How Can Dual-Task Working Memory Retention Limits Be Investigated?

Abstract

Resource limitations are assessed by examining performance on concurrent tasks. Pitfalls in attributing dual-task conflicts to central attention rather than specific processing conflicts (Navon, 1984; Norman & Bobrow, 1975) are often underestimated. In this report, we identify a limitation in the interpretation of existing dual-task studies of working memory and describe a dual-task procedure that may be the first to assess concurrentretention costs fairly. Our results support claims of a central capacity (e.g., Baddeley, 2001; Cowan, 2001) and challenge evidence for certain task-specific limits. In dual-task procedures for studying working memory, two stimulus sets are presented in succession, to be retained concurrently, and memory for one or both sets is tested. Concurrentretention costs are decrements in memory performance on one set caused by the need to retain the other set concurrently. Concurrent-retention costs are larger when the two sets share many features (e.g., two spatial arrays or two verbal lists) than when they share few features (e.g., one spatial array and one verbal list; Baddeley & Hitch, 1974; Cocchini, Logie, Delia Sala, MacPherson, & Baddeley, 2002; Fougnie & Marois, 2006). Does this mean that working memory storage is largely domainspecific? Perhaps not. Although the theoretically relevant demand in dual-task studies of working memory is concurrent retention in two tasks, performance also could suffer from the inevitable overlap between retention in one task and encoding or responding in the other task. Set 2 encoding occurs in the presence of Set 1 retention, and responding to whichever set is tested first suffers from concurrent retention of the other set (which, in turn, suffers from response interference). If encoding or responding depends on the same resource as retention, dual-task conflicts are not solely due to the difficulty of concurrent retention. We sought to isolate effects of dual retention without any possible contribution of conflicts with encoding or responding, and to assess whether the effects of dual-task retention depend on intertask similarity. Because we collected only one response per trial, the overlap between responding and retention was eliminated. A more difficult problem was the overlap between Task 2 encoding and Task 1 retention. Our solution was to present four events on every trial in which two stimulus sets were included: Stimulus Set 1; Stimulus Set 2; a postcue to retain Set 1 only, Set 2 only, or both sets; and a memory test on one retained set. With this procedure, demands during encoding and responding are identical on trials with two stimulus sets when the postcue instruction is to retain a single set and when the postcue instruction is to retain both sets.