Constraints of attention, stimulus modality, and feature similarity in working memory

Two of the most important concepts in working memory that differentiate many theories are the role of attention and similarity between items. Investigators have debated whether there is a central, general resource of attention, and whether interference between items depends mostly on their modality and type of code (i.e., verbal/acoustic versus visual/spatial coding) or upon multiple dimensions of feature similarity even within a modality. Here, we examine results from three experiments in which the features of items to be remembered differed for visual objects in color or orientation, or for acoustic objects in noise duration or tone pitch. There were one or two of these sets on a trial and, when there were two sets, the similarity between their features varied: there were sets in different modalities, sets with different feature types within a modality, or sets of the same feature type. One-set trials consistently produced superior performance. For two-set trials, dissimilarity of the sets mattered only when both sets had to be attended, compared with attention to only one set. Feature differences within a modality mattered at least as much as between-modality differences. The findings conflict with what would be expected if modality were the sole organizing principle and support a working memory model in which a capacity-limited attention is constrained by the feature similarity of task-relevant items.