that baggage X-ray security screeners should specialize in one type of threat, or be trained to conduct 2 independent searches, 1 for each threat item.