Four experiments examined individual differences in working memory (WM) capacity and how those differences affect performance on retrieval from both primary and secondary memory. The results showed that WM differences appear only in retrieval from primary memory and then only under conditions that lead to interference or response competition within the task. This suggests that WM capacity is important to retrieval that is based on controlled effortful search but not search that is based on automatic activation. A view is presented suggesting that individual differences in attentional resources lead to differences in the ability to inhibit or suppress irrelevant information. The paradigm also allowed more general comparisons between the processes involved in retrieval from primary and secondary memory. As expected, it was found that retrieval from primary memory was a function of set size. However, for sets larger than 2 items, retrieval from secondary memory was independent of set size.

Ongoing debate surrounds the capacity and characteristics of the focus of attention. The present study investigates whether a pattern of larger recency effects and smaller primacy effects reported in previous working memory studies is specific to task conditions used in those studies, or generalizes across manipulations of task-demand. Two experiments varied task-demands by requiring participants to remember lists of letters and to then respond to a subsequent two-item probe by indicating either the item that was presented later in the list (judgment of recency) or the item was presented earlier (judgment of primacy). Analyses tested the prediction that a WM task emphasizing later items in a list (judgment of recency) would encourage exaggerated recency effects and attenuated primacy effects, while a task emphasizing earlier items (judgment of primacy) would encourage exaggerated primacy effects and attenuated recency effects. Behavioral results from two experiments confirmed this prediction. In contrast to past studies, fMRI contrasts revealed no brain regions where activity was significantly altered by the presence of recency items in the probe, for either task condition. However, presence of the primacy item in the probe significantly influenced activity in frontal lobe brain regions linked to active maintenance, but the location and direction of activation changes varied as a function of task instructions. In sum, two experiments demonstrate that the behavioral and neural signatures of WM, specifically related to primacy and recency effects, are dependent on task-demands. Findings are discussed as they inform models of the structure and capacity of WM.