Mixing it up: Intermixed and blocked visual search tasks produce similar results

We have decades of visual search data from experiments where observers look for targets among distractors. Typically, observers are tested in blocks of several hundred trials, and conclusions about underlying mechanisms are inferred from Reaction Time × Set Size functions and errors. However, in the real world, searchers almost never search for the same target or the same type of target hundreds of times in a row. You search for cereal, then milk, then a bowl. Do the rules derived from blocks of trials apply when search tasks are mixed? Here, we compare mixed and blocked conditions in five experiments. In Experiment 1, four different feature searches are tested. In Experiments 2 and 3, the target was the same in four tasks that were defined by different distractor sets. In Experiment 4, different targets are searched for amongst distractors that remained constant across trials. Finally, in Experiment 5, we allowed participants to choose which of four tasks to perform on each trial. In each experiment, there was no qualitative change in search behavior as a function of the mixed/blocked manipulation. The results support the generality of rules of search learned from blocked trials. However, these results do pose a challenge to simple adaptive models of search termination.