Colors, characters, locations, and shapes: The capacity of working memory for multiple, dissimilar sets of items.

Working memory (WM) often includes heterogenous items, as when one uses it while assembling a desk from sets of boards, knobs, bolts, and washers. Here, we investigate how WM capacity is limited when recalling multiple sets of items, for which performance surpasses the usual limits observed in single-set procedures. We presented participants (N = 181) with up to four sets of items for serial recall, usually of different stimulus types in the same trial (colors, characters, locations, and/or shapes). Conditions differed in the total number of items, the number of sets, and/or item types across sets in a trial. For uniformity in analyses, Set 1 was kept constant at three items of a type and was usually recalled first, free of output interference. In Experiment 1, recall of Set 1 was not only limited by the total number of items but also by the number of sets in a trial. Experiment 2 ruled out interference as an alternative explanation. Experiments 3-4 showed the dependency of the results on clearly grouped presentation of the sets. The results suggest that groups of items are associated as newly formed, often incomplete chunks offloaded from the focus of attention (FoA) to an activated portion of long-term memory (aLTM) for later retrieval. This offloading process would spare capacity but not without cost; a fraction of an item was lost from Set 1 for each subsequent item recalled. We present a dual-stage theory in which pointers held in the capacity-limited FoA allow retrieval of chunks from aLTM.