The role of object categories in hybrid visual and memory search

In hybrid search, observers search for any of several possible targets in a visual display containing distracting items and, perhaps, a target. Wolfe (2012) found that response times (RTs) in such tasks increased linearly with increases in the number of items in the display. However, RT increased linearly with the log of the number of items in the memory set. In earlier work, all items in the memory set were unique instances (e.g., this apple in this pose). Typical real-world tasks involve more broadly defined sets of stimuli (e.g., any "apple" or, perhaps, "fruit"). The present experiments show how sets or categories of targets are handled in joint visual and memory search. In Experiment 1, searching for a digit among letters was not like searching for targets from a 10-item memory set, though searching for targets from an N-item memory set of arbitrary alphanumeric characters was like searching for targets from an N-item memory set of arbitrary objects. In Experiment 2, observers searched for any instance of N sets or categories held in memory. This hybrid search was harder than search for specific objects. However, memory search remained logarithmic. Experiment 3 illustrates the interaction of visual guidance and memory search when a subset of visual stimuli are drawn from a target category. Furthermore, we outline a conceptual model, supported by our results, defining the core components that would be necessary to support such categorical hybrid searches.