The detail is in the difficulty: Challenging search facilitates rich incidental object encoding

When searching for objects in the environment, observers necessarily encounter other, nontarget, objects. Despite their irrelevance for search, observers often incidentally encode the details of these objects, an effect that is exaggerated as the search task becomes more challenging. Although it is well established that searchers create incidental memories for targets, less is known about the fidelity with which nontargets are remembered. Do observers store richly detailed representations of nontargets, or are these memories characterized by gist-level detail, containing only the information necessary to reject the item as a nontarget? We addressed this question across two experiments in which observers completed multiple-target (one to four potential targets) searches, followed by surprise alternative forced-choice (AFC) recognition tests for all encountered objects. To assess the detail of incidentally stored memories, we used similarity rankings derived from multidimensional scaling to manipulate the perceptual similarity across objects in 4-AFC (Experiment 1a) and 16-AFC (Experiments 1b and 2) tests. Replicating prior work, observers recognized more nontarget objects encountered during challenging, relative to easier, searches. More importantly, AFC results revealed that observers stored more than gist-level detail: When search objects were not recognized, observers systematically chose lures with higher perceptual similarity, reflecting partial encoding of the search object's perceptual features. Further, similarity effects increased with search difficulty, revealing that incidental memories for visual search objects are sharpened when the search task requires greater attentional processing.