The mechanisms of working memory capacity: Primary memory, secondary memory, and attention control

• We draw a distinction between several popular measures of working memory capacity.
• We relate working memory to primary and secondary memory, and attention control.
• We examine the relationship of working memory to reasoning ability.
• Many working memory tasks strongly reflect attention control.
• The best predictors of fluid intelligence have strong relationships to primary memory.

Working memory capacity is traditionally treated as a unitary construct that can be explained using one cognitive mechanism (e.g., storage, attention control). Several recent studies have, however, demonstrated that multiple mechanisms are needed to explain individual differences in working memory capacity. The present study focuses on three such mechanisms: Maintenance/disengagement in primary memory, retrieval from secondary memory, and attention control. Structural equation modeling reveals that each of these mechanisms is important to explaining individual differences in working memory capacity. Further analyses reveal that the degree to which these mechanisms are apparent may be driven by the type of task used to operationalize working memory capacity. Specifically, complex span (processing and storage) and visual arrays (change detection) performance is strongly related to a person’s attention control, while running memory span (memory for last n items on a list) performance has a relationship to primary memory that is apparent above-and-beyond other working memory tasks. Finally, regardless of the working memory task that is used, it is found that primary and secondary memory fully explain the relationship of working memory capacity to general fluid intelligence.